Propeller Diameter & Speed

Bob Axsom · #1 11-09-2011, 09:29 AM

Something to chew on and if some good inputs come in maybe I'll get a little smarter.

Originally built the RV-6A with O-360-A1A and 72" dia Hartzell prop with 7666 blades. First flew it in 2004 and raced with it until November 1, 2009.

Installed a 72" blended airfoil Hartzell with F7496 blades and picked up 3 kts as reported in Van's testing article in the RVator (sigh) several years ago.

Recently cruising at around 6,000 ft d alt. at 2630 rpm I was seeing 193 kts GS on the GPS so I ran it up to my top 2720 rpm and the speed faded around 3 kts. I played with the mixture to see if I could get it back but I could not. When I pulled back the prop to 2630 again the speed came back to 193 kts. A plane can't fly faster than the prop pitch angle in level flight so to get the rpm the pitch is assumed to have dropped off - thus the slower a/c GS. The efficiency of the prop tips my be less at the higher rpm as well - not calculated yet.

If the diameter were incementally reduced by small amounts it seems to me it could be tuned to achieve a higher aircraft speed at the higher rpm - of course if I had more power the prop could be driven to the higher pitch as is.

Thinking only at this point.

Bob Axsom

Toobuilder · #2 11-09-2011, 09:45 AM

I think that every propeller has a sweet spot. 2700 RPM is not the fastest airspeed with the BA prop on the -8 either. There comes a point where the extra HP that comes with RPM is more than offset by blade drag, so you see a net reduction in thrust.

rocketbob · #3 11-09-2011, 09:45 AM

Not necessarily. I had a lot of discussions with Paul about this very same subject when he designed a blade mod for my Hartzell (oops let the cat out of the bag.) Reducing the blade diameter will reduce the critical mach losses but there are more variables to deal with....airfoil CL, tip shape, sweep, etc. Also the RPM/speed coefficient will vary with DALT so try your experiment at 8000 and see what happens.

C-FAH Q · #4 11-09-2011, 09:47 AM

Quote:

Originally Posted by rocketbob

Not necessarily. I had a lot of discussions with Paul about this very same subject when he designed a blade mod for my Hartzell (oops let the cat out of the bag.)

ok Bob, now ya gotta tell us what the mod is !

sailvi767 · #5 11-09-2011, 09:55 AM

That brings up a question I have had about the difference between aero and auto engines. Most auto engines hit a peak HP at a specific RPM and then often lose power. I have been told that within the acceptable RPM ranges on a LYC 360 more RPM is always more HP. I wonder if this is actually true. I know that those racing often twist the props to 2800 RPM or more. Do most show a gain at the higher RPM? I do gain a couple of knots bringing the prop up to 2700 RPM from my norm of 2500 if looking for top speed at low altitude.

George

Toobuilder · #6 11-09-2011, 10:32 AM

Quote:

Originally Posted by sailvi767

I have been told that within the acceptable RPM ranges on a LYC 360 more RPM is always more HP. I wonder if this is actually true...

True... The published "peak" HP for an aircraft engine is based on hitting the limit of propeller RPM, not the peak (max) output the engine is capable of. Spin a 360 Lycoming to 4000 RPM and it will make a lot more power than 200HP.

rocketbob · #7 11-09-2011, 11:10 AM

Quote:

Originally Posted by C-FAH Q

ok Bob, now ya gotta tell us what the mod is !

I could show you, but then I'd have to kill you.

C-FAH Q · #8 11-09-2011, 11:53 AM

Quote:

Originally Posted by rocketbob

I could show you, but then I'd have to kill you.

Well, maybe Tom and I will come down for a quick visit this winter. Going to do a short flying trip down to Memphis or Nashville, or somewhere!

rv7charlie · #9 11-09-2011, 02:02 PM

Quote:

Originally Posted by Toobuilder

True... The published "peak" HP for an aircraft engine is based on hitting the limit of propeller RPM, not the peak (max) output the engine is capable of. Spin a 360 Lycoming to 4000 RPM and it will make a lot more power than 200HP.

Which is why the old hangar tale 'a/c engines are designed to run full power, & car engines are not' is, in the immortal words of Harry Morgan, 'Horse Hockey'.

A/c engines are massively de-rated below what they are actually capable of producing, to ensure longevity & reliability.

One reason so many auto 'conversions' fail is trying to exceed the auto engine's factory full power rating & then put it in an a/c.

Charlie

flynwest · #10 11-09-2011, 02:17 PM

Are any of the race guys using the Whilwind 200 RV propeller with any sucess?