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Altitude sensitivity....

pierre smith

pierre smith

Well Known Member
...in my-10 really shows itself more so than the -6A.

By that, I mean, I see a lot more speed response as I get higher in the -10 than I did in the -6. I can true over 200 easily at 6500 and up but only 196 or so at 4500'.

Is it the fact that the airplane is much larger and has a greater net loss of drag than a smaller airplane like the -6?

Dan? Smoky?

Thanks,
 
pierre smith said:
Is it the fact that the airplane is much larger and has a greater net loss of drag than a smaller airplane like the -6?
Click to expand...

It has more to do with the engine then the drag due to size. Maybe the wing design difference.

Skin friction drag is a function of the Reynolds Number which has a "characteristic" length term in it calculation. The higher the "characteristic" length, the higher the Reynolds Number, and therefore the lower the skin friction drag coefficient. The density term though is much more dominate then the size term.
 
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Pierre, I noticed the same with my last 10. TAS was much higher above 6000. The sweet spot was between 6K' and 10K', depending on temps. It really stood out to me on the first flight, as I stayed at 3000' and thought I had a really slow RV-10!

Vic
 
I thought I read that the RV-10 wing is like the RV9 wing and more efficient at higher altitudes?
 
From a technical point of view, it has to be less drag relative to the -6.

The engine produces power based on density altitude, they all do. It certainly does not produce more power relative to the 0320 unless intake air flow is not constant. I believe the Lycomings are consistent with regard to HP and BSFC, it does not matter if it is a 0235 or the 0540.

The -7 definitely does better at 8500' than at 4500', maybe 6-8 knots, for the same fuel flow. Less resistance going through thinner air. Even with less power, drag reduction wins. It does slightly better yet at 10,500'.

It would appear the -10 sheds more drag up high relative to other models - or maybe it has more drag down low relative to other modes. :)
 
The Wing's the Thing

Pierre,

Good lesson here in wing design. I'd venture to say that what you're seeing is mostly related to differences in the wing design between the two planes. And of the design differences, the added aspect ratio the RV10 has will no doubt make a difference in cruise drag numbers.

Aspect ratio is defined as the wing span squared, divided by the area:

A = b^2 / S

It plays a major role in how induced drag (Cd_i) is produced by the wing - that's the drag due to producing lift. An often used expression for Cd_i is:

Cd_i = Cl^2 / pi / A

Note the effect changing aspect ratio would have on the induced drag. Even though Cd_i is diminished at cruise, it still plays a significant role in the overall drag picture for the airplane. The wing is the biggest drag producer we have.

Now for the difference in wing design for the 6 and the 10:

RV6 Aspect Ratio = 23^2 / 110 = 4.8

RV10 Aspect Ratio = 31.75^2 / 148 = 6.8

The RV10 has 42% more aspect ratio than the RV6, which will drive the induced drag values down, relative to the total drag the airplane sees in flight.

Ok, so why the difference at altitude? You probably know that induced drag increases as airspeed decreases. At high altitude, the airplane is operating at much lower indicated airspeeds - lower dynamic pressure - which requires the wing to operate at a higher CL to offset the weight of the airplane.

So, when looking at the total drag picture on the airplane, induced drag is a greater factor at higher altitudes than when flying down low. The RV10's wing handles this issue much better than the short wing the RV6 has, and you're seeing the difference in performance.

There may also be significant propeller differences between the two planes, and the RV10 has an airfoil that's light years ahead of the old NACA 5-digits we have on most RV's. Pure size differences really do not play a role here.
 
My "Guess"

The RV-10 is designed to carry 4 people, baggage and fuel for normal cruising tavel and non supplemental oxygen altitudes. That requires more wing than is required for a 2-place airplane that spends a lot of time flying at lower altitudes and doing arerobatics. The RV-10 compromise penalty is excessive drag at low altitude where the big wing is not needed. You do lower the angle of attack an have less induced drag but you still have the parasitic drag of the unecessary wing that must be pulled through the dense air.

All of my formal testing of speed mods on my RV-6A are done at 6,000 ft density altitude and the fastest configuration which I have come up with has the wing span reduced to 21.5 ft and yielded 184.4 kts or 212.2 mph. In this same configuration last June I flew the 2-way speed run at Courtland Alabama at 200 ft AGL and recorded 220.107 mph.

Bob Axsom
 
One more thing, Pierre: 6500 MSL is still kinda low for this wing. Try cruise at 10000 to 12000 feet and see what you get.
 
Thanks Bill....

...and everyone for the insight. The more I fly this airplane, the more in love I am with it!

Bill, the highest DA I've seen with this airplane was 8000' a few weeks ago on a trip from Florida home and I couldn't believe the extra added airspeed.....I've never noticed this much gain in my -6A to my recollection.

Incidentally, the -10 also has 3 degree reflex (up) in the flaps at cruise. That super efficient wing and the fowler flaps make landing in under 1000' a snap, now that my speed management has improved in this airplane.


Thanks again,
 
Yep. We noticed about the same takeoff and landing performance as our 6, even with 3 people and 45 gallons of fuel on a hot August Atlanta afternoon!
We miss ourt 10, but our inspection on the new one is scheduled for Saturday, so we should be flying again. :)
We never should have sold the first one, but you don't know you are between jobs until after the fact. It's been a long hard year building this one. Once I got employed again and we decided to get another 4 place, I was ready to just get a Bonanza and avoid all of the hard work. But Carol was adamant on how much she liked the 10. I think she just likes keeping me in the hangar! I was determined to put a bow on this one for Christmas, but I am going to miss it by ONE day.
Hoping for nice weather on Sunday. :)

Merry Christmas to all!

Vic
 
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