Flaps to aid climbing - truth or fiction

[FastEddieB]

For now, can we focus on positive flap settings? I have zero experience or knowledge of the effects of negative flaps.

[FastEddieB]

Here's the math: 

Yes. One key is that any time you increase lift, you increase drag via the rearward component of that lift. Some thrust must be used to counter that increase, leaving less excess with which to climb.

 

Unless some small flap settings actually do reduce drag, as may be the case with the CT.

 

Leaving the question, why not just leave them down? If they provide a real drag reduction, Flight Design would have had incentive to beef up the flap mechanism to handle higher speeds and get a drag reduction across the board - even in cruise.

 

Right?

[Anticept]

Nevative flaps go above the chord line. It's called reflexing flaps. Large camber wings can recover a little bit of drag at speed with reflexing flaps when done right, yet have little impact at slower speeds. Large camber wings also provide a bit more lift at the low end of speed, which is why FD chose them. Europe is a lot of grass fields and the low performance is necessary. It gives the airplane a little more versatility but sacrifices a little top end speed, if all else is equal.

 

I'm going to be straight with you guys; you're going into aeronautical engineering territory with a lot of fluid dynamics in play. The best bet is to throw the pilot book out of the window because stuff can get weird, and you can discuss it until blue in the face and still not scratch the surface.

 

And I wouldn't jump on the idea that something is a design flaw just because it doesn't fit the convention. Engineering has come up with a pretty good idea of what an average, typical airplane would look and act like, but I would say there is no such thing as completely typical!

[Ed Cesnalis]

Yes. One key is that any time you increase lift, you increase drag via the rearward component of that lift. Some thrust must be used to counter that increase, leaving less excess with which to climb.

 

Unless some small flap settings actually do reduce drag, as may be the case with the CT.

 

Leaving the question, why not just leave them down? If they provide a real drag reduction, Flight Design would have had incentive to beef up the flap mechanism to handle higher speeds and get a drag reduction across the board - even in cruise.

 

Right?

The Answer is easy, we can only go 120kts+ TAS with the flaps up in the reflex position.  With the flaps down one notch to the 15 degree position speed is limited by Vfe and physics.  At 15* you would be able to pass us in your pusher!

 

So something is wrong, possibilities:

 

• My test was flawed - perhaps I spent too little time at 15* and only thought I had stabilized to a steady state
• The 'excess thrust' concept is flawed
• The small number concensus here is wrong

[Anticept]

Oh by the way CT, speed matters too. Try again at a higher and lower airspeed and see what happens to your numbers.

[Ed Cesnalis]

Oh by the way CT, speed matters too. Try again at a higher and lower airspeed and see what happens to your numbers.

 

I had to vary my speed to seek best climb at that setting.  -6 speed was 49kts, other speeds were in a more normal range.  

[ctfarmer]

Surely speed is the huge variable.

 

My thoughts.

Drag is drag of the whole air frame. That will drop markedly at a lower speed. Proportional to V^2. So if one flap configuration gives higher lift at 80kts and than another at 100kts and the overall drag of the air frame is less at 80kts, then there is more power available in climb at the lower speed. So the rate of climb will be better for the low speed high lift flap configuration.

 

Or am i wrong here?

[Ed Cesnalis]

Surely speed is the huge variable.

 

My thoughts.

Drag is drag of the whole air frame. That will drop markedly at a lower speed. Proportional to V^2. So if one flap configuration gives higher lift at 80kts and than another at 100kts and the overall drag of the air frame is less at 80kts, then there is more power available in climb at the lower speed. So the rate of climb will be better for the low speed high lift flap configuration.

 

Or am i wrong here?

 

I think your right.   15* climbs really well but with a big speed penalty.

[FastEddieB]

Surely speed is the huge variable.

 

My thoughts.

Drag is drag of the whole air frame. That will drop markedly at a lower speed. Proportional to V^2. So if one flap configuration gives higher lift at 80kts and than another at 100kts and the overall drag of the air frame is less at 80kts, then there is more power available in climb at the lower speed. So the rate of climb will be better for the low speed high lift flap configuration.

 

Or am i wrong here?

You're wrong in that you're only considering parasitic drag.

 

Induced drag is the culprit at lower speeds.

 

 

Total drag goes up both at high speeds and low speeds.

 

I have learned from this thread that very small flap settings in some planes may slightly reduce drag, which I did not know. The CT appears to be one of these if Ed's figures are correct. Still working on whether that means the wing could have been designed for a bit less drag with a bit more curvature, and a few degrees of flaps are just getting it where it could have been in the first place - with a slightly lower drag profile.

[Cluemeister]

What I just noticed in all of this discussion is that Eddie has 3,330 posts, and CT has 3,322 posts. I'm wondering if Eddie is responding to CT every time to keep an 8 post lead.

 

Just my theory. 

[Tom Baker]

You're wrong in that you're only considering parasitic drag.

 

Induced drag is the culprit at lower speeds.

 

 

Total drag goes up both at high speeds and low speeds.

 

I have learned from this thread that very small flap settings in some planes may slightly reduce drag, which I did not know. The CT appears to be one of these if Ed's figures are correct. Still working on whether that means the wing could have been designed for a bit less drag with a bit more curvature, and a few degrees of flaps are just getting it where it could have been in the first place - with a slightly lower drag profile.

 

What if the reduction in parasitic drag is greater than the increase in induced drag?

[FastEddieB]

It Ed's figures are correct that may be what we're seeing in the case of the CT. I still hold that that's an unusual circumstance in the GA fleet at large

[Tom Baker]

It Ed's figures are correct that may be what we're seeing in the case of the CT. I still hold that that's an unusual circumstance in the GA fleet at large

 

I don't disagree. That is why I have told you before that the CT is not like every other airplane, and should not be treated as such.

[FlyingMonkey]

I don't disagree. That is why I have told you before that the CT is not like every other airplane, and should not be treated as such.

The CT is manufactured to exacting specifications using a combination of pixie dust, dreams, and unicorn tears...It's magic!   

[FastEddieB]

The CT is manufactured to exacting specifications using a combination of pixie dust, dreams, and unicorn tears...It's magic!   

Wait! I thought that was the Cirrus!

 

Seriously, in the final analysis they're all just planes.

[John Vance]

Setting the flaps at 15 degrees results in a higher drag coefficient than at 0 or -6.  So if you compare actual drag force at 15 degrees flaps vs the lower flap settings, it will always be higher at 15 given the same airspeed.  For example, I wouldn't anticipate getting a 120 kt cruise with 15 deg flaps (plus you don't want to try).  However, if you use a lower airspeed when climbing with 15 deg. setting vs a climb at a 0 degree setting (which is typically the case), drag could be lower when climbing with 15.  That being said, the airplane doesn't climb because of an excess of power, it climbs because of an excess of lift. 

 

John

[FastEddieB]

That being said, the airplane doesn't climb because of an excess of power, it climbs because of an excess of lift.

 

John

That has been contradicted by numerous sources both online and hard copy.

 

Most recently by Rod Machado:

 

"One of aviation's biggest misconceptions is that airplanes climb because of excess lift. This is similar to believing that putting hand lotion in your airplane's fuel tank will make your landings smoother, softer, and younger looking.

 

Airplanes climb because of excess thrust, not excess lift. Let's return to the example of a car on the road to learn a little bit more about why this is."

 

Before that by Peter Garrison.

 

Pretty sure the PHAK and Aerodynamics For Naval Aviators agree.

 

Please cite your source - maybe we can have dueling sources!

[gbigs]

Setting the flaps at 15 degrees results in a higher drag coefficient than at 0 or -6.  So if you compare actual drag force at 15 degrees flaps vs the lower flap settings, it will always be higher at 15 given the same airspeed.  For example, I wouldn't anticipate getting a 120 kt cruise with 15 deg flaps (plus you don't want to try).  However, if you use a lower airspeed when climbing with 15 deg. setting vs a climb at a 0 degree setting (which is typically the case), drag could be lower when climbing with 15.  That being said, the airplane doesn't climb because of an excess of power, it climbs because of an excess of lift. 

 

John

 

Correct on the flaps/drag relationship.  A powered aircraft climbs when changing the DIRECTION of thrust.  

 

Flap setting of 0 in the CT still has a tiny bit of drag...the -6 setting the least drag for all flap settings.  That's why -6 works best in cruise and in climb as long as you have enough power to climb at a given angle of attack.

[John Vance]

The comment I made about climb and excess lift just confuses the issue.  It's true in a sense, but depends upon how you draw the coordinate frame (thrust/drag parallel to the horizon or parallel to the longitudinal axis). 

[Andy]

I'm with FastEddieB on this one, and my experiences with the CT match up as well. Climb performance is only a factor of excess power.  From chapter 9:6 Climb Performance, (The Static Performance Problem) "Airplane Aerodynamics",Dommasch, Sherby, Connoly (which we used for aircraft design performance studies in our aero engineering classes): Rate of climb (in ft/min) = 33,000 (excess HP)/W where W = weight.  (Assuming flight path angle is small, which holds for flight path angles less than 15 degrees to the horizontal.)  Notice the controlling term is excess HP, i.e, the power required beyond that necessary to maintain level unaccelerated flight.  Power required is going to be driven by drag, and I think Eddie might be right about the factor primarily being changed with flap setting being induced drag.  Going to play with Charlie Tango's numbers and see what I can back out in my spare time (which is very limited).  

[FastEddieB]

As I hypothesized, to get Ed's numbers, maybe a small flap setting in the CT changes the pitch angle just enough to "nudge" the fuselage or some component - wheel pants? - into a more streamlined attitude, decreasing form drag just a bit more than the increase in induced drag caused by the flaps.

 

You've obviously delved far deeper into the engineering aspect of this than have I. Can you recall any exercise or real world example where flaps helped either rate of climb or service/absolute ceiling?

 

IMWTK!

[Ed Cesnalis]

I'm washing my plane this morning and discovered that my flaps are out of adjustment.  I have 2 reflex settings -6 and zero. hmmmm

 

What does this mean for my test?

 

We have said a lot about what flaps do, they increase the camber of the wing and add lift and drag.  What about reflex?  What do they do?  We know they minimize drag but instead of decreasing camber I say they shorten the chord producing a smaller wing with less drag and less left.

 

If my take on what reflex flaps do then all my test did was to determine that a bigger wing climbs better.

[FastEddieB]

Reflex flaps are pretty new to me.

 

Might they be causing a slight nose up moment, which would lead to the need for less tail down-force? Which would reduce the lift the wings needed to produce which would reduce their induced drag?

 

If that's the case, going to reflex flaps should require nose down trim. Does it?

 

Just thinking out loud.

[Ed Cesnalis]

A photo of my CT flying in formation over Tahoe for Outside Magazine shoot did show an extra high nose up attitude on my CT.

 

The trim is nose up for less flaps and nose down for more flaps including reflex.  

 

Looks like all my settings slipped about 5*  -6 measures -10; zero measure -5; 15 measures 10; 30 measures 25 and 40 measures 35.

 

I'll have to get that fixed, kind of a shame to fix it since every actual setting is more useful to me than the indicated settings.

[FastEddieB]

So, to make sure I'm digesting this right:

 

In your test, when you thought you had a better climb rate with flaps, you were actually pretty close to clean?