Question about variable-pitch propeller aircraft & slow flight

[steve181]

This question is specific for a normally aspirated Cessna 172RG with variable-pitch prop.

First & foremost I know the aircraft flight manual will shed some light on this but I wont get a chance to have a good read of it until next week, so I'm just after a generalized answer until then.

Say I have pax on board & we are sight seeing over a small city. I want to fly at a slow speed suitable for this situation, I would even put on a stage of flap.

What is better? Reducing the manifold pressure then bringing the pitch control to full fine (Just like a finals configuration) & hence treat the aircraft as fixed pitch (throttle controls RPM etc) OR find the MP/RPM configuration suitable for the purpose of sightseeing at slow speed given the aircraft weight etc, for example 20"/2050RPM or 2100"/2150RPM etc etc.

The first option I think would be more suitable, especially if the turns are steep enough to require more power in the bank. Imagine doing a 45 degree turn with the second option, I'd need to increase the RPM then increase the MP & vice versa once I've leveled off, so much work compared to the first option!

[toolowtoofast]

first, if you're low and slow, don't be doing too many steep turns. will be uncomfortable for your sightseeing pax (is a very quick way to make them feel queasy), and there's not enough fat in the airspeed/stall equation, especially as you're obviously not overly experienced in the aircraft. very easy to get into a stall in the turn situation. personally i'd be setting 2200rpm, whatever MAP it takes to maintain 85-90kts, maybe even 10 deg flap if you're that way inclined (will give better forward vision at least) and keep the bank angle down to 30deg max.

my 2c........

[steve181]

Thanks for the replies VK2TVK & toolowtoofast. I agree about the not getting too stressed about the RPM in this context, that compliments what I've learned so far more or less.

Yes more than one or two 45 degree turns would be pushing it pax comfort wise so I wouldn't get too carried away with those. I would be flying 1500 feet above sea level so worst case scenario I would be a safe height to recover from the onset of a stall. :)

[ReverseFlight]

Looking at the Arrow POH, at 2000' P/A, you'll get 110 hp (55% rated) with either of these settings:
2100 rpm / 22.4" MAP
2400 rpm / 20.0" MAP
I've even seen guys do 2200 rpm / 19" MAP but that's just as fine.

I agree that you should avoid steep turns (includes 45 degrees AoB). Even with a low wing on a scenic flight (plank obstructing views), you should not bank any more than 15 degrees AoB in slow flight. You don't want to stall as you are likely to experience wingdrop - how current are your spin techniques ? You will appreciate why we practice HASELL checks and terminate practice recoveries at minimum of 3000' AGL.

[Captain Nomad]

Have you ever stalled out of a turn before? :suspect: You might find you would have preferred a bit more than 1,500ft... When you did your stalling lessons you would have/should have been over twice that high.

I think you will find that with a variable pitch prop, people usually leave the RPM set at the cruise setting and then just adjust MP accordingly to achieve the speed/performance required (adding power through a turn usually not a problem).

If you are hiring from a Flying School, have a chat to the instructors/CFI on the preferred methods of operation for their aircraft and of course as you have said, the Operating handbook is the bible document. Be careful to abide by any special requirements such as time limits associated with certain RPM ranges (some aircraft have such limitations - for example a small range where long term operation in that range can have damaging vibration consequences).

[steve181]

Do you guy's think a 45 degree bank at full power & at 1500 feet AMSL with pax briefed about the turn is reckless?

[Joker 10]

In one word YES reckless

[steve181]

On the very few occasions I have done a 45 degree banking turn on a sight seeing flight at 1500 feet it hasn't been a 360 degree bank more like a 100 degree turn & at full power. That to me isn't reckless & if peoples safely margins are such that they see it as dangerous then I'm surprised they can even get out of bed in the morning & face the danger's of day to day living.

Years ago my instructor made a sharp bank over his friends house when he spotted it, I can't recall what angle of bank it was but close enough to 45 degrees & around 1500 feet. With stall practice I've always known I need to recover by 2500 feet & not to perform stalls over built up areas etc, same deal with max rate turns etc. I can't recall ever being told to take the same precautions with 45 degree banks.

I've just never looked at them as being potentially dangerous maneuvers like stall practice, max rates etc.

[rcoight]

What are you going to achieve by banking at 45deg instead of limiting your turns to 30deg or less?

You are in a high-wing aircraft on a sight-seeing flight (ie. the view will be fine).

Why would you reduce your margins?
Low speed, low altitude, low experience level, steep turn...
:confused:



Must be a bloody small city if you need to bank at 45deg to stay over it!!

[knox]

"Do you guy's think a 45 degree bank at full power & at 1500 feet AMSL with pax briefed about the turn is reckless?"

IMHO - no, not reckless.
What about a 40+ deg AOB, 360 deg turn (any where between 5 - 10 consecutive times) at 500ft.... is that reckless? I'd still have to say no.
However this is all dependent on level of pilot experience.

Knox.

[steve181]

Don't get me wrong I don't make a habit of performing 45 degree turns at any stage of or on any type of flight given they're not the most gentle way of banking an aircraft. I more or less mentioned the 45 degree bank for arguments sake with relation to my question about MP & RPM.

I felt like a kamikaze pilot with some of the replies I've gotten here & I pride myself on being the best/safest pilot I can be so was surprised to hear a 45 degree bank through 100 or so degrees at 1500 feet is seen by some as being dangerous.

I can bet some of you guy's imagine me flying over a city with pax & strictly performing 45 degree turns, that's not how it is at all, as mentioned I have made the odd 45 degree bank through 100 degrees or so on the odd occasion, but who hasn't right?

rcoight I agree with you about the margins thing but where do we draw the line? Only having Cessna's taking off on 4km runways because their safer than 500m ones? Obviously that's at the extreme end of the scale but you can see what I mean.

[Captain Nomad]

Do you guy's think a 45 degree bank at full power & at 1500 feet AMSL with pax briefed about the turn is reckless?
No, not necessarily reckless. But if mishandled/unbalanced and with the idea that I would be flying 1500 feet above sea level so worst case scenario I would be a safe height to recover from the onset of a stall. is what concerned me...

[das Uber Soldat]

Its not necessarily 'reckless'.

More 'pointless'. Why do you need to turn so steeply? Passengers in general won't enjoy it and you're erroding safety margins for no reason. What kind of ground feature requires such a steep angle of bank?

45 deg = 1.41g = Vs + 20%

If you're sightseeing, you've probably slowed it down to help people see things. You've eroded this safety margin by a further 20%. This might be fine when everything works perfectly. What if something doesn't? You overbank slightly, you get a decent gust. What if you need to turn more steeply due reason x?

In my personal opinion, decision making is about maximizing options, and by doing something like this, you are reducing them for no reason at all.

I reiterate though that I wouldn't call it 'reckless'. Just unnecessary. All the above is my personal opinion only.

[steve181]

I don't know what planes you fly Captain Nomad but I've never stalled or come close to stalling a plane doing a 45 degree bank. Let's say in the unlikely event I'm doing a 45 degree bank the stall warning chirps & the plane feels like it's on the edge of a stall... ease off pressure on the control column now I'm out of the near stall roll wings level. Maybe I lost 100 feet or so, what a rebel & a danger to society I am!

[Captain Nomad]

I've never stalled or come close to stalling a plane doing a 45 degree bank Hence that's why you were happy with 1,500ft for recovery? :E

Chill mate, I'm sure you can probably handle a 45 turn at 1,500ft fine. BUT, in a past life I have seen the shock on people's faces as they experienced practicing stalls out of turns and the little warning and vicious onset of the stall itself - it is NOT something you want to do at 1,500ft especially if the outer wing stalls... I have also had a previous life maneouvering at those angles of bank regularly around intimidating terrain and wx and in a loaded aeroplane it can become very uncomfortable VERY quickly...

[ForkTailedDrKiller]

Do you guy's think a 45 degree bank at full power & at 1500 feet AMSL with pax briefed about the turn is reckless?

In one word YES reckless

Oh dear!

Dr :8

[rmcdonal]

45deg turn with a high wing doing a scenic? Now all they get is a pic of the wing not the ground. Having flown around a bit at 1000-1500ft on scenics I can recommend S+L for just about everything.
As far as stalling in a turn... having done a bit of instructing I have deliberately stalled many times in turns (power on-off-flap-no flap) I would go with steve181 ease off pressure on the control column now I'm out of the near stall roll wings level.

:ok:

[sixtiesrelic]

OK theory of flight covered!
How are you paying ? Tacho hour or hour meter?
If it's tacho; it most probably divides the revs by some number like 2400 and registers it as minutes
OK guys ... yes they're in tenths of an hour.
What I'm getting at is; fly for an say, an hour, at 2100 RPM; the tacho says 52minutes while the hour meter comes up with 60 minutes. From start to stop the difference is even greater... low RPM on approach and taxiiing while the clock ticks on relentlessly..
At 200 bucks plus, per hour, you pay over thirty bucks less for the same flight and bung the sixty five or seventy minutes total flight time in the log book.
Cheating ??? NO! The tacho time is what the maintenance is done on and the flying schools get paid by the hirers, for the time the clock runs... probably get paid for 110 hours in the 100 hourly period.
You can open the window in a Cessna and get good clear photos or see clearly in medium turns and aviod the need for forty five degree turns.

[VH-XXX]

I've never stalled or come close to stalling a plane doing a 45 degree bank

I love that comment and I used to make similar ones myself. How would you know that? I guess you only find out when it's too late.

FYI - Stalling is not an aerobatic maneouvre and you DO NOT have to be at 3,000ft to do it.

[steve181]

Oh I completely agree that 45 degree turns in a high wing aircraft on a sight seeing flight is not necessary (wing might block the sight, less comfortable for pax etc etc). The handful of times I have performed one in this circumstance was more so the passengers could experience a G or 2 as opposed to performing the turn for the purpose of viewing sights on the ground.

In hindsight if I knew my original comment about the 45 degree turn was going to be the focus of replies then I would have said a 30 degree turn just as an example of how an increase in power would be needed to perform the turn if going from slow flight re: juggling the throttle & pitch lever.

[steve181]

Remember VH-XXX stalls are not the silent assassin, you get many clues one is about to happen & therefore the opportunity to avert one developing. ;)

[Fratemate]

So, away from the aerobatics, aces, and back to the question :ok:

I've never flown sight-seeing trips per se but I do have a fair few hours with twisty props. At the risk of delving into egg-sucking lessons, one of the important things to remember with VP props is not go get 'over-square' or, put another way, have too much MAP for the RPM. To keep it simple in my beer-oddled brain, I always likened it to trying to drive your car up a hill in too high a gear. You can put your foot down all you like but it's still going to judder away and not accelerate.

So, in your scenario, firstly I would reduce the MAP and then increase the RPM (23-2400 sounds about right, having never flown a 172 with VP prop) and then you will have the ability to increase power during your turns, without having to worry about the nasty chugging sound if the RPM was too low.

My time in the military showed me that they always enjoyed their rules-of-thumb and many of these seemed to work very well. It's probably old-hat to many of you but we were always taught to take the first 2 numbers of the RPM, add 2.5 to that and that was the max MAP you should apply. So, RPM of 2400 = max MAP of 26.5in.
Of course, this doesn't apply to all aircraft, e.g. those big-engined things with boost gauges, but it does apply to the vast majority of engines that are found in light aircraft IO-360s etc.

That rule + "Rev up, Throttle back" and that's the end of the VP endo :}

PS: When it does stall in the turn, depending on which way the prop spins, remember the quickest recovery to the horizon might be rolling under....might upset the fare payers though :eek:

[Pilotette]

What is better? Reducing the manifold pressure then bringing the pitch control to full fine (Just like a finals configuration) & hence treat the aircraft as fixed pitch (throttle controls RPM etc)

If you try to do this, remember that the gear warning horn will start to go off and that will be sure to scare your pax more than a 45deg bank turn! :uhoh: Not to mention the fact that you wouldn't want to be in finals configuration while trying to maintain altitude :ok:

[The Green Goblin]

one of the important things to remember with VP props is not go get 'over-square' or, put another way, have too much MAP for the RPM.

Not this old wives tale rearing its head again.

Over to chuck :E

On the very few occasions I have done a 45 degree banking turn on a sight seeing flight at 1500 feet it hasn't been a 360 degree bank more like a 100 degree turn & at full power. That to me isn't reckless & if peoples safely margins are such that they see it as dangerous then I'm surprised they can even get out of bed in the morning & face the danger's of day to day living

No wonder the bungle bins in Kunus used to have so many spew bags at the end of a session :cool:

Don't these city instructors teach Airmanship anymore?

Slow down and a 15 AOB is sufficient for seeing anything that is worth seeing on a scenic! (your passengers may even give you a tip)

[j3pipercub]

stalls are not the silent assassin, you get many clues one is about to happen & therefore the opportunity to avert one developing.

You tell em biggles, I'll even let you borrow my white silk scarf to furl out the window and slap the vertical stab while you fly...

j3

[VH-XXX]

Remember VH-XXX stalls are not the silent assassin, you get many clues one is about to happen & therefore the opportunity to avert one developing.

Remind me to not let you on the controls when I get my new Lancair!



What do you mean VH-XXX?


That means I don't agree about your stall philosophy! Enough said.

[steve181]

Lets take away what I've said because some people are missing the point of my original question. :ok:

I'm cruising (24"/2450RPM, which is the control configuration for my aircraft) in the middle of nowhere at 3000 feet flying solo in the same aircraft as described in my first post.

I randomly decide I want to slow the aircraft down in straight & level flight to say 70 knots, from your experience what would you do to achieve this? I would just reduce the manifold pressure with the throttle until I reached 70 knots.

Now I want to do a turn where by I need maximum power (say a max rate turn); should I bring the pitch control to full fine first before I increase the MP to full (full throttle). I think I would need to but what would you guys do from your experience?

Sorry to those who have answered me on my original question, I'm just trying to find a consensus.

[steve181]

What do you mean VH-XXX?

[The Green Goblin]

I'm cruising (24"/2450RPM, which is the control configuration for my aircraft) in the middle of nowhere at 3000 feet flying solo in the same aircraft as described in my first post.

I randomly decide I want to slow the aircraft down in straight & level flight to say 70 knots, from your experience what would you do to achieve this? I would just reduce the manifold pressure with the throttle until I reached 70 knots.

Now I want to do a turn where by I need maximum power (say a max rate turn); should I bring the pitch control to full fine first before I increase the MP to full (full throttle). I think I would need to but what would you guys do from your experience?

Reduce the power until you have the desired Airspeed. (If I'm flying something a tad bigger such as a 520 i'll plan it to reduce an inch a minute or every 500 feet for my own warped ideas on engine management) If I'm straight and level I'll work that on miles per minute traveled so if I'm doing a 140 knot GS that will be around 2.5 miles a minute. If I'm cruising in a single 24" 2400RPM and want to lay off 4 inches Ill be reducing power 10 miles out etc.

If in the turn I need to get out of there quickly (say a potential collision) I'd fire wall it maneuver as required (paying attention to collision avoidance and who has right of way etc) and worry about the pitch later.

Also note that in "most" CSU equipped machines 24" 2400rpm cruise power is more than enough performance for a 45 AOB turn.

I would also talk to your instructor and take what is written on the internet with a pinch of salt (a slice of lemon and a shot of tequila also helps)

[steve181]

Thanks The Green Goblin that's one of the best replies I've got so far. :ok:

Obviously the flight manual will help answer my questions on engine management but as I said it's worthwhile getting information on how others go about it (with a grain of salt) in the mean time. :)

[SM227]

This situation happens every flight in PJE ops, reduce the MAP to around 16-17 inches and bring back the RPM to about 2200RPM, that usually gives you 70-80kts. Increasing the MAP up to 22in will give you more than enough power for a turn, you wont even get that above 7,000ft in a normaly aspirated engine, and its quite common to do a 'wing over' (not really :E ) as the last person jumps out to watch them fall away, thats easily a 60 degree turn at low speed and you would be lucky to loose more than 500ft through the whole thing!

In a Cutlass, I imagine that the configuration given for slow flight (its in the POH somewhere) would give you what you need, im pretty sure that calls for gear down though. Just try it and see what works, but you will be looking at close to the settings listed above.

[snoop doggy dog]

From memory, we use to keep the RPM where it was for cruise, about 2400 rpm, throttled back to about 15-18 inches with the first stage of flap out and this usually gave us about 70 kts (I think :uhoh:.?) and a bit more margin over the stall. This way you could push and pull on the throttle as you needed for power.

That was for mustering and other low level stuff. Just try it out and see if it works mate :ok:. It was a while ago and I'm sure Cessna fly the same as they did 10-15 years ago. We flew C182 and C172 with variable props.

I'm no expert and I'm no egg sucking teacher. Just know the aircrafts', your limitations and all will be safe as you already know. Most of all, enjoy your flight :)

[Howard Hughes]

one of the important things to remember with VP props is not go get 'over-square'
Is that right? :rolleyes:

Better not let all the WWII pilots hear you say that!:eek:

For example the P-51 Mustang has a maximum MP of around 61 inches! I bet the prop is not turning at 6200 RPM...:E

In the days when I flew scenics, I quite often had the RPM 2-300 less than the cruise setting in the 210, made for a very quiet cabin. Can't say I would be recommending it in 172RG though!;)

[toolowtoofast]

any turbo piston will cruise @ 30"/2500rpm all day.

[LeadSled]

Remember VH-XXX stalls are not the silent assassin, you get many clues one is about to happen & therefore the opportunity to avert one developing.

Steve181,
I suggest the following: Get a C-172, as per this discussion, a sightseeing flight, make certain you are loaded close the the aft C.of G., and with some flap extended, and in the aforementioned 45 degree angle of bank, slowly reduce the speed until "something" happens.

It might not "legally" be an aerobatic maneuver, but please don't try the first one at 1500'. Then you will find your statement above is NOT based on training or experience, preferably in that order. Just a hint, I usually went to 5-6000 ft for that exercise.

Suddenly the staid and steady, soggy as a blancmange good old C-172 behaves like a Yak 52 in the same circumstances. Oh! Boy! can it flick over the top fast and into an (incipient) spin, it always comes as a great surprise to the student.

I have had some wonderful reactions from instructors, when I ask them to demonstrate this on a check ride, it gives me a good idea what kind of a pilot (let alone instructor) they really are.

Tootle pip

[Fratemate]

Is that right? :rolleyes:

Better not let all the WWII pilots hear you say that!http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/eek.gif

For example the P-51 Mustang has a maximum MP of around 61 inches! I bet the prop is not turning at 6200 RPM...http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/evil.gif

HH,

Please, if you're going to cast aspersions then at least do me the favour of reading my whole post, instead of selectively quoting. I thought I made it clear about such things when I wrote

Of course, this doesn't apply to all aircraft, e.g. those big-engined things with boost gauges, but it does apply to the vast majority of engines that are found in light aircraft IO-360s etc.

I've never been lucky enough to sit in the front but I have experienced, from the back seat, the operation of BIG engines in the Sea Fury and Firefly, so I'm not completely dim about such things (many other things though, that is true).

And it is very true to say that you CAN have RPM low and, in a normally aspirated engine this is arguably the most efficient way to fly it but that's in the cruise, whereas I suggest you don't want really want to be flying around slow speed in this configuration 'cos when you need the engine to respond to your firewalling the throttle it ain't gonna happen.

Not this old wives tale rearing its head again.

I apologise, GG, if this has been done to death before. I wasn't aware it had but I do appreciate how tedious some of these threads get when someone trumpets stuff that has already been covered a gazillion times.

I randomly decide I want to slow the aircraft down in straight & level flight to say 70 knots, from your experience what would you do to achieve this? I would just reduce the manifold pressure with the throttle until I reached 70 knots.

Now I want to do a turn where by I need maximum power (say a max rate turn); should I bring the pitch control to full fine first before I increase the MP to full (full throttle). I think I would need to but what would you guys do from your experience?

This is what I was trying to address when I blahed on about 'rev up, throttle back'. If you want to increase power, for instance, to climb, you first increase revs i.e. increase RPM and then increase MAP (so you rev up power) and then vica versa, i.e. to reduce power you first reduce MAP and then reduce the RPM (throttle back power). So, I would suggest you're almost there with what you wrote, however, I can certainly see no need to go full fine for max rate turns, in fact any manoeuvre really apart from a go-around. In most of the aerobatic aircraft I've flown the recommendation of between 2400-2600 RPM has allowed full throttle to be used throughout the engines' envelopes and I would think the same to be true of yours.

I enjoy mucking around on sunny days in light aircraft and boring my kids with how many vertical rolls (up) we can squeeze in before running out of ideas and becoming a brick but, despite my teachers being very capable and knowledgeable chaps, I think it's best to keep it simple and stick to jet engines :ok:

[trimotor]

Minor point to start -the prop on the 172 you mention is not 'variable pitch', it's actually 'constant speed' - there is a notable difference between the two, with variable pitch not havig been used (to my knowledge - go on someone, correct me) since during WWII. Anyway, moving right along:

As has been done to death already, what you are proposing is all ok in theory -a bit of flap, slow-ish speed and some bank for visibility. All you wanted to know what how much MAP and RPM would be a good idea.

First, I'd probably not bank excessively - the poor old 172 ain't travelling that fast that high angles of bank are required to required for a good look as you sail past those on the ground who are probably awaiting your arrival.

On that basis, high power settings should not be required. Regardless, you expose yourself to the possibility, at higher angles, of the chance of some stall symptoms: here's where a little bit of covering one's backside might help if you do plan high angles and flap usage- have the prop in 'full increase' so that full power is available so you can do the pukka stall recovery should you need.

The 'need' might be greater than you think: one danger of orbitting around a ground feature at low level (though 1500ft is not THAT low), is that you tend to be looking at it too much (target fixation?), the result being bank angles greater than intended, perhaps an unitentional descent (which would actually help), possibly decaying airspeed and the effect on your ground track in strong wind conditions (perhaps causing you to pull a little tighter to achieve the desired ground track, wih the effects as previously discussed). The attention to the task, rather than the pure flight path control can also add to the distraction, and result in not being so sensitive to to the symptoms of stall onset.

So, protect yourself from yourself and avoid low level aerobatics with pax: set the rpm so power is available, put the boost pump on, do HASSELL checks (they are not just a routine for aerobatics), fly the aeroplane first and demostrate your professionalism not your daring. Try it without flap (keep it simple) -I can't think of a time, in a clear area, where the turn radius was so m uch better with a little flap, compared to clean, or that the slower speed (between what 90 kias with 10° and 110kias clean?) made a jot of practical difference in the time over the target..

TM

If you were thinking of using a lot of bank to show the 172 off I wouldn't bother -if you don't have smoke you need noise; unfortunately the 172, great as it is, has neither!

[Polymer Fox]

At the moment I'm building my command time for my CPL in a C72RG.

Over the weekend I was doing the Harbour Scenic one with me and two friends (220kg) and about 180 litres of fuel on board. 20"/2300RPM with one stage of flap was sufficient to stay straight and level and slow enough to have a nice look around during the orbits and on the transit to and from the bridge. I didn't pull more than 20 degrees of bank as I wanted to give my friends a chance to enjoy the view and to not have them worrying about what the plane was doing. The controls were sloppy enough at that speed and power setting as it was so I don't think it would have been wise to go and push the envelope.

[Sunfish]

Steve, you do have a constant speed propeller endorsement don't you?

[The Green Goblin]

Sunny, I've never heard of a constant speed propeller endorsement. Can you point me in the direction of where to find it in the regs?

[HarleyD]

Oh My Goodness,,,

I would never have thought that such an up front and seemingly sinscere question could possibly bring such a load of absolute nonsense from the self informed, which represents many, though certainly not all of those posting in response.

Steve 181, reduce both RPM and MAP, keep them about square, if anything RPM slightly below MAP, this is much better for the engine. Fratemate is totally wrong about using high RPM all the time, there is no good reason to do this other than ignorance of how the engine works (which is actually not a good reason at all) sorry fratemate but you are wrong as has been pointed out already. I have flown some of those high performance engines as well as many hours on Turbo-charged and many more on normally aspirated piston engines from A65’s to IO720’s and there is no imperative to keep the engine oversquare, otherwise you could not take off in a normally aspirated aircraft with a fixed pitch prop!! Though ,of course, the absence of a MAP gauge eliminates any concern in that instance. Low rpm will also make the flight cheaper if you are paying by tacho. Standard engine handling procedures of the correct use and sequence of cowl flap mixture, pitch and throttle should be observed. I am not sure of why the philosophy of keeping RPM at max continuous just in case some MAP is needed, and keeping the cowl flaps open for the same reason is still espoused by some who should know better if their self promoted experience levels are to be believed.

Angle of bank won’t necessarily kill you and 45 deg is not a huge increase in Vs, however, as has been noted in earlier posting, you will not need, nor is recommended such bank angles for a scenic jaunt. Radius of turn is dependent upon speed and AoB, in a directly proportional relationship, therefore you will require less AoB at low speed, 15 to 20 will be heaps for casual punters to get an excellent view from a high winged platform. Shallow bank angles at low altitude, where bank is sometimes held off and the turn promoted with yaw/rudder because some pilots don’t like steeper turns when close to the ground is a significantly more lethal and can readily promote a loss of control situation with the controls set just right for having a bad day. “see him crash, see him burn, he held off bank in a gliding turn”, you should have learned at your CFI’s knee. Keeping the aircraft in balance is the most important aspect of any such turns. Ag pilots use out of balance turning and deliberately make gross yaw induced wings level turns at a couple of feet, but you sure aren’t one of them. At 1500 ft for sightseeing purposes when just popping along at reduced power/speed shallow AoB balanced turns are the go especially if it’s a warm and bumpy day and the punters are a bit new to it ( I was going to say a bit green, but that’s not exactly what I had in mind).

Have fun mate, I think you will do well.

Polymer Fox, you are not pushing the envelope, this has been well and truly done for you during certification, even if to very old and outdated standards. If you operate the aircraft within the limitations specified in the AFM/POH the aircraft will “behave in a conventional manner” and will not need “excessive force or exceptional piloting skills” it’s a Cessna 172 FFS. Read the book, get checked out by a reputable school that has senior instructors for this exact purpose and then do as you have been shown.

HD

[Fratemate]

HarleyD,

I'm genuinely interested in one of your comments and would like, for my own education, to take advantage of your experience in the matter. However, let me just clear up a couple of things before getting to that.

First of all, I am only talking about normally aspirated engines here. I thought I had made that clear in one of my posts but this obviously isn't the case. I have no claim to be a whizz on turbo-charged engines because my only experience on those is the Seneca II 200T....and the less said about those, the better :)

keep them about square, if anything RPM slightly below MAP, this is much better for the engine

Again, we're essentially talking the same thing here. Did I not say something about the first two digits of RPM + 2.5? Therefore RPM, say 2300 would be MAP 25.5.

Fratemate is totally wrong about using high RPM all the time

I didn't say use high RPM all the time. I suggested he increased the RPM, once at a slower speed, in order that he had better engine response when he needed it during his manoeuvering. Again, I typed some numbers but qualified that I did not know the exact ones as I'd never flown that type. The 2400-2600 numbers I quoted were for aerobatics and were the figures from the POH for those aircraft/engine mixes that I flew. When touring around in an A36 or whatever, I'm never near those sort of numbers. My engine knowledge is reasonable, not expert, but I wouldn't say I'm 'ignorant' about engines as I didn't say what you claimed I did.

there is no imperative to keep the engine oversquare, you could not take off in a normally aspirated aircraft with a fixed pitch prop

In a normally aspirated engine you'll never be able to develop that high a MAP, so "over-boosting" (note the quote marks) is not a problem.

So, having got the semantics out of the way, over to my question. You said:

there is no imperative to keep the engine oversquare.

Now, I don't know whether it was a typing error, but I am suggesting you DON'T want to over-square the engine, agreeing with your statement. Just to make sure we're both talking about the same thing; I think over-square is when the MAP number is greater than the RPM number and I have always been taught not make that difference greater than 2.5 (on normally aspirated, 'normal' sized engines that don't have Merlin written on the side).

If there was a typing error then you are suggesting that over-squaring the engine is okay and, in this, you're in agreement with Green Goblin. Now, just one more time for the record, I accept that over-squaring is okay to a limit (+2.5) but why does Green Goblin call this an old chestnut and why do you think it's okay to manoeuvre (not cruise) with an engine more over-square than this. I'm not sniping, it's a genuine question.

Cheers,

Fratemate

[The Green Goblin]

From memory the PA31's limit was 46" 2550 on Takeoff and you wound the props back to 2400rpm and the MP to 38" on climb 31" 2200 rpm cruise.

What about little RA engines? they run some pretty decent MP compared to RPM :)

There is nothing wrong with running an engine 'oversquare' in fact if you look in the POH under different cruise configurations it is recommended.

Now lets just hope someone doesn't add another lever in to the equation like the mixture and the ROP/LOP debate :cool:

[c100driver]

1 You can never "over boost" a normally aspirated aero engine, because it is not supercharged or turbocharged and can never physically exceed the pressure of the day. The term "boost" is correctly only ever used for piston engines capable of greater than atmospheric in the induction.
Unless your engine is capable of at least 3000 RPM then you will be running "over-square" at every takeoff.
It is possible to exceed recommended MP v RPM settings set out in the aircraft flight manual, but not advisable to do so.

TCM has just issued a service bulletin for IO520 engines not to exceed +4" MP v RPM (i.e. 26" v 2200 RPM) for long periods due to reported service difficulty reports of premature engine failure in a number of engines.

[FGD135]

The "oversquare" thing is a complete and utter MYTH. Can we please drop it now?

On the subject of myths and constant speed propellers, another one you will hear frequently (when you get onto twins) - especially when approaching an aerodrome, is:

"Push the props up - it'll help you slow down".

What they are saying is that, by increasing prop RPM, somehow you will increase the drag - and that will assist you to slow down.

This is total rubbish but is surprisingly deeply entrenched. In my experience so far on twins, it is a rarity to NOT hear somebody espouse it.

The truth is that, increasing RPM could help you slow down - provided it causes the torque to go negative. The same thing can be achieved by pulling back the throttles (or power levers) until you get negative torque (also known as "wind milling" and sometimes "backdriving").

steve181, you said

bringing the pitch control to full fine (Just like a finals configuration) & hence treat the aircraft as fixed pitch (throttle controls RPM etc)

Perhaps I have misunderstood you, but it sounds like you think that the constant speed prop will behave like a fixed pitch prop when max. RPM has been selected.

That is not true. At max RPM, the governor will still be operating to vary the blade angle (dependent on how much torque you have, of course). The blade angle will only be at "full fine" for low power values.

If you are just beginning on constant speed props you may be interested to know that their purpose is much more about engine efficiency than propeller efficiency.

[Atlas Shrugged]

Now lets just hope someone doesn't add another lever in to the equation like the mixture and the ROP/LOP debate

Can we have the debate on stalling instead? :ugh:

[Brian Abraham]

A picture is worth a thousand words.

http://i101.photobucket.com/albums/m56/babraham227/l0009.jpg

This is for the IO-720. You can see that at sea level with 1,800 RPM the limiting MAP is 25", and at full throttle you can reduce RPM from the max of 2,650 down to 2,300. As a pilot you need easily memorised figures, or simple tabulated charts, to use without resorting to the sort of graphs we see here.

[The Green Goblin]

What I wouldn't give to have spent a bit of time with a couple of 720's lighting the fire! (Almost better than a Garrett :cool:)

[ZEEBEE]

Coming back to the stalling debate;

Even though a 172 has reasonably benign stalling characteristics, it WILL bite if pushed.
however, long before that, there should be some serious warning bells going off to indicate that there's a situation occurring.

Even if we discount the stall warning starting to scream, few people understand that a fixed wing aircraft will NOT stall unaccelerated unless there is a significant amount of rear stick applied..AT ANY SPEED.
Therefore, if you are in a turn of any angle and are pulling a bunch of back stick then the situation is getting sticky and you're going to have to stop doing it or the scenery is going to take a turn for the worse.
Applying power at this late stage is not a lot of good in an anemic thing like a 172, as the backside of the power curve is fast upon you.

So, as has been said, use moderate power and enjoy.... Take note that if you're starting to have to pull back on the stick to maintain height, then the prudent thing is to roll off a bit of bank rather than pull back some more.

Any pilot should be able to execute a 45 deg banked turn in a 172 safely at 1500ft or they shouldn't be flying . := Not reckless at all.

[glekichi]

ZEEBEE,

I need to add to your statement.

It should read:

"Any commercial pilot should be able to execute a 60 deg banked turn in a 172 safely at 500ft or they shouldn't be flying ."

KiwiHater (GG),

Constant speed units ARE referred to (albeit briefly!) in the NZ regs. :}
How many aircraft have you flown that ACTUALLY have manual propeller pitch control???? (As per our out-of-date regulations here!)

[Brian Abraham]

TGG, you need to get hold of a modded Queen Air or Commander. :)
ZEEBEE has it baggeed. Any aircraft will bite if mishandled. Before we went to the aircraft carrier we were shown a movie of f*ck ups, and a majority of them were stalls and resulting spins on the approach turn. Read the crash comics and NTSB reports and approach turn stall/spins feature with monotonous regularity. As has been said a scenic flight in a 172 need minimal bank.

[The Green Goblin]

KiwiHater (GG),

Constant speed units ARE referred to (albeit briefly!) in the NZ regs.
How many aircraft have you flown that ACTUALLY have manual propeller pitch control???? (As per our out-of-date regulations here!)

My licence is endorsed Constant Speed Unit or CSU. If I bothered to pay CASA for a new licence it would be endorsed as Manual Propeller Pitch Control.

Our out of date regulations were changed a couple of years ago to incorporate the two classes of CSU that now exist since the introduction of aircraft such as the Cirrus which has Automatic Pitch control. Some time ago (around 2006 I believe) a Cirrus Pilot issued a mayday who did not understand the Automatic Pitch Control System and believed the throttle was jammed at cruise power. Upon landing and closing the throttle he believed the throttle had become 'unstuck'. After reflection and subsequent retraining with the particular flying school he did feel mighty silly! (and hence the need for separate design distinctions in the regulations)

Perhaps NZ should change their modern regulations to become 'out of date' too so they can satisfy this anomaly :cool:

Oh...... and if you read my original post - I was referring to a Constant Speed Propeller Endorsement (which does not exist, at least in Australia) :E

[glekichi]

Oh what, so this MPPC thingy they've stuck on my licence isn't for those really old planes where you set the pitch directly? God damn killjoy you are! :}

Well you wouldn't have that problem in NZ anyway coz they use the even more out of date system of using a type rating for each plane, even for singles <5700 :eek:

Jokes aside, and while it is a pain in the arse at times, I still don't think its a bad idea. The MAUW handling part, especially.

[Polymer Fox]

For all of those who are talking about more advanced aircraft types, it is lovely that you are so smart and experienced that you can fly such lovely aircraft but it only muddies the water for someone asking a question for a simpler aircraft type.

The same can not be said for a turbo aircraft as for an NA type.

From the POH (notice the absence of 40" of MP..... or over 25"?):

http://williams.best.vwh.net/172RG_POH/5-17.jpg


You're going to be going mightly slowly and wasting a lot of people's time if you're running the engine with MP over the RPM @ 12000ft:

http://williams.best.vwh.net/172RG_POH/5-22.jpg

[FRQ Charlie Bravo]

Over-square schmover-square, hogwash. Take one legitimate meter (Revs per minute) divided by 100 then match it up so some antiquated standard (like inches of friggin' mercury) then try to make a rule-of-thumb like keep one bigger than the other. OK for Johnny McStudent on his first three CSU flights but poppycock for this discussion.

Get yourself into a Chinese or Russian bird (or a bloody US built Cessna in Vladivostok) and try to tell me what a "square" setting is... 2300 rpm and 750 hPa?:yuk:

Flight manuals rule-OK! After Goodies of course.

Sorry that I have nothing else to add to the debate. Over-square jut ****s me.

FRQ CB

[ZEEBEE]

Sorry that I have nothing else to add to the debate. Over-square jut ****s me.

Off the track a bit there FRQ, but really, the inches of MP aligned against rpm/100 has served us well.
It's a good guide as long as the religious zealots don't misconstrue the possibilities.

As a quick guide of operational acceptability, there really isn't a better rule of thumb.

As for oversquare/undersquare, yes...consult the manuals but they don't tell the WHOLE story.
Not one I know tell you why it's better to run at lower rpm's in the cruise.
Sure, it may be implied in the better specific fuel consumption etc, but say nothing about the less wear and tear that the ancillary components like alternators, vac pumps, magnetos undergo at the lower end of the operational envelope.
Propellers too are more efficient at the lower revs so there really isn't much to be said for making noises at 2500 rpm when 2250 does better.

[Atlas Shrugged]

Look up John Deakin's articles on engine management, best explanation of MP and wobbly props you will find anywhere.

[FGD135]

Propellers too are more efficient at the lower revs ...

And there is less internal friction in the engine. That friction is an enormous detriment to the efficiency of the engine (second only to heat loss). Of course, we are talking about piston engines.

So, how much friction are we talking about?

Think about when you are driving your car and wish to slow to a stop. If you "use the gears" to slow you down (as many drivers do), you will repeatedly shift gear in order to get that high RPM braking effect.

That "braking" effect you use is provided by the friction in the engine. The friction is between the piston rings and the cylinder walls and is most pronounced at the higher RPMS.

You would agree that it has a significant braking effect (at the higher RPMs).

Of course, that friction-induced braking effect is there all the time. You only notice it when you are using it to slow the car down but it is always there. It is there when you are accelerating and there when you are cruising.

The lower the RPM, the significantly more reduced is this friction effect.

[The Green Goblin]

For all of those who are talking about more advanced aircraft types, it is lovely that you are so smart and experienced that you can fly such lovely aircraft but it only muddies the water for someone asking a question for a simpler aircraft type.

The same can not be said for a turbo aircraft as for an NA type.

From the POH (notice the absence of 40" of MP..... or over 25"?):

Thats because the pressure at sea level is around the 30" mark and a NA donk can't produce more pressure than that! AND without forced induction you will be at full throttle height around 5000 feet making it pretty obvious that you cannot run it at a higher MP.

Interesting to note that at 12000 feet in the type you have posted you will need a combination of full throttle and 2600 rpm to achieve 65% power!

So from your figures I would be running 2300/23" at 2000 feet and trying to maintain 65% power at any altitude with the corresponding combination of RPM and MP.

[ZEEBEE]

Although over-square settings may not hurt the engine, I think it's important to note that some mp/rpm settings, including some over-square settings, can cause propellers to fail due to vibration. Further, the destructive vibrations may not be perceptible to the pilot. This is a consequence of the physics of the propeller / engine combination.

yep...Good point Clint....That's why it's so important to use the POH (where there is one).

[The Green Goblin]

hmmmmm

Very interesting stuff.

Thanks for that link!

[ForkTailedDrKiller]

For what its worth, I generally look to the POH for advice on power settings. If I want to just stooge around slowly, I also tend to wind the RPM back (cause I like to have a quiet life) toward the bottom end of the RPM green arc and then set the MP I need to maintain the target speed (eg 2100/18" with 10-20o flap).

Dr :8

[bushy]

And there are some dead people who did 45 degree turns at 100 feet while checking the water level in a dam etc.
Pax don't like 45 degree turns.
Beechcraft have printed some safety information, and the one I like says many accidents are caused by pilots stalling their aeroplanes, so they suggest we take great care to NEVER allow our aircraft to stall. (when it is more than two feet off the ground)
Think about it.

[ZEEBEE]

Stabilized operation is prohibited above 25" manifold pressure between 2300-2350 RPM and below 15" manifold pressure above 2600 RPM

How do you EVER get stabilised operation above 2600 rpm with less than 15" MP with any of those engine/prop combinations listed ???

[The Green Goblin]

Not that it matters guys but you won't be taking a small donk like this up to 12000 feet unless you are a VFR pilot stuck in cloud trying to get on top in an abnormal situation or a meat bomber with oxygen supplies. (who would usually be using something with a snail on it anyway)

I don't want to clean up a VFR 172 on the way down who is playing test pilot for the day in class E/G airspace.

Stick to your 2000-8000 feet charts and you won't get into trouble :ok:

[Jabawocky]

.enough from me......read here >>>>>

Pelican's Perch #15:<br>Manifold Pressure Sucks! (http://www.avweb.com/news/pelican/182081-1.html)

then

Pelican's Perch #16:<br>Those Marvelous Props (http://www.avweb.com/news/pelican/182082-1.html)

then

Pelican's Perch #18:<br>Mixture Magic (http://www.avweb.com/news/pelican/182084-1.html)

then

Pelican's Perch #19:<br>Putting It All Together (http://www.avweb.com/news/pelican/182085-1.html)

and just something extra for you!

Pelican's Perch #1:<br>Throw Away That Stupid Checklist! (http://www.avweb.com/news/pelican/182037-1.html)

J:ok:

[bushy]

You are quite likely to find small aircraft well above 8000 feet, usually because they are trying to get out of the thermal turbulence that occurs over much of our country. And they have a perfect right to be there.

[ForkTailedDrKiller]

Stick to your 2000-8000 feet charts and you won't get into trouble

??

Pardon my ignorance, but what charts are those?

Dr :8

[squawk6969]

Joker 10

In one word YES reckless

Very good sir :D

And very well spotted, you were probably aware of the fatigue issues of large spans of aluminium as in wing skins.

At low levels and very high air desities those pesky air molecules create far greater force at high angles of incidence and in an older airframe this over time creates severe fatigue cracking around the ribs and rivet heads.

Of course renters would never care and its only the owners that suffer in the long term.

SQ

[The Green Goblin]

Come on fellas a lighty shouldn't be over 10,000 feet without oxygen for the Pilot and once over FL140 oxygen for the punters too.

I was merely pointing out that providing a power chart for what looks like an IO-360 at FL120 and saying that you could not run it 'oversquare' at this altitude is a little pointless.

And yes I'm hearing you about them dreaded thermals especially over red country having spent many hours there myself. If you have a bottle with you and your machine is capable at those altitudes then it's a big sky and we are all welcome!

[Jabawocky]

Forkie, bring the O2, see how far up the Retard Vehicle will go! :ok:

Better still, :E I should get my own, and you bring the Bo :} and no Turbo's allowed to be fitted!

[baron_beeza]

That "braking" effect you use is provided by the friction in the engine. The friction is between the piston rings and the cylinder walls and is most pronounced at the higher RPMS.

FGD135

Have you got any references for your comments. I have been in this game for a long time and I am sure I have never heard any of this before.

This engine braking effect you speak of .............
many here have ridden motorcycles, - I am sure they will agree that a Yamaha 500 4 stroke twin will have far more engine braking than a Kawasaki 500 triple. And the Two stroke triple would normally be pulling far more revs.

I admit this was certainly true in the 1970's ........

I chose this mainly because every rider of the period knew only too well the differences in the 2 strokes power, - but also incidentally it was the same period as the engines in our aircraft were produced.

Please don't try and tell me that the 2 stroke has ports in the bore so the rings are not strictly against the cylinder walls the entire time.....




Engine braking - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Engine_braking)

Please don't tell me that your are involved in aircraft engineering in any capacity also.

[The Green Goblin]

Of course, that friction-induced braking effect is there all the time. You only notice it when you are using it to slow the car down but it is always there. It is there when you are accelerating and there when you are cruising.

The lower the RPM, the significantly more reduced is this friction effect.

This is called "friction"

If your pumping in the gas to keep it turning you'll loose some of the ponies to mechanical friction and receive the rest as torque through the shaft which produces thrust from the propeller. If you lay off the gas the airflow will drive the propeller and the mechanical friction will absorb the energy in the airflow. If the engine has failed you feather the propeller to preserve this energy, if you have a live engine you use it to your advantage to slow down!

Your argument is flawed. Higher rpm equals the propeller taking smaller bites of air with a finer blade angle at a higher rate. Lower RPM equals the propeller taking larger bites of air at a slower rate with a coarser blade angle. The engine does not know any difference! It's still producing 65% power in the cruise via torque from the shaft which is feeding the propeller governor. The prop governor is doing all the work converting the torque from the engine shaft into a set RPM as per what is set on via the CSU which in turn produces thrust from the engine via the propeller.

EDIT

Hmmmm

Having a look at the power graphs from the previous page I pulled two examples.

2500 rpm 22" 66% 124 KTAS 8.8 GPH

2200 rpm 24" 66% 124 KTAS 8.9 GPH

The engine is producing more MP at a lower RPM for the same percentage of power output. I would assume the engine is working a little harder to turn the prop via the governor at a higher RPM loosing about 2" in the process.

[Flopt]

I got a spare one Jab....I'll loan ya one......The Mo is quite happy at FL130 coming home from YWTN...usually gets 190 g/s...

Flopt

[Flopt]

You can check the BOM for CBs [with 3G and a laptop] ,and plan to avoid them, chat with FTDK on skype [ get his opinion about avoiding CBs if you need to!], email the missus with eta, check your bank account , transfer some money, read PPRUNE and post photos , get latest notams &tafs ,lodge fplans/amendments on NAIPS, maintain coms without HF, and VOIP your Airbus Captain mate in Qatar........all the while maintaining a good watch for traffic of course.....and monitoring your panel.

And it's usually as smooth as a baby's bum!!!!!!! 3.5hrs later you're home. And if there is a bit of weather you are usually on top of the benign stuff and you can see the really crappy stuff and compare it with the WX10+/BOM/WZ [wish you had one eh Doc!].

Flopt

[snoop doggy dog]

A fella ask for some advise on doing a scenic flight.

It some how turns into a Wank fest :mad:

Get over yourselves :*

[das Uber Soldat]

Thanks for your entirely useful input.

The thread posters question was answered some time ago. The thread has now turned into an interesting discussion / debate about what I regard is a fairly poorly understood concept.

I think its good.

[ZEEBEE]

That "braking" effect you use is provided by the friction in the engine. The friction is between the piston rings and the cylinder walls and is most pronounced at the higher RPMS

I was hoping that Harley would weigh in on this one, but since he's allowed it to go through to the keeper, here goes;

While it is true that the higher the engine speed, the greater the friction, it is only a small part of the "braking effect" that one uses with motor vehicles.

In motor vehicles the braking effect is the combination of the gearing of the engine to road speed and the resulting retardation provided by compression pumping within the engine without fuel addition.
If one wants to control the braking effect, then a small amount of accelerator will allow some fuel to arrive at the combustion chamber and use the compression products to deliver power according to the fuel supplied.
With no fuel, the compression developed is still there but it is ALL wasted in the form of retardation provided to the wheels that are driving the engine.

As for you Snoopy if you don't like the discussions then go and play somewhere else.:=

[Jabawocky]

FLOPPY
I got a spare one Jab....I'll loan ya one......The Mo is quite happy at FL130 coming home from YWTN...usually gets 190 g/s...

Flopt

:eek:

You sure???? I think you should get your eyes checked!
:sad:

[bushy]

You have to be nice to them. But they will go there, and higher.

[Flopt]

Jab,

Even Qantaslink plan slow going out , fast coming home and their blokes have got Class 1s !!!!!

It's not always there but there is usually a tailwind [westerly ,that is] in the FLs....tonight Area 41 14000 230/30 area 40 240/35.......

Flopt again ....now a CASA DAEE !!!!!!!!!!!!

PS special rates for prunes...

[FGD135]

Have you got any references for your comments. I have been in this game for a long time and I am sure I have never heard any of this before.

baron_beeza, the reference behind this assertion of mine would have to be the Val Dyson-Holland textbook for the CPL syllabus subject "Engines and Systems".

This is by far the most comprehensive book on the theoreticals of piston engines that I have ever seen. I still have the book but it is in storage in a different state to that where I am living now.

It was 20 years ago that I did the CPL subjects so the significance of the friction, in my mind, may have changed over that time. I would really need to go back into that book and check that.

It was also from this reference that I made the statement that "heat loss is the source of greatest efficiency loss" in the reciprocating engine.

I thank you and ZEEBEE for raising the alternative of vacuum being the main driver of engine braking. I was unaware of this and have learned something new.

2500 rpm 22" 66% 124 KTAS 8.8 GPH
2200 rpm 24" 66% 124 KTAS 8.9 GPH

The engine is producing more MP at a lower RPM for the same percentage of power output. I would assume the engine is working a little harder to turn the prop via the governor at a higher RPM loosing about 2" in the process.
The Green Goblin,
Engines do not "produce" MP. The higher fuel flow at the lower RPM (for the same power) is because with aircraft piston engines, the lower the RPM, the less fuel efficient is the engine.

Petrol burning engines get the most work from the fuel at RPMs around the 3,000 mark. Diesel engines are thus more efficient for aircraft as they are far more fuel efficient in the RPM range that is required for aerodynamically efficient propellors.

The "solution" for petrol burning aircraft engines is the gearbox, which allows the engine to run at the higher speeds it likes (in terms of fuel efficiency) whilst allowing the prop to turn at the lower speeds it prefers (around 1,700 RPM - depending on typical cruise airspeed).

The gearbox brings weight and complexity penalties however.

[43Inches]

Petrol burning engines get the most work from the fuel at RPMs around the 3,000 mark. Diesel engines are thus more efficient for aircraft as they are far more fuel efficient in the RPM range that is required for aerodynamically efficient propellors

Diesel engines are more suited to aircraft application because of the following;

Fuel - Diesel fuels contain more energy per unit and burns slower so lower consumption can be achieved for the same power output. The fuel is also generally cheaper than gasoline (dependant on goverment tax policy). Due to the high flashpoint they are also safer to handle and less likely to ignite in an accident.

Simple and Robust - Diesel engines have less componants (no ignition source required). The fuel also has better lubricating properties and the nature of construction and operation allows longer time between overhaul.

Why are they not in all aircraft?

Because diesels are inherently heavier than gasoline engines and the fuel itself is heavier. Weight has been the enemy of aircraft designers since day one. The cost of producing diesels is generally higher as well and once you start getting past about 500hp the jet based engines easily win the weight/power/reliability/consumption/cost argument.

The "solution" for petrol burning aircraft engines is the gearbox, which allows the engine to run at the higher speeds it likes (in terms of fuel efficiency) whilst allowing the prop to turn at the lower speeds it prefers (around 1,700 RPM - depending on typical cruise airspeed).


Thielert diesel engines are geared from 3900 engine rpm to 2300 prop rpm so not sure that argument works. Diesel, petrol and turbine engines running props can have gearbox reduction and still be very efficient.

[bushy]

Fgd
I suggest you have a look at some Lycoming engine manuals. You will find that lower RPM settings DO result in lower fuel consumption for the same horsepower.
A long time ago I was told that Rolls Royce advised that maximun endurance is achieved with "high boost and low RPM." The Lycoming manuals and my own experience definitely confirm this.

[steve181]

das Uber Soldat the good majority of the posts here are rubbish or off-topic. There is only a handful of decent posts that help to answer my original question.

Since the time I made the thread I've scanned the flight manual & got my books open on constant speed propellers so I'm going back to basics & clearing up the gaps in my knowledge that way. At least these two resources were definitely written by adults so can be trusted.

Having left aviation for 4 years & only just getting current again & rated in a Cessna 172RG with constant speed prop I have proven to my instructor I am a capable pilot. Surely I can be forgiven for having some gaps in my knowledge but I am making every effort to fix that before I next take to the skies.

[Brian Abraham]

As Bushy says, high MP and low RPM win the fuel consumption stakes. It took Charles Lindbergh to drive home this point to WWII P-38 pilots so that they could extend their area of operations in the Pacific.

http://i101.photobucket.com/albums/m56/babraham227/l0012.jpg

[MakeItHappenCaptain]

Research topic for the day....

VOLUMETRIC EFFICIENCY.:ok:

[FGD135]

43Inches,

You disagreed with my statement about diesel engines being a better match for aircraft than petrol burning engines. You said:

Thielert diesel engines are geared from 3900 engine rpm to 2300 prop rpm so not sure that argument works. Diesel, petrol and turbine engines running props can have gearbox reduction and still be very efficient.

I was actually referring to diesel engines burning diesel fuel. The Thielert engines burn jet fuel so this is nothing like the situation I was alluding to.

As I said in my previous post, avgas burning engines get peak work from the fuel (maximum torque) at an RPM value somewhere between 3,000 and 4,000. This is because of the (relatively rapid) rate at which the fuel/air charge burns.

At issue is the relative geometry of the con rod and crankshaft during the period of maximum pressure in the cylinder (on the power stroke). The arrangement is well less than ideal at RPMs in the 2,000 range as the peak pressure occurs too early. Get the RPM up to around 3,500 and the peak pressure now occurs across the period where the con rod and crankshaft "arms" are now at 90 degrees to each other.

Car engines are virtually identical to aircraft engines in this respect. If you have a tachometer in your car, you will notice that you get peak torque at somewhere around the 3,500 RPM mark (allowing for some variations between engine makes/models of course). (Cars have the luxury of a gearbox so they can afford to run the engine at these speeds).

This is all due to the rate at which the fuel/air charge burns. If that rate could be controlled then this issue would not exist.

Run your engine at 2,000 odd RPM and you won't be getting anywhere near the same bang out of your fuel.

Diesel engines, burning diesel fuel, are an entirely different proposition. They get their maximum torque at much lower RPMs - the RPM range that is well suited to driving an ungeared prop on an aircraft (memory limitations prevent me from quoting any rough figures on this).

Another reason you don't see aircraft with diesel engines that burn diesel fuel: there are no aerodromes with bowsers that supply diesel fuel!

bushy,

Until now, I have been talking only about what is going on inside the engine. If you now start talking about the performance of the aircraft as a whole (e.g range, endurance), then you are talking about the powerplant (which is the engine/prop combination) and the airframe.

We all know well that aircraft design is about choosing compromises. The powerplant is a collection of compromises where the optimum performance may be at a point well removed from where any individual efficiencies may peak.

The two most significant efficiencies on the reciprocating aircraft
powerplant are:

1. Engine specific fuel consumption (what I have talking about above), which is best somewhere around 3,500 RPM but drops off as RPM reduces, and

2. Propellor efficiency, which is best somewhere around 1,800 RPM but drops off as RPM increases.

At some point of compromise - somewhere in the middle - you get the most economical performance for the powerplant as a whole. For the typical powerplant at a typical cruising speed, that could well be, for example, an RPM of 2250.

There are limits to the general statement "the lower the RPM the better". When people espouse this (e.g. Rolls Royce, Charles Lindbergh) they are referring only to the RPMs in the "useable" (or permissible) range. I would wager that an RPM value of 1,000 for example, would be hopelessly inefficient.

[bushy]

FGD
Have a look at Brian Abraham's Lycoming graph which shows that Lycoming also believe that their engines are more economical at low RPM.
If you want there are other charts which tell us the same thing.

These engines are designed to run at low RPM.

[Brian Abraham]

FGD135, to take up some points, whether a diesel engine uses diesel or jet fuel (kerosene) is immaterial. The original diesel engine was in fact to use the oil coming straight out of the oil well. Ship diesels use bunker oil which has to be heated in order to get the viscosity low enough in order to pump the stuff. You can not compare aircraft piston engines to motor vehicles. Aircraft engines are big bore and one of the reasons for dual plugs is so that the charge in the cylinder all gets burnt. This is because of the slow speed of the flame front. With only one plug not all the charge would be burnt. The DB engine as fitted to the Me 109, although it had dual plugs, both were installed on the same side of the head, rather than on opposite sides, and the engine suffered some thing like a 10% loss of power as a result. The RPM of an aero engine is limited by the prop aerodynamics (Mach number of the tip), extra horse power can be extracted by increasing RPM, but that then necessitates a gear box to reduce the prop RPM. Added complexity. The Bear bomber has turbo props putting out some 15,000 horse power each with a prop RPM of only 750 and is capable of near jet speeds. Peak pressure does not occur at 90° crank on any internal combustion engine.

[FGD135]

These engines are designed to run at low RPM.

Perhaps you speed-read my post but that is basically what I was saying. The designer will "match" a particular propeller with the engine he wants in order to achieve the most economical cruise - and that cruise will be with a lowish RPM (nowhere near as economical as what could be obtained from a diesel engine running diesel fuel).

To illustrate the point I was making in my previous post about RPM that is TOO low, I will again quote these figures that The Green Goblin originally extracted and posted in this thread. Note that the fuel economy is starting to go to mud as the RPM gets below 2200:

2500 rpm 22" 66% 124 KTAS 8.8 GPH
2200 rpm 24" 66% 124 KTAS 8.9 GPH
If there was a published set of figures for 2100 RPM I'm sure it would show fuel economy that is beginning to get dramatically worse.

[43Inches]

Diesel is a process, the generally available fuel happens to be named as such.

Jet A fuel is virtually identical to diesel and can run in normal diesel engines provided a lubricant is added (check with your manufacturer first).

Obviously the forum here deals with aircraft engines and any mention of diesel refers to those that may be run on JET A. These engines however may be fueled by either Jet A or straight diesel (Diesel #1, Diesel #2, bio-diesel EN 590) as they are diesel engines.

If a direct drive diesel in th 1500 - 2000rpm range was suited to aviation then why have geared higher reving diesels been developed and produced?

[FGD135]

The Bear bomber has turbo props putting out some 15,000 horse power each with a prop RPM of only 750 and is capable of near jet speeds.


Brian, I am referring only to the typical piston types that are currently flown in Australia - not such rare turboprops - I thought that was obvious.

Peak pressure does not occur at 90° crank on any internal combustion engine.Nitpicking.