Low RPM, Slow RV-6

[Bob Axsom]

When I built my RV-6A there was a shim I had to make out of some fairly thick aluminum as I recall and it went under the forward part of the horizontal stabilizer. If that isn't in there it could be forcing you to compensate with down elevator in cruise. Another thing to recheckis the wing incidence angle. The method specified seems a little crude but it works fine - level the canopy deck (x and Y plane) stand the specified block over the rear spar and lay a 48" level on the forward spar location and the block and the bubble should center. If that is off there is a way to fix it without touching the wing but I hesitate to mention it. I will say that the relative fwd and aft angle between the wing and horizontal stabilizer is what provides the pitch stability and I suspect you have an overly stable airplane.

Another thing I did on my gear leg fairings to fuselage interface was to seal the interface and only that (top) interface with silicone RTV. Since one fairing was "hanging down" you may have some big gaps in this area.

Bob Axsom

[Kevin Horton]

Originally Posted by Mark7986

Garmin 430 is an IFR Cert. GPS Capable of Calulating TAS.

But, what data does it use to calculate TAS? It doesn't have pitot and static lines going in the back, so it doesn't know what the IAS is. Do you manually input the IAS, temperature, etc? If so, have you done testing to determine what the error is in your indicated airspeed system? It almost certain has some error. It might only be 1 kt, or it might be 10 kt. If you are using the TAS as a measure of goodness of your engine, prop, airframe, etc, then it is important to know what error the TAS value has.

[gmcjetpilot]

Originally Posted by Kevin Horton

But, what data does it use to calculate TAS? It doesn't have pitot and static lines going in the back, so it doesn't know what the IAS is. Do you manually input the IAS, temperature, etc? If so, have you done testing to determine what the error is in your indicated airspeed system? It almost certain has some error. It might only be 1 kt, or it might be 10 kt. If you are using the TAS as a measure of goodness of your engine, prop, airframe, etc, then it is important to know what error the TAS value has.

Not sure about the 430 Garmin. The Dynon EFIS has TAS if you have the OAT probe attached. However I would not rely on that for flight test data. For cruising around TAS on the Dynon is fine.

To get good TAS, the Temp has to be perfect, Baro setting perfect and IAS, which we know has some error, needs to be corrected (CAS). I get what you are saying Kevin, ground speed average (per your three leg constant course method) is more reliable and accurate. We will never get 0% error, but when you are hunting down 1 or 2 mph the ground based TAS is best.

However I see the other point, in the context of this discussion the speed started off being way off. Good news is he found 13 kt laying around with some tape in fairings. So for gross changes, calculated TAS was good enough to find a big change.