Go Back  PPRuNe Forums > Aircrew Forums > Flying Instructors & Examiners
Reload this Page >

Best Glide Speed change with flap?

Flying Instructors & Examiners A place for instructors to communicate with one another because some of them get a bit tired of the attitude that instructing is the lowest form of aviation, as seems to prevail on some of the other forums!

Best Glide Speed change with flap?

Old 14th Dec 2011, 05:07
  #1 (permalink)  
Thread Starter
 
Join Date: Aug 2009
Location: europe
Posts: 6
Likes: 0
Received 0 Likes on 0 Posts
Best Glide Speed change with flap?

Hey!

Does the speed for best glide change with flap?
makelove is offline  
Old 14th Dec 2011, 06:35
  #2 (permalink)  
 
Join Date: Jun 2001
Location: UK
Posts: 3,982
Likes: 0
Received 1 Like on 1 Post
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!
fireflybob is offline  
Old 14th Dec 2011, 09:53
  #3 (permalink)  
Moderator
 
Join Date: Feb 2000
Location: UK
Posts: 14,181
Received 41 Likes on 20 Posts
Of course it does, you've just changed the aerodynamic characteristics of the wing. It would be pretty odd if it didn't.

G
Genghis the Engineer is offline  
Old 14th Dec 2011, 10:21
  #4 (permalink)  
 
Join Date: May 2001
Posts: 10,815
Likes: 0
Received 0 Likes on 0 Posts
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.
mad_jock is offline  
Old 14th Dec 2011, 10:24
  #5 (permalink)  
 
Join Date: Oct 2004
Location: UK
Posts: 6,558
Likes: 0
Received 2 Likes on 2 Posts
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.
Whopity is offline  
Old 15th Dec 2011, 01:08
  #6 (permalink)  
Thread Starter
 
Join Date: Aug 2009
Location: europe
Posts: 6
Likes: 0
Received 0 Likes on 0 Posts
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?
makelove is offline  
Old 15th Dec 2011, 02:00
  #7 (permalink)  
 
Join Date: Jan 2004
Location: Canada
Age: 63
Posts: 5,103
Received 129 Likes on 56 Posts
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.
Big Pistons Forever is offline  
Old 15th Dec 2011, 03:18
  #8 (permalink)  
Moderator
 
Join Date: Aug 2006
Location: Ontario, Canada
Age: 62
Posts: 5,586
Received 55 Likes on 39 Posts
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....
Pilot DAR is offline  
Old 15th Dec 2011, 05:22
  #9 (permalink)  
 
Join Date: Jan 2004
Location: Canada
Age: 63
Posts: 5,103
Received 129 Likes on 56 Posts
Originally Posted by Pilot DAR

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.
Big Pistons Forever is offline  
Old 15th Dec 2011, 10:03
  #10 (permalink)  
 
Join Date: Jan 2008
Location: London UK
Posts: 517
Likes: 0
Received 0 Likes on 0 Posts
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?
24Carrot is offline  
Old 15th Dec 2011, 13:54
  #11 (permalink)  
Moderator
 
Join Date: Aug 2006
Location: Ontario, Canada
Age: 62
Posts: 5,586
Received 55 Likes on 39 Posts
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.
Pilot DAR is offline  
Old 15th Dec 2011, 16:05
  #12 (permalink)  
 
Join Date: Jan 2004
Location: Canada
Age: 63
Posts: 5,103
Received 129 Likes on 56 Posts
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 is offline  
Old 15th Dec 2011, 16:26
  #13 (permalink)  
 
Join Date: Jan 2004
Location: Canada
Age: 63
Posts: 5,103
Received 129 Likes on 56 Posts
Originally Posted by 24Carrot
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.
Big Pistons Forever is offline  
Old 15th Dec 2011, 16:45
  #14 (permalink)  
 
Join Date: Aug 2000
Location: UK
Posts: 3,648
Likes: 0
Received 1 Like on 1 Post
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?
bookworm is offline  
Old 17th Dec 2011, 14:33
  #15 (permalink)  
 
Join Date: Jan 2008
Location: London UK
Posts: 517
Likes: 0
Received 0 Likes on 0 Posts
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.
24Carrot is offline  
Old 19th Dec 2011, 11:07
  #16 (permalink)  
 
Join Date: Sep 1999
Location: UK
Posts: 89
Likes: 0
Received 0 Likes on 0 Posts
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!!!
FOUR REDS is offline  
Old 19th Dec 2011, 12:37
  #17 (permalink)  
 
Join Date: May 2001
Posts: 10,815
Likes: 0
Received 0 Likes on 0 Posts
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.
mad_jock is offline  
Old 19th Dec 2011, 13:08
  #18 (permalink)  
 
Join Date: Feb 2009
Location: England
Posts: 858
Likes: 0
Received 0 Likes on 0 Posts
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.
Pull what is offline  
Old 19th Dec 2011, 13:19
  #19 (permalink)  
 
Join Date: Sep 2003
Location: UK,Twighlight Zone
Posts: 0
Likes: 0
Received 0 Likes on 0 Posts
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?
S-Works is offline  
Old 20th Dec 2011, 02:25
  #20 (permalink)  
 
Join Date: Jun 2001
Location: Australia
Posts: 889
Likes: 0
Received 0 Likes on 0 Posts
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.
Oktas8 is offline  

Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service

Copyright © 2024 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.