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Best Glide Speed change with flap?

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Best Glide Speed change with flap?

Old 20th Dec 2011, 13:16
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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!
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Old 20th Dec 2011, 13:45
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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.
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Old 20th Dec 2011, 22:25
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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.
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Old 20th Dec 2011, 23:01
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@ 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.
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Old 21st Dec 2011, 08:58
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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.
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Old 27th Dec 2011, 15:47
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go find out!

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
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Old 3rd Jan 2012, 22:18
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Flaps will lower those speeds

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.

Last edited by huv; 3rd Jan 2012 at 22:35.
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Old 4th Jan 2012, 05:45
  #28 (permalink)  
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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.
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Old 5th Nov 2023, 17:34
  #29 (permalink)  
 
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NO FLAPS for best GLIDE Range

Originally Posted by makelove
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.
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Old 7th Nov 2023, 09:55
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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.
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Old 7th Nov 2023, 17:48
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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.
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Old 7th Nov 2023, 18:05
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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.

Last edited by Fl1ingfrog; 7th Nov 2023 at 18:26.
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Old 7th Nov 2023, 18:22
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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.
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Old 7th Nov 2023, 19:56
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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.
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Old 7th Nov 2023, 20:36
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Originally Posted by Fl1ingfrog
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.


Last edited by EXDAC; 7th Nov 2023 at 20:46.
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Old 7th Nov 2023, 20:52
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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.

Last edited by Fl1ingfrog; 7th Nov 2023 at 22:38.
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Old 7th Nov 2023, 21:47
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Originally Posted by BigEndBob
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
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Old 9th Nov 2023, 00:34
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Originally Posted by Fl1ingfrog
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.

Last edited by EXDAC; 9th Nov 2023 at 02:50.
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Old 9th Nov 2023, 05:31
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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
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Old 9th Nov 2023, 13:50
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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/cher...ed-wing-float/




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