fixed pitch vs. constant speed

[Bill Dicus]

Watching videos on this site of various RV's with FP props doing take offs suggests rather long ground runs and flat initial climb segments. My limited experience as either pax or pilot in FP RV's confirms this impression. I've not measured take off distances. My RV-8 with a Lycon IO-360M1B is off the ground by 5 seconds after throttle forward (solo) and reaches 1000' AGL between 35 and 40 seconds after throttle going forward and at that point hasn't reached the end of a 3500 foot runway. This results in a good glide cone back to the airport in well less than 1 minute of full throttle operation. If the engine continues to operate normally for a few minutes after beginning the take off both CS and FP airplanes are fine. Should the engine quit at a critical moment I think the CS prop delivers a safety advantage. Does anyone have measurements for some RV's take off distances with FP and CS props? And horizontal distance to achieve reasonable glide cone altitude? What airplanes (CS or FP) produced Vans' figures? Intuitively I think there is an advantage to the CS prop with engine failure at a critical point in the take off. Other thoughts or hard data would be welcome. Thanks.

[brad walton]

Bill, I live on a 3600 foot grass strip at about 155 msl. In the winter, with colder temps my RV 8 Superior IO360 and a Catto 3 blade gets to 1000 agl by the end of the runway. I haven't timed the climb to 1000 agl, but this is where I relax a little. Within the last few days, I have left full throttle in for one minute to see what altitude I get and it is about 1350 agl from a standing start.

[N941WR]

Bill, I think your safety assertion my not be accurate. It really depends on what the prop does when the oil pressure goes away. You could find yourself with a big air break up front and have to push the nose down to keep your airspeed up. With a FP prop you will glide. (It is even better, if the engine stops.)

The performance of my FP O-360 powered -9 matches yours. Although the initial acceleration doesn't match that of a CS prop equipped RV, once I'm flying it is hard to tell the difference.

[Bill Dicus]

Brad and Bill - Thanks for your replies and your experiences. Seems my intuitive estimates are wrong! Any other input is welcome as well in the future. It seems that much might depend on prop pitch, climb speed etc. Am glad your planes perform so well. Thanks again.

[N941WR]

Bill, I understand you love of the CS prop. I flew a -9A the other with an O-320 and CS prop. It had been a LONG time since I had flow an RV with anything but a FP prop.

I had forgotten how quickly they will accelerate (and slow down). It is very impressive.

Somewhere I have the data for max rate climb from 1000 to 11000 but I can't find. If I can find that file, I'll post it, or parts of it anyway as it is rather large. It will give you an idea of what a FP can do.

[Christopher Murphy]

I have a 0320 160 hp fixed pitch -4

Anyone have a 160 hp cs -4 nearby?

We can do some standing start take offs and climbs to see how they compare.

Cm

[plehrke]

If anyone has the urge to do some plotting of data, this thread from 6 years ago has a whole lot of apples and oranges to compare climb rates. In Post 39 I plot and compare 4 prop/engine/model variation. More data in post that are after post 39.

[Saber25]

Chris,

Check out my speeds in the SARL race at KPSO last year. This with a Catto FP from a standing start at field elevation over 7500MSL. A 160HP C/S simply will not beat a comparable RV4 with an efficient F/P especially for top speed. I've out climbed and fly faster than a stock RV8 with 180 HP C/S so the challenge is out for the non believers. Any takers are welcome and I'll supply the coffee and donuts :-)

Cheers, Hans

[Neal@F14]

Originally Posted by Christopher Murphy

I have a 0320 160 hp fixed pitch -4

Anyone have a 160 hp cs -4 nearby?

We can do some standing start take offs and climbs to see how they compare.

Cm

Just make sure you do that comparison test from Johnson Creek's runway with both RV4s loaded full of camping gear at noon on a warm summer day

[meloosifah]

This is definitely something that every builder wrestles with and there is no right answer. But I still assert, to someone on the fence, that other that a very small weight benefit (many people end up adding extra weight to the nose with a FP anyways) that there is very little to be gained in FP over CS but a lot to be gained in CS over FP. If I was paying new prices you can bet I would consider Catto because I am building on a super tight budget.

It simply comes down to whether you are willing to live with the restricting nature of a FP or not. Despite all the arguments both ways, the reality is that a CS is a compromise in that it cannot be finely dialed in to optimize that one sweet spot, BUT a FP is a bigger compromise in that it will never have the flexibility of the various pitches - it's not rocket science - I really don't know why this debate is endless.

Do you want to fly as fast as possible at 8000 ft? Get a FP. Do you want to climb like a homesick angel? Get a FP. Do you want to do anything within 98% of that range? Get a CS. If you are on a budget, get a Sonex.

But again, I have yet to see anyone switch from CS to FP - but TONS go the other way.

Not the same as primer, simply because you CANT switch even if you want to.