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Speed Test Conditions?

BillL

Well Known Member
I have read the RVator article where Vans talks about the condition for the cruise speed test is 8000'DA, max RPM (or best) and ROP for max power at that altitude.

But . . . I can not find where the conditions of "top speed" testing have been stated. So, just where does the normally aspirated airplane make its highest TAS? Is it down low where the engine makes the most power, but the airframe drag is also way higher or some odd place (yet to be found) in between sea level and the specified cruise condition?
 
Kitplanes Designers Notebook

The January 2018 issue of Kitplanes magazine has what you are looking for:

Wind Tunnel
Design process, part 2?how high should you fly?

By Barnaby Wainfan

My quick takeaway is 8,000 to 9,000 is the best compromise for my normally aspirated RV9A.
 
Bill,

I believe max speed runa are done at sea level and full throttle and leaned for max power.

This isn't done typically because I believe you can run past Vne.
 
The January 2018 issue of Kitplanes magazine has what you are looking for:

Wind Tunnel
Design process, part 2—how high should you fly?

By Barnaby Wainfan

My quick takeaway is 8,000 to 9,000 is the best compromise for my normally aspirated RV9A.

And typically, for a fixed pitch prop, full throttle should produce 2700 rpm at this 8K to 9K altitude. This also will probably work out to 75% power.

A 'cruise' prop will have less rpm and a 'climb' prop with have more rpm.
 
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Bill,

I believe max speed runa are done at sea level and full throttle and leaned for max power.

This isn't done typically because I believe you can run past Vne.

I did my max speed run about 50' over the Pacific Ocean. I believe everything was forward so may have been a little on the rich side.
 
I had an informative exchange with the late Bob Axom on the subject of performance testing. I captured his methodology which adjusts for pressure and temperature. Bob tested his race mods at an adjusted 6000’ DA.

You can find the write-up here ...

http://elder.ninja/blog/p/4459
 
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I've never seen a recommendation for "top speed" testing. Have only seen the max cruise speed testing referenced several times here with altitude in the vicinity of 8000' density altitude.

Would be interesting to make two runs, preferably on the same flight, one as low as safely/legally possible, and the second run at 8000' density altitude. Each one using the same max power setting, flying the 3 heading gps tracks, fed into the spreadsheet to calculate TAS. In central Illinois you could probably make a run at or near to 1500' density altitude, depending on local elevations.

Wonder how much difference there would be in the two numbers? If you ever do this, please post the results.

I would try it myself the next time I'm out in my RV6, but today, as I write this, the density altitude at Sedona is 7628'!

Glen, thanks for the link to the write up from Bob Axsom. He was a special guy.
 
This is actually basic physics. Pick a power setting: e.g., 75%. At sea level there is a lot of air, so a lot of drag. Go up. Air density drops, so drag drops, so you go faster. Finally, at 8000?(DA) it takes full throttle to get 75% power. Now if you go still higher, keeping full throttle, both power and parasitic drag will decrease, so an ideal airplane will maintain the same true airspeed. But all real airplanes also have induced drag, and as you go higher the induced drag slowly increases (due to the higher angle of attack needed because of the decreasing air density), so the true airspeed will slowly fall off as you go above 8000?(DA).
You can reverse this arguement and go down from 8000?, maintaining full throttle. The increasing air density increases both drag and power, balancing out, but the decreasing induced drag causes true airspeed to increase, maximizing at sea level and 100% power.
 
I think this group is missing BillL''s question.

It is not what conditions CAN one use for top speed testing but rather what conditions WERE used, by Vans, to justify the top speed numbers on the website.

In my case, you?re right. I missed the point.
I would guess, looking at the numbers, that the top speed tests were done where they would give the best results: as close to sea level as they could safely get.
 
Bob,
I just deleted my message before you posted because I reread Bills post and realized he asked both. Ignore me please.
 
I had an informative exchange with the late Bob Axom on the subject of performance testing. I captured his methodology which adjusts for pressure and temperature. Bob tested his race mods at an adjusted 6000? DA.

You can find the write-up here ...

http://elder.ninja/blog/p/4459

Well that's because he lived in Colorado, IIRC, so he couldn't test much lower than that even if he wanted to. Does not address what altitude to get the best TAS.
 
This is actually basic physics. Pick a power setting: e.g., 75%. At sea level there is a lot of air, so a lot of drag. Go up. Air density drops, so drag drops, so you go faster. Finally, at 8000’(DA) it takes full throttle to get 75% power. Now if you go still higher, keeping full throttle, both power and parasitic drag will decrease, so an ideal airplane will maintain the same true airspeed. But all real airplanes also have induced drag, and as you go higher the induced drag slowly increases (due to the higher angle of attack needed because of the decreasing air density), so the true airspeed will slowly fall off as you go above 8000’(DA).
You can reverse this arguement and go down from 8000’, maintaining full throttle. The increasing air density increases both drag and power, balancing out, but the decreasing induced drag causes true airspeed to increase, maximizing at sea level and 100% power.

This logic would be correct except for two factors. First, as you change altitude, engine power changes in proportion to density. But Profile drag changes in proportion to the square root of the density. So the two changes do not balance out. (edit: I stand corrected on this -- at constant true air speed, the profile drag also varies in proportion to density, as Bob said) For an airplane with a constant speed prop, I think the conclusion is still correct.

Second, for airplanes with a fixed-pitch prop, as you change altitude, it is not possible to maintain both WOT and constant RPM. So, the engine power changes not only because of the density change, but also because of the RPM change. Further complicating this is the fact that you are no longer operating the prop at a constant propeller efficiency. As your true airspeed increases, you move along the efficiency--advance ratio curve in a way that decreases prop efficiency fairly quickly. At lower altitudes, the prop may be more efficient, but the pitch may prevent reaching maximum allowable RPM. So in that case, the best altitude for top speed may be the lowest altitude for which maximum RPM can be achieved.
 
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As Bob explained, maximum available power will produce the highest TAS at sea level. RV’s with recommended engines will not exceed Vne in level flight, that is part of the design process. If you wait for a cold day, you can do speed runs at a lower than sea level DA!
 
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I just put together a quickie spreadsheet to calculate top speed at various altitudes.

Assumptions:
Span=23 ft
Weight = 1550
Max power =180 Hp
prop efficiency 0.85 (constant)
span efficiency 0.95 (constant)
engine RPM = 2700 (constant)

Assumes engine power at WOT at constant RPM varies in direct proportion to density.

Assumes equivalent flat-plate profile drag area of 1.30 sq ft - this was tuned to give a sea level top speed of something close to 200 mph. This "knob" can be adjusted to match your actual top speed. Everything else is physics-based, with the listed assumptions.

Results: The answer is that it almost doesn't matter - the top speed varies only 1.2 mph between sea level and 8000 ft. I don't have a way to quickly post a graph or a link to a hosted spreadsheet, but I can summarize in a table here and someone else can plot it.

Altitude (ft), Speed (mph, rounded to 1/10)
0, 203.4
1000,203.3
2000,203.1
3000,203.0
4000,202.9
5000,202.7
6000,202.6
7000,202.4
8000,202.2
9000,202.1
 
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Steve, cool, thanks for the analysis. Van's published numbers for the various models show about a 10mph difference between top speed at sea level and 8000 ft. ?? Here are my own empirical data for max performance runs, GPS NTPS spreadsheet data, speed in knots:



About a four knot difference between sea level and 8000 ft. I've been wondering why the slope was so low, but perhaps I should have been wondering why it isn't even lower.
 
Steve, Did you include induced drag (e.g. as air density drops the wing AOA has to increase, increasing induced drag)? I believe this is the main ?non-ideal? factor which causes WOT true airspeed to drop with altitude.
PS. Yes, I only considered ideal (constant efficiency) props. Real fixed pitch props are too complicated!
 
Andy, the TAS data you show is about 10 knots faster than Vans ?book? speeds at 8000?. Are you flying a non-stock -10, or have some other explanation?
 
Bob, stock engine and attention to detail on the airframe. I fussed over details that most don't, like the cowl exit. Another example is that I would cut a hole in my arm before I cut a hole in my wing for a landing light or stall warning tab. Even bugs on the wing have a measurable effect, keep it clean and polished. Mine isn't the fastest though, others have posted higher speeds:
http://www.vansairforce.com/community/showthread.php?t=95952
After four years and nearly 400 hours I am still amazed nearly every time I fly it. I think we owe Steve a big thanks for that amazing wing. :)
 
Steve, Did you include induced drag (e.g. as air density drops the wing AOA has to increase, increasing induced drag)? I believe this is the main ‘non-ideal’ factor which causes WOT true airspeed to drop with altitude.
PS. Yes, I only considered ideal (constant efficiency) props. Real fixed pitch props are too complicated!

Yes, of course! Since induced drag is actually my area of specialization!

Thus listed in the assumptions a constant span efficiency, and why I listed the span and weight in the assumptions. In dimensional terms, the induced drag = Di=W^2/(pi * q * b^2 * eta) where W is the weight, q is the dynamic pressure, b is the span, and eta is the span efficiency. Note that eta is similar to, but not identical to the so-called Oswald efficiency factor. For a rectangular wing, there is also some variation in the span efficiency as angle of attack (lift coefficient) changes. I did not account for that- but generally the span efficiency improves as CL increases for the very low CL we are dealing with at top speed.

The induced drag (in lbs) for my analysis was about 7.3 lbs at sea level and 9.3 lb at 8,000 ft.

What I did not do was make any adjustment to the profile drag as angle of attack changes. There is some dependence of profile drag on lift coefficient. I just treated the profile drag (all the drag that is not induced drag) as an equivalent flat plate drag area. This is likely the reason that real-world tests see more altitude dependence than my simple model.

And it is also likely that the prop efficiency is not very constant even for a constant speed prop. The blade pitch has to change as the power drops off, so even though the advance ratio is almost the same, the pitch change moves the peak in the efficiency curve.
 
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Bob, stock engine and attention to detail on the airframe. I fussed over details that most don't, like the cowl exit. Another example is that I would cut a hole in my arm before I cut a hole in my wing for a landing light or stall warning tab. Even bugs on the wing have a measurable effect, keep it clean and polished. Mine isn't the fastest though, others have posted higher speeds:
http://www.vansairforce.com/community/showthread.php?t=95952
After four years and nearly 400 hours I am still amazed nearly every time I fly it. I think we owe Steve a big thanks for that amazing wing. :)

You are more than welcome, I'm glad it turned out so well too!
 
Thanks to all. BillR, Gary, Dave, BobT, rzbill, and Steve. Great to have experience, analysis and data (Andy). Lower DA the better, apparently. I had thought that the Vans top speed numbers were at SL, standard day, but could not find any documentation. Steve, thanks for the analysis, and Andy for corroborating data.

BillR, according to the SL gain over max cruise, I think Vne (200KTAS) won't be passed, but closer than I thought. Pretty happy for a 180 hp engine.

Winter weather should bring some test opportunities, but maybe I should go find a beach:D.
 
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