If you're flying them side-by-side at the same
fuel flow then you're probably making about the same horsepower on each engine. In that case I'd expect the taildragger to be a few knots faster since it has less drag. If you were instead flying the same
percent power (for example, wide open throttle at 8500' and using the same leaning procedure to get 70% power on both engines), the 390 has got to be burning more fuel and making more power than the 360.
http://www.vansaircraft.com/public/rv14perf.htm Note that the specs show the taildragger to be faster on the smaller engine at the same
percent power settings...
Mark,
I think you are "picking nits" (as some would say it).
I don't know how much experience you have doing detailed speed flight testing, but when you start talking about performance numbers and attempting to measure down to the difference of 1 or 2 kts it is a challenge.
Not impossible, and at Van's we have always strived to give honest confirmed via testing performance #'s, but that level of accuracy is difficult. I think the thousands of customers that state that their airplane meets or beats the published #'s substantiates that we at least got it close.
I agree that the larger displacement engine, at the same exact percent power setting should burn a little bit more fuel and have a little higher power output. The difference between the 360 and 390 would be less than 10 HP.
It is simple physics that shows that small changes in HP have relatively small effects on speed
(A good tutorial I found a few years ago is this one previously published in an EAA chapter news letter
http://www.eaa691.net/images/technotes/Tech_Note_5_Aircraft_Performance.pdf)
A less than 10 HP difference would result in only about a 2 kt theoretical speed difference. As already mentioned, that is getting into the realm of difficult to measure.
So if you want to argue theoretical vs actual, go ahead. If you want to question the data published for the 14 and 14A, that is also your prerogative.
I can't explain the reason for the data currently posted, but I don't believe it is because there is a larger speed delta between the tri gear and tail dragger 14, than there is with the RV-7 and 7A.
As mentioned previously, I flew in one flight test where both airplanes were configured at the same MP, RPM and mixture (as much as possible, mixture is a difficult one to match) setting and the airplanes were dead even speed wise at a DA of about 3000'. There are a lot of different factors that can have little influences on the
actual power output (the 390 engine had 600 hrs, the 360 was brand new, etc) and there are numerous other things that could account for a couple kts one way or the other (I don't think both airplanes were washed just before the test
) so I believe that there is not much more speed difference between a 14 and 14A, than there is between a 7 and 7A... if there is, I don't think it is more than a couple kts.
In my mind, considering the many other influences that builders can have on actual speed performance, a couple kts is just a couple nits.