Question about variable-pitch propeller aircraft & slow flight
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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
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
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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
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.
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.
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:
.
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
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
Fratemate is totally wrong about using high RPM all the time
there is no imperative to keep the engine oversquare, you could not take off in a normally aspirated aircraft with a fixed pitch prop
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
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
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
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.
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.
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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
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.
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)
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.
Now lets just hope someone doesn't add another lever in to the equation like the mixture and the ROP/LOP debate
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A picture is worth a thousand words.
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.
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.
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Flying 45 at 1500ft is reckless ???? gimme a break
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.
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.
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!)
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!)
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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.
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.
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!)
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!)
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
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)
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
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.
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
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.
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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"?):
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:
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"?):
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:
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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?
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
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?
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
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Sorry that I have nothing else to add to the debate. Over-square jut ****s me.
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.
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Propellers too are more efficient at the lower revs ...
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.
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"?):
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"?):
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.
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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.