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fixed pitch vs. constant speed

Dbro172 said:
There is not much cost difference between a 320 and 360, maybe $1500. The difference from FP to CS is $5k and up.
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Exactly why I went 360 with fixed pitch. (And I am not a "financial sub-standard member of the community" but may resemble one at first impression)
 
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real money

"There is not much cost difference between a 320 and 360, maybe $1500. The difference from FP to CS is $5k and up."

To paraphrase Everett Dirksen," Ya a thousand here, a thousand there, pretty soon your talking real money"
 
n5lp said:
Boy do I ever feel the fool. The enjoyment of the last 17 years of flying an elegantly simple and efficient design was just an illusion.

Maybe with a heavier, more expensive and more complicated propeller that wouldn't let me fly any faster but would accelerate better during the takeoff roll, I would actually be having fun.
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Awesome. I hope I say the same thing...when I install my fixed pitch. :)
 
I have 7A fixed pitch. Came into the pattern a few evenings ago on a 45 degree downwind at 168mph. No problem getting speed within flap range on downwind. Don't think ability to slow down should influence CS vs FP.
 
rightrudder said:
Not with a -7, but it would come in handy on a -9! I still like my FP on my -9A, even though it does a Schweizer sailplane impression on final. If I needed to get into shorter airstrips, I'd consider the CS.
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I'm still in Phase I, but yes the C/S prop on my 9A definitely does give the ability to bleed energy quickly in the flare. I love it, wouldn't trade for it.
 
Dbro172 said:
There is not much cost difference between a 320 and 360, maybe $1500. The difference from FP to CS is $5k and up.
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At sea level, take off power
An IO-320B CS 2700 RPM produces 162 HP
An IO-360A FP 2250 RPM produces 163 HP

At 8000'
An IO-320B 2700 RPM produces 130 HP
An IO-360A 2700 RPM produces 163 HP

The FP is lighter, cheaper, climbs better, is faster and produces more HP.
Big problem with the IO-360 FP will be exceeding VNE, and you'll have to pull the throttle back above 8000'. The emperor has no clothes.

If you are an aircraft designer, you don't want an engine that wiil exceed the airframe Vne in normal flight. A way to get the climb performance without increasing top speed is to use a CS prop. In fact, this is the major reason why the RV-9 is limited to a 160 HP engine, but adding a lightweight CS prop is OK.
 
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The designer advised to build light and keep it simple. Doing this will maximize the wonderful handling characteristics of the design. Heavy and complex may be advantageous for those interested primarily in crosscountry.
 
brad walton said:
The designer advised to build light and keep it simple. Doing this will maximize the wonderful handling characteristics of the design. Heavy and complex may be advantageous for those interested primarily in crosscountry.
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I agree with Brad on this and my Catto F/P on a light RV4 has absolutely no problem surmounting the 14teeners around here nor landing in short back country strips. She'll turn on a dime and bite your six. Been that way for 27 years and I'm still grinning.

Cheers, Hans
 
Light is right...

rockit said:
I am look at purchasing a RV-7 and it has a fixed pitch prop. I have low tail wheel time in a champ but lots of time in a RV-9A. Someone mentioned a CS prop is better because it will help you slow down the airplane and better for low TW time pilots.
I was wondering if anyone has an opinion on this.
Thanks.
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Ravi,
Age old question. F16 Fighter weapons school answer? It depends. :)
Long ago in a galaxy not so far away I built a light, simple RV4 that weighed in at 925lbs and cost me $21K in 1994 dollars. This was before pre-drilled, pre-punched, pre-fab RV's, the internet or chat groups and social media or even this wonderful site "DR" built. Back then RV builders (RV3,4) bragging rights weren't the HP of the engine or amount of glass in the panel but the empty weight, total cost and efficiency! Also a prime topic for discussion was utilizing clean up mods, the coolest personal innovations and of course Propellers as most folks used wood, FP props.

I began flying my 4 with a wood Sterba FP prop and 0-320 and found the RV4 to be an easy tail-dragger compared to our family Taylorcraft. I also found adjusting to a FP wood prop on a clean airplane wasn't difficult at all, just required practice. Later I would install and test a one-off Craig Catto masterpiece prototype prop that would become the RV standard. Much improvement and performance.
Useful tips:
1. Reduce engine idle to 600RPM. (why? less rpm equals less thrust in pattern)
2. Pre-planning. Slowing down earlier, planning descents or using overhead traffic patterns with no traffic to bleed off energy.
3. Practicing both straight in flame out (engine failure) and standard traffic patterns at best glide speed (85 KIAS/100mph)
4. Challenge yourself to fly final approach at 65KIAS/70MPH with flaps. (I use 60KIAS)

Constant Speed RV's are nice for their own reasons not to mention flexibility, a bit shorter takeoffs and the air brake option you mention. Their drawbacks though (cost,weight,maintenance,engine failure glide distance, etc) don't add up for a Jurassic builder/flyer like myself.

The lighter they are, the better they fly...
V/R
Smokey
 
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I've read a fair amount of work being done to improve on FP prop designs (Catto and other) over the years but had any of this flowed to the CS airfoils? Hartznell has their blended, which has a tough to swallow premium, but does it represent a similar leap in performance for CS type props? Whirlwind props are even more expensive, but any better tech? IDK.

Seems like we should have already seen more improvement to CS airfoils than is apparent, at least to me.
 
Speed Brake?

Not to go too far off this thread, but the big issue I see between CS and Fixed is slowing down. Especially with the 9. I just read an article in Kitplanes about the CX5 Thatcher and it has a speed brake that drops down between the gear legs. Maybe that is the answer.
http://www.kitplanes.com/issues/33_4/flight_reports/Building-the-Thatcher-CX5_21502-1.html

I am a low time pilot and the thought of one more thing to worry about with a CS prop helped my decision to go with a Catto. All the planes I am taking lessons in have a Fixed prop too. I did put the recess in my firewall and my engine can be converted. Maybe some day I will change. It has been an interesting discussion and learning experience.
 
One thing my CFI reminded me with the FP planes was you can always trade airspeed for alt to slow down. Of course not in the pattern, but way before - which brings us back to planning. After all you can slow down a lot on a 5mi approach to the down wind.

Remember that mooneys all have CS props. But they still need to plan their arrivals not to be speeding around the pattern etc. Some even have speed brakes, and so the same arguments about props and planning being made here (ie slowing) revolve over at their sites about speed brakes and planning.....
 
So a friend and I both built RV7's with o-360's. He went with CS, I opted for FP. We were chatting recently, when I made a comment about kind of wishing I had gone with a constant speed. He replied that he was thinking he should have went FP. Go figure!
 
The decider for me was that in all the FP/CS debate I have yet to see one documented instance of someone voluntarily trading CS for FP. I am certain my -8 will outperform my cub, even with a 2x4 on the nose, but I got a deal on a CS and the overall VAF response makes me believe that if you can do CS you should. If I ever build a -3 it will have a Catto for weight purposes only. On an -8 I can save the difference elsewhere.
 
meloosifah said:
The decider for me was that in all the FP/CS debate I have yet to see one documented instance of someone voluntarily trading CS for FP.
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To really pull all the debates into one big ball, I have never heard any one with glass voluntarily trade it in for steam. Nor have I heard of some one that primed decide they wanted to un-prime. Sunk cost.
The debate is not if primed constant speed glass is better, the point is if unprimed fixed pitch steam is good enough for me the builder or buyer. That is an individual decision and one of the big perks of Experimental Amateur Built.
 
plehrke said:
To really pull all the debates into one big ball, I have never heard any one with glass voluntarily trade it in for steam. Nor have I heard of some one that primed decide they wanted to un-prime. Sunk cost.
The debate is not if primed constant speed glass is better, the point is if unprimed fixed pitch steam is good enough for me the builder or buyer. That is an individual decision and one of the big perks of Experimental Amateur Built.
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Vlad is FP, steam, (not sure about primed), and I'm pretty sure he has more fun than any of us...

Chris
 
rockwoodrv9 said:
Not to go too far off this thread, but the big issue I see between CS and Fixed is slowing down. Especially with the 9. I just read an article in Kitplanes about the CX5 Thatcher and it has a speed brake that drops down between the gear legs. Maybe that is the answer.
http://www.kitplanes.com/issues/33_4/flight_reports/Building-the-Thatcher-CX5_21502-1.html

I am a low time pilot and the thought of one more thing to worry about with a CS prop helped my decision to go with a Catto. All the planes I am taking lessons in have a Fixed prop too. I did put the recess in my firewall and my engine can be converted. Maybe some day I will change. It has been an interesting discussion and learning experience.
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Not to worry. I went from a 65 T-Craft to my FP -9 and didn't have any issues. The only time slowing down is an issue is when flying formation with a CS prop and he pulls all the power off at once.

Other than that, don't come ripping into the pattern, running over Cubs and such at 150+ knots, and pull the power expecting to slow down to flap speed. It won't happen.

Plan on being at pattern altitude three miles before the airport and slowing. By the time you are ready to put out the flaps you should be at the correct speed. On final, speed is critical, do not let it speed up, even 5 knots!
 
CS vs FP

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.
 
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.
 
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.
 
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FP vs CS

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.
 
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.
 
Lets have a test

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
 
A whole lot of unscientific data

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.
 
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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
 
Christopher Murphy said:
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
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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 :D:D:D:D:D
 
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.
 
plehrke said:
To really pull all the debates into one big ball, I have never heard any one with glass voluntarily trade it in for steam. Nor have I heard of some one that primed decide they wanted to un-prime. Sunk cost.
The debate is not if primed constant speed glass is better, the point is if unprimed fixed pitch steam is good enough for me the builder or buyer. That is an individual decision and one of the big perks of Experimental Amateur Built.
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I agree. But that is not what the OP asked. He essentially asked which is better. And the tribal village of VAF, by and large, has uniformly stated that they would never trade back to FP, given the option of CS. One is not BETTER, but you are making my point. The VAST majority of people, given a choice where price and effort are not an issue, will choose to put in glass and CS. The very few that diverge, do so, typically, for a VERY specific reason.

So, to the OP, if you can, buy CS. You won't ever wish you hadn't. Or buy a 152 and then an RV with a pair of rowboat paddles will seem to be a rocket ship.
 
Deviating even more from the OP, what about complexity of installation? To install a FP prop, all I'd need is a spacer, a prop, and about a dozen bolts.

I'm a big fan of CS props, but is the added work for installation significant?
 
StuBob said:
Deviating even more from the OP, what about complexity of installation? To install a FP prop, all I'd need is a spacer, a prop, and about a dozen bolts.

I'm a big fan of CS props, but is the added work for installation significant?
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Up front at the prop, it's the same - spacer, prop, bolts just like a FP. I added maybe 2 hours hanging the governor and running the lines forward, and maybe 2 hours running the cable to the panel.

Where I added a whole BUNCH of time was at my office, earning the dollars to pay for the constant speed - but it's so worth it!! :D
 
meloosifah said:
I agree. But that is not what the OP asked. He essentially asked which is better.
Click to expand...

Better. There's that word again. I consider my FP Catto to be better because of lighter weight, lower cost, less complexity and very little maintenance (wiping the bugs off the nickel leading edge and retorquing the hub bolts occasionally).

Better can vary greatly according to your mission profile. I don't need to be pinned back in my seat on the takeoff roll. I land at paved strips that probably average 3000', so I don't need prop braking to clear 50' trees and a kamikaze dive down to a 1200' grass strip. Climb performance is still excellent compared to the rental bug-smashers I've flown, and to tell you the truth, I prefer a climb rate of less than 1000 fpm most of the time so I have improved visibility over the nose and increased airflow over the cylinders. Less workload in the cockpit is always a good thing, to maximize eyes-outside time.

A CS prop in an engine-out scenario goes to high-drag configuration, right? I'd rather have an FP's better gliding distance for more landing options in an emergency. As for increased float on landings, I'm learning to manage the energy better, and becoming more comfortable with a slight lower over-the-fence speed that allows me to hit my mark.

So, better depends...
 
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airguy said:
Up front at the prop, it's the same - spacer, prop, bolts just like a FP. I added maybe 2 hours hanging the governor and running the lines forward, and maybe 2 hours running the cable to the panel.
Click to expand...

Really? That's hardly worth talking about in the big picture.

Where I added a whole BUNCH of time was at my office, earning the dollars to pay for the constant speed - but it's so worth it!! :D
Click to expand...

There is that part.....
 
Prop Conversion

Hi all,

I recently converted my RV 8, IO360M1B from a three blade Catto to a Hartzell composite. There have been a number of pluses and and number of minuses.

On the plus side, markedly better acceleration. Much shorter take off run. About 20 kts improvement in top speed (I would overspeed the prop with much more than 19" MP at cruise). Aerobatics work better as it climbs better and the flat prop also provides a little more wind resistance in the down line. I can make substantially shorter landings now with a giant air brake in the front.

On the minus side: Definitely louder, especially at cruise speeds. More vibration although not uncomfortably so. My CG moved quite a bit forward, which sometimes a plus and sometimes a minus. I usually keep #25 ballast in the rear luggage compartment if I don't have a passenger or baggage.

In general, I thought the airplane was more ladylike with the FP prop: smooth, maneuverable, easy to land and take off. With the CS, it is much more of a tiger. More torque on take off to adjust the rudder for, need much more care in landing as I can't tell how much power I have by listening to the engine--the RPM is pretty high until the throttle is way reduced. Also, if you come completely off the throttle at speed, there is kind of a backfiring that goes on. Doesn't sound like it could possibly be good for the engine. Glide ratio was better with FP.

In the final analysis, I am glad I made the transition. I think the overall performance improvement was worth the time and expense to make the change. The actual installation with pretty simple and straightforward.
 
Latest on FP vs CS and purchased

I just purchased my engine and prop. I currently drive a Piper Dakota with an O540 engine and CS prop. I use the CS prop per pilot handbook. But benefit is marginal, if detectable. Weight is high. Maintenance is high. Doesn't seem worth it. So I was definitely going Fixed Pitch on my RV-7A. I've been building since 2008 and no way am I using CS prop. Dah. . .

So at Sun-n-Fun I filled in my engine order form and left prop type blank.
Then Van's called me and asked which prop?

I buckled and ordered the Constant Speed. Why? Weird. Because . . .


1. If cost were no object, CS does give a little performance improvement.
2. Cost at this point is mute. I'm building a new plane. Might as well put in the type of prop future buyer will want.
3. Weight of CS for RV-7A is actually beneficial because it needs forward weight.
4. I want a plane configured like most everybody else

Good luck deciding. No remorse.
 
sglynn said:
I buckled and ordered the Constant Speed. Why? Weird. Because . . .

1. If cost were no object, CS does give a little performance improvement.
2. Cost at this point is mute. I'm building a new plane. Might as well put in the type of prop future buyer will want.
3. Weight of CS for RV-7A is actually beneficial because it needs forward weight.
4. I want a plane configured like most everybody else

Good luck deciding. No remorse.
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You have the freedom to make the choice and use what ever reason you want BUT I would have thought 1 of the 4 reasons would have been "it is what you wanted" instead of 2 of the 4 being about what others would want.

Best advise I ever got is that I should build what I wanted.
 
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mlwynn said:
Hi all,

I recently converted my RV 8, IO360M1B from a three blade Catto to a Hartzell composite. There have been a number of pluses and and number of minuses.

On the plus side, markedly better acceleration. Much shorter take off run. About 20 kts improvement in top speed (I would overspeed the prop with much more than 19" MP at cruise). Aerobatics work better as it climbs better and the flat prop also provides a little more wind resistance in the down line. I can make substantially shorter landings now with a giant air brake in the front.

On the minus side: Definitely louder, especially at cruise speeds. More vibration although not uncomfortably so. My CG moved quite a bit forward, which sometimes a plus and sometimes a minus. I usually keep #25 ballast in the rear luggage compartment if I don't have a passenger or baggage.

In general, I thought the airplane was more ladylike with the FP prop: smooth, maneuverable, easy to land and take off. With the CS, it is much more of a tiger. More torque on take off to adjust the rudder for, need much more care in landing as I can't tell how much power I have by listening to the engine--the RPM is pretty high until the throttle is way reduced. Also, if you come completely off the throttle at speed, there is kind of a backfiring that goes on. Doesn't sound like it could possibly be good for the engine. Glide ratio was better with FP.

In the final analysis, I am glad I made the transition. I think the overall performance improvement was worth the time and expense to make the change. The actual installation with pretty simple and straightforward.
Click to expand...

I think you summarized the positives and negatives very well. Assuming you are injected, it is typical to have that "popping" when dumping the throttle. I don't think it hurts anything.
 
Get it balanced to smooth it out. Two blade has a larger diameter than a three blade configuration, so a higher tip speed equals more noise. I have the WW150 three blade and am loving it. I too get the popping if I come off the throttle at a high speed, but as the plane slows down the popping goes away.
 
Did you pull the prop back when comparing the glide ratio? not flying my 7A yet but our 172 would accelerate when I pulled the prop to course pitch from an idle setting when I would practice power out landings.
 
CS Adventures

Hi all,

I did dynamic prop balancing before the first flight with the new propeller, so I think that issue has been addressed. I have tried glides both with the propeller in high RPM (where it would go with engine failure and loss of oil pressure to keep it closer to feather) and with the prop control pulled back. Makes a really big difference to have the prop pulled back, but I can't necessarily expect that to be possible in an engine failure situation. So, there are really two best glide speeds. Still working on defining those. The folks who make my AOA indicator say that the best glide speed is one green bar. That seems to be pretty accurate, in so far as I have been able to measure thus far. The nice thing about AOA is that it takes into account weight as well as drag. Anyway, more data to follow.
 
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