CS vs. FP Props

[Adam Oke]

Clearly with a constant speed prop out front

Why?

Ive flown both CS and FP,, and a good FP prop is better.. Lighter, simpler. and FASTER!

by good FP I mean CATTO and the likes...

That is an interesting statement Strega. I'm not saying your wrong by any means. I would hate to answer with a question .... but can you back up your response? Your thoughts are kind of backwards to what I have been taught. My understanding is that a constant speed propeller is more beneficial because changing the blade pitch allows you to take full advantage of engine power at various stages of flight. As opposed to a FP prop which is fixed to perform best at only one stage. This does not allow one to take full advantage of the engine and what it produces.

Also comparing CS vs FP, I'm assuming you've flown them on the same type of aircraft in order to have a fair comparison?
.

Adam,,

When you have a plane as the RV-8 with TONS of excess power, you can size a fixed pitch prop for high cruise e, the trade off is reduced take off performance, but as mentioned, with a 200hp engine, the rv will easily get airborne in a reasonable time with a "cruise" fixed pitch.

you will always be able to make a prop more efficent by designing it for ONE rpm/torqe/airspeed than trying to to it all (as done by a CS prop)

not to mention the CS prop and govener adds like 50 lbs to the aircraft, and is just more crap to break.

I'm still confused as to how one "gear" vs. multiple "gears" is more beneficial. Maybe this should be opened up for discussion in a new thread. CF vs FP.

So I'm making this a break off from another thread. What is everyone's take on the discussion? I feel that a CS prop would be more beneficial due to its overall performance. It takes advantage of the full engine potential. Strega seems to think that a FP prop is better because you can tune it for a good cruise. Someone lead me to knowledge!

[iflyforpie]

It depends on a ton of factors including the aerodynamic and economic variety.

In my experience I've found that the break-even point is around 180-200hp. Below 180hp, you are adding weight, complexity, and cost and only adding minimal performance. Same goes with retractable gear. A 150 hp 172 would probably cruise slower with the added weight of a 210 style retractable gear.

There are exceptions of course. Your Pawnee spends most of its time taking off and climbing so cruise performance isn't really that important, hence a relatively large engine driving a fixed-pitch prop. Same with the L-19. Then on the other side there are the few planes that have small engines and constant speed props, mostly with newer technology engines and light weight electric or mechanical props.

A CS prop will give you better performance over a wide range of speed and more importantly let your engine run at its optimal speed (slow for economy or high for power). A 180hp RV-8 with a fixed pitch prop will get its doors blown off by a similar one with CS. The only fixed pitch props you see in the air races are in Formula One where CS is not allowed. But for the private guy who's already got a fast plane, four figures for a fixed pitch sounds better than five for a CS.

[Matthew]

It depends on a ton of factors including the aerodynamic and economic variety.

In my experience I've found that the break-even point is around 180-200hp. Below 180hp, you are adding weight, complexity, and cost and only adding minimal performance. Same goes with retractable gear. A 150 hp 172 would probably cruise slower with the added weight of a 210 style retractable gear.

There are exceptions of course. Your Pawnee spends most of its time taking off and climbing so cruise performance isn't really that important, hence a relatively large engine driving a fixed-pitch prop. Same with the L-19. Then on the other side there are the few planes that have small engines and constant speed props, mostly with newer technology engines and light weight electric or mechanical props.

A CS prop will give you better performance over a wide range of speed and more importantly let your engine run at its optimal speed (slow for economy or high for power). A 180hp RV-8 with a fixed pitch prop will get its doors blown off by a similar one with CS. The only fixed pitch props you see in the air races are in Formula One where CS is not allowed. But for the private guy who's already got a fast plane, four figures for a fixed pitch sounds better than five for a CS.

Uhhh, I'll just add a +1 to that, you pretty much said what I was thinking.

Matt.

[xsbank]

Even the latest-generations of jet engine, like that proposed on the 'C' series will have variable pitch fans to give about 15% more efficiency over a fixed-pitch fan.

Fine pitch for take-off, coarse for cruise and quiet, what could be better?

+2.

[Strega]

CS props are used when high prop e is needed over a large range of airspeeds..

with the RV-8, high e is needed at high speed, but at low speed, if one has a prop e of 65% who cares.. you still have TONS of thrust to take off.

I could go on in depth with the "math" behind this, but not many members of this forum would appreciate my words..

Bottom line,, dont worry about the CS prop/gov on your project..

[Strega]

A 180hp RV-8 with a fixed pitch prop will get its doors blown off by a similar one with CS

yes and no,, the CS version will "blow the doors off" the FP version on TO inital climb, but the FP will "blow the doors off" the CS in cruise (if a cruise prop is specified). I have flown both.

[Strega]

The only fixed pitch props you see in the air races are in Formula One where CS is not allowed

use google..

[cgzro]

Not sure what the original context was but constant speed is more or less manditory for aerobatics these days.

Noise is a major concern and a constant speed will not wind up like a fixed pitch. Also zero workload while flying, just set the RPM and forget.

You do see fixed pitch props on the 180HP and less biplanes and they fly beautifully but they are soooooo noisy I don't know where you could actually practice with one, at least anywhere near a built up area. You could fly a 400HP Sukhoi and not bother anybody (sounds like a big tractor in the air) but wind up a 180HP Lycoming to 3000 RPM with a fixed pitch prop and the noise complaints will start comming in.

Be nice not to have the extra weight though for sure. Its about 60lbs or so difference on a 200HP engine.

[fl80]

This looks like a good place to ask a question I've been wondering about for several years now...

Why is it that the fp prop requires inspection every 5 years vs. the more complex CP prop requring inspection every ten years?

[Strega]

Why is it that the fp prop requires inspection every 5 years vs. the more complex CP prop requring inspection every ten years?

Well it was not that long ago that CS props required 5 year inspections as well, but it had been shown from years of data, the 5 year period was too short, and was increased to the current 10, this was a huge economic benefit. With FP props, the 5 year inspection is simple and cheap,, could it be pushed to the 10 year cylce as well? sure, but there would be no economic or really any other benefit.

[Strega]

The big flaw in the CS prop is you can only "twist" the entire blade.
There are even some props (warpdrive) that "twist" only the tip!

it has to do with the prop "washout" or the relationship between rotational and forward speed. Ever wonder why a helicopters blades are the same pitch all the way along the blade profile? (yes I know there are some new fancy ones that arent, but think of BH 05 06 etc)

I supposed one could design a CS prop, that has its blade profile optimized for
high speed cruise, but the reality is it almost NEVER is.

[black hole]

I have found that the fixed pitch is the most efficient for a specific pupose. If you have a slick little RV and like to travel long distances at a higher altitude you should have a courser prop. If you are stooging around the farm at less than 1000' than something finer would be better. My rule of thumb is to pick the prop that would give you maximum rated RPM in straight and level flight, full throtle, at the altitude you fly at most.


BH

[Strega]

I find it strange that as soon as I provide some technical detail,, the post dies....

Hmmm...

[iflyforpie]

Perhaps you could explain why a Cessna 177A with 180hp fixed pitch has a top speed of 130 knots and the 177B with 180hp and a constant speed has a top speed of 139 knots?

[Strega]

Perhaps you could explain why a Cessna 177A with 180hp fixed pitch has a top speed of 130 knots and the 177B with 180hp and a constant speed has a top speed of 139 knots?

you are comparing apples to oranges,

the 177A if equiped with a FP prop that was set up for max e at high speed, would need a 10k foot runway to get airborne..

not too mention, what are the other airfram differences betweent he 177a and b? Also I would be very skeptical if a cardinal could go that fast ever!!

if you understood the theory behind a prop, you would understand my point.

[Hedley]

the 177A if equiped with a FP prop that was set up for max e at high speed, would need a 10k foot runway to get airborne

Doesn't sound like anyone could ever use that prop, then.
Not very practical.

Bottom line is that a constant-speed prop provides better
thrust at slow speed, which results in better takeoff and
climb performance, and a c/s prop also provides for reduced
fuel burn in cruise. It costs more, and weighs more, however.

[Strega]

see below again:

The big flaw in the CS prop is you can only "twist" the entire blade.
There are even some props (warpdrive) that "twist" only the tip!

it has to do with the prop "washout" or the relationship between rotational and forward speed. Ever wonder why a helicopters blades are the same pitch all the way along the blade profile? (yes I know there are some new fancy ones that arent, but think of BH 05 06 etc)

I supposed one could design a CS prop, that has its blade profile optimized for
high speed cruise, but the reality is it almost NEVER is.

[iflyforpie]

the 177A if equiped with a FP prop that was set up for max e at high speed, would need a 10k foot runway to get airborne..

Thanks for proving my point. A CS e will be much higher at top speed than the majority of FP designed for cruise.

not too mention, what are the other airfram differences betweent he 177a and b?

Hardly any. Not enough to explain a 9 knot difference!!

Also I would be very skeptical if a cardinal could go that fast ever!!

So am I but it is right out of the POH. I don't have any other empirical data at the moment but I assume that since the planes were made and tested by the same manufacturer that the same errors are present in each.

I understand that under a very restricted set of circumstances a FP is more efficient than a CS. The lighter weight of the FP makes these set of circumstances a bit wider. But outside of these circumstances the cavitation of the FP at low speed and the possibility of overspeed (solved by running at reduced power) at high speeds makes the CS a winner by a long shot if you are trying to make your plane faster.

[mag check]

not too mention, what are the other airfram differences betweent he 177a and b? Also I would be very skeptical if a cardinal could go that fast ever

Well, the 'b' model is longer, has a slightly shorter wing, has a shorter vertical tail, and came standard with wheel pants, while the 'a' didn't. All these things will definately add up to alot of extra speed. Just the wheel pants are good for 5 mph.

An interesting point is that the fixed pitch 'a' model has a 1200' higher service ceiling than the 'b'. 15800' vs. 14600'.

[Strega]

circumstances the cavitation of the FP at low speed

we are talking about an airplane prop, not a boat prop.

mr pie, you clearly do not understand the concept I am trying to teach you.
Buy a book on props, read it, then come back to the discussion.

Answer me this? why does an airplane prop have "washout"?

[MichaelP]

There's theory and then there's reality.

The first time I flew the Wassmer Pacific I wrote "could benefit from a CSU" in my logbook... I was wrong, and when I used the aeroplane numbers rather than those given to me by the instructor checking me out this aircraft performed well.
I was taught: rotate at 75KIAS and climb at 95KIAS... later I rotated and was airborne at 45KIAS, into ground effect soft field style and accelerated to 75KIAS for the climb.

The Wassmer WA51A Pacific is a composite 2+2 seat low wing aircraft with a 150hp O-320 engine. It cruised at an easy 120 knots behind a fixed pitch propeller. G AZYZ.
IMHO this aircraft had almost as good a performance as the DA40-180 does now because I believe the aerodynamics of the Wassmer are much better than the Diamond. (The plastic Wassmer Pacific was built in 1971 - 1973).

Then I flew the Wassmer WA52 Europa, G BDSN.
This was an identical airframe to the Pacific but with a 160hp O-320 engine and a constant speed propeller.
I reckon that the CSU robbed this engine of all the additional 10hp and more!
It was a metal (Hartzell?) propeller that was basically the same one used for the 180 - 200 hp O-360 engines, and it takes power to turn these propellers and provide the oil pressure to run them.
So with the CSU and 10 more horsepower the WA52 was slower in cruise, and I did not notice any improvement in takeoff performance.

The Wassmer WA54 Europa has the O-360, 180hp engine but I never got to fly one

Then came the Slingsby T67M with the same 160hp O-320 but this time the CSU was a Hoffmann composite propeller and this aeroplane performed well, even being able to do outside loops!
Now a lightweight propeller is used that takes less energy loss to turn than the additional thrust developed.

Likewise, the Katanas have the light Hoffmann CSU propellers, but I wonder how they would do with fixed pitch props?

The C1 would be nice with a CSU but the big heavy lump of old technology Continental means the CG is already too far forward.

Better than the DA40-180? This is the 150hp F/P Wassmer WA51A Pacific I used to fly:

[iflyforpie]

Answer me this? why does an airplane prop have "washout"?

To account for the different angles of airflow due to the different linear velocities along the span of the prop blade in relation to the relative airflow. Piece of piss Strega. A helicopter blade doesn't have washout because the relative airflow through the rotor is minimal.

Why is it that you can never give a straight answer in spite of your obvious credentials?

There is a distinct difference between knowledge and wisdom Strega. When you learn that difference and how to apply it you can come back and have an intelligent discussion.

[Jungle Jim]

I've been looking at Thorp T18's lately and have found that the C/S versions have the advantage of climbing faster, can use a shorter runway for takeoff and also allow the plane to slow down faster, are easier to control approach speed and use less runway on landing. The top speeds are about the same.

My choice is to go with the C/S version once the Stinson is sold.

Jim

[Strega]

I've been looking at Thorp T18's lately and have found that the C/S versions have the advantage of climbing faster,Why do you want to climb faster than 500fpm in an unpressurized aircraft? can use a shorter runway for takeoff yesand also allow the plane to slow down fasterNegative trust on the small lycomings is not "good", are easier to control approach speed huh?and use less runway on landingwe are talking a prop without beta. The top speeds are about the same.

My choice is to go with the C/S version once the Stinson is sold.

Jim

[Strega]

Adam wanted to know why a CS prop is less e than a fixed p at high cruise,, NOT why some POS cardinal is faster than the other POS, or have "speculative" info from all.

Adam, if you want a plane that is very efficient at cruise speed/high speed, buy a proper FP prop, Catto or Prince or Lightspeed, or Hertzler etc.

if you want a plane that accelerates like mad, and climbs so fast it makes your ears bleed, and can carry like 50 lbs less and costs 10k more,buy the CS one.


MR pie, why is it that the AG-5B Tiger with a fixed pitch prop and 180 HP have a top speed of 148 knots? perhaps the grumman Engineers need to learn some things...