Fixed-pitch vs. Constant-speed

G-Man

G-Man

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What are the trade offs re: greater efficiency (lower fuel burn, better climb, more speed, etc.) of a constant-speed prop versus cheaper cost (lower maintenance, fewer parts) of the fixed-pitch prop?

I've seen discussions on this and other forums about 'first' and/or 'simple' planes, and I'm curious about where the advantages and disadvantages lie.

What's it work out to in the real world? Thanks!
 
CS is the way to go, climb prop in climb, cruise in cruise, MX wise there really isn't all that much to do, just do a IRAN if you see any issues.

Anything 180hp or better I'd really want to see a CS prop. Smaller stuff not as important.
 
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Performance vs. lighter weight, low cost and simplicity. It just depends on what appeals to you as a pilot and airplane owner.
 
Constant speed gives you access to maximum engine power during all phases of flight. With a fixed pitch prop you're making a compromise somewhere - either you're losing performance in takeoff, climb, or cruise.

The trade-off is that fixed pitch props are effectively maintenance free, and constant speed props are expensive when they go wrong. We have to buy new props for the 414 (over $20k) now. However, I've also flown over 2,000 hours with constant speed props, which amounts to over 4,000 hours of engine time since most of that's in twins. This is the first time I've had to spend any money on a constant speed prop.

It just depends on what you're going for. If you want a simple fun machine and performance isn't a factor, go fixed pitch. If you have a go places plane, you probably want a constant speed.
 
Which prop type an airplane has is usually not your choice. You might target buying a plane that has one or the other but the prop type probably won't be important in your decision.
 
What type of flying will you be doing and from what types of airports?

Are you flying relatively medium length flights from typical GA airports? A fixed pitch set at a cruise setting would be fine.

Are you flying short flights out of short strips with obstacles? A fixed pitch with a climb prop would be better.

Do you make longer flights out of short fields with obstacles? A constant speed would be better.

A mixed bag? Constant speed is better.
 
James331 said:
Anything 180hp or better I'd really want to see a CS prop. Smaller stuff not as important.
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CS props come with a weight penalty. On smaller engines, where it would be really nice to be able to use 100% of power for takeoff and cruise, CS props are sadly not a good choice, because the extra weight in these smaller airframes diminishes your load carry capacity more than the CS prop will help. As @James331 notes, 180 horsepower is about the dividing line between CS ad FP prop installations.

-Skip
 
There is no comparison - CS. You need the ability to adjust blade pitch for optimal performance. Why do all the high performance and larger prop planes only come with CS? There is a reason.

If you want a 172 fixed pitch is fine. Beyond that CS only.
 
In an equivalent GA sport airplane, having a CSP is sort of like deciding to have a stick shift instead of an automatic. Only in a plane's case, the stick shift if better. It will save on fuel and give a wider area of performance. And they give a different "feel" (a better feel), when you pull the power back. More deceleration smoothness too. Subtle but there...
 
I always thought stick was better in cars too?
 
They make light airplanes a bit more nose heavy, and just heavier as well. Thats the only performance negative. Although for any one mode of flight, a fixed pitch prop can perform just as well, sometimes better. The fixed pitch wont perform as well at the other modes of flight (climb, top speed, cruise, take off) because its optimised for one. The CSP can be pulled back to 40% power and get better gas mileage. So there is that. In an aerobatic plane they are necessary so you dont overspeed the prop.....
 
CS is the only way to go. Maximum performance in all aspects, newer composite ones are pretty light as well. Can be used to slow airplane down, no need to reduce throttle on decent. Cruise flight in turbulent conditions is much easier as you don't need to adjust power settings for thermals or sinks. Constant speeds are really durable now a days.

I'm assuming you are talking experimental as with most certified planes you don't have a choice. If you are spending 60+ to build an airplane another 10k shouldn't break the bank.
 
Thanks, everyone, for interesting information and discussion. I'm not airplane shopping, just trying to learn.
 
My new experimental Cub will have 220hp and an experimental CS prop. My prop weighs 47#. The hot option for fixed pitch weighs 13#. My prop will utilize my horsepower better than fixed pitch but my primary advantage is big performance from a smaller diameter prop. There are lots of factors to consider.

When considering opinions ask how many of the responders have been presented with the choice and which decision they made. Very few guys have ever made the decision you're asking about. Most planes have prescribed props and even when an approved CS option exists not many guys will spend $20K to get one.
 
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Brand new two blade Hartzel Top Prop for my plane is only about $9,000. Since we are on the subject, has anyone ever made an electric CS prop? I know electric variable pitch propellers used to be common, but CS? If so, I wonder if there would be any weight savings?
 
coloradobluesky said:
In an equivalent GA sport airplane, having a CSP is sort of like deciding to have a stick shift instead of an automatic.
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As an automotive analogy, I always think of it as having a continuously variable automatic transmission set to maintain a selected (optimum for the phase of operation) engine RPM vs. having a direct drive without transmission but being able to select the gear ratio (pitch) of the rear end for a compromise leaned toward acceleration from a stop light (take off and climb) or cruising.
 
A CSP changes the pitch of the prop without adjusting the prop knob to keep the same rpm with different power settings and airspeeds.

So it IS automatically changing the pitch but you dont always notice it because the rpm stays the same.
 
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Graueradler said:
As an automotive analogy, I always think of it as having a continuously variable automatic transmission set to maintain a selected (optimum for the phase of operation) engine RPM vs. having a direct drive without transmission but being able to select the gear ratio (pitch) of the rear end for a compromise leaned toward acceleration from a stop light (take off and climb) or cruising.
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Basically that's it. It's the same as a CVT vs. a one speed direct drive. The only place I can think of that direct comparison is in 1970s era mopeds. The typical European moped of the time had one speed and some kind of automatic clutch, but the Honda moped of the same era was light years ahead and had a CVT. What a huge difference!
 
Dav8or said:
Brand new two blade Hartzel Top Prop for my plane is only about $9,000. Since we are on the subject, has anyone ever made an electric CS prop? I know electric variable pitch propellers used to be common, but CS? If so, I wonder if there would be any weight savings?
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Yes. MT makes an electric prop that you can pair to a CS controller.
 
Ted DuPuis said:
Yes. MT makes an electric prop that you can pair to a CS controller.
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The old Beech electric props could be set up that way as well, circa the late '40s. I'm sure there are others as well.:)
 
mondtster said:
The old Beech electric props could be set up that way as well, circa the late '40s. I'm sure there are others as well.:)
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I'd recalled that about some of the early V-tails, although I don't know enough about them to declare with any certainty. I do know about the MTs, and would actually be all in favor of that option, especially since it makes having a reversing prop easier (something I'd love to have).

We'll be going with the 4-bladed MT MTV-14 hydraulic, full-feathering, constant-speed propellers on the 414, though.
 
G-Man said:
What are the trade offs re: greater efficiency (lower fuel burn, better climb, more speed, etc.) of a constant-speed prop versus cheaper cost (lower maintenance, fewer parts) of the fixed-pitch prop?

I've seen discussions on this and other forums about 'first' and/or 'simple' planes, and I'm curious about where the advantages and disadvantages lie.

What's it work out to in the real world? Thanks!
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Weight is the big difference. many small aircraft will not tolerate the extra weight out front.
 
Here's a real world comparison, and another reason to consider the CS prop.

When we were shopping for RV-8s, I test flew an -8 with a fixed pitch prop on a 180 HP engine. It was just as fast as the CS prop versions, but to achieve those speeds it had to be at full power, which meant the prop was spinning at ~2700 RPM.

If you've ever flown in an RV-8, you know that the back cockpit (especially) is one of the loudest environments in aviation. There is something about the shape of the canopy that seems to act like a parabolic reflector, focusing the engine sound with mind-numbing loudness on the center of your brain. At 2700 RPM, even with Lightspeed Zulus, that fixed pitch prop was like the hammers of hell.

We didn't reject the plane on that basis, but in hindsight we are very glad that we ended up buying one with a CS prop. With the CS prop, we can dial back to 2350 RPM (while cruising at 170 knots) which dramatically lowers the noise level in the cockpit.
 
Ted DuPuis said:
Yes. MT makes an electric prop that you can pair to a CS controller.
Click to expand...

I had no idea, so I went to their website to learn more abut their electric props, but their website is pretty poor. What I could gather is it is not STC'd for very many airplanes. The electric controller mounts where the old hydraulic one does, but has no mechanical connection to the engine. I'm guessing the advantages might be lighter weight, safer operation and the ability to add a CS propeller to an engine not traditionally designed for one. These are guesses though on my part as I couldn't get much out of their site.
 
mondtster said:
The old Beech electric props could be set up that way as well, circa the late '40s. I'm sure there are others as well.:)
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My understanding is, the old Beech electric props are just variable pitch and not constant speed, is that not so?
 
Dav8or said:
I had no idea, so I went to their website to learn more abut their electric props, but their website is pretty poor. What I could gather is it is not STC'd for very many airplanes. The electric controller mounts where the old hydraulic one does, but has no mechanical connection to the engine. I'm guessing the advantages might be lighter weight, safer operation and the ability to add a CS propeller to an engine not traditionally designed for one. These are guesses though on my part as I couldn't get much out of their site.
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That's more or less correct. I believe they have an STC on the 172. You see them on experimentals more.
 
An optimized fixed pitch prop is optimized for both pitch and TWIST RATE.
A CS prop can only change the pitch, the TWIST RATE stays the same no matter what the rpm or airspeed is.

And there are TWO airspeeds behind the prop in moving flight. The airspeed from the prop is usually different in angle than the airspeed due to the plane's movement. Those two air movements add in a complicated way. It gets pretty complicated, but propellor design is pretty well worked out by the mechanical engineering cad and design equipment. A real brain trust worked on it too. Froude was a english naval propeller designer, one of the first to make it a science.
 
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Dav8or said:
My understanding is, the old Beech electric props are just variable pitch and not constant speed, is that not so?
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No, there were constant speed controllers available for them. They could be operated either way.
 
I really enjoy flying constant speeds, no way do I want to pay $$$$ when it comes due.

Prop shop already has you by the short hairs.
 
Most of the factory airplanes that were certified with both CS and FP props have a slight speed advantage with the FP prop. ie the Cherokee 235, Cherokee 6-260 and the BE-24 fixed gear model. Takeoff and climb performance is a bit better with the CS prop. While the CS prop will usually turn rated RPM the performance enhancement is limited by the lower efficiency of the CS prop. The FP prop also provides better engine cooling because the design of the blade is pulling more air close to the shank. The round portion of the CS blade near the shank pulls no air. Charlie Melot Zephyr Engines
 
FP made a lot of sense for me. Climb performance of the RVs is already excellent, so no need for the drag-strip launch. But lower initial cost, zero maintenance cost, less weight on the nose gear, lower noise on takeoff and reduced complexity/fewer things to go wrong were all contributing factors. I went with a composite Catto prop, and in the event of a prop strike they come apart nicely and often save the crank/flange from damage. Also, as said before, smoother running than a metal prop and significantly less "wet dog shake" on shutdown, so easier on the motor mounts. Down side: Less flywheel effect, so idle speed needs to be a little higher.

But...If I lived back east and was always flying into 1400' grass strips with tall trees on either end, I'd have a CS for sure.
 
So, would say a Grumman Tiger benefit from a CS? I think most are configured for a cruise prop as their short field performance isn't mind blowing. But they typically cruise comfortably in the 130s with a useful load around 950 lbs.

Seems to me that the Tiger is sort of in the sweet spot.
 
I think most airplanes that have a fixed pitch prop won't benefit a great deal from a constant speed from a performance perspective. The engineers did a pretty good job of figuring out which option was better when they designed it. I doubt the Tiger would benefit much, although perhaps on takeoff.
 
bnt83 said:
I really enjoy flying constant speeds, no way do I want to pay $$$$ when it comes due.

Prop shop already has you by the short hairs.
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Prop re-seal? Hub rebuilds? Yup, been there. I really miss my friends Benjamin.
 
FP - Chasing engine RPM on a turbulent day really sucks but I'll live with it since my prop is only about $3k brand new, and will never pop a seal and spew red dyed oil or engine oil all over lol.
 
Ted DuPuis said:
I think most airplanes that have a fixed pitch prop won't benefit a great deal from a constant speed from a performance perspective. The engineers did a pretty good job of figuring out which option was better when they designed it. I doubt the Tiger would benefit much, although perhaps on takeoff.
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It's more likely the accountants and marketing folks rather than the engineers.
 
There really is no "right" answer on FP versus CS, but a good discussion here to let the individual weigh the pros and cons, and make a decision appropriate for their mission.
 
mondtster said:
No, there were constant speed controllers available for them. They could be operated either way.
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The early Beech Bonanzas (1947-1956) had an electric prop. The design only provided manual control of the prop pitch via a switch in the cockpit so it wasn't "constant speed" at all. In the early 50's an electronic control became available (I think it was originally an aftermarket product) using vacuum tubes and relays to operate the pitch change motor in response to RPM changes. Normally this was only used when large changes in airspeed were anticipated such as when taking off and landing or when leveling off from a climb. Leaving the controller in auto mode all the time tended to wear out the pitch change mechanisms which weren't designed for continuous adjustment. Later a "modern" transistorized version of the controller became available well after the Bonanza line had converted to hydraulic props for new production.
 
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