High DA; no flaps

[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...

 

[lawfly]

Don't know if I agree with holding it on the ground to get the greatest lift-off speed possible. In a Part 25 airplane, you'll have a no-flap set of V-speeds, including Vr, if a no-flap take-off is an option. Maybe a slightly less brisk rotation, if you prefer, but you are committed at V-1. Remember that the true airspeed at a high density airport is higher. So, you are flying "faster", in terms of obstacle clearance, runway needed to achieve V-1, V-R, etc. According to an excerpt from the 8400-series Handbook at the FAA website, less flaps at high density altitudes means a longer takeoff run, but an improved climb gradient. Aerodynamic drag is cited in the excerpt as one reason for the effect of flaps on climb performance in such a scenario. (Another way to say it might be less lift, or an unacceptable lift/drag ratio.) Climb gradient is a function of both rate of climb and groundspeed. As noted, at high density altitude, TAS (and, hence groundspeed) is already higher, and if the flaps are detracting from climb capability more than usual (they usually detract from climb rate capability, but allow instead for a steeper gradient during the initial climb), there is a point at which a no flaps takeoff could produce a better climb gradient, despite the higher takeoff V speeds invloved (for a given weight).
To say that an airfoil is insensitive to air density seems intuitively at odds with an AOA readout at high altitude (and relatively high true airspeed) in various configurations, as compared to AOA readout in the same configurations at much lower altitudes. Take the extreme hypothetical: zero or "trace" air density. Will the wing fly? (Loss of thrust is significant, too, of course, but focusing for the moment on the other aerodynamic effects.)
Incidentally, I have begun to review limitations for an airplane I'm about to train in--there is a max altitude for flap extension, not just a max airspeed.

 

[lawfly]

A couple of additional observations: If it was just about engine output, then what of turbocharged piston engines? Airplanes so equipped are still susceptible to some effects of density altitude and, possibly, to the flaps or no flaps question.
And as to the high altitude vs. low altitude AOA indications I mentioned, can be high pressure altitiude vs. lower pressure altitude, or, in effect, same pressure altitude but different density altitudes, i.e., different SAT. For me, effects of temperature and altitude (principally density, I would think) were learned again in high altitude flight. I defer to the many more experienced high altitude fliers here, but to be at FL430 one day, well within the envelope, and to be at or climbing to the same flight level a few days later, with temps 6 to 7 degrees C warmer and to see the difference in AOA and Mach number, was quite instructive. That is not a takeoff or flaps secnario, but still instructive on temperature/density, I would think.

 

[midlifeflyer]

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...

Well, there comes a point in some airplanes when the density altitude becomes high enough to degrade the engine power and thrust to the point where the increased form drag of flaps will extend rather than shorten the takeoff roll. And you can prove this to yourself with airplanes with high drag flaps, a low power setting to simulate the altitude, and a long enough runway.

But repetition becomes a bit silly after a while, don't you think? We're certainly allowed to disagree with each other on this, even if we agree on related things. After all, the initial post asked why a POH was recommending it. No one is suggesting that the POH be discrearded; we are just arguing about why.

 

[UndauntedFlyer]

Originally Posted by 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...

I guess I should have said that if it's going to get airborne at all, that the use of some degree of flaps, maybe just 1 degree, will get the airplane airborne in a shorter distance. Certainly with too much flap, like 40 degrees, that isn't going to help.

But the general rule for flaps is high density altitude use minimum or no flaps. At lower density altitudes use flaps or no flap depending on runway length. Short runways use takeoff flaps. And for obstacles ahead, usually it is about a wash, but I tend to believe that with say 10 degrees that would help if the airplane was really flown at max performance airspeeds after liftoff. Of course I'm talking about typical GA aircraft where some experience is part of the process.