Constant Speed Prop vs. Controllable Pitch
 

I have thought about the constant speed prop for some time (which I have on my RV-6A) and I have a hard time accepting it as the best way to obtain optimum performance - you can read that as best cruise speed if you like because that is what is important to me. I just seems to me that I could select a fine pitch for climb and fine tune a high pitch setting for maximum speed. I know that the constant speed setup works well for all reasons, etc. but sometimes I would like to dial in the pitch and peak for speed. I assume this is a theoretical subject but I would like to get your thoughts on this.

Bob Axsom

Bob,

If you had a controllable pitch prop, you would set full throttle and then vary the pitch as desired. The rpm would go to whatever rpm was required to make the torque required to spin the prop equal to the torque produced by the engine.

If you had a constant speed prop with the exact same blades as your controllable pitch prop, you could achieve the same effect my making small movements of the prop lever. This would change the rpm, and the pitch would go to where ever pitch was required to hold the rpm constant. But, if you had the same rpm as with a controllable pitch prop, the pitch would be exactly the same, the prop efficiency and engine power would be exactly the same, and the aircraft speed would be exactly the same.

A controllable pitch prop has no advantage in this scenario.

Interesting thought. A constant speed prop might actually be giving you a bit of an advantage here, maybe. As you slow down, say, in a slight climb or a turn, the constant speed prop would tend to go to slightly finer pitch as the prop starts to load up. Depending where you're flying in terms of engine/prop efficiency at that time, it could give you an extra little bit of acceleration right when you need it the most.

It'd be neat if someone actually had any numbers showing if this helps, hurts or is just a wash.

If you want to optimize cruise speed, I would think that the fixed pitch prop would be the best choice....assuming you really didn't care about climb performance.
The CS prop provides a better combination of climb and cruise, but at the penalty of cost and weight, of the prop itself, not to mention the governor, oil lines and additional oil and controls.
My personal preference is for the CS prop, since it converts HP to power more efficiently in all regimes. But, for pure speed, I'd bet on the FP prop and the lightest airframe possible.

More RPM
 

Bob do you have a Blended airfoil Hartzell, the fastest prop. Still the HC-C2YK/F7666-4 is no slouch. Of course 72" is better than 74" for high speed cruise.

I will trade a little speed secret, tweak your governor (at your own risk) to 2750 or 2800 or even 2850 rpm. You will make an extra 2, 5, 7 HP. I personally don't like to run above 2800 rpm. Fixed props can be faster because they OVER turn the RPM. With a governor you are limited to 2,700 rpm automatically. It also matches Lycomings rpm RED line.

I adjusted my RPM to 2,800 rpm intentionally. You will gain speed based on the cube root of HP ratio. Say your 180 HP speed was 200 mph, your new speed assuming a +5 HP gain for 100 RPM increase would be:

= 200 * ( 185 / 180 ) ^ (1/3) ≈ 202 mph. (a 2-3 mph gain depending on altitude)

To avoid 2,800 rpm on takeoff I'd intentionally pull the prop back just a little on takeoff. When racing its prop and throttle wide open. MORE RPM = MORE HP.

More RPM does reduce prop efficency at high speed a little, but it's a small fraction of a precent loss in efficiency, verses the gain in thrust from RPM induced HP increase. Of course it plays heck with your fuel burn, but you will go a little faster.

You could be bold and bump it to a full 2850 rpm or 2900 rpm but I am too conservative for that. However if 2,800 RPM is good, 2,900 RPM is better, race wise. Now Lycoming will not be happy but it has been done. I am also talking about only doing it for a race, an hour or two once a year or so, not all the time. How much faster? Well if 100 RPM gave +2 mph, you can expect +4 mph more for +200 RPM.

Good Luck, I don't give out all my secrets. :D but for you buddy. Don't tell anyone. :rolleyes:

I'm Thinking
 

What you say makes sense but what I see in my airplane doesn't. Since I do not have direct control of propeller pitch I have no confidence that the pitch is increasing linearly as I dial in the prop and the RPM is a function of load balancing. I would not be surprised to see the same RPM produced at more than one prop pitch angle because of different loads, different operating procedures, system inefficiencies, environment, fliuds, etc. Direct pitch control would result in whatever RPM the engine could drive it to. I am mainly interested in the "bite" and the thrust produced by the turning prop and the resulting true air speed. The "small movements" may be a clue to working around this system that I have to deal with. If I can get a good calm air day I may set up test with RPM increments of 10 from 2400 to 2700 and record the GPS and TAS results. With enough data I can come up with an operational work around for some of the inefficiencies in this indirect system.

Bob Axsom

Thanks George
 

I did adjust the governer up over 2700. When I first flew it the out of the box it maxed out at 2630 RPM. I have an EI tach and, like you, when that red light comes on at 2700 it gets my attention and I back it off a bit. I have some more cooling drag ideas to work through but I will keep the 2800 race only option in my "to do" list.

Bob Axsom

P.S. The prop is the older C2YK-1BF/F7666A4 so it is the 72" non-blended prop.

Bob

Very good point Steve
 

That Sensenich for the 180HP RV-6 looks mighty good. The problem with the fixed pich prop is what you see is what you get. If it isn't right you are stuck with it and it won't soften the load for a stressed engine.

Bob Axsom

Race prop
 

Bob,

There is a prop shop at Hayward, CA if memory serves me right that has done some prop testing and mods for guys running the sport and Bi Plane classes at Reno. An aquaintance from work had them work up a Hartzell for his Lancair based on research that they had previously done for others. I don't know which engine and prop combos they have worked with, but the 0-360 Lyc is pretty common and chances are good thay they know the magic. The way they described the process to me when I was shopping for a prop was that they take a sacrificial set of blades (most likely just out of specs) and play with the twist. Testing is done to see when it optimizes in flight. At this point they start whacking some of the length off while doing more tests. When they get to the point where the speeds taper off they know they went to far. At this point the good blades come out and are set up to the optimums. For the 0-320 and the Hartzell they end up putting a "D" twist to the "C" twist blades to make it similar to a Baron prop which has the "D" but is much longer. Then they shorten it to their sweet spot. They claim 15-20 knots on a Lancair and seemed to think that on an RV it should be good for at least 10 knots. I was also told to assume the climb would go down 300 or so fpm. They offered to do this to my new Hartzell, or to get a set of Baron blades that were no longer servicable for a certified plane and work them up for $700 each. The process was just to much for me and I gave up before taking the chance on them ruining my new prop. My old co-worker swears by his.

Regards,

Wow thanks Brian
 

That is some fresh input! My chicken hearted response is what you would expect but that definitly is something I want to know about. Some brave soul may have taken them up on it and will report the results. The next time I see Cris Ferguson I will ask him about it as well. He has a Lycon prepared engine in his Pitts that he finished second with in the biplane silver class but I never really looked at or asked about his prop. He designed and built the plane that finished second in the gold class also but they have one of those Paul Lipps design props made by Cato on it.

Bob Axsom