Guys! You with the reflexible flaps! Try this to get more mph(sorry; knots!). Get a level bubble at your local RV (not Van's) supply store. They have a package with two, one for sideways, one for fore and aft, made by Hobby. Mount the bigger one inside your canopy in a fore-aft direction. I mounted mine on the bottom of the canopy just forward of my arm so I could see it easily in flight. I set it zeroed-out in line with the wing's chord-line at about 40% span, but the zero reference really doesn't matter.

    When you are flying, especially at high density altitude and/or at high weight, start with your flaps fully reflexed, and note the bubble reading. Decrease the reflex slightly to get the bubble reading about one degree less. Watch your TAS reading for an increase of one or two mph/kts over the next four or five of minutes. Note, too, that the nose is slightly lower giving better visibility. Repeat this until lowering the flaps more gives a speed decrease. Go back to the previous bubble setting and enjoy about 2-4 mph/kts more speed, and better visibility over the nose. And you with a fixed-pitch prop and an altitude hold, when you are flying over slightly rising or descending air, you will see the bubble change 1/2 or one degree, the rpm go up or down 10-30 rpm, and TAS/IAS change 1-3 mph/kts.

 Paul,

 

You are absolutely right about the trim results in the conditions you set out.  I have used slightly different equipment in a slightly different way.  I have a digital level on a thin aluminum bracket that fits on the longeron and is clamped in place by the canopy when it is closed.  Planes are usually designed by establishing power and performance targets in some weight and altitude range.  Then the longeron line is drawn level and all other things grow from there, including the wing and horizontal stabilizer incidence.  At my Lancair 320's cruise speed and between 7000 to 9000 feet, the longeron angle is zero.  I did report some numbers relating AOA indications, Longeron angle, speeds and flap deployments a few years ago - it was very interesting.

 

Yes, the reflex flap is a great trim device for rearward CG conditions.  I have not experimented with performance/speed improvements at high altitudes.  At low altitudes and high speeds some have even tried reflexing the flaps further up but no benefit was seen.  Taking them out of reflex slightly does not improve speed at low altitudes even though one might expect the down tail forces and drag to be reduced because of the pitch change.  Of course at higher than normal speed, flaps fully reflexed, more nose down trim is required perhaps apparently also reducing H. stab drag (asking the upside-down wing to "lift" less).

 

I agree with you and others that point out that a component of uncommanded speed changes can be related to entering rising/falling air masses while using an autopilot to maintain a constant pressure altitude.  I.E. In rising air, the airplane must dive a bit to stay at the same pressure.