What is the best flap configuration for a short take off for a RV-9A

Folks I would like to post a question and hear opinion from your experience.

In experimenting and trying to decide what flap configuration and roll speed would yield the shortest take off am stuck between 10 or 20 degrees...

Anyone have data about that?

In theory there is the exact flap for a short take off , many think that the best would be start without flaps and apply them just before roll out. But that is untrue and a big mith.

There is a best flap configuration that even with small speeds would provide lift enough to counter the landing gear friction forces on start where offering almost no drag in that small speeds ... so the best flap configuration is a balance/compromise.

Anyone has actual data from experience or opinion in the matter? 10 or 20?

Many thanks.

degrees of t/off flap

I too wanted to know this. I was simply trying to establish the best glide speed and minimum sink; and thought if part flap provides more lift, you should use this setting anytime you want or need to maximize lift!
Van's response was to refer my question to John Roncz. He did not have exact data, but offered exactly what you suspect, it will be somewhere between 10 and 20 degrees, and you must establish that by testing.

How? Good question. I suspect timed glides using GPS to measure distance travelled, and then some triginometry at the kitchen table later!

I expect there might be small variations with different CG loading, but for the most part, I think there would be a practical 'best lift' setting.

I'm sure a lot of much smarter areo-gurus out there will chime in with some good feedback!

flyboy1963 said:

I too wanted to know this. I was simply trying to establish the best glide speed and minimum sink; and thought if part flap provides more lift, you should use this setting anytime you want or need to maximize lift!

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More lift also means more induced drag. Your best glide is going to be clean, at the bottom of the total drag curve. Where that is, I couldn't tell you. There are plenty of NTPS grads on here that could tell you how to figure it out.

In proper terminology, it's not a best glide speed, but angle of attack. That speed varies with weight and CG, in an RV it's probably not by enough to really matter unless that piece of pavement you're dead engine gliding to is no kidding in the last mile or so of your glide range.

Caveman method to figuring it out... setup a GPS point. And from that point at the same altitude and heading, do an idle decent at say 100 knots. See how far you get from that point in say 2000' of altitude loss. Then climb back up and go back and to it again at 95. Then again at 90, and so forth. The speed that gets you the most distance over the 2000' of altitude will get you in the ball park for that given weight. It's not an exact science, but a good way to spend some flight time, and will give you a good idea of your dead engine performance for energy managment in the event of an engine failure.

Your ailerons are typically rigged so that in the "down" position, they will provide the optimum lift/drag ratio. Usually this is somewhere around 15°. My recommendation is to push the stick to one side and align the flaps with the "down" aileron. That's about as close to optimum as you will get.

I have 3 notches on my flaps. 13°, 26°, and 39°. I use 13° for take-off. It shortens my take-off roll noticeably.

Mel said:

Your ailerons are typically rigged so that in the "down" position, they will provide the optimum lift/drag ratio. Usually this is somewhere around 15?. My recommendation is to push the stick to one side and align the flaps with the "down" aileron. That's about as close to optimum as you will get.
I have 3 notches on my flaps. 13?, 26?, and 39?. I use 13? for take-off. It shortens my take-off roll noticeably.

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Exactly what I do. Easy and repeatable method to set your electric flaps.

Well I think you folks completely missunderstood my question, maybe due to my bad english.

I would like to know what is the best flap configuration (if i should apply 10 or 20 degrees) to peform the shortest possible take-off.

Nothing about glide.... only a discussion about take-off distance and that implies/concerns lift / drag / reducing ground friction by applying more or less flap on roll out.

Anyone has any data or experience on that?

If you want data, you are going to have to add more info...

What is the limiting factor? TODA? TORA? Obstacles? Surface? Slope? HW? TW?

My own view, is that unless you are a qualified test pilot and willing to employ various specialist techniques (establish and fly VMU / accelerate in ground effect, climb on back of drag curve), it matters not a lot... and the safest technique will not involve minimising TO roll...

Andy

ALMARTON said:

Well I think you folks completely missunderstood my question, maybe due to my bad english.
I would like to know what is the best flap configuration (if i should apply 10 or 20 degrees) to peform the shortest possible take-off.
Nothing about glide.... only a discussion about take-off distance and that implies/concerns lift / drag / reducing ground friction by applying more or less flap on roll out.
Anyone has any data or experience on that?

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Pretty much the same thing applies to glide as to take-off run. Bill R. & I recommended "flaps" equal to the "down" aileron for short field take-off. Personally I use this same technique for all take-offs.

ALMARTON said:

Well I think you folks completely missunderstood my question, maybe due to my bad english.

I would like to know what is the best flap configuration (if i should apply 10 or 20 degrees) to peform the shortest possible take-off.

Nothing about glide.... only a discussion about take-off distance and that implies/concerns lift / drag / reducing ground friction by applying more or less flap on roll out.

Anyone has any data or experience on that?

Click to expand...

There has been plenty of "experience" reported here indicating 10-13 degrees works very well.

If you want hard "data" to back it up, you may have to gather it yourself.

I use 10 and it works well, 20 works also, 40 is too much drag. The forces of lift verses drag seem to be best around 10. No data to back it up, just experience. Measuring "ground friction by applying more or less flap" would require some very fancy acceleration instrumentation and that just does not happen with these very simple airplanes.

In any event, if the best flap setting based on engineering data and flight testing turns out to be say 13.85 degrees, it will be difficult to notice a difference from a 10 degree take off. There are too many variables in every take off including pilot technique to be that finite.

I would still say there is no "best setting"....

• Best RoC = Flaps Up
• Best Angle of Climb (still air) Flaps = X degrees
• As Headwind increases / AuW increases... best Angle of Climb will be <X but >0

That's for obstacle clearnance. Min Ground Roll will almost certainly involve:

1. Full Flap
2. Full Power, held against brakes (which will be trashing your Prop with FOD)
3. Full Backstick (or sufficient to get NW off ground)
4. Brakes off, stick/pitch a/r for NW just off
5. Rotate to Tail Skid touching ground at VMU

Trouble is, in some types this will get you airborne, but with more drag than thrust In (almost) any type, you now need to accelerate at low alttitude, in ground effect and clean up to some extent.

The latter is poor practice unless really required (and requiring it is poor practice)...

See Link for some unofficial guidance

Andy

Thanks Mel and thanks David that is what I was looking for I too tested (not with fancy instruments but by measuring take off distance) and found that something between 10 - 20 is the best , I even think that best is this magic 13.85 (can't apply it precisely though) !!!

Well that solved what I was looking for a "rule of thumb" flap configuration for short take offs.

Andy I got the link you sent very informative but way too theoretical too me... I think I can go with a little above 10 degress and do the shortest take off .

Again thank you all !

Nice Flying this weekend, regards from Brazil.