Prop Design for Dummies

Google struck out on this one, so it must not exist. Hopefully, some of you prop experts will see this and comment.

I have a Catto 2007 model two blade 68x76 on my 160 hp RV-3B. Static rpm on a cool day at SL is 2200. Unloads to 2320 on takeoff roll.

WOT at 1000' DA is 2980 rpm and 219 mph TAS. Pulled back to 2700 at the same DA speed is 197 mph TAS.

WOT at 8000' DA is 2890 rpm and 210 mph TAS. Pulled back to 2700 rpm speed is 197 mph TAS.

My bud has the almost identical prop (2" less pitch) on his 160 hp RV-4. Static is 2200 and WOT at 1000' DA is 2800 rpm.

Craig Catto is happy with the prop as-is. So am I as long as I remember to pull the throttle back when the nose comes down. Craig says he can repitch the prop and reduce WOT rpm by 150, but I'm guessing that will bring the static way down as well. So for now I'm sticking with what I have.

All numbers are from a Dynon D180 and confirmed correct.

Bart Lalond says it won't harm the engine to occasionally kick it into RV-4 passing gear, but it will wear faster when operated at the higher rpm's.

Comments welcomed!

Tony

Tony, your effective pitch numbers obtained by multiplying TAS by 1056 then dividing by rpm keep coming out to about 76"-77" which is good. Yes, your takeoff rpm will suffer if you go to a higher pitch, but why do that. With the numbers you have you get great takeoff and climb and excellent cruise. Will higher rpm cause your engine to wear out sooner? Probably, but by how much? 5%? 10%? How old are you and how long do you plan on keeping the airplane? How many hours do you fly each year? If you do your cross-country flights at 11.5' to 12.5' baro, which in the summer is about 14k' to 15k' dalt, you'll probably run right at 2700 at WOT, and have good speed. If it was me, I'd live with it! My engine turns rated 2800 rpm at 10k dalt and gives me 201 mph TAS, and at 1k dalt I get 2950 rpm and 214 mph TAS, but I do most of my flying cross-country and would like to turn up just the rated 2800 rpm at 14.5k dalt, instead of my 2760. To each his own, but don't let the wear thingy upset you!

Additional weight?

Are you fellows using additional weight on the prop mount somewhere? Like a heavy crush plate, Landoll balancer or weight ring, etc. If so, I'd appreciate your experience and recommendation. I am going with a lightweight fixed pitch prop on my 4. I could use a little extra weight up front but not necessarily. Would you suggest starting out without weight and going from there? I hate to bolt on discretionary weight of any kind since I have worked hard to keep the whole airplane light. What say ye?

Sincerely,

elippse said:

Tony, your effective pitch numbers obtained by multiplying TAS by 1056 then dividing by rpm keep coming out to about 76"-77" which is good. Yes, your takeoff rpm will suffer if you go to a higher pitch, but why do that. With the numbers you have you get great takeoff and climb and excellent cruise. Will higher rpm cause your engine to wear out sooner? Probably, but by how much? 5%? 10%? How old are you and how long do you plan on keeping the airplane? How many hours do you fly each year? If you do your cross-country flights at 11.5' to 12.5' baro, which in the summer is about 14k' to 15k' dalt, you'll probably run right at 2700 at WOT, and have good speed. If it was me, I'd live with it! My engine turns rated 2800 rpm at 10k dalt and gives me 201 mph TAS, and at 1k dalt I get 2950 rpm and 214 mph TAS, but I do most of my flying cross-country and would like to turn up just the rated 2800 rpm at 14.5k dalt, instead of my 2760. To each his own, but don't let the wear thingy upset you!

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Thank you, Paul! I was sure hoping you would pop up on this one. I have no plans to ask Craig to change anything on the prop. As to the engine, I'm not afraid to apply the whip a bit when required and am confident it will be going strong long after I'm not.

Tony

If your CG is a little too far to the rear, adding crush-plate weight is the way to go, since it is the farthest forward and you get the most CG change for the least weight increase; you also get a little more rotary momentum to help smooth out the pulses with the lighter prop. I have the Mark Landoll damper, and have had it since day one, so I can't tell you the change when adding it. If you put one on, make sure you put on the aerobatic stiffener with it so as not to stress it with gyroscopic forces. The Lancair tended to have a rearward CG, but mine doesn't, so I guess the MLD helped. I can idle down to 650 rpm with my 12lb 3-blade, 'course it does shake a little there. I've actually been able to fake-out my dual Plasma EI to get it to fire at TDC rather than advance, and could keep it running just below 400rpm. Klaus didn't think it was possible 'til I showed him!

Thanks Paul

I have not heard of anyone using a wood or composite FP prop without some added weight on the front of the crankshaft somewhere. I just took delivery of a new MT wood/composite fixed pitch prop. At 10.9 pounds, it's one pound less than the old Pacesetter woody I have. Thanks for your comments.

Hi Brian,
Interesting to hear about your observation on weight forward on woody powered RV-4. I have no extra weight on my prop. I am flying a 10 year old RV-4 with an O-320 E2D 150 horse power, Hendrickson 68/76 prop on a 2550 hour engine which was topped when installed 550 hours ago and the tall man option (automatic aft CG). 2200 static, 2300 climbout at 120 with 160 when level. 2400 gives me 170 and WTO at 3000 ft gets me 195 mph at 2650 RPM. All numbers are IAS in MPH. I can easily keep up with all the 160 hp constant speed RV-4s in all regimes except climb at altitude. I idle at 650 with no ill effects. As an aside, the prop is desinated by Ted Hendrickson as a 160 hp prop, the 68/74 being his prop for the 150 hp. Sorry. Ted is out of the prop business.


Keep it light and a little aft CG lets it flys faster........... Also keeps the really big guys out of the back seat.