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What is the performance hit of fixed pitch prop vs C/S?

O

OldAndBold

Well Known Member
I looked around and didn't see if this had been discussed.

I am building a 7A. How much of a performance hit would I take if I chose a fixed pitch prop instead of a CS? FP would be so much simpler to build and to fly it would seem...

Thanks in advance

John Babrick
 
OldAndBold said:
I looked around and didn't see if this had been discussed.

I am building a 7A. How much of a performance hit would I take if I chose a fixed pitch prop instead of a CS? FP would be so much simpler to build and to fly it would seem...

Thanks in advance

John Babrick
Click to expand...

It all depends on how you pitch your fixed pitch prop.

A general statement that I don't think anyone would disagree with, is that a fixed pitch prop can give you the same cruise speeds as a CS prop, but you compromise by giving up some rate of climb.

However, with regard to the RV series specifically, the worse rate of climb on an RV using a cruise pitched FP prop will still out climb any GA aircraft by a 2 to 1 margin.

Do a search here on VAF using "Catto", and you'll get some of the more interesting performance reports.

Highflight
 
I'll get pounced on by all the C/S guys, but the only performance hit you will take is that you can't get MAX takeoff performance AND MAX cruise performance with a F/P. You choose which is more important to you. I fly a Catto 3-blade F/P RV-6 off a 1500' strip in Texas and have no problems. Yes a C/S will give you better performance at both ends of the scale, and they are great. But unless you are flying off a short strip at high altitudes, a F/P will work fine. The great thing about home-builts is, we have a choice. If the added benefit of a constant speed prop is worth the cost, weight and maintenance to you then go for it.
 
Yes

Hi John,
As Mel said, you can have the same or higher cruise speeds with a FP if you don't mind turning it 2600 RPM or more. We had our three-bladed Catto pitched for max cruise because we have a 180 Lyc. We only get 2200 static on initial takeoff and use around 600 feet to get in the air....still not shabby.
At 135-140 MPH I get close to 2000 FPM and at 160 MPH cruise/climb, she'll still show me 1000 FPM or more. At that speed it's around 2450-2500 RPM.

You don't get any braking on final or during the flare as you would with a CS but it's not difficult. The airplane is light, nimble and very smooth and we have no regrets.

Regards,
 
OldAndBold said:
...FP would be so much simpler to build and to fly it would seem...
Click to expand...
I agree with what the others said about performance and am happy with my fixed pitch prop, however I can't really agree that it is simpler to fly. I would say there is a bit more fiddling necessary with the fixed pitch. You have to do quite a bit of throttle manipulation to manage the RPM.
 
Vans can help

Call Ken Scott at Van's. He flew a fixed pitch for years on his six and then "upgraded", changed, whatever, to a CS prop. He can give you first hand comparisons.
I like the versatility of the CS and the ability to "throw on the brakes" when it is time to slow down.
 
Is there any advantage/disadvantage of using a C/S arrangement as opposed to an adjustable pitch propeller w/o the governer?
 
Fixed pitch vs.CS

My RV-6A has a 160/Sensenich FP cruise prop. It is a very good combination when flying by myself. But when I get into a formation of 180-200 hp with CS, my climb rate and wing work become very frustrating. For that reason I hope to sell my firewall forward this year and install the 180/CS so I can keep up with the others all the time.

Another way to look at FP is to compare a racing go-cart with only one gear-fast! They have to be pushed off to get started,but then lookout! (like 4th or 5th gear in a car) That is how a FP performs.The pitch is set to climb, midrange, or cruise. So think about how your car would perform if you only had one gear.

However, money is a big factor. In addition to the initial cost of purchasing, you add the complexity of a governor, various lines, another cockpit control and guage. Then add in maintenance. Three of the CS guys in my hanger have had big time prop mx in the last 2 years. One required a new hub, one an overhaul, and the other a return to factory for required mods. My FP stayed bolted on the nose and ready to go. An 8A in our group is doing a prop change today because of AD's.
 
Sorry,
But comparing a F/P prop to a car with only one gear doesn't wash. If this were a fair comparison then there would be single gear cars running around or ONLY C/S props. There a plenty of F/P props performing just fine. How many boats have transmissions? They seem to get along with only one gear. The difference is that boats and airplanes work in a fluid media. Cars do not. Yes C/S props have their advantages, but to say a F/P is like a car with only one gear is not a valid comparison.
 
My three-blade fixed pitch has better efficiency in cruise than any CS on the market, by 4-8%. It also gives me good climb performance; how's 1550 fpm at sea-level, 1350 lb, 125 HP O-235F at 2410 rpm! The secret to getting good all-around performance with a fixed-pitch is to properly characterize the kind of flying you are going to do. First off, so many pilots are afraid of that little red line on their tach. The engine won't blow up if you exceed it by 10%! That's 2970 RPM for a 2700 RPM rated engine. And if you think that by operating it at red-line that your engine won't get to TBO, let me ask you: how old are you and how many hours do you fly in a year? Let's say you're 55 and you fly 65 hours a year. That means that if operating at red-line drops your TBO to 1800 hours, you won't reach that until you are over 82! You'll probably sell the plane after about five years. And won't the buyer be impressed by your plane's performance. If you've still got a mechanical, eddy-current tach you most likely don't really know what your RPM is anyway! And if you have a 200 mph airplane, you'll spend a lot less time flying your plane on trips than when you flew that 172!
So if your plane is for cross-country, my recommendation is that you get a prop that will give you rated rpm at the highest density altitude you will fly in the summer. For me on the west coast, flying at 11,500 -12,500, that's 14,000 to 15,000 dalt. Typically, your rpm will change about 0.6%/1000'. That means that if you get 2700 RPM WOT at 14,500' dalt, you'll run about 2940 RPM at sea-level. Guess what that higher rpm will do for you on take-off and climb? You'll be almost matching those CS guys in a climb, and you'll be out-running them at cruise, especially if it's one of my high-efficiency designs! Remember, you can always throttle back at the lower altitudes. You'll only be turning 2800 RPM at 8000' dalt WOT, and believe me, when you turn higher RPM your engine is much smoother! But if getting rated rpm on takeoff is your main criterion, then by all means spend the extra buck$ for a C$ and its maintenance!
 
Elippse props

Where can an Elippse prop be ordered from ? I did find a few articles about it on the web, and I'm interested.
Gary
 
More ways of killing a cat than drowning it in cream.

elippse said:
For me on the west coast, flying at 11,500 -12,500, that's 14,000 to 15,000 dalt. Typically, your rpm will change about 0.6%/1000'. That means that if you get 2700 RPM WOT at 14,500' dalt, you'll run about 2940 RPM at sea-level.
Click to expand...

Paul,
Very interesting information.

On the cruise, how do you establish your % power to ensure it is below 75% for leaning.
Do you have a MAP guage, or do you just lean it until it's rough and then shove it in a bit.

Pete.
 
I only design props; I don't make them. However, I'm in contact with a principal from Vari-Prop. I'm going to design a three-blade prop for their hydraulic CS hub that will be a good match for the planes with 150-220 HP, 180 mph to 240 mph TAS. If all goes well, you hould be seeing this introduced at Sun 'N Fun in April. there might also be a ground adjustable.
 
On any trip of 100 miles or more I fly at 11.5k' or 12.5k', so % power is no problem. That gets me above most of the traffic around 8k', the glide range is greater, plus it's cooler in the summer! At 14,500' dalt, with 198 mph TAS, power should be about 60% at rated rpm and full dynamic pressure recovery. The density altitude changes 113.4'/C or 63'/F above or below standard baro altitude temperature. Once baro altitude is corrected for temperature to obtain dalt, the density ratio may be obtained from DR= (1-6.88E-6 dalt)^4.256. I correct DR for dynamic pressure from IAS. DP = rho * IAS^2/2 where rho = 0.00237689. Corrected DR = (DP +2116)/2116 * DR. Altitude Power % = (CDR -.15)/.85 * RPM/Rated RPM. With MAP, it's approximately MAP/29.92 * RPM/Rated RPM. That makes it easy to set up a little table of values for your console or instrument panel. Fred Felix of Felix props has expressed an interest in making my prop design. Contact him if you are interested. 800 776 7357
 
My RV-6 has a 160 hp 0-320 turning a two blade Catto Propeller. Yesterday, a friend took off in his T-18 powered by a 180 hp 0-360, and we departed right behind him. There were two of us in the RV-6. He was alone in the
T-18. He out climbed us, but as we leveled off I left the RV at full throttle, trying to back him up a little. The indicated airspeed worried it's way up to 195 mph. The engine was turning 2760 RPM. The whiz wheel put our true airspeed at about 202 mph. We did catch up.
Sometimes, on hot days at high elevation airports, I wish for a constant speed prop, but it isn't worth $10,000 to me for the mission I have for this airplane.
 
% Power calc - more

fodrv7 said:
Paul,
Very interesting information.

On the cruise, how do you establish your % power to ensure it is below 75% for leaning.
Do you have a MAP guage, or do you just lean it until it's rough and then shove it in a bit.

Pete.
Click to expand...
According to Walter Atkinson in previous posts, when running LOP, power is purely a function of fuel flow times a multiplier which is a constant for a given engine. In his posts he gave some examples, varying with compression ratio. This makes power calculation in cruise - below 75% - a snap. For instance, for my engine, it's based on the BSFC of the engine. For example in the case of 8.5:1 compression ratio..
"14.9 * FF in GPH = Hp produced."
I am very comfortable using this when LOP.

http://www.vansairforce.com/community/showpost.php?p=52095&postcount=12

When ROP, it's based on MAP and RPM and in my humble opinion, a very inexact science. My reason for this statement can be found by comparing the various formulae and/or tables offered by acknowledged experts, including Walter, Lycoming , Superior, K. Horton, all of which disagree to some extent with all the others. Lycoming will even disagree with itself for the "same" engine in different airframes.

But maybe there is another way. Superior says the BSFC for their IO-360+ (8.5:1) when 50 degF ROP is 0.50. You could use this along with Walter's formula (he's using 0.40 in this example). Then you can go directly from fuel flow IF you are sure where your EGT's are. This idea is purely mine and no expert has supported it that I know of. I would do it thus: 14.9 * .4 / .5 = 11.9 factor. So, 11.3 gph at 50 ROP is 75% for my engine. (11.3 x 11.9 = 135 which is 75% of 180).

This all leads me to the conclusion that there is no precise way to determine 75% ROP and caution is appropriate. JMHO. Polite arguments invited. This is almost as much fun as building and flying.
 
More good stuff.

H.,
Whilst I agree there is variation between what different sources claim as the settings required for a given % of power, the ones I have only vary by a few percent.

For this reason I cruise ROP for what Kevin's H's spreadsheet/Ms. Lycoming figures assure me is 75% and LOP only back at what the same data assures me is 65%.

So, back to my query; whilst Paul has given an excellent method of calculating the power being extraced under a given set of conditions for his FP prop, having a CSU where I can set the MAP/RPM mix as I like, I now know that at 65% my Aero Sport IO-360 burns 40litres/hr ROP adnd 31 l/hr LOP. It certainly is simple and clear confirmation as to what is going on in the combustion chamber. Though a very expensive way to be able to do it.
Pete.
 
OldAndBold said:
I looked around and didn't see if this had been discussed.

I am building a 7A. How much of a performance hit would I take if I chose a fixed pitch prop instead of a CS? FP would be so much simpler to build and to fly it would seem...

Thanks in advance

John Babrick
Click to expand...

And it weighs less, too, John. That's an important consideration with these airplanes.

My C/S Subby is a delight to fly but there is a weight penalty with it and all Lyc hydraulic systems, plus it costs more.

The decision is a personal one and won't suit everyone. I'd like to have both. :)
 
I don't agree that a FP is easier to fly, depending on pitch, you may have to worry about throttle position in different phases of flight. CS, full fwd, takeoff. Climbout, reduce to 2500rpm, doesn't get touched again until pre-landing checklist if I don't want to. Don't have to worry about throttle position or overspeeding the engine/prop. Mechanically not as simple, but flying I sure think the CS is workload reducing.
 
Check out Dan's W&B database at www.rvproject.com. Last I checked, the -7's tend to be tail heavy. 7A's are a little better, I think...you'll need to check this yourself. A CS might help a -7 with it's balance. Incidentally, the 8's have the same problem when flying solo (only it's nose heavy in this case). It makes me wonder why more people don't put a FP prop on their -8's. It looks like it'd balance better AND be lighter.

A CS is much heavier, though, so what you get back in load due to balance, you may be giving away due to heavier gross anyhow.

It's just one more thing to consider. If you can add some weight up front and increase the utility of the aircraft, plus get better performance all around as a bonus, that seems worth considering at least.
 
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Let's see......Airplanes flying with F/P props..............Millions
....Cars on the highway with 1 speed transmissions..........?
Yep, looks like a pretty good comparison! I hope Sensenich and Catto don't find out about this. Most flight schools could be in trouble too.
 
Boats, cars, airplanes!

My ride has a stir-it-yourself gearbox. Third only will not win a drag race. It is a compromise.

The boat at least has trim adjustment so the F/P prop is less likely to cavity from a standing start.

A C/S prop gives you more control and options. That's why we have C/S props. They do cost more. And just like a gearbox, you can "down shift". You can get a steeper approach with C/S drag. I've seen a whole bunch more overshoots into a tight strip on F/P than C/S RVs.

A C/S prop is probably more c.g. friendly in a side-by-side, also.

If all you do is cruise like Rosie, go F/P. You want performance, get all the gears. That's my take.

John Siebold
 
My 7A's mission is...

...mostly to be a cross country aircraft that also serves as an avionics testbed. So I can see economizing and not doing the constant speed prop. It sounds like I won't really lose too much by going FP, so I think that's the direction I am going to go.

Thanks for the input, all.

JCB
 
Constant speed props have definite advantages. I never said they didn't. My point was and still is; An airplane with a fixed pitch prop may not be optimal but it IS practical. A car with only one gear is NOT!
I understand how automatic transmissions operate. I've been a master mechanic since the early '60s and a certified aircraft mechanic since the early '70s.
 
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Regarding the original post, you should check Van's website and construction manual for the designer's thoughts on it. My understanding is that this plane was originally designed with fixed pitch in mind. Furthermore, I've got a feeling that a lightly built 150hp, fixed pitch RV is an f18 compared to the spam cans that most of are stuck with. Sure the CS is better, but I'm sure I'll be thrilled to death when my simple wood prop'd bird is humming along at 200 mph.
 
Mel said:
Constant speed props have definite advantages. I never said they didn't. My point was and still is; An airplane with a fixed pitch prop may not be optimal but it IS practical. A car with only one gear is NOT!
I understand how automatic transmissions operate. I've been a master mechanic since the early '60s and a certified aircraft mechanic since the early '70s.
Click to expand...

tell me, what else is there to compare them to? the reason cars do not have one speed is quite simple, stop and go. the airplane usually only takes off once per trip.. the car 20-30 times.
fixed pitch = max RPM cannot be acheived at a stand still therefore all horsepower is not availiable at launch, as speed increases max rpm can be obtained therfore max power output acheived. is this correct?
same with a single speed direct or fluid coupled. rpm limited at launch yet can fully develop as speed increases. really, i cant think of a better comparison. however this is my opinion.
on a side note because i'll only be taking off once or twice a day i'll go sensi fixed pitch.
william 843-687-8093
 
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cytoxin said:
fixed pitch = max RPM cannot be acheived at a stand still therefore all horsepower is not availiable at launch, as speed increases max rpm can be obtained therfore max power output acheived. is this correct?
same with a single speed direct or fluid coupled. rpm limited at launch yet can fully develop as speed increases. really, i cant think of a better comparison. however this is my opinion.
on a side note because i'll only be taking off once or twice a day i'll go sensi fixed pitch.
william 843-687-8093
Click to expand...

Apart from developing full power with a fixed pitch prop, that's properly sized and pitched, which is usually a compromise between takeoff & cruise power; the advantages to both takeoff and landing with a C/S prop are overwhelming compared to a fixed. Having flown four different RV's with both, I'd never go fixed pitch if I didn't have to. I'd even sacrifice something else for a C/S. And this especially apply's at higher altitude airports that I commonly use. Can't think of anyone around here that would change their C/S prop for a fixed, either! :D
 
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tall gearing.

I don't know Mel, Those old Norton Commando's with the 3 speed transmission, I could have commuted to work and never get out of 1st. :)

I think I'll stick to my 1 speed RV-4 over the constantly variable transmission (C/S).
 
yes

L.Adamson said:
Apart from developing full power with a fixed pitch prop, that's properly sized and pitched, which is usually a compromise between takeoff & cruise power; the advantages to both takeoff and landing with a C/S prop are overwhelming compared to a fixed. Having flown four different RV's with both, I'd never go fixed pitch if I didn't have to. I'd even sacrifice something else for a C/S. And this especially apply's at higher altitude airports that I commonly use. Can't think of anyone around here that would change their C/S prop for a fixed, either! :D
Click to expand...
yes you would select the ratio/pitch for what you are most likely to be doing
and there would be a trade off. im at sea level and dont plan on short strips and mountain flying. i did however have my engine set up for c/s so i can upgrade later if its that bad. i dont think it will be .guess im just hard headed. it should be like a rocket ship compared to the spam cans ive been flying. and maybe thats where mels opinion comes from . flying light high power planes with f/p props. they are so over powered they still climb like mad.
 
maybe i should have just said

the performance loss you'll see, will be not being able to acheive full power on take off due to the inability to downshift cause you only got one gear and when you come in for landing you will not have engine braking due to the inability to downshift cause you only have one gear. however because the hp to weight ratio is such that you wont really care.
 
cytoxin said:
flying light high power planes with f/p props. they are so over powered they still climb like mad.
Click to expand...

What's very noticeable between fixed and C/S with a "slick" RV, is how far away from the airport, that you must begin slowing down to get to reasonable pattern speeds. With a fixed, it's way out, perhaps six to ten miles, while a C/S allows higher airspeeds to the pattern, as well as steeper approaches if desired.

The constant speed guys/gals will have re-fueled and eaten their $100 hamburgers while you're still slowing for the approach. They'll have to leave without you! :D

I am at 4600' msl to start with, and am biased towards C/S because of the altitude conditions. Although a fixed RV at this altitude, still has no real takeoff problems.
 
boiled down

If you can afford the cost both in weight and dollars go c/s. If you can't afford the cost go fp you will not be sorry either way. Both are great. :)
At the drag strip the rails have one gear, they both start out and end up very fast. :) ;) I know I can't compare a rail to an RV, but almost.
My 2 cents used up.
 
L.Adamson said:
What's very noticeable between fixed and C/S with a "slick" RV, is how far away from the airport, that you must begin slowing down to get to reasonable pattern speeds. With a fixed, it's way out, perhaps six to ten miles
Click to expand...

I've never flown one so forgive my ignorance, but is it really going to take ten miles to slow down?! What am I not understanding?
 
Harvey L. Sorensen said:
At the drag strip the rails have one gear, they both start out and end up very fast. :) ;) I know I can't compare a rail to an RV, but almost.
My 2 cents used up.
Click to expand...

While I've never driven a dragster, I did have a 2 speed car. It was a lime green 1965 (I think) Toyota Corona 4-door. It came with a 2 speed automatic that would shift into second about half way through an intersection. Definitely no dragster.
 
No, it doesn't take 10 miles to slow down with a fixed pitch propeller. I usually start backing off the power a mile and a half from the downwind, and I'm down to flap speed when I'm opposite my landing spot. If I want to set the airplane on the number on the end of the runway, that can be done too. All it takes is a little practice. Even though I have a fixed pitch prop, I use manifold pressure to set the desired power rather than chasing the tach, which has large swings in RPM with airspeed changes.
There was a learning curve when I first began flying my RV-6, but it didn't take long to figure it out.
 
Slowing Down Thoughts

I fly a FP prop RV-7A, 180 HP. Slowing down and losing altitude are two aspects of the same thing as we all know. Power is the variable, since until you can deploy flaps, drag is not very changeable. Regardless of prop type, the limiting factor in reducing power enough to slow down and/or lose altitude at the desired rate is cylinder cooling. Now that means we have to talk about shock cooling.

I don't want to start a religious war about it and I'm not an expert, but a good 4-probe EGT monitor can help a lot. If you use reasonable technique while descending from cruise altitude to near pattern altitude, the engine will likely be cool enough that you can slow down by radically reducing power and raising the nose. If you can't monitor CHT, then such techniques are not a good risk. I use the GRT EIS (and EFIS) with cooling rate alarms programmed. Comments invited.

Personally, I don't find slowing down my RV to be any big deal and my other airplane is a C-150 - drag heaven. I admit to all the advantages of C/S; I chose FP for all its advantages. Just personal preference, as it should be.
 
Prop testing on the same airplane

Everyone that wants to know how the different props perform on the same airplane needs to find a copy of November 1990 'Sport Aviation' and read the article. It has the report title "First Annual CAFE Propeller Competition" and lists performance that was obtained on 'Old Blue' the prototype RV-6.

If you have the magazine, the article in on page 76.

8 DIFFERENT prop manufactures props were flown. The two best performing props tested are sold by Van's Aircraft.

According to the article, the Hartzell has 260 fpm better rate of climb at 105 IAS and 13 mph Cruise [(TAS-8000') at 2600 RPM] advantage over the Sensenich prop. Old Blue was a 160 HP RV-6.

If you find an on line copy of the article, post a link here. I have my copy. As far as I know, the article is copyrighted by EAA. The article was written by Dick VanGrunsven.
 
Fiixed Pitch vs. CS

I just converted my 79" pitch Sensenich FP to a Hartzell CS on my 160 hp RV6 for reasons I don't really understand myself (just say because CS are cool!!!). I was very happy with the FP, although I only got 2200 rpm on takeoff, it still operated very well in all phases of flight, flew delightfully but was difficult to slow down to flap speed. With experience this problem became very manageable along with a flat glide which was just a matter of better speed control. This is a smooth, well balanced combination and I would not by any means be critical of this combination.
Now the CS was just installed about 8 hrs ago and I had almost no CS experience as my background was in light 65 to 180 hp taildraggers so I am still learning. Take off is very impressive, while I do not have actual numbers, it reduces the roll considerably and it climbs like a tiger. Cruise at the lower altitudes appears similar but at the higher altitudes the CS prop seems to do better. Runway handling on takeoff & landing is improved except for torque on takeoff which is more pronounced but manageable (tail dragger here). Those are the positives I have heard would occur but there are negatives. The RV6 flies more like a "bigger" airplane, is more complex, and the glide is not as good...it drops like a rock power off. There are some minor viabrations at certain power/prop settings and the light, nimble glide is gone. The heavier prop, I believe, seems to add some gyroscopic stability which takes this lightfeel away which I do miss. Power on the RV6 still handles like a minifighter.
synopsis: I would do it again although I would not retrofit as it was a complicated conversion discovered after purchasing the prop. Installation during construction would be so easy. The 0-320 engine and Hartzell is a problem free combination as all the AD problems the counterpart 0-360/Hartzell has do not apply plus there are no cautionary operating ranges. The added performance is expensive...I always say $1000 per knot, which is sadly, very appropriate but maybe not precisely correct since other handling changes (the word "improved" was purposely not used here). It is, like always, a performance compromise and a cost of 3 to 1, $2000 for new Sensenich FP vs $6000 for CS used with 0 SMOH including all hardware...add about $2000 more for new from Van's. I hope this rather unscientific perspective helps.

Dick DeCramer
RV 6 N500DD
RV8 wing kit
Northfield, MN
 
Dick,

For the glide, try bringing the prop control back with the engine at idle, you should find that the prop goes to a courser pitch, and that the aircrafts drag will be greatly reduced. Do use caution the first few times you do this, making sure you don't drop off too much RPM (The engine in theory could die, depending on prop settings, i've never had that happen.) This increases the engine off glide ratio of Piper Arrow's and others very effectively.
 
For you guys with a fixed pitch prop; OK, you've been going to fly-ins or running around with your buddies who have an O-360 vs your O-320. You like the extra performance they've been getting, and have been giving serious thought to upgrading to an O-360 yourself. Let's see, how much is that going to cost? $20k; $40k? Oh yes, and when you do, you're also going to have to put on a different prop. More $! I've mentioned that dreaded RED-LINE before, that strikes fear in the hearts of even the staunchest and strongest among us! But just for kicks and giggles, what if instead of going to the expense of another engine AND a new prop, instead we went out and only got a new prop that would let us turn 12.5% more rpm to give us the same power we would get with the O-360! That means instead of 2700 rpm at WOT in cruise, we would turn up, if you're standing, please sit down for this, 3037.5 rpm! I know, I know! By saying this I've probably put a tear in the space-time continuum that will never be repaired in our lifetime. But, consider this: you will go 4% faster, turn 4% more rpm on takeoff for more acceleration and better high-hot performance, and all of the extra power will translate into a higher climb-rate. And this for just the cost of a new prop. Does it get any better than this? But this isn't for the faint of heart, the traditionalists who won't turn more than 2500 rpm, who want their engine to last much more than their lifetime, for those who know that an aircraft engine, unlike your car's engine that red-lines at 6k or 7k, will blow up the moment you go 1 rpm over that little line. Ok! Save your engine that additional 100 or 200 hours for the next owner, and sweat out that next marginal performance takeoff or seeing your buddies leave you behind. Better safe than sorry, I always say! How about this; call Lycoming and ask them if their engines will blow up if operated beyond the red-line on the tach. Push them for a good answer on this and TBO. 'Bet all you'll get is obfuscation and CYA! You do know that the geared Lycomings operate in excess of 3000 rpm! Ask them about that and why those engines can take it but yours can't. Lycoming states for the GO-480 and GO-435. "In the climb configuration, we recommend full throttle throughout the climb...with RPM reductions initially to 3000 RPM and then 2750 RPM for prolonged climb." (Lycoming Flyer Key Reprints, top, P.43) This IS EXPERIMENTAL aviation!
 
Paul/Elippse,
Your comments sound as though your prop design is ideally suited for the Rotary engines- linear power curve and no problems with high rpm operation. Do you happen to have some idea about your prop's efficiencies/performance at different rpms and best diameter to plan around?

Im trying to decide which redrive ratio (2.18 or 2.85) would be best to use with a new 200 hp Renesis engine- assume likely target of ~6000rpm (180hp) service with a reasonable top end around 8000 rpm (210hp)? The current preference seems to involve the 2.85 gear with a big 74-76" FP prop.
 
Sound barrier?

Ellipse, I didn't quote your post - too long - but I'm curious at what RPM and prop diameter will the tips get into a mach number that's a structural or even a noise problem? Can you do this with a composite-over-wood prop do you think?
 
Spin

Paul

But then my buddies would just spin up their O-360s to 3000 and outrun me anyway! I'd then have to go for 3200. Cold war all over again. We should stop this flying arms race now to avoid mutually assured destruction!

Kidding, I get your point and I like it but I suspect it won't appeal to everyone! A small diameter, three blade prop with skinny tips would seem to be the enabling technology. Otherwise, noisy and the drag would eat up a lot of the benefit - if conventional props could be spun that fast.
 
zav6a said:
Paul

But then my buddies would just spin up their O-360s to 3000 and outrun me anyway!
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An O-360 is just an O-320 with 1/2 inch longer stroke. Therefore, an O-360 at 2,700rpm has about the same piston speed as an O-320 at 3000.
 
I think you have it backward- the longer stroke will have the higher piston speed (goes farther per unit time).
 
Speed

I think that is what BOBM said. Piston speed in an O-360 matches that of a O-320 at a lower RPM. At same RPM, O-360 would have higher piston speed.

I'll remind my buddies of that when they are tweaking their governors.
 
As I've previously posted, you can reduce diameter when you add more blades and still have the same or more mass-flow. That, plus the lower tip speed when you reduce diameter for the same rpm, results in a lower tip speed with less loss.
 
rtry9a: 6000 rpm, 76" dia, 2.85 redrive, 200 mph, would give M0.68 tip speed. But if you wanted a smaller 3 or 4 blade to give additional ground clearance, you could go with the 2.18. BTW, I got an e-,ail from Pat Panzera stating that the alternate engine round-up will be at Jean, NV on April 27-29, www.contactmagazine.com/roundup.html . Bill Grimm, a hangar neighbor and RV9 builder and pilot, is unhappy with his Sensenich, so this coming weekend we are going to try my original two-blade on his plane and measure the performance. If all goes well, I'll post the comparative numbers of the two props.
 
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