Flaps and Performance

spiderweb

spiderweb

Final Approach
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Ben
I have a question about flaps and performance.

On youtube, there is a long video of a complete oral exam. At one point, the examiner asks the applicant about flaps and performance at a high-elevation airport. The student says, he wouldn't use flaps, as that would decrease performance.

Is there something I am missing here? I would have answered that I would use the recommended takeoff configuration for maximum performance, as specified in the POH. Often, as you know, that would include the use of takeoff flaps.

Thoughts? Anyone see that video?
 
Flaps extended (to takeoff setting) will generally decrease your takeoff distance required but you will get better climb performance flaps up. In the Citation jets that I fly, flaps up numbers are better for second segment climb. It requires a greater takeoff distance, but you get better climb performance with a clean wing (flaps up).

On my type ride, the examiner asked me to run the second segment climb numbers for a simulated high density altitude departure. We were able to meet the required climb gradient flaps up, but not flaps 15. Check the performance charts for the airplane you fly.
 
As an example, running airport analysis for runway 33 at Aspen at 32 deg C in the CE-510, our flaps up max takeoff weight is 7,321 lbs vs flaps 15 max takeoff weight of 6,746 lbs.

The flaps 15 max T/O weight is more restrictive due to the required climb gradient but taking off flaps 15 yields a shorter takeoff distance. This is even taking into account an alternate departure procedure off of runway 33.
 
HPNPilot1200 said:
As an example, running airport analysis for runway 33 at Aspen at 32 deg C in the CE-510, our flaps up max takeoff weight is 7,321 lbs vs flaps 15 max takeoff weight of 6,746 lbs.

The flaps 15 max T/O weight is more restrictive due to the required climb gradient but taking off flaps 15 yields a shorter takeoff distance. This is even taking into account an alternate departure procedure off of runway 33.
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OK, so my answer would have been best. If the POH recommends flaps for reduced T/O distance, that would be better, as a high-density airport means a longer run. After takeoff, and obstacles cleared, set flaps to recommended cruise climb configuration (and in a simple single-engine airplane, that'd probably be flaps-up).

So, that student's comment that flaps reduce performance is perplexing.
 
spiderweb said:
OK, so my answer would have been best. If the POH recommends flaps for reduced T/O distance, that would be better, as a high-density airport means a longer run. After takeoff, and obstacles cleared, set flaps to recommended cruise climb configuration (and in a simple single-engine airplane, that'd probably be flaps-up).

So, that student's comment that flaps reduce performance is perplexing.
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If you are trying to clear terrain off of the departure end of the runway and have a long enough runway to takeoff flaps up, that would likely be a good option.

However, if you can't takeoff flaps up with the runway distance available, then you either have to consider a different flap configuration or wait until it cools off (morning or evening) when you have a lower density altitude that may allow you to takeoff flaps up.
 
HPNPilot1200 said:
If you are trying to clear terrain off of the departure end of the runway and have a long enough runway to takeoff flaps up, that would likely be a good option.

However, if you can't takeoff flaps up with the runway distance available, then you either have to consider a different flap configuration or wait until it cools off (morning or evening) when you have a lower density altitude that may allow you to takeoff flaps up.
Click to expand...

In the video, the examiner says that there are no takeoff obstacles. So, that removes the consideration of angle of climb, and leaves it to rate, solely.

In that case, I want min takeoff roll. Once I am in the air, I can remove flaps, as recommended in the POH for efficient climb. Does that sound correct?
 
I am by no means a high DA expert, but I believe that at some point the flaps add more drag than lift.The specific effect will probably varies between airplanes.

To give you an example, I was out one day doing slow flight in my 145 hp 170 at 8000' and at full power and full flaps, I was descending and my AFM calls for full flaps as recommended short field takepff configuration. In my case, full flaps might shorten the ground roll, but I'd never be able to climb away from the runway.
 
spiderweb said:
OK, so my answer would have been best. If the POH recommends flaps for reduced T/O distance, that would be better, as a high-density airport means a longer run. After takeoff, and obstacles cleared, set flaps to recommended cruise climb configuration (and in a simple single-engine airplane, that'd probably be flaps-up).
Click to expand...
This is aircraft-specific, and not a "one-size-fits-all" situation. In some cases, the decreased takeoff roll might be outweighed by the extra distance for flap retraction before climb is achieved. In others, it might be the other way around. There's just no one single answer for all aircraft and DA's.
 
spiderweb said:
I have a question about flaps and performance.

On youtube, there is a long video of a complete oral exam. At one point, the examiner asks the applicant about flaps and performance at a high-elevation airport. The student says, he wouldn't use flaps, as that would decrease performance.

Is there something I am missing here? I would have answered that I would use the recommended takeoff configuration for maximum performance, as specified in the POH. Often, as you know, that would include the use of takeoff flaps.

Thoughts? Anyone see that video?
Click to expand...

Maybe he was flying a cub on his check ride and adding flaps to the plane would truly decrease performance. Plus, I don't think you can get a field approval for that from the DPE.
 
spiderweb said:
In the video, the examiner says that there are no takeoff obstacles. So, that removes the consideration of angle of climb, and leaves it to rate, solely.

In that case, I want min takeoff roll. Once I am in the air, I can remove flaps, as recommended in the POH for efficient climb. Does that sound correct?
Click to expand...

Yes. As the others have mentioned, it is airplane specific.
 
Fearless Tower said:
I am by no means a high DA expert, but I believe that at some point the flaps add more drag than lift.The specific effect will probably varies between airplanes.
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It shouldn't. Drag and lift forces are both proportional to density for a given configuration, so L/D is the same for all altitudes.

The only relevant variable is reduced engine power. So, if the climb rate ever gets to zero, you're SOL. But that's what takeoff performance tables are for. And they always specify short field configuration -- e.g., 20 deg flaps on a C182.
 
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MAKG1 said:
The only relevant variable is reduced engine power. So, if the climb rate ever gets to zero, you're SOL. But that's what takeoff performance tables are for. And they always specify short field configuration -- e.g., 20 deg flaps on a C182.
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That makes sense. At 8000' on a warm day I'm not even close to getting 145hp. The challenge for those of us who fly older aircraft is that the performance tables are pretty minimal.

My airplane AFM is a 2 page document (1 page front and back). There is a basic takeoff performance table, but no short field - just an arbitrary note to 'decrease calculated takeoff by X percent' for short field technique, but as mentioned at the higher DA, short field technique isn't going to get you anywhere but into the trees.
 
Changing the configuration after takeoff doesn't provide any net benefit insofar as close-in terrain clearance is concerned.
spiderweb said:
In the video, the examiner says that there are no takeoff obstacles. So, that removes the consideration of angle of climb, and leaves it to rate, solely.

In that case, I want min takeoff roll. Once I am in the air, I can remove flaps, as recommended in the POH for efficient climb. Does that sound correct?
Click to expand...
 
Fearless Tower said:
That makes sense. At 8000' on a warm day I'm not even close to getting 145hp. The challenge for those of us who fly older aircraft is that the performance tables are pretty minimal.

My airplane AFM is a 2 page document (1 page front and back). There is a basic takeoff performance table, but no short field - just an arbitrary note to 'decrease calculated takeoff by X percent' for short field technique, but as mentioned at the higher DA, short field technique isn't going to get you anywhere but into the trees.
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Then you're doing it wrong. Vx is higher at high DA (and Vy is lower). But Vx is still Vx. L/D is not a function of air density for a given configuration (though it IS a function of air speed -- so get the numbers right!).

And you must take off already leaned to best power. If performance is critical, during a full-power run-up.
 
What are you after? I get my best climb angle with flaps 20 but best rate with no flaps
 
Ron Levy said:
This is aircraft-specific, and not a "one-size-fits-all" situation. In some cases, the decreased takeoff roll might be outweighed by the extra distance for flap retraction before climb is achieved. In others, it might be the other way around. There's just no one single answer for all aircraft and DA's.
Click to expand...

That's why I would either have gone with POH recommendation for conditions, which that student should have had memorized. I -think- but am not sure that he was flying a Cessna 172.

If so, I can't figure out his response. The examiner specified no obstacles, so even if climb rate would initially stink, that rate wouldn't matter (except in the case of engine loss).
 
wabower said:
Changing the configuration after takeoff doesn't provide any net benefit insofar as close-in terrain clearance is concerned.
Click to expand...

Right, but the examiner specified no obstacles. Maybe it was understood that there was terrain further ahead, though.
 
spiderweb said:
I have a question about flaps and performance.

On youtube, there is a long video of a complete oral exam. At one point, the examiner asks the applicant about flaps and performance at a high-elevation airport. The student says, he wouldn't use flaps, as that would decrease performance.

Is there something I am missing here? I would have answered that I would use the recommended takeoff configuration for maximum performance, as specified in the POH. Often, as you know, that would include the use of takeoff flaps.

Thoughts? Anyone see that video?
Click to expand...

You are correct about reference to the POH. It is airplane specific - even the C-172 and C-152 are different:

If the airplane happens to be a Cessna 172M, then you'll find these statements in the "Normal Procedures", "Takeoff", "Wing Flap Settings" paragraphs:

"The use of 10 degree flaps will shorten the ground run approximately 10%, but this advantage is lost in the climb to a 50-foot obstacle." ... "If 10 degree of flaps are used for the minimum ground runs, it is preferable to leave them extended rather than retract then in the climb to the obstacle." ... "During a high altitude takeoff in hot weather where climb would be marginal with 10 degree flaps, it is recommended that the flaps not be used for takeoff."

But in the Cessna 152, specifically the 1985 POH, it states:
"Using 10 degree flaps reduces the total distance over an obstacle by approximately 10%."
The "Takeoff Distance [Short Field]" chart for the C-152 has entries for altitudes from sea level to 8000ft - with the condition always being that flaps are at 10 degrees. I could find no recommendation of zero degree flaps for hot and high short field takeoffs in a C-152.
 
Interesting.

Jim Logajan said:
You are correct about reference to the POH. It is airplane specific - even the C-172 and C-152 are different:

If the airplane happens to be a Cessna 172M, then you'll find these statements in the "Normal Procedures", "Takeoff", "Wing Flap Settings" paragraphs:

"The use of 10 degree flaps will shorten the ground run approximately 10%, but this advantage is lost in the climb to a 50-foot obstacle." ... "If 10 degree of flaps are used for the minimum ground runs, it is preferable to leave them extended rather than retract then in the climb to the obstacle." ... "During a high altitude takeoff in hot weather where climb would be marginal with 10 degree flaps, it is recommended that the flaps not be used for takeoff."

But in the Cessna 152, specifically the 1985 POH, it states:
"Using 10 degree flaps reduces the total distance over an obstacle by approximately 10%."
The "Takeoff Distance [Short Field]" chart for the C-152 has entries for altitudes from sea level to 8000ft - with the condition always being that flaps are at 10 degrees. I could find no recommendation of zero degree flaps for hot and high short field takeoffs in a C-152.
Click to expand...
 
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