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Reflex Flap Setting

Bob Axsom

Well Known Member
If you go to http://www.ouchrockgallery.com then click on "Events" then click on "Taylor 100 Air Race" then scroll down the thumbnails at the left side of the display until you come to the red Lancair Race #36 in flight in turn 3, you will see an interesting thing (in addition to the rudder and elevator gap seals). If you look at the left aileron you will see it is down and think OK, he is rolling out of the turn but if you look at the other aileron it appears to be down also. Then after some thought realize that the flaps up above the normal airfoil configuration. Has anyone tested this on an RV to determine the best speed position of the flaps? In my RV-6A, except for the lower flap skin extension under the fuselage to form a gap seal when the flaps are up, this would be easy to implement by adjusting the linkage.

Bob Axsom
 
Bob
Many of us west coasters have tried this. If you have manual flaps you can figure it out for yourself by going flying and releasing pressure on the flap handle @ high speed until it is in trail with zero pressure. Note that position and that'll get you close.
Tom
RV3
 
I played around a few months ago with XFoil and AVL trying to see what they predicted about reflexing the flaps on an RV. XFoil provides predicted 2D airfoil lift and drag, and AVL predicts 3D lift, drag, etc on a wing and tail.



XFoil, which calculates 2D airfoil performance, does show a small reduction in drag by reflexing the airfoil. For example, at a CL of 0.15 (typical RV CL at high speed), the airfoil section CD is about 0.0062 with flaps at 0 degrees, and about 0.0052 with flaps reflexed up 5 degrees. The lift for a given angle of attack is decreased too, as you might expect. This means the inboard wing will make less lift, so the outboard wing has to make more lift. You would think this would mean that the outboard wing would see a drag increase. But, the NACA 23013.5 airfoil actually has its minimum drag at a CL of about 0.45. So increasing the CL on the outboard wing might decrease the drag a bit. Of course there is always a pay back, and in this case the additional lift on the outboard wing will lead to stronger wing tip vortices, and that means more drag.

AVL does rudimentary 3D analysis (i.e. it attempts to account for the variation in CL along the span, wing tip vortices, etc). It predicts that the whole aircraft CD is:

RV-8, 193 kt, MSL, aft CG, flap 0 deg -> CD = 0.03424
RV-8, 193 kt, MSL, aft CG, flap -5 deg ->CD = 0.03651
RV-8, 193 kt, MSL, aft CG, flap + aileron -5 deg ->CD = 0.03875

Note: AVL was originally created to do analysis of RC gliders, so it assumes that the fuselage is of circular cross section. This means that the fuselage drag, and fuselage/wing interference drag is not correct, so the absolute values of the whole aircraft CD are probably not correct. The variations between conditions is probably reasonable though.

Summary - there is probably little benefit from reflexing flaps and/or ailerons on short wing RVs.
 
Not necessarly. the above calculations are assuming you have a perfect airfoil. Do you? Are your flaps in perfect trail accross the flap span of the wing? Any twist? Top skin tweeked up a little bit at the rear spar ? how big of a gap betwen the top of the flap skin and the bottom of the wing skin? everything perfect?
One beautiful thing about homebuilts is you can go and try a little tweak here and there. Don't depend on someones first hand experience or someone elses
formulas and calculations. Go out and try it yourself. Don't remember if it was the above poster but someone tried to convince this board that a piper pitot static mast wouldn't work on an RV wing. He was wrong as many posters included myself had perfectly good results with said Piper mast.
Cheers
 
Thanks Kevin

I am always looking for something and this looks promising. I will have to worry this one for a while because there are some implementation complexities on my plane at the flap and fuselage interface. I need to have everything tested before the AirVenture Cup race on July 22. Not that I expect to win anything but I would like to improve over last year once again.

One thing that was mentioned by Marc Frederick at Taylor Texas recently was that the gap between the ailerons and flaps is a significant source of drag. I have a couple of half baked ideas but nothing really useful - this is a real back burner possibility.

Bob Axsom
 
tin man said:
...Don't remember if it was the above poster but someone tried to convince this board that a piper pitot static mast wouldn't work on an RV wing. He was wrong as many posters included myself had perfectly good results with said Piper mast.
...
Well I don't know if it was the above poster either, but the above poster is professional test pilot Kevin Horton, for my money the most knowledgable person on the forum for matters like flight testing and aerodynamics. Look through his posts. You will find that he relies on facts but has the background to sometimes hazard educated opinions, which will be clearly labeled as such.

I really hate to see him thrown in with garage tinkers, such as myself.
 
Well Said

Well I don't know if it was the above poster either, but the above poster is professional test pilot Kevin Horton, for my money the most knowledgable person on the forum for matters like flight testing and aerodynamics. Look through his posts. You will find that he relies on facts but has the background to sometimes hazard educated opinions, which will be clearly labeled as such.

I really hate to see him thrown in with garage tinkers, such as myself.

Well Said Larry. Tinkering is fun and fine, but Kevin keeps us grounded in the science of it all. He is a much appreciated resource here.
 
I believe...
That the Lancair has a laminar flow airfoil with a high pitching moment. At high speed this requires a lot of trim drag. Reflexing the flaps relieves this condition somewhat. RV's on the other hand have an airfoil with a near zero pitching moment, thus no such benefit will be realized.

I've been wrong before, but...
 
No disrespect ment to Kevin Hortren or any other posters but I'm here to tell you that a small change in flap position on an RV wing will make a measureable difference in top end speed. Its all gonna depend on where you start from and the condition of your wing as I posted earlier. A couple of us have spent many test hours and gallons of gas trying to fine tune our airplanes, some things working, some things not. If you don't want to hear the good or the bad from personal experience don't ask the question.
Tom
 
tin man said:
...If you don't want to hear the good or the bad from personal experience don't ask the question.
Tom
I didn't ask the question but I am sure interested and I'm sure many others are.

Did you find that reflexed (negative relative to the normal flap position) had any significant benefit on top speed? How did you determine this? How did you determine the flap angle relative to the normal retracted flap position? How did you determine the top speed at the normal flap position? How did you determine the top speed at the optimum flap position? What is the optimum flap position for speed?

Many of us have found that getting good numbers is very difficult and so we want a little more than just saying that it works, especially in light of the opinions of knowledgable people that the RV airfoil is not one that would be expected to benefit significantly from negative flaps.

As for the benefits of negative flap with some airfoils, there is no question at all that that is the case. This has been proven many times in real live aircraft using carefully thought out performance measuring techniques.
 
tin man said:
No disrespect ment to Kevin Hortren or any other posters but I'm here to tell you that a small change in flap position on an RV wing will make a measureable difference in top end speed. Its all gonna depend on where you start from and the condition of your wing as I posted earlier. A couple of us have spent many test hours and gallons of gas trying to fine tune our airplanes, some things working, some things not. If you don't want to hear the good or the bad from personal experience don't ask the question.
Tom
Real world results trump technical opinions any time.

One small caution though. Playing around with flap angle will be changing the airflow and air pressures behind the wing. There is a chance that this could affect the pressures seen at the aft fuselage static port. Thus the airspeed system errors could change slightly - i.e. any increase in ASI reading could be due to a change in airspeed system errors rather than an increase in TAS.

I would rely on a good GPS-based test method (like the NTPS Spreadsheet, using the four leg option), rather than just look at ASI readings. Some people are not aware that it is not as simple as just averaging GPS readings on different headings. This approach only works if the wind is zero.
 
I have the lift and drag polars of the NLF(1) 0215F airfoil, the so-called "Eagle 1" from Dan Somers, both with the flaps in trail and reflexed -10 deg. I looked at the CL requirements of my Lancair vs weight and density altitude, and saw that it might be possible to adjust the reflex to optimize CL and CD for any given flight condition. On a flight from Casper to Santa Maria, I tried varying the reflex, and lo and behold, I could change deck angle and speed by reducing reflex. How much? in going from -8 deg to -6 deg, TAS increased 3 mph and the nose dropped perceptibly. From -6 to -4 it dropped 3 mph. From -4 to -2 it dropped 4 mph, and from -2 to 0 it again dropped 4 mph. I have since found that there is a particular deck angle that gives me the best TAS, regardless of dalt or weight. Why? 'Don't know! I set an RV level in my cockpit side rail so that zero corresponds to the wing's chord line at about 40% span; 'had to account for the twist, don't you know! When the bubble is around +0.5 to +1.0 seems to be where I get the best TAS.
 
Poor Man's Method for electeric flaps

Fly the 3 or 4 course NTPS speed check method promoted at this site many times by K. Horton and J. Huft. I fly my tests at 6,000ft density altitude as described in the US Air Race Handicap procedure at http://www.us-airrace.org, wide open throttle, max RPM, leaned for best speed/power. Take GPS ground speed recordings at 20 sec. intervals for five consecutive recordings that do not have a variation indication of more than 1 knot (with 1 knot precision GPS ground speed) with constant track and altitude. Just for good measure record the observed RPM, manifold pressure, CHTs, EGTs, oil pressure, and oil temperature. Average the ground speeds of each track and enter them and the track angles into the applicable NTPS spreadsheet to establish the unmodified baseline speed.


If your plane is properly rigged in flaps up neutral aileron position the wing tip, aileron trailing edge and flap trailing edge will form a straight line if the tips conform to the airfoil design and are put on straight.

Mark the trailing edge of the flap position on the fuselage and use a protractor or the sine of 5 degrees times the flap chord to determine the trailing edge position at the 5 degree reflex position above the normal airfoil position marked above and mark this position on the fuselage.

Remove interference with the fuselage (this is the tricky part) then remove the inside flap linkage cover panels and adjust the linkage until the flap aligns with the reflex position mark paying close attention to any interference as the flaps move to the higher position. This is the test and possibly the high speed cruise and race position.

Close everything back up and and repeat the original speed test with with the flaps in the max up (reflex) position. and compare the results.

Once the flap linkage is changed, an operational change will have to be made to neutralize the ailerons and toggle the flaps to the aileron aligned position standard (take-off, etc.) operations. If you have the lower flap skin extending under to fuselage and covering the actuator hole some uncomfortable changes will have to be made.

Let me know how your test comes out.

Bob Axsom
 
elippse said:
I have since found that there is a particular deck angle that gives me the best TAS, regardless of dalt or weight. Why? 'Don't know!
In level flight, the angle of attack is equal to the pitch attitude (plus or minus the incidence angle of the wing). You found that there is a certain angle of attack that gives the highest ASI indication at the CL required for cruise. Makes sense.

You might achieve even more benefit if you could also tweak the aileron angle a bit.

Of course, keep in mind that whether your static source is on a wing-mounted pitot-static tube, or it is an aft fuselage static source, that changing the flap angle may have changed the pressure at the static source. So there is a chance that the real change in TAS is different from what your ASI suggests.
 
In my prior life as a Mooney 201 driver the "hot setup" for cruise speed was with the flaps and ailerons in the reflex position -- don't remember how many degrees---seems like it was no more than 2.

FWIW.

Cheers,

db
 
db1yg said:
In my prior life as a Mooney 201 driver the "hot setup" for cruise speed was with the flaps and ailerons in the reflex position -- don't remember how many degrees---seems like it was no more than 2.
Mooneys have NACA laminar flow airfoils (see the Incomplete Guide to Airfoil Usage), with a significantly larger pitching moment than the airfoil on the short wing RVs. Reflexing the flaps and ailerons on a Mooney would reduce the pitching moment of the airfoil, and and thus could reduce the downforce that the tail needs to create. This could reduce drag.
 
Kevin,

Thanks for that info--I wondered what the difference was. My Mooney was maintained by Dugosh Aircraft in Kerrville, Texas (just across the field from the Mooney plant) and they are the ones who rigged mine with the reflex---I just never asked them why this worked on a Mooney and not on other aircraft.

Thanks again.
Cheers,

db
 
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