PDA

View Full Version : Best Glide Speed change with flap?


makelove
14th Dec 2011, 04:07
Hey!

Does the speed for best glide change with flap?

fireflybob
14th Dec 2011, 05:35
Does the speed for best glide change with flap?

I presume by "best glide" you mean minimum height loss versus horizontal distance travelled, as opposed to minimum rate of descent?

Assuming you mean the former, then you need to be gliding at the speed which gives you the best Lift/Drag Ratio and, as we know, extension of flaps will decrease the Lift/Drag Ratio and therefore result in a steeper angle of descent.

It's an interesting question and am not sure of the answer!

Genghis the Engineer
14th Dec 2011, 08:53
Of course it does, you've just changed the aerodynamic characteristics of the wing. It would be pretty odd if it didn't.

G

mad_jock
14th Dec 2011, 09:21
There is a sweet spot AoA for every configuration which gives you the best L/D ratio.

The speed is dependent on your weight as well.

Its alot more involved than the books make out. But for light aircraft the variation with weight is minimal compared to what the pilot error in airspeed.

Whopity
14th Dec 2011, 09:24
Why would you want to glide with flap? Usually, we fly at the best glide speed following a power failure to achieve the longest time in the air to allow us to find a suitable landing area. Flap is preserved until just before landing when we can use it effectively to steepen the glide angle to remove the little bit of extra distance we have built in as safety margin. In that case, the same speed is maintained but the distance reduces considerably due to the extra drag. The min drag speed will change with flap as will the glide distance which is why we do not glide with flap.

makelove
15th Dec 2011, 00:08
Yes, I'm refering to best glide in terms of range, ie. after engine failure I want to glide furthest distance to give me more options, in terms of landing sites.


Why would you want to glide with flap? Usually, we fly at the best glide speed following a power failure to achieve the longest time in the air to allow us to find a suitable landing area. Flap is preserved until just before landing when we can use it effectively to steepen the glide angle to remove the little bit of extra distance we have built in as safety margin. In that case, the same speed is maintained but the distance reduces considerably due to the extra drag. The min drag speed will change with flap as will the glide distance which is why we do not glide with flap.



So, in theory, with each selection of flap, do I increase or decrease my speed to maintain speed for best glide?

But, in practise, I'm assuming this speed change is so small it's not relevant?

Big Pistons Forever
15th Dec 2011, 01:00
For light aircraft the general rule is the best glide speed decreases with flap. For example the C 172 has a no flap best glide speed of 65 kts and a full flap best glide speed of 60 knots. If you are doing a forced approach all other things being equal you should slow down from 65 to 60 when on final with the field made and you have gone to full flaps.

Pilot DAR
15th Dec 2011, 02:18
I am not an instructor, so do what your instructor says. Since taking flying instruction I have seen other aspects associated with gliding powerplanes.

I want to glide furthest distance to give me more options, in terms of landing sites.

Inspired by experience (four times), and John Farley's excellent book on test flight, I'm going to present a different point of view. I do not seek to glide the farthest, I'd rather do a good job of a closer site (even is less perfect as a spot). Too much can affect "making" the far site, and then you're changing your plan at the last minute for a much less well thought out spot. Avoid avoid changing your plan during emergencies! I'd rather pick the closer spot, not have to stretch the glide, allowing the use of sideslip (which is easily undone as required), then flap when you've got it made (and you can still sideslip). Most light planes can be sideslipped into a crosswind, right into the flare - if the speed is right!

If you're busy trying to stretch the glide, then changing over to be busy making a good landing in a never before seen landing spot of unknown condition and dimension, you're increasing your workload at the very worst time. Keep it simple, pick the best spot closer in, get the pattern correct around it, get prepaired, and the pax properly briefed, with less rush, and have the perfect power off touchdown set up.

If you're not convinced, test yourself, at your home runway. fly a "normal" final approach power off, and watch yourself spend time simply worrying if you'll make the end of the runway at all - result will be a poor landing, which might even be short. Then try a little high, and ten knots fast, in a steady sideslip, adding flaps as you think you need. You can always lessen the sideslip if you mis judged, or set too much flap. The correct sideslip will help you correct for a cross wind too.

So what does this have to do with glide speed and flap setting? if you're gliding to a planned landing other than making landfall straight ahead, don't worry about speed precision, as much as approach precision. If the approach is good, and stable, the speed can be fixed at the last minute with a sidesli.

If your glide is low, and you change to stretching your glide, you'll be surprised to find that the plane will glide at a sped 5 knots too slow, and seem not bad - but, there'll be nothing left to flare with, and you'll just slam in while trying to arrest your descent rate. To get a true feeling of this, watch videos of full on helicopter autorotations. You will see the point every time, where the purposeful excess in speed (rotor RPM) is bled off just before touchdown, maybe held off for a bit, or carried right through if the speed was right on already. You're really trying to do the same thing in the plane.

Now, if you've run your Airbus full of Europe bound passengers, out of fuel, and the Azores has the only runway you can make, look up the correct glide speed, fly it exactly, leave the flaps up, and hope like hell, until you start to worry about overshooting! Then slip and flaps....

Big Pistons Forever
15th Dec 2011, 04:22
If your glide is low, and you change to stretching your glide, you'll be surprised to find that the plane will glide at a sped 5 knots too slow, and seem not bad - but, there'll be nothing left to flare with, and you'll just slam in while trying to arrest your descent rate.


DAR

Your point about the importance of making the field is a very good one. The less than good field that is well within gliding range is always a better choice than the ideal field that is near the limit of what you can make. However I must disagree with your point about not having enough energy to flare. In a for real forced approach the point is not to flare to a nice touch down it is to touchdown at the chosen point at ideally a lower airspeed because low airspeed equals lower energy which means a shorter ground run and lower deacceleration forces. What shape the airplane is left in when it stops is utterly irrelevant after a real forced landing, the only thing that matters is that nobody is hurt.

I tell students that hitting the touch down point is critical and if the aircraft is going too fast, which is what usually happens with students as they tend to be high and fast on most PFL's, then they should drive the aircraft on to the ground at a good spot even if it means a nose wheel first touchdown.

In any case pilots should know the best speed for gliding for all configurations so that they can fly the correct speed for what they need the aircraft to do.

24Carrot
15th Dec 2011, 09:03
Why would you want to glide with flap?

Prompts another question:

If your engine fails on short finals, and you are not going to make the runway any more as configured, but you do have maximum flap, should you retract all flap, and what is the best technique?

I can imagine a similar situation if you mess up the approach to a forced landing.

I would have thought that removing "drag flap" was OK, and further flap if you carefully maintain the same airspeed on the ASI (which means you will be pitching up to maintain lift).

However, I have often heard from FIs that there is an inevitable one-off height loss if you do this. I have never understood why though. Can anybody help?

Pilot DAR
15th Dec 2011, 12:54
Yes, I agree Big Pistons, sometimes I think more than I should about saving the plane. If I have to risk me or the metal, it'll be the metal!

However, then they should drive the aircraft on to the ground at a good spot even if it means a nose wheel first touchdown. is not something I favour. When I think of some of the wrecks I've helped to clean up, there were many more which were over runs of small landing areas (intended or forced), than undershoots.

Pilots under stress seem to have trouble stopping planes once on the surface. I recall the TSB investigator telling me of the question of a non aviation witness, to an over run crash on a short runway (pilot was a thousand or so hour instructor). The witness asked the investigator: "don't the main wheels have to be on the ground for the brakes to work?" I've watched pilots decide that the aircraft is going to "stay" now, and the nose pitches down.

Once you've crossed the "fence", obviously, slowing is the primary objective. (descent and touchdown should naturally occur at the right time in that deceleration). There are three basic ways to slow a "normal*" GA aircraft: Wheel brakes, aerodynamic braking, and hitting obstructions. The latter obviously implies damage, or at the very least less opportunity for the pilot to control the rate of deceleration. Aerodynamic braking is the best, as it costs nothing, but takes some skill and judgement. Wheel brakes work well, when the wheels are working well on a suitable surface - friction good. Off airport landing - you can't count on the surface being suitable for wheels or brakes - or similar to a ski landing - poor friction.

So for the off airport landing, you're reduced to aerodynamic braking, and/or hitting obstructions for stopping. I opine that once you have decided to force the nose down, you have given up flying the plane, and surrendered nearly all aerodynamic braking opportunity, and put yourself completely at the mercy of the obstructions - be they furrows or trees.

Flying the plane till it stops is the safest thing to do. At the very least, keeping the nose as high as possible keeps the strongest landing gear taking the brunt of the obstructions, and lessens the chance of a nose over.

Therefore, planning a forced approach so as to optimize the speed for a touch down as intended, then using aerodynamic braking to minimize the need for obstruction braking as much as possible.

* I was lucky enough to be flying a two seat turbine taildragger last week. Whe a strong gusty crosswind made directional control uncertain, and braking useless on snowy wet grass, a large dose of reverse got the plane stopped in feet - what a luxury!

I would have thought that removing "drag flap" was OK, and further flap if you carefully maintain the same airspeed on the ASI (which means you will be pitching up to maintain lift).

Without being able to quote a lot of aerodynamics to support what I say, I will assert that there is no benefit in retracting flaps to correct an approach, which is still intended to result in the landing. If you do, the plane will settle, you'll raise the nose to compensate, increase drag again, and slow and settle more. With the single exception of sideslipping (or I suppose spoilers), a good, stabilized approach will be one with steady descent and deceleration with the drag being either maintained, or incrementally increased, but not decreased, if the drag is associated with lift.

Big Pistons Forever
15th Dec 2011, 15:05
Pilot DAR

My point with forcing the aircraft on the ground is to avoid it floating in ground effect past a landable area and then hitting a solid object at high speed. If you smash the aircraft on the ground with forward stick it will break the nose wheel off and then rapidly decelerate as the front of the aircraft crumples and digs into the earth. Obviously this is a last ditch desperation move. But the killer crashes are ones where the aircraft hits an immovable solid object like a stone wall, deep ditch, tree etc at high speed. If smashing it on the ground is what it takes to make the aircraft come to a stop in the flat middle of the field rather then hit the wall at the end then IMO you do what you have to do.

Big Pistons Forever
15th Dec 2011, 15:26
Prompts another question:

If your engine fails on short finals, and you are not going to make the runway any more as configured, but you do have maximum flap, should you retract all flap, and what is the best technique?

?

I tell my students that to delay any flap until short final and they are sure they are going to make the field. If there is any doubt about easily making the field an early diversion to a closer field is the best option.

bookworm
15th Dec 2011, 15:45
Does the speed for best glide change with flap?

Difficult to generalise but the answer lies in the drag polar for the aerofoil (and aircraft) and how it changes when flap is extended.

It's hard to find much published data, but Abbott and von Doenhoff's 'Theory of wing sections' has a plot on p214 with a double slotted flap, and on p220 with a double slotted flap, showing the profile drag coefficient against the lift coefficient. Those curves suggest that best L/D is achieved at a higher lift coefficient as flap is extended, in other words that best glide is at a lower speed.

However that's just the aerofoil and doesn't take account of the effect of the pitch angle on the drag from the fuselage and the rest of the aircraft. That might complicate things, since with flap extended the same lift coefficients are achieved at much lower (more nose down) pitch angles, though my instinct is that it should exacerbate the effect.

That's the theory, why don't you measure it for your aircraft in practice?

24Carrot
17th Dec 2011, 13:33
Thank you to those who responded re flap retraction.

Pilot DAR, if I maintained attitude while retracting flaps, I completely agree with your assertion. I had hoped that maintaining speed would avoid the settling, etc, but I dare say I am still missing something. When I get the a chance I will try it, (at a safe altitude).

Big Pistons Forever, I see what you are saying - it's best to avoid the scenario!

--

Bookworm, I think I can add another complicating "whole aircraft" factor to your list.

In the Abbott book, the plots you mention show an aerofoil pitching moment coefficient of -.35 with 30 degrees of flap. If that applied to a C172 at 65kts, then the extra downforce needed at the tailplane to balance the pitching moment would be around 10% of the aircraft weight. The wing would have to increase its lift coefficient by 10% to compensate, so Induced Drag would rise by around 20%. As Induced Drag and Parasite drag are supposed to be equal at best glide speed, this also changes the speed where they balance.

FOUR REDS
19th Dec 2011, 10:07
24 Carrot

Whilst doing my CPL a long while back, an ex military instructor showed me an 'Engine Failure' on finals. He raised the flap and did dive towards the ground to increase speed, which he then used to 'soar' in ground-effect.

Lo-and-behold we did touch down on the threshold.

For good order he then let me fly this, with the same outcome. I was impressed with myself.

Obviously this is non-standard, but might one day save your bacon!!!

mad_jock
19th Dec 2011, 11:37
Gliders get taught to do this ground effect thing as well.

Another one for your tool box is if your short coming into a forced landing field and there is a fence or hedge in the way clean up and dive down into ground effect then just before your about to hit the fence put your flap all the way down/in and you will bounce up over the object. At this point pitch to the stall attitude and you will drop with virtually no ground speed at the otherside. If your in a sprung steel undercarrage (C150/152/PA38) aircraft they can be dropped from silly heights and just suck it up. The more delicate UC like PA28's you will break it but so what as long as the people have survived.

Pull what
19th Dec 2011, 12:08
Another one for your tool box is if your short coming into a forced landing field and there is a fence or hedge in the way clean up and dive down into ground effect then just before your about to hit the fence put your flap all the way down/in and you will bounce up over the object. At this point pitch to the stall attitude and you will drop with virtually no ground speed at the otherside. If your in a sprung steel undercarrage (C150/152/PA38) aircraft they can be dropped from silly heights and just suck it up. The more delicate UC like PA28's you will break it but so what as long as the people have survived.

That technique is probably safer in a tool box.

S-Works
19th Dec 2011, 12:19
pull what,

you wanna share with us the driver behind your desire to enter intoa pissing contest with everyone? You seem hell bent on mixing it up with MJ!!

How much Instructional time do you have out of interest?

Oktas8
20th Dec 2011, 01:25
Pilot DAR, if I maintained attitude while retracting flaps, I completely agree with your assertion. I had hoped that maintaining speed would avoid the settling, etc, but I dare say I am still missing something. When I get the a chance I will try it, (at a safe altitude).

24Carrot, perhaps this explanation may help a little.

When retracting flap in a glide, there is a reduction in (coefficient of) lift assuming all else remains the same. Due to inertia, it takes time for the aircraft to accelerate to restore the lift force. As the flaps are retracting and the aircraft is accelerating, the aircraft will descend more steeply due to the loss of lift. There is therefore a momentary sink associated with flap retraction.

If you are close to the ground, avoid the momentary sink associated with flap retraction as it will be far more significant than the slightly improved glide angle associated with less drag.

If you are still quite high, you might accept the (relatively) minor sink associated with flap retraction in favour of improved glide performance for a prolonged glide to a still-distant landing site.

24Carrot
20th Dec 2011, 12:16
Thanks again for the responses.

Four Reds and MJ, that ground effect approach sounds like fun!
I would certainly want an instructor on board before I practiced that one, preferably pre-warned!;)

Oktas8, I agree there is a reduction in lift if all else remains the same.
But my intention was to make sure that all else was not the same!

In effect, flap makes the wing behave like it has some extra angle of attack. Flapless, you need to pitch the aircraft up to replace the missing "Flap AoA" with real AoA. If you can pitch up as the flaps retract, the lift coefficient need not fall. Maintaining speed seemed the easiest way to control the pitching up, with the extra benefit of avoiding a stall.

I should add that I have been thinking in terms of Cessna-style electric flaps rather than the PA28 handbrake type.

If I am at 65 kts, and pull the flap switch fully up, then I think I could keep my speed under 70 kts by pitching up to control speed in the usual way, though I accept there would be quite a lot of pitching up as the flaps travelled, and it might not feel right pitching up that much that slow when close to the ground. Also if I mess up the manoeuvre and speed up more, I will surely sink more.

A 5 knot speed excursion from 65 knots implies (in theory) only a 25 ft height excursion below my glide path, which is recovered when I get the 65 kts back.

I agree with everything else you said.

Also, I had better add that I am perfectly happy to accept what I am taught by FIs, even when I can't understand it. The world is full of things that I don't understand, but remain true regardless.

I'm just happier when I do understand!:)

mad_jock
20th Dec 2011, 12:45
I would suggest the ground effect stuff you go to somewhere with a glider or TMG.

The gliding fraternity is more switched on when it comes to this sort of training than the powered instructors.

Oktas8
20th Dec 2011, 21:25
Hi again 24carrot.

What you say is true, in terms of pitching up as you retract flap. You will indeed minimise height loss. Unfortunately, you will end up quite slow - avoiding the stall perhaps, but certainly well below best glide speed for the new configuration. A wing that goes from a more-curved state to a less-curved state will require a disproportionately large pitch change to maintain lift, if no acceleration occurs.

For me, the key to understanding the problem is to consider acceleration and what provides it, when retracting flap. Either it's engine power or gravity - either way results in a transient loss of performance.

Well, I hope you find an explanation that clicks for you - all the best with the search.

Tarq57
20th Dec 2011, 22:01
@ 24Carrot,

Some years ago I flew alongside a ridge to test the theory that flaps could be raised without height loss (momentary.)

It was in a 152.
I configured at full flap, and around 65kt, power to maintain level flight, then selected the flaps to up.

After several test runs, I concluded that one can avoid a momentary height loss, but only with some rather ham-fisted up-elevator as the lift from the flaps decreased. I actually doubt anything was saved by doing this (unless you're at sufficient altitude that the trade-off is worth it), because the elevator movement required probably creates quite a bit of induced drag at the tail in order to effect the pitch change needed. I wouldn't be surprised if the drag created nullified (briefly) the drag decrease from raising the flap.

There is quite a well researched story about a pilot who found good cause to raise the flaps on his airliner from 35 to 25 degrees at Heathrow some time ago....worth a read.

24Carrot
21st Dec 2011, 07:58
Thanks again for the responses.

In particular, I hadn't thought of transient effects of the control movement. As the flaps move, the air will strike the tailplane at a changing angle, so there is a lot going on.

I am quite keen now to try this out! I think a safe height, with a GPS in the back, could work. During the flight, I can fly the aeroplane, and afterwards I can look at the vertical GPS tracks. Flying a straight track, the GPS will probably not switch satellites on me, and I might get some useful data.

Unfortunately, I am really trying to get something else done at the moment, despite the weather, so it might be a while. I will post again if I find anything useful.

ouaviator
27th Dec 2011, 14:47
I'm surprised no one has suggested making a project out of this!

Next time you fly, climb to a safe altitude, pull the power to idle and pitch for best glide. Note your descent rate on the VSI. Then drop the nose to pick up about 5 kts and look at the result on the VSI. Repeat for 5 kts below best glide.
Now add some flaps and do it again.

Not only will you find out how it changes, but you'll know the best air speeds for your different flap settings.

Everyone is right though, it's not a simple problem.
-Chip

huv
3rd Jan 2012, 21:18
Back to the original question. I risk my neck with a rather definite answer.

The speed giving best L/D (flattest glide) decreases when flaps are deployed. Always.

Even if flaps did not add drag, this would be true. When the drag of the flaps is added, it is even more true.

An example of flaps giving very little added drag is setting 10 degrees of flaps on a single engine Cessna. The flaps move back, reducing stall speed mostly by adding wing area. Only little drag is added.

In this case, the speed giving best L/D will decrease by approximately the same percentage as the stall speed.

Now think of a case in which only drag is affected. How about extending the landing gear of your Piper Arrow? In this case the speed giving best L/D is also reduced. I'll get back to why.

Now this, I imagine, could be useful. There could be a number of scenarios where the pilot would wish to maximise a glide with gear down.

Generally lowering flaps will add lift capacity as well as add draq. Both will work towards a lower speed for best L/D.

One more thing. The speed giving best L/D is always very close to the speed giving best rate of climb (Vy) (not true for jet aircraft, never mind that). So how about getting max climb rate after a balked landing with flaps down? How about needing to climb when the undercarriage will not retract?

In both cases use a lower speed than the book Vy speed for a clean aircraft.

Back to flaps; in most cases flaps deployment will increase wing camber and thus both lift capability and parasite drag; and so the speeds for best climb and best glide performance will decrease by a significant amount.

The general conclusion even holds true for glide with minimum sink and for best climb gradient (Vx). With flaps and/or gear extended, use lower speeds than those given for a clean aircraft, either for best glide or climb performance.

Of course this may not be good advice in other respects than to performance - watch the engine temps, keep a good lookout and do not fly at unsafe speeds...

The rest of this post is theory only.

A basic point in the theory behind is that the speed giving best L/D is the speed for which the lift induced drag is equal to parasite drag (all the rest of the drag).

Imaging gliding a clean aircraft at best L/D and then extending the undercarriage. Before lowering, induced and parasite drags are equal. Lowering the undercarriage increases parasite drag (significantly), but do not change induced drag. So, now parasite drag is greater than induced drag. To equalize the two it is necessary to reduce airspeed; this will increase induced drag and decrease parasite drag. Hence best glide with undercarriage down will be established at a lower airspeed.

How much lower? Not easy to work out, but if the gear adds (say) about a third to the parasite drag (not unreasonable, may be even more), it can be calculated that speed for best glide (or best rate of climb) is now reduced by 9%, or from (say) 85 knots to 77 knots. Unfortunately, the amount of drag from the undercarriage is generally not available information. But (hopefully!) it makes it clear (somewhat?) that the key glide/climb speeds are always lowered, when either flaps, or undercarriage, or both, are lowered.

Pilot DAR
4th Jan 2012, 04:45
I certainly agree with HUV, and appreciate the clear explanation. This concept was proven to me during test I did with a Cessna Caravan carrying a very draggy external load. The aircraft could not achieve the required rate of climb performance. By reducing the climb speed, the aircraft did meet the climb requirements, and all seemed well.

But then it was required that I demonstrate a land back after engine failure from 50 feet, at the now lower climb speed. This was very difficult to achieve, as there was just no reserve energy with which to flare at the bottom. It was similar to a helicopter autorotation entered from within the height velocity curve "unsafe" area, there's just not enough left at the bottom.

Similarly, aircraft equipped with STOL kits can be glided at slowed speeds, but when you do this, again, there is nothing left at the bottom with which to flare. You flare, stall right away, and settle in without arresting the rate of descent.

From this I learned that the speeds chosen for certain operations are often a compromise, considering more than just the obvious objective. Yes, you can fly at different speeds, but you are probably reducing a margin of safety elsewhere, which may go un-noticed, until something else goes wrong.

maddabbo
5th Nov 2023, 16:34
Hey!

Does the speed for best glide change with flap?

Best glide is going to be achieved with no flaps configuration. Any flaps will add lift and drag and decrease range and decrease glide ratio. Flaps by adding lift decrease sink and by increasing drag lower speed all to give a slower stall speed. When you want to glide far you'd like the fastest speed with the best glide ratio. As you can make your runway you want to add flaps and keep that same speed dropping the glide ratio and landing distance to not overshoot. Speed stays the same until flare and the increased drag allows for the shortest landing distance possible. No flaps until on final and can make all the way through landing with no power. Then flaps to make that landing as needed with side slip as needed to lose altitude as needed.

ZeBedie
7th Nov 2023, 08:55
If you're gliding with flap, then presumably, you only extended flap because you were in an overshoot situation and therefore don't require best gliding performance. But if the glide turns into an undershoot, then a more important question might be, is it viable to retract some or all of the flap?
But yes, best glide speed will be lower with flap.

Another important factor is wind - if you're gliding into a headwind, best gliding angle will be achieved by flying above best glide speed - add half the headwind to best glide speed. In case of a tailwind, it may be better to fly at the speed for minimum sink, a few knots below best glide speed.

Fl1ingfrog
7th Nov 2023, 16:48
then they should drive the aircraft on to the ground at a good spot even if it means a nose wheel first touchdown.

No, no, no! Your likely to porpoise, wheel barrow or flip over doing this plus you will certainly lose control and very likely loose the nosewheel; I've known the fuselage to be ruptured doing this particularly should the nosewheel dig in. You can always lose more distance in the air by inducing drag. At the slowest possible speed the brakes will be the most effective on touch down.

Fl1ingfrog
7th Nov 2023, 17:05
But yes, best glide speed will be lower with flap.

The best glide speed is distance over time. Drag must always be balanced with speed. If you increase the drag you must increase the speed. Therefore the speed will be higher with flap to achieve the best glide in this abnormal condition.

Fl1ingfrog
7th Nov 2023, 17:22
I should add that I have been thinking in terms of Cessna-style electric flaps rather than the PA28 handbrake type.

It is worth noting that many aircraft, that are still commonly in use, pitch down when flaps are lowered and pitch up when they are raised; the PA28 Cherokees with the slab wings as opposed to later models with the extended tapered wings all do (PA28-140 and PA28-180 etc)

​​​​​​​Take care not to introduce a rule when there isn't one.

BigEndBob
7th Nov 2023, 18:56
I don't worry about glide speed, glide above to give a margin, aim for where you want to land and if speed excessive, lower flap to act as air brake, even side slip all the way down if you have to.
Flying at glide speed too easy to try and stretch a glide end up on the back of the drag curve.

What worries me is what is taught, 70% of test candidates in my experience, when told sim. eng. failure, trim, look around and aim for a field 4 miles odd ahead.
I wouldn't mind but i put them next to some good, text book fields.

EXDAC
7th Nov 2023, 19:36
It is worth noting that many aircraft, that are still commonly in use, pitch down when flaps are lowered and pitch up when they are raised; the PA28 Cherokees with the slab wings as opposed to later models with the extended tapered wings all do (PA28-140 and PA28-180 etc)

Take care not to introduce a rule when there isn't one.

Isn't that what you just did? My PA-28-180 does not pitch down when I extend flaps. What happens in mine is that the increasing pitch force that I was holding as I slowed goes away as the flaps are extended. Some students, and maybe some instructors, have to learn not to chase the trim when slowing to flap extension speed.

Fl1ingfrog
7th Nov 2023, 19:52
Now if you had specified "stick free" response I might have agreed with you.

What I described was without any input from the pilot. Sorry I didn't make that clear enough for you. On review, I didn't introduce an input from the pilot, but you have.

Instructors are taught only to teach trim feel although this is extremely important it is not the whole story, I agree. Trim technique is skimmed over in my experience and therefore there is a lot of trim chasing. This not the fault of the syllabus.

Genghis the Engineer
7th Nov 2023, 20:47
I don't worry about glide speed, glide above to give a margin, aim for where you want to land and if speed excessive, lower flap to act as air brake, even side slip all the way down if you have to.
Flying at glide speed too easy to try and stretch a glide end up on the back of the drag curve.

What worries me is what is taught, 70% of test candidates in my experience, when told sim. eng. failure, trim, look around and aim for a field 4 miles odd ahead.
I wouldn't mind but i put them next to some good, text book fields.

And about another 20% will pick a mediocre nearby field, but then absolutely refuse to change their plan away from trying to make exactly that field, and no other, even when things don't turn out quite right, and half a dozen better fields present themselves around it.

G

EXDAC
8th Nov 2023, 23:34
What I described was without any input from the pilot. Sorry I didn't make that clear enough for you. On review, I didn't introduce an input from the pilot, but you have.

Instructors are taught only to teach trim feel although this is extremely important it is not the whole story, I agree. Trim technique is skimmed over in my experience and therefore there is a lot of trim chasing. This not the fault of the syllabus.


Well, I had deleted "Now if you had specified "stick free" response I might have agreed with you", but not before you had quoted me.

My PA-28-180 doesn't get flown much now because the FX-3 is a lot more fun. Today I took it out for an airing and got one test point trimmed in a constant speed departure climb. Stick free my PA-28-180 pitched up when flaps were extended and pitched down when retracted. Not a strong response in that test condition but certainly there. Now it may pitch down with flap extension in some other conditions, I really don't know for sure. I have so much time in the airplane it just does what I want it to without my thinking about it.

TheOddOne
9th Nov 2023, 04:31
One of our PA28 is a '74 PA28-180 Archer. It's got the longer fuselage and extensions to the slab wing and stabilator. It wants to pitch up with flap extension, just like later PA28s such as our Warrior 2.
Now we also had for a while a slightly earlier PA28-180 with the longer fuselage but the original 'Hershey Bar' wing and stab. The first time I flew it, I was slightly taken by surprise when it pitched DOWN with flap selection.
So, you can have either reaction with apparently small changes to the airframe. Here's the takeaway:
We teach our students to trim for the cruise, 100kt, observe the attitude out of the window. An early exercise, having previously explored the handling at 75 kt and flap zero with a very nose-high attitude we then learn how to extend flap and set up for the approach. With carb heat hot, reduce power whilst maintaining pitch attitude. In a PA28, this invariably means accepting an out-of-trim load requiring back pressure on the yoke. Glance in and note reduced speed, when in the white arc, extend one stage of flap WHILST MAINTAINING THE PITCH ATTITUDE. Expect a change in amount/direction of force required on the yoke, but maintain attitude. Select second stage of flap, set new attitude to maintain 75kt, adjust power to maintain. As aircraft settles to new attitude, note force required on yoke and re trim. DO NOT trim during all these changes. You will probably find that the net trim change with the re configured aircraft is very small or zero. With this technique, there shouldn't be surprises and the speed should remain above the minimum safe speed.
TOO

EXDAC
9th Nov 2023, 12:50
I had not realized that the PA-28-180 fuselage and wing extension did not happen at the same time. Mine is the last year of the straight wings (1975).

Discussion of the various PA-28 wings, with dimensions, here - https://charles-oneill.com/blog/cherokee-tapered-wing-float/

BigEndBob
10th Nov 2023, 22:15
As a training aircraft Cherokee 140 and 160 are much better than the optimised for cruise warrior and archer.
Older aircraft pitch down with flap requiring nose up trim, which with experience you van put on a bit extra up trim to assist with the flare.
The warrior is not so nice in the hold off students struggle to flare especially the heavier max wt. 2440 variants and because they pitch up with flap application, too much nose down trim can be applied.