High DA; no flaps

[VW Pilot]

Ok I got cha.

 

[void]

C-172RG is kind of a bad example. It does say in the POH 0 degrees for normal and short field take offs. But for Soft field at anything less than 2550 lbs, (2650 is MTOW) you can use 10 degrees for take off to decrease ground roll. Regardless the Cutlass is a pig in climb.

I've flown a BE-76 and it says 0 degrees for all take offs. However a DE told me that it used to authorize flaps for short field and it was taken out for some strange reason. I couldn't confirm that anywhere. He then demonstrated a short field with 10 degrees for me.

 

[midlifeflyer]

I'm having a hard time with the logic that the use of flaps can increase the takeoff roll...induced drag doesn't happen until an airfoil creates lift. Using flaps during the takeoff will enable the wing to create lift at a lower airspeed which would equate to the airplane breaking ground in less distance.

It's not the induced drag. It's the form drag of those barn doors hanging down being pulled by a weak engine. It will simply take much longer to get to that airspeed.

You can test it empirically. A Cessna 182's standard short field takeoff configuration is with 20° flaps. So, set the flaps at 20° and the power at 22" MP. See how much runway you use to get to rotation speed. Now, do the same with no flaps. I think you'll find that it will take longer to get to the target rotation speed with flaps than without. In a real 10,000' density altitude situation the performance will be even worse.

 

[Freight Dawg]

In jets it is to insure second segment climb performance, if you lose an engine going out of MMTO (Toluca, Mexico). It has a field elevation of roughly 8650' imagine that on a hot day. Same principle applys to all aircraft.

 

[Doc Holiday]

Flaps add both lift and drag. When the air is thinner, like at high density altitudes where both the engine and the airfoils (including the propeller) are less efficient, the net effect is that the increase in drag of even minimal flaps is greater than the lift benefit. The drag will actually extend the takeoff roll under these circumstances rather than decrease it.

This is where I disagree with your logic. Airfoils do not care what the density altitude is. All they know is how many molecules of air are moving over them. What comes into play at high DAs is how much power the engine can produce. When engines become less efficient(including the prop), they cannot overcome the normal induced drag created by lift. Therefore, the wing must be operated at a reduced angle of attack to match the engine's power. Simply being at a higher DA does not mean that a wing creates more drag.

 

[midlifeflyer]

This is where I disagree with your logic. Airfoils do not care what the density altitude is.

I guess it depends on you're definition of "care." Propeller efficiency cares because there is more slippage when the air is thinner and the wing cares in the sense that it will take more true speed and hence a longer amount of time to reach the speed necessary to have the required AoA.

But, it's not really about lift and drag. It's about =power= and drag. And the engine definitely cares about the density altitude. It's operating at a reduced power output. Things hanging down from the airplane increase drag and make the airplane more difficult to pull. How much runway will it take to get a 152 into the air if you pull it with a bicycle? Will it take more or less time or make it harder or easier for the bicycle to pull it over the ground if the flaps are deployed than if they are not?

I won't quibble with you on a theoretical basis. As I said, you can show this empirically. Come on up with me to Leadville CO in a 182, and we'll test it out together.

 

[UndauntedFlyer]

The use of flaps will always get the airplane airborne in a shorter distance, even at a high DA. The problem when operating at a high DA airport is climbing out of ground effect. Ground effect will allow an airplane to become airborne when it actually has so much drag that it can not climb above this cusion of air that has helped it. Flaps in this situation only allow the airplane to become airborne sooner and with even more drag, preventing a climbout to any higher than above just a few feet AGL.

So at high DA, it's 0-degrees of flap if allowed in the POH, hold on the ground as long as possible to gain the greatest lift-off speed and rotate to a shallow pitch angle that will allow the fastest airborne acceleration, hopefully before the airplane strikes anything solid.

Your further questions or comments are always welcome. ...

 

[midlifeflyer]

hold on the ground as long as possible to gain the greatest lift-off speed and rotate to a shallow pitch angle that will allow the fastest airborne acceleration, hopefully before the airplane strikes anything solid.

One of the things we teach out here is a little different than that. Since even the friction of the tires on the ground can be a bit too much extra drag for the engine, many of us teach a modified soft-field style takeoff, getting the airplane off the ground at as low an airspeed as possible and the level off into ground effect to accelerate to normal climb speed.

 

[UndauntedFlyer]

One of the things we teach out here is a little different than that. Since even the friction of the tires on the ground can be a bit too much extra drag for the engine, many of us teach a modified soft-field style takeoff, getting the airplane off the ground at as low an airspeed as possible and the level off into ground effect to accelerate to normal climb speed.

This sound like a very good technique.

 

[Doc Holiday]

midlife, i'm at a loss here...undaunted said that "use of flaps with always get the airplane airborne in a shorter distance, even at a high DA". you didn't pull any "empirical" arguments with that...