Flaps On T/O (obstacles)

[uwochris]

Hey guys,

Is it a common practice to use minimum flap setting on t/o from a runway with an obstacle nearby?

I understand that flaps should be used on short fields (i.e. they allow a shorter t/o run by allowing the a/c to lift off at a lower IAS because of the higher lift coefficient).

However, on a rwy with an obstacle nearby, we are concerned with our angle of climb (Vx). The best angle occurs at the point where we have the highest excess thrust (thrust available versus required to overcome drag... Thrust-Drag).

Is it correct to say that flaps REDUCE the angle of climb, because of the extra drag they create? Flaps should have no effect at all on the props ability to produce thrust, so it seems the excess thrust will be reduced, for any given IAS.

If this is true, why should we use flaps on t/o from an obstructed runway?

Perhaps the effect of a shorter t/o run and climbing out at a lower IAS with flaps extended offsets the reduced angle of climb effect? (i.e. benefits outweight costs).

Thanks in advance.

ps- for any regional/jet pilots, is it common practice to always t/o with flaps extended? When would you elect not to?

 

[Axel]

Flaps will shorten the ground run but will reduce climb performance. Your understanding is essentially correct that the reduced speed with flaps extended will serve to offset the reduced climb rate to a degree. Another factor is that the reduced ground run will result in a lift off point that is further from the obstacle than without flaps, thereby providing a greater distance in which to climb.

 

[dmspilot00]

If this is true, why should we use flaps on t/o from an obstructed runway?

The POH for the older Cessna 172s (M and earlier, not sure about others) specifically states that flaps should not be used for short-field takeoff because, although shortening the ground run, total takeoff distance to clear the obstacle would be longer with flaps. However, if flaps are used, for example because of a soft field, it is recommended to leave them extended and not retract them. Lesson: follow the POH.

 

[Joe Jet Pilot]

Depends On The Aircraft

for any regional/jet pilots, is it common practice to always t/o with flaps extended? When would you elect not to?

It depends on the airplane and what you are going to do with it. For example the airplane I fly has approved takeoff data for flaps 0, 10, and 20 degrees. Normal takeoff is with flaps 10. But lets say I need the climb gradient I might consider a flaps 0 takeoff (would need more runway to do this). On the other hand if a specific climb gradient is not required and I'm trying to get out of a short runway at the max. weight I can, I may need to use flaps 20.

Really it is no different then flying single engine or light twins in principle. We just have more to consider when it comes to climb performance.

 

[blueridge71]

When in doubt, read the instructions. The POH has the best information for your specific airplane.

 

[bobbysamd]

Obstacle takeoffs

Is it a common practice to use minimum flap setting on t/o from a runway with an obstacle nearby?

What the others have said is true. It depends on your airplane and POH recommendations.

Originally posted by blueridge71
When in doubt, read the instructions. The POH has the best information for your specific airplane.

Sadly, some POH recommendations are nearly worthless. I learned how to fly in an early '70s vintage 172. Not only was the ASI calibrated in miles per hour, the POH was the size of a pamphlet! The instructions about using flaps for takeoff were disturbingly vague: "Use the minimum flap setting for runway length." Thanks for all the guidance, Mr. Cessna. In any event, my instructor taught me to use zero flaps for an obstacle takeoff. Later-model 172s offered more guidance and recommended something like 10 degrees of flaps.

By the way, congratulations on your progress! I remember that not too long ago you had listed "student" in your profile.

 

[avbug]

It's all airplane-dependent. I fly some airplanes that use full flaps for takeoff; others use takeoff flaps settings, others are optional, some use some, all, or none depending in the circumstances.

The resulting use of flaps varies accordingly.

I've flown a few aircraft that saw 50 knot stall speed changes with 50% flaps; they were essential for takeoff.

In our STOL equipped agtrucks, we couldn't get the tail up without going to full flaps, that also drooped the ailerons. It practically shot the tail into the air, and then with full aft stick we came off the ground and were out of runway...and then it took a mile before we had adequate airspeed to bring the flaps up and make a turn to clear the parallel powerlines, and go to the field.

I failed to extend flaps last year on a shorter mountain airfield in a turbine dromader, and met the end of the runway without feeling ready to fly. I horsed it off and ran the trim full back, and it finally dawned on me as I was still debating dropping part of the load, that I had no flaps on. Too big a rush to get out, I guess. I applied some flap, and it felt a whole lot better, and I climbed out a whole lot better, too.

It all depends on the airplane.

 

[dmspilot00]

Re: Obstacle takeoffs

I learned how to fly in an early '70s vintage 172. Not only was the ASI calibrated in miles per hour, the POH was the size of a pamphlet! The instructions about using flaps for takeoff were disturbingly vague[/B]

The POH was rather brief, but the L and M models of the early 70s did in fact specifically state that flaps should not be used on a short-field takeoff where an obstacle was involved. I think this changed in the N model (but I'm not sure), perhaps because of the new engine, or maybe they just changed their mind. But it's better to follow the book, even if it may not be that great of a help, than nothing at all or guessing.