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Old 08-18-2008, 04:36 PM
MSFT-1's Avatar
MSFT-1 MSFT-1 is offline
Join Date: Feb 2006
Posts: 276
Default Reflex flap position = slower speed?

On my RV-10, the full up flap position (reflex position) seems to reduce speed by about 1 knot. I "think" Vans says that the reflex position should yield a couple knots of extra speed in cruise. Anyone with the 10 getting that performance?

Richmond, VA (KFCI)
RV-10 (520+ hours since first flight in Nov 07)
RV-8 (500 hours, sold Sept 07)
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Old 08-18-2008, 09:06 PM's Avatar is offline
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Location: Ogden Utah
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Default Flap Reflex

I have not measured it, but it feels like it accelerates when the flaps come up. I am going to Casper this weekend, I will measure it.......
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Old 08-19-2008, 05:07 PM
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sonny junell sonny junell is offline
Join Date: Aug 2007
Location: Midlothian
Posts: 147

What is the reflex postion, I am starting my qb in sept (x) (X)
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Old 08-19-2008, 08:33 PM's Avatar is offline
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Location: Ogden Utah
Posts: 155
Default Reflex

The flaps, ailerons and wing tips are all set in a neutral position that is 3 degrees reflex as compared to the wing. If you use the vans flap positioning system, it has 4 positions......Up (3 degrees reflex), 0, 1/2 and full flaps. Normally take off at the 0 degrees, cruise at the up (reflex) position.
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Old 08-20-2008, 02:47 AM
Bob Axsom Bob Axsom is offline
Join Date: Mar 2005
Posts: 5,685
Default Interesting subject

As I understand it from this thread the RV-10 flaps have a design position that is 3 degrees above the ailerons when they are both aligned with the tips. I do not know what the flap chord is on an RV-10 but the sin of 3 degrees is ~0.05234 so for each 12 inches of flap chord the misalignment would be a little over 6 tenths of an inch. Assuming the design airfoil is achieved when the flaps are in the zero position then this would be expected to be the most efficient shape for nominal cruise speed. If you do not need all of that lift and you have adequate power (thrust) you might experimentally find an optimum upward angle of the flap (reflex) from conformance to the design airfoil that sacrifices some lift and yields a maximum speed above that achieved with the design airfoil. If in this experimentation the designer found that the optimum reflex was 3 degrees but an operator flew at a slower speed than that required to produce sufficient lift to sustain level flight then the loss of lift would have to be compensated for with a higher angle of attack which would increase the induced drag and result in slower flight. There is absolutely no reason to reflex the flaps other than to increase the maximum speed. If your plane is flying slower with the flaps reflexed per the aircraft design you may need more thrust or less weight or a careful check of the rigging. If it were my airplane the flaps would never be placed in the reflex position unless this resulted in a higher maximum speed.

Bob Axsom
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Old 08-20-2008, 01:59 PM
TimO TimO is offline
Join Date: Feb 2005
Location: Wisconsin
Posts: 358

The reflex position is -3 degrees of flaps. It is in this position that the flaps and ailerons and wingtips are in alignment. I have found an approx. 2kt increase in speed with the flaps in reflex. In smooth air, you definitely feel the difference when you move them to this position. The zero degree position will not align with the ailerons and wingtips, but does give the standard airfoil shape.
Tim Olson - CFI
RV-10 N104CD - Flying 2/2006 - 1300+ hours
RV-14 N14YT - Flying 6/2016 - 175+ hours
RV-10/14 Transition Training:
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Old 08-22-2008, 01:41 PM's Avatar is offline
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Location: Ogden Utah
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Default Reflex

I did some testing today, 13500, smooth air with autopilot flying. I saw a 2-3 knot increase in airspeed with the flaps in the reflex position.

Last edited by Mike S : 08-22-2008 at 05:18 PM.
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Old 08-22-2008, 11:17 PM
RBurn RBurn is offline
Join Date: Apr 2007
Location: Lexington Park, MD
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Default Reflex decreases drag!

From an aerodynamic point of view putting in reflex does a few things...

1) Reduces the effective angle of attack on that portion of the wing.
2) Causes a pitch change (although minor).

Reducing the angle of attack on the wing reduces the amount of drag on the wing. If you keep the same engine settings the aircraft will accelerate until a new equilibrim is reached (unaccelerated flight).

Other possibly reasons are that reflex will change the pitching moments on the airplane and may result in an elevator position that causes less drag, further increasing speed.

To put it in math terms...

CL=coefficient of lift
roe=density of air
S=wing area

CL gets reduced when reflex is applied


So if CL gets reduced something in the denominator needs to increase for the equation to remain balanced. Density is constant and so is the wing area. Therefore, velocity is the only thing that is able to increase.

Hope this helps a little bit.

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Old 08-23-2008, 12:30 AM
Bob Axsom Bob Axsom is offline
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Posts: 5,685
Default CL has to assume some fixed conditions

I know I'm going to regret this post but here goes:

CL expressed in the CL=2W/(roe*V^2*S) has to assume certain constants to have meaning such as steady state level flight with a given airfoil. When the flaps are reflexed the airfoil is changed, adjustments will have to be made to the test vehicle to re-achieve steady state level flight and the dependent variable V will change producing a new CL for the modified airfoil. The statement "CL gets reduced when reflex is applied" sounds reasonable but that is a test determination for a specific reflex and a specific starting airfoil.

Bob Axsom
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