fixed vs. constant speed

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?

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

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.

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

Harvey said:

.....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

Click to expand...

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

Rick6a said:

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.

Click to expand...

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

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

I vote for CS

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

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).

Don't forget about the RV Speed Range!

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

Crude but simple analogy----------------imagine a car with only one forward gear in the transmission--------you have to use it for acceleration, cruse, hill climbing, and top speed. Now think how much better the 4,5,or 6 speed you drive is.

Mike

Mel:

Thanks for the note about glide and stopped prop. I'm going to play with that this weekend. My engine out emergency proceedures have me pull to coarse pitch but not to stop the prop. Can't recall an engine out procedure on any airplane telling me to stop the windmilling.

Is the effect dramatic? If it's the area ratio of the full disk to just the prop it sounds like it would be very large.

Chuck

"...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."

Serious question: If the engine/prop were able to safely spin faster, to say 3200 rpm assuming the tips stay less than sonic speed, is your argument still valid?

What about Maint?

One thing I'm not seeing mentioned here is prop maint.

I don't know much about which prop uses more or less fuel. However the issue of a constant speed saving enough fuel in 2000 hours has too many variables. For example if the CS costs 6K more than the FP than you would have to save $3 an hour in fuel pay for the differance. This does not take into account the cost for overhaul. Most planes never make it to TBO before the prop reaches the reccomended overhaul based on age.

Lets face it a CS prop and gov overhaul is much more costly than a FP service.

Also what about insurance? I havn't checked into this and it may not even be a factor. Do insurance companies charge more or less depending on what prop?

I'm not taking either side here. Just giving some food for thought.

The increase in glide is pretty dramatic, but stopping the prop is not usually practical. If the prop has lost some mass or is out of balance causing a lot of vibration, then yes, stop the prop if you can. Otherwise, to stop the prop usually requires almost stalling the airplane. You already have a problem. Don't push for another one.
Mel...DAR

200HP RV-4, fixed pitch, 170 MPH 1700 FPM climb...the only thing a CS would benefit me is slowing this darn thing down on landing, and with aerobatics...the climb performance is fine for me.

Fixies Unite!

care to guess?

pbesing said:

200HP RV-4, fixed pitch, 170 MPH 1700 FPM climb...the only thing a CS would benefit me is slowing this darn thing down on landing, and with aerobatics...the climb performance is fine for me.

Click to expand...

Assuming a limited amount of capital to invest - does anyone want to guess at the climb rates of say, a 160 HP CS vs maybe a 210 HP fixed set up for max cruise? In other words, where should I put my first "extra" $6K?

Newbee w/ -7 plans, but still thinking about the -8

B. Hoover said:

Assuming a limited amount of capital to invest - does anyone want to guess at the climb rates of say, a 160 HP CS vs maybe a 210 HP fixed set up for max cruise? In other words, where should I put my first "extra" $6K?

Newbee w/ -7 plans, but still thinking about the -8

Click to expand...

Personally I would say put in the prop. Increase in speed will be at the cube root of the added power, which is not a good return on $$, not even factoring in added weight. Increasing prop performance seems to be a good idea unless you intend to fly mostly at the optimum speed your FP is configured for (i.e. cruise). If that is the case invest the money instead, or buy snazzier avionics. From what I have read, it is a personal preference question where it would be nice to have C/S but is it worth it based on your intended flying.....
Advice from one who has never flown an RV....
You know what they say about advice!

I don't have an ore in the water on this race, so I think I am fairly objective.

wingtime said:

One thing I'm not seeing mentioned here is prop maint.

I don't know much about which prop uses more or less fuel. However the issue of a constant speed saving enough fuel in 2000 hours has too many variables. For example if the CS costs 6K more than the FP than you would have to save $3 an hour in fuel pay for the differance. This does not take into account the cost for overhaul. Most planes never make it to TBO before the prop reaches the reccomended overhaul based on age.

Lets face it a CS prop and gov overhaul is much more costly than a FP service.

Also what about insurance? I havn't checked into this and it may not even be a factor. Do insurance companies charge more or less depending on what prop?

I'm not taking either side here. Just giving some food for thought.

Click to expand...

I hit 1,842 hours last weekend. My CS prop has cost around $15 for maintenance over the 8.5 years I have been flying. I am on my second tube of grease. My insurance is $311 per year for $1 Million coverage through NationAir. Am expecting an increase this year.

I do NOT want an RV without a constant speed prop. Ken Scott once said that you can only get 85% of the performance out of an RV without the constant speed prop.

There is a fuel savings enough to more than pay for the constant speed prop. See another post that I have in the forums that show it will pay for itself and the overhaul costs over it life time.

Gary A. Sobek

To the nay sayers...

Get a ride in an RV with a C/S prop. Go to 8,000 ft with the throttle wide open and the prop at max rpm. Pretend you?re on a cross country flight. After a couple of minutes pull the prop back to 2300rpm. Now sure you may loose a couple of kts, but ask your self, ?Do I really want to cruise cross country at 2600 or 2700rpm or do I prefer 2300 or even 2200rpm?? I think you will then understand a major advantage of a C/S prop. It?s a no brainer.

If you can't get a ride in an RV then take your car up to 55mph. Take it out of over drive, then put it in second gear. Now you have a fixed pitched trans. Do you really want to go all the way to grandma's house in second gear?

RV-9
160hp, C/S

Fixed pitch

Mr Ping, you're a little off.
In the first place, Lycoming shows performance tables in all the Skyhawk and 150 applications for % of power at the different altitudes. Just because the engine turns 2700 RPM is meaningless. Going to ..."Grandma's house in a car with a 'fixed pitch' transmission" is not quite the same thing since you can still pull a lot more manifold pressure.

I fly an Airtractor 502 with a reversible, full feathering prop on the PT6 engine and yes, it's nice and handy and pulls strong with a 500 gallon load of chemicals, but we're comparing apples to cumquats here.

Our RV6A has a fixed Catto three bladed prop and doesn't climb nearly as well as the CS guys do but we're as fast in cruise, if not faster, than the CS guys are and 35 lbs lighter and thousands of dollars ahead, with a climb near 2000 FPM. The smoothness and quieter operation is also appreciated by us and the public. We run the engine according to Lyc specs and burn under 10 GPH while trueing 202 MPH at 8500' with two people aboard.
Pierre

To each his own

pierre smith said:

Mr Ping, you're a little off.
Going to ..."Grandma's house in a car with a 'fixed pitch' transmission" is not quite the same thing since you can still pull a lot more manifold pressure.

Click to expand...

Thank you, that?s exactly my point. In order to pull MAP with a F/P you have to increase RPM. You can leave the throttle all the way in AND reduce RPM with a C/S.
Sure, a fixed pitch can be a few knots faster at altitude but I?ll take less rpm, less vibration and less noise when I?m traveling cross country and I?ll take the extra 5 minutes on a two hour leg too. I?m not trying to win a race. I can also push everything forward (even back off from max rpm by 100rpm to run with the Lycoming/Cessna pwr settings) if I want max speed, but to me it?s like running in second gear.

Also, I have a -9 and weight & balance couldn't be better with a C/S prop up front.

That?s just the way I prefer to travel. I know there are a lot of FP RVs out there and they are fine airplanes. This is just the set up I wanted. That?s why I built my RV. And that?s why I?m on this forum so I can express my opinion. To each his own.

I?m just having fun, just like you.

So what RPM are you running in cruise at 8000 ft and 11000 ft.


RP

to constantly speed or not

Last year I went to Van's and took a ride in the RV-7A. Gus Funnell was the PIC and I asked him about the CS prop. He told me that, in his opinion, the extra WEIGHT and extra COST of the constant speed prop out weighed any benefits on that aircraft.

Note: At the time I was planning to build the -7A.

I'm now going to build a -9, O-320 with fixed pitch - I may go with an IO just to avoid carb heat. Call me cheap but I don't want the added expense now or long term. The O-320 with a good prop will give me all the performance I want.

This is my opinion and my choice. Building your own plane allows you to make these (and many other) choices based on the flying you do. I kinda had the opinion - opinion - that the CS props were for the higher altitude (flight level) type flying - I could be wrong this.

To me it depends on the type of flying you plan to do. I'm planning on a serious cross-country machine. This to me means no less than 180HP (currently an TMX-IO-340 is my minimum engine) and a CS prop. This is for climb-cruise-decent performance that will blow away any fixed pitch in flexablity. If your building a -8 look very closely at the weight of a TMX-340 with Hartzell blended airfoil, and compare to a IO-360 with a Sensenich. I think the weight penality is worth the difference in performance. But if your looking to do a lot of local "down low" flying then the Sensenich might be a better deal. I fly a lot above 10K feet, so the climb performance is a big plus for me.

Robert M said:

I'm now going to build a -9, O-320 with fixed pitch - I may go with an IO just to avoid carb heat. Call me cheap but I don't want the added expense now or long term. The O-320 with a good prop will give me all the performance I want.

Click to expand...

How do you avoid carb heat with an IO? Is it injected? What is the price savings?

I am working on a constantly changing "wish list".....

Ward Johnson said:

How do you avoid carb heat with an IO? Is it injected? What is the price savings?

I am working on a constantly changing "wish list".....

Click to expand...

Yup, it's injected. That's what the "I" in IO means.
And an injection system is going to cost a bit more than a carb setup.

There are proponents of both for different reasons other than just price.

Fp vs CS

Mornin' y'all,
One point to remember is that when you reduce RPM on a CS prop, you're also making less horsepower, since more RPMs mean higher horsepower and a resulting faster cruise. When I turn my Catto 2700 RPM and the CS guys turn 2400 or 2500 RPM, it's not the same thing. They are making LESS horsepower because of the lower RPM and therefore go slower, even though both of us are wide open throttle and consequently pulling the same manifold pressure for a given altitude.

As for cruising above 10,000 feet, why do it if it's not necessary to avoid clouds or rocks? Granted, an 80 knot tailwind would be reason enough but even the fixed pitch Catto or Sensenich will get you there in 7 minutes or less.
Pierre

I have a fixed pitch prop on my RV-6 and I fly another 6 with the same engine and a constant speed. The constant speed is a lot more fun for about 30 seconds of each flight, the takeoff roll (really not flying) and the initial climb up until the power reduction.

Now this power reduction is not required, but I have found that just about everyone reduces the RPM on the constant speed to about 2,400 shortly after takeoff. At lower altitudes they may also reduce MP.

So the situation is max MP and 2,400 RPM with the constant speed and max MP and about 2,400 RPM with my fixed pitch, except the fixed pitch plane is lighter. Guess which one climbs better? The same one that climbs better flies a bit more pleasantly, probably because it is lighter and the CG is not so far forward.

In cruise it is necessary to run higher RPM in the fixed pitch plane if you want to go really fast. I prefer the more relaxed feel of 2,400 RPM, but doen't find it a very big deal to rev a bit faster. In normal practice I usually run about 2,450 with the throttle back quite a bit and true around 160 knots or a bit better.

The takeoff roll in my airplane, with the fixed pitch, feels like it takes forever at the high density altitudes I usually fly in, but that is a relative thing. To people observing, it looks like it pops off the ground with an amazingly short run. Also the takeoff performance does not limit the airplane in any way, as the landing distance is longer than the takeoff, at least for the condtions I tested in. I did not believe this when I heard someone at Van's say it, so I tried real hard during my initial testing to land shorter than I could take off. Couldn't do it. Maybe it would be different with a heavy load, but it is still hard to think of a situtation where the fixed pitch would limit what I could do.

I have operated from Leadville, with a load, in the middle of summer and the fixed pitch did very well. What more can one ask?

Larry

Ward Johnson said:

How do you avoid carb heat with an IO? Is it injected? What is the price savings?

I am working on a constantly changing "wish list".....

Click to expand...

My wish list is the exact same - fluid.

From what I've gathered about carb versus injection - the "I" in "IO" ranges from between a few hundred and about a thosuand more $$$ - all things considered. I believe I'm comparing rebuilt when I'm talking the few hundred and a grand for a new engine.

Somebody jump in here and clarify if they can....

Mike,
I am not sure what the question is. I wasn't arguing actually, just informing. I have probably flown farther and faster with propellors than most on this forum, having flown the Lockheed Electra at over 350 knots at a prop speed of 1020 RPM all the way across the Indian Ocean. (Perth to Johannesburg) For some reason they did not outfit this airplane with a fixed pitch propellor?

The Allison 501D develops 4000 shaft horsepower, and with the constant speed propellor you get to use it all on takeoff. I have taken a picture of the prop in cruise, and the blades appear to be feathered at 350 knots. The reason for the low RPM is that the blades have a 14 foot diameter. If the prop turned any faster, the blades would go supersonic! The cool thing is, the blades only turn 1020 RPM during takeoff as well.

The intrepid Jon Johanson started his long distance flying with a fixed pitch propellor. He now uses a constant speed propellor. I am sure he would want the most efficient and the best propellor that he could get.

My point is, that propellor airplanes that haul the mail, have constant speed propellors. (Super Cub excepted) So, if you want to get the max out of your RV, get one.

Cheers, Pete

RV6_flyer said:

Ken Scott once said that you can only get 85% of the performance out of an RV without the constant speed prop.

Click to expand...

That's interesting. I recently sat down with the whiz-wheel and calculated the %HP I get on take-off with my fixed pitch, and it was nearly exactly 85%. Somehow I figured it would be less than that.

And, in an RVator article a couple years ago, Ken wondered aloud if the switch to a C/S had been worth it on his airplane.

I am building an 8 and I had the same question myself, I got lots of opinions at Sun N Fun this year, most in favor of th CS prop. Finally I asked Van himself who suggested if I could afford it the CS prop is the way to go.

I have a XP360 on my RV4 with a Fred Felix 3 blade wood prop and am very happy. I get 170 to 180 indicated with 2000 feet per minute climb. I personally feel a constant speed prop on a Lycoming is a waste of money it is required on an Egenfelder Sabru though to get any performance out of the machine. The only real advandtage to a constant speed prop in Lycoming installations is the ability to slow down. I had a former member of the US Aerobatic team advised me to go fixed pitch for aerobatics as less chance of prop problems. He threw a prop in a Pitts on takeoff and had it lodge in the lower wing. Oh, yes he got it back on the runway. Roger Moore N67PM

I agree

pierre smith said:

Mr Ping, you're a little off. Pierre

Click to expand...

No I think Mr. Ping is dead nuts on. In a previous discussion PROP efficiency goes WAY WAY WAY up with lower RPM. With a constant speed prop you control RPM independent of throttle position. Example: If you are 9,500 ft and WOT with a fixed pitch prop you get what you get, RPM wise. To lower the RPM you have to close the throttle. A partially closed throttle you have to suck past the throttle plate, which produces "pumping losses". With a Constant Speed prop you can reduce RPM with throttle Wide Open.

So yes a fixed prop wide open high RPM may match a constant speed prop in speed for one flight condition, but often at a higher RPM. A C/S prop is more efficient over a wide range of conditions, especially for low RPM / power efficient cruise. With gas going so high I can see guys slowing down to 135 mph to save gas. The constant speed will do this better, meaning slightly more efficiently.

DISCLAIMER: I am not anti-fixed props, I am just about the facts. However since the Sensenich fixed prop came along, I think that is a fine, excellent way to go for the money. That is the fixed pitch prop true advantage, economy. However for overall performance the C/S prop is the way to go. As far as TOP speed many fixed can get high speeds by OVER REVING the engine. More RPM means more HP. Since C/S props have a RPM governors they are max RPM limited, which is great for aerobatics but not for HOT ROD racing and over revving the engine. Of course over revving is not great for the engine. However if "You want to RACE" the fast C/S planes all adjust their governor to allow hight RPM's.

Cheers George