fixed vs. constant speed

[Ward Johnson]

This may be a bit premature (OK, alot premature) but I am curious nonetheless. I have seen some things on performance vs cost analysis of the two prop systems, but beyond a mention of "slightly better fuel flow" with the constant speed prop I can't find anything more in depth on that issue. With fuel costs going ever higher, this could easily turn into a significant decision. Is there any data on this?

[flyeyes]

Hi Ward.

The fuel-flow with constant-speed vs. fixed pitch is IMHO a negligible factor, compared to many other things.

The CS prop and governor will add 3 (yellow tagged, scrounging) to 9 thousand dollars (new, top-of-the line) to your building cost. Even at a slightly more efficient cruise, that's a lot of 100LL.

What the CS prop gives you is dramatically better takeoff and climb performance. It is also easier to perform aerobatics and easier to land with a CS prop.

The CS prop also allows you to choose a quieter (lower RPM) cruise if you wish.

Formation flying is much easier with a CS prop.

For cruise efficiency, the fixed-pitch Sensenich props are every bit as good as the Hartzells, and much cheaper. Jon Johansen flew his RV around the world behind a FP Sensenich, although he later upgraded to a CS prop.

There are currently six RVs that I know about based at M01 (Dewitt-Spain). Four of these have CS and two are FP. At 2M8 (Charles Baker) there is one FP RV-6, and at least three more CS.

For me personally, the CS prop was a no-brainer.

PM me and I'd be happy to give you a ride in a constant-speed equipped RV-8, and I'm sure one of the FP owners would do the same.

James Freeman
based at M01

[jcoloccia]

James makes some great points. Here's a couple other thing to consider:

Everyone (at least everyone I've read on these forums and spoken to in person) who has flown a LIGHT RV says the same thing: it's a different animal when it's light and it's way more pleasant than a heavy RV. That said, I'm building a -7 and will likely throw a constant speed on it for weight up front. If it adds performance AND helps get the CG where it belongs, seems like a clear win to me.

If I were building an -8, I would have to think long and hard about the advantages of a CS prop vs. the advantages of having a light, nimble RV. By the time I factored in the cost, I'm thinking the FP would probably win me over.

So anyhow, I guess what I'm saying is there are potentially advantages and disadvantages and they change depending on the type of flying you do AND the specific aircraft you build.

[Harvey]

Hi Ward,

Regarding the FP / CS question, it basicly comes down to mission profile combined with cost. For me it was an easy one. I was on a limited budget and wanted to build light, hence a simple FP Sensenich mated to an O-320.
The result is a nice light ( 1023 pound) RV6 that climbs at 1500 - 2000 fpm and cruises very economically at a TAS of 150 kts at 8500 ft on 55% power, sipping fuel at 6.3 US gallons per hour.
If short field performance is critical or your airfield is at a high DA, then by all means go for a CS. I aerobat my 6 and fly formation without a problem.

Cheers

Martin in Oz

[Rick6a]

Originally Posted by Harvey

.....it basicly comes down to mission profile combined with cost......a limited budget and wanted to build light, hence a simple FP Sensenich mated to an O-320. The result is a nice light ( 1023 pound) RV6 that climbs at 1500 - 2000 fpm and cruises very economically at a TAS of 150 kts at 8500 ft on 55% power, sipping fuel at 6.3 US gallons per hour.
If short field performance is critical or your airfield is at a high DA, then by all means go for a CS. I aerobat my 6 and fly formation without a problem.....
Martin in Oz

Martin pretty much summed up the high points. I would only add that with a Sensenich fixed pitch prop, you will not experience the pressed-against-the-seat take off acceleration and dramatic increase in initial rate of climb as you will with constant speed setup but if you are patient, the Sensenich will eventually overtake an equally powered constant speed RV in level cruise.
Also, I have noticed...but never see mentioned by others......when you chop the power at altitude (such as when practicing engine out procedures), best glide speed performance is significantly less.....maybe up to 200 FPM higher rate of descent with a constant speed setup as is with a Sensenich. Anyway...that has been my perception. Like Martin alluded to, if short field performance is at all critical, then by all means spend the extra dollars for the undeniable safety margin a constant speed setup will buy you.
Rick Galati RV-6A "Darla" Sensenich equipped

[chuck]

Originally Posted by Rick6a

Also, I have noticed...but never see mentioned by others......when you chop the power at altitude (such as when practicing engine out procedures), best glide speed performance is significantly less.....maybe up to 200 FPM higher rate of descent with a constant speed setup as is with a Sensenich. Anyway...that has been my perception.

Speculation follows:

Is this really true? I don't have FP experience in an RV but with the CS on my 4 if I cut the power and leave the prop full forward (fine pitch) the descent rate is pretty dramatic, but if I move the RPM lever to coarse pitch my glide improves dramatically. I don't recall the sink rates off the top of my head but I'd be surprised if the coarse pitch sink rate on a CS prop was worse than that of just about any FP.

Now of course it depends on how your engine dies, if it siezes then you will have lousy performance, if you run out of gas (or have a failure where the prop is windmilling and you have oil pressure) you can probably get the propeller into coarse pitch and have better glide performance.


Chuck

[Mel]

Chuck,
One minor error here. If your engine siezes, you will have much better glide performance. A stopped prop has MUCH less drag than a windmilling one. With a windmilling prop you have the drag of the entire prop disc. This was proven in an old video put out by AOPA. The video also demonstrated the reduced drag of course pitch over fine pitch.
Mel...DAR

[R.P.Ping]

If you can afford it, go with a constant speed prop. I must also say all of the above is true. But, let me expand on a couple of points. Cost: Over a period of 2000 hours you will save enough fuel to offset the price of a constant speed prop. Lower cruise RPM: Besides lower fuel flow you also have reduced wear and tear on the engine and something you don?t measure is the comfort level; reduced vibration and noise.

Either prop will work but my vote is for a constant speed.

Rv-9
C/S 160hp 86 hrs

[cobra]

Seems to me be biggest advantage of the adjustable prop involves its ability to act as a speed brake while landing, particularly with the 9's that like to float. All the rest is nice, but not as big a deal.

The downside is the extra expense and weight penalties, particularly with alternative engine choices that tend to force expensive MT electronic units- the IVO adjustble might be the exception there (not usable with Lycs vibration, so data is limited and varied).

[Mustang]

Guys,

My partner in the RV-8 was adamant about having a fixed pitch prop. I had to convince him that the constant speed was a GOOD idea. This is how I explained it.

For a Super Cub or other conventional light aircraft like a Citabria etc, a fixed pitch prop works fine. The reason for that is that the speed range of these aircraft is only from, say, 40 knots to 100 knots. That is 60 knots of airborne operational speed.

The RV on the other hand, operates from 55 knots to 175 knots. The airborne operational speed range is 120 knots. Twice as much as a "normal" light aircraft. It becomes obvious then, that no fixed pitch propellor will be able to be anywhere near efficient throughout the RV speed range, and only at one cruise speed.

My partner said he understood this but I could tell, he was still not convinced.

Then, we went flying in a friends fixed pitch RV-4. I told him, "Notice the RPM during the takeoff roll" "Hmmmmm. Only 2350 on takeoff." I said "Yeah, guess what that works out to for horsepower?"

We looked it up and it came to around 125 hp out of the 150 available. So then I asked him, "How much horsepower would be available for takeoff from a 4000 foot elevation airstrip?" The answer was obvious and he decided that maybe a constant speed prop might be a good thing.

During the same RV-4 flight we bumped the speed up to 175 mph indicated. The RPM was nudging 2600 at this point. It was obvious the airplane wanted to go faster but the engine RPM was getting into the limiting range.

The big trade-off with the fixed pitch propellor is takeoff performance vs. high speed cruise performance, and on the RV, the trade-off is aggravated by the large operational speed range

So, I guess my point is, that if you want to exploit the true potential of your RV, then get a constant speed propellor.

If you will be happy with limited takeoff and climb performance and good, high RPM cruise performance, then, get a fixed pitch propellor.

If you do get a fixed pitch propellor, and choose a wood prop, be prepared to throttle back to 2000 RPM in any kind of rain. Sad, but true, the rain will eat a wood prop on a speedy RV. However 2000 RPM and 110 knots will keep your leading edges OK.

Hope this will help with your choice.

Cheers, Pete