Optimum Conditions for Speed Runs?

[Kyle Boatright]

What are the optimum conditions to test the top speed of our normally aspirated aircraft? Obviously a physically clean aircraft, but beyond that, I assume ideal conditions would be a calm day with the speed runs conducted a very low density altitude.

Is that fair/reasonable/accurate?

The reason I ask is I ran the RV-10 out to full throttle the other day and saw 181 knots (TAS) in level flight with a nasty dirty airplane, light turbulence, and at something like 5,000' density altitude. Then I wondered, hmm how much faster would it be under optimum conditions?

 

[blaplante]

Top Speed or Top Cruise Speed respecting the 75% power limit?

For the latter, you want the lowest altitude where 75% power is not exceeded at full throttle. That gives a balance between least air resistance (better higher) vs. max power (better lower). This is typically found about 7500' DAlt.

If you just want big numbers, look for updrafts so you can point the nose down to maintain altitude....

 

[Lucmac]

https://www.perplexity.ai/search/What-are-the-Rc5EiJSwQsGEx2xRQHaX5Q

 

[Kyle Boatright]

If you just want big numbers, look for updrafts so you can point the nose down to maintain altitude....

.. Or put it in a vertical dive. ;-)

No, the question is top speed in level flight.

 

[Kyle Boatright]

https://www.perplexity.ai/search/What-are-the-Rc5EiJSwQsGEx2xRQHaX5Q

The first two points here are in opposition: 1) Fly between 5 and 10K'. 2) Go for a day with cool temperatures so the air is denser. (If dense air is the key, why not fly at the lowest density altitude (i.e. low altitude)?)

 

[RV6_flyer]

For a Top speed test, I wanted a standard temperature, standard pressure day at sea level. Since I was on the west coast, I went out over the Pacific ocean and did a run at full power. (Max manifold pressure and max RPM.) I may have adjusted the mixture slightly to make sure that I had a smooth running engine. At the time, (1990's) Van's aircraft published 201 MPH as top speed of the RV-6 with 160 HP. I recorded 175 KIAS, 175 KTAS, and 175 KTS GS on GPS.

 

[turbo]

snf 'race' 6a o360, fp, wot, 2950 rpm, alt off, on the deck, triangle course, full rich, average calculated speed, 208 mph.