[albertaflyer]

Hi,

I bought an RV6A with a sensenich climb prop 81" pitch prop. Im looking for a good cruise prop. Or should I wait and spring for a constant speed? Any suggestions would be greatly appreciated! It sounds like the catto props are quite impressive?

 

[turbo]

the sensi 85" prop is a great cruise prop for the 180 rv6. been running one for 10 years. high altitude or short field might need cs.

 

[Ron Lee]

Is the crank hollow to allow a typical CS prop setup?

There are lots of advantages to a CS prop. Just depends on whether you feel the benefits are worth the cost.

 

[Neal@F14]

Is the crank hollow to allow a typical CS prop setup?

There are lots of advantages to a CS prop. Just depends on whether you feel the benefits are worth the cost.

And even if it is a hollow crank... make certain the engine case halves were assembled with the correct oil plug(s) in place for running a CS prop. In the past two years, there have been at least two or three reports of engines being assembled for CS props that the engine builder left those out... and at first startup, no prop cycling because all the high pressure oil was getting simply dumped back down into the sump instead of getting routed out the crank to the prop. A quick search on the forums will retrieve those discussion threads... one of them was my friend JetJ01 and I was there at startup to witness the disappointment firsthand. The engine has to be split open to install the missing plug.

 

[Mel]

Not all engines with a hollow crank are adaptable to a constant speed prop. It depends on the front crankshaft bearing configuration. This is an engine design feature and is not an assembly issue.

 

[albertaflyer]

It's an O360 A1A. It foes have the hollow crank as well as the oil port for the oil line. I'm just curious if it's worth the extra money to go with the cs prop?

 

[plehrke]

the sensi 85" prop is a great cruise prop for the 180 rv6. been running one for 10 years. high altitude or short field might need cs.

I agree. the 85" Sensenich is great on 180hp. Climb is excellent in STL (400ft MSL) even in the hot humid summer.
I would guess you will have all the CS people say CS and all the FP people say FP. My choice was the lower weight of FP and lower maintance cost.

 

[L.Adamson]

I would guess you will have all the CS people say CS and all the FP people say FP. My choice was the lower weight of FP and lower maintance cost.

That's true.....

I've used C/S for a long time, and much prefer the additional takeoff power, quieter cruise, and braking power. I'd never want an RV without one!

L.Adamson RV6A --- Lycoming 0360AIA/Hartzell CS

 

[Mel]

Not Quite!

I would guess you will have all the CS people say CS and all the FP people say FP. My choice was the lower weight of FP and lower maintance cost.

There are a few C/S guys that think their way is the only way.
On the other hand there are many of us who have flown both ways and will tell you that there are advantages and disadvantages on both sides of the equation.
Constant speed props are nice, and if that's your choice, great. But an RV will fly superbly with a F/P prop. I've flown my -6 out of Leadville, CO. with a "tired" 150 hp and a F/P wood prop. No problem at all. I also fly out of my 1500' home strip in Texas summers with no problems.

 

[Sam Buchanan]

That's true.....

I've used C/S for a long time, and much prefer the additional takeoff power, quieter cruise, and braking power. I'd never want an RV without one!

L.Adamson RV6A --- Lycoming 0360AIA/Hartzell CS

If given the choice between no RV or an RV with a fixed pitch prop........guess which I would take...........

 

[Flybuddy2]

The BIG benefit to a CS is long distance, high altitude fuel burn. I did a long trip yesterday with a FP cruise Catto. The Catto is a great prop but, like all FP, it will rev too high as altitude increases unless you pull way back. I had a hard time staying under 10 gph for long first leg. 2nd I ran 23" and was 11+gph. I had this identical engine in a Mooney and could get the burns down around 8 gph. With a CS on plane yesterday I could have made it to destination without a fuel stop.

 

[Ron Lee]

If given the choice between no RV or an RV with a fixed pitch prop........guess which I would take...........

That is a valid argument as is the Cessna 172 vs RV argument.

But let's discuss RV versus RV and then merits or negatives of FP/CS.

The BIG benefit to a CS is long distance, high altitude fuel burn. I did a long trip yesterday with a FP cruise Catto. The Catto is a great prop but, like all FP, it will rev too high as altitude increases unless you pull way back. I had a hard time staying under 10 gph for long first leg. 2nd I ran 23" and was 11+gph. I had this identical engine in a Mooney and could get the burns down around 8 gph. With a CS on plane yesterday I could have made it to destination without a fuel stop.

This is opposite of my experience with a FP prop (currently a Catto 3 blade). At 16000' full throttle I get 2550 RPM. I do not get to 2700 RPM until around 13000'. I did not check the fuel flow but on most flights I run about 2550 RPM and am under 8 GPH. This is with one Lightspeed ignition and cylinders ported and flow matched.

With a CS prop I could probably drop the RPM, have lower fuel flow, similar airspeed and lower noise.

 

[PCHunt]

The BIG benefit to a CS is long distance, high altitude fuel burn. I did a long trip yesterday with a FP cruise Catto. The Catto is a great prop but, like all FP, it will rev too high as altitude increases unless you pull way back. I had a hard time staying under 10 gph for long first leg. 2nd I ran 23" and was 11+gph. I had this identical engine in a Mooney and could get the burns down around 8 gph. With a CS on plane yesterday I could have made it to destination without a fuel stop.

What engine and prop are you running?

Usually the FP and CS props are very similar in cruise performance, and the advantage for the CS prop is in takeoff and climb performance, and ease of throttle management during aerobatics. A slight advantage in formation flying, as well.

Sounds like you might have the wrong FP prop installed.

 

[Flybuddy2]

O360 A1A with a 3blade cruise Catto. Even with that, at altitude I run 2600 rpm (pulled back). For cruise, high rpm is simply inefficient. Lindbergh proved that in WWII. I've flown RVs with CS and the fuel burn at similar speeds is significantly less when you can maintain power and decrease RPM. It's simply more efficient.
That said, given that long trips are now only a small % of my normal flying, I prefer the smoothness and simplicity of the Catto.

 

[elippse]

Typically, an FP prop can be more efficient in converting horsepower into thrust, because it can have a more efficient tip and, especially, root planform. But as you say, by using the prop control to keep rpm low with high MAP can give more efficient flight. The trick with an FP is, instead of reducing power with the throttle, which gives less efficient engine operation due to the pressure drop across the throttle plate, is the use carb heat, if available, to reduce engine power while at the same time increasing the engine efficiency due to the higher input temperature.
The heated induction also allows better fuel atomization and better leaning. On my plane I designed my latest prop to give 3000 rpm at WOT at 14,500 dalt. This gives me much more rpm for takeoff and climb. My climb rpm at best rate is now 2750, 50 rpm below rated. So by designing for more rpm at high altitude, I get all of the benefits of the CS for takeoff and climb, and by use of carb heat I can still keep the rpm down in cruise. On two recent flights, I was getting 4.9 gph at 180 mph TAS at 14,000 dalt! I know that this FF and TAS was real because my GPS said that it took a total of two hours for 341 miles and I used 10 gallons.

 

[gasman]

The trick with an FP is, instead of reducing power with the throttle, which gives less efficient engine operation due to the pressure drop across the throttle plate, is the use carb heat, if available, to reduce engine power while at the same time increasing the engine efficiency due to the higher input temperature.

So with the use of carb heat, does this mean that the intake snout and ram effect is not needed?

The heated induction also allows better fuel atomization and better leaning. On my plane I designed my latest prop to give 3000 rpm at WOT at 14,500 dalt. This gives me much more rpm for takeoff and climb. My climb rpm at best rate is now 2750, 50 rpm below rated. So by designing for more rpm at high altitude, I get all of the benefits of the CS for takeoff and climb, and by use of carb heat I can still keep the rpm down in cruise.

How do you keep your rpm down in cruse without colsing the throttle if your engine can turn 3000 rpm at 14,500 feet? does carb heat reduce it that much? Again, do we need ram air?

On two recent flights, I was getting 4.9 gph at 180 mph TAS at 14,000 dalt! I know that this FF and TAS was real because my GPS

and this was a "no wind" condition?

said that it took a total of two hours for 341 miles and I used 10 gallons.

Thank You for your reply........

 

[elippse]

Thank You for your reply........

The "ram" effect, that is the conversion of dynamic pressure to increased manifold pressure, is necessary when you want to get more power and decrease induction losses. My curved divergent submerged NACA duct is giving me total pressure recovery when it's open, even with a K&N filter. When I close it carb heat enters the engine to reduce power but increase effciency.
To keep the rpm at or below rated requires more pitch, which means less static rpm and thrust, a longer take-off, and reduced climb rate. So I chose the route of lower pitch for better take-off and climb, and more speed if I want it, but when I want more economy I use the carb heat. On both of those two way trips wind wasn't a factor. I'm just trying to get the best all- around performance from the plane and prop.
I haven't tested for this yet, but according to my estimates, I will get 220 mph TAS at 3250 rpm at 1000' dalt if I want. My ROC is over 1700 fpm at 2750 rpm, 1000' dalt, 1350 lb, 105 mph IAS. Not too shabby for 125 HP O-235! With my previous 3-blade I was getting 1550 fpm at 2410 rpm at the same conditions, and a measured 213 mph at 2950 rpm, 1000' dalt. This prop design lets me trade off speed and performance or economy. Using the 10% above rated rpm rule, I should be able to turn 3080 rpm all day with no problem.

 

[GSchuld]

Paul,


Great info. Could you tell us more about your new propeller. A picture or two perhaps(or link to one). Does the planform resemble the prop you designed for Tony Higa with the more swept tips?

Regarding carb heat rpm control, do you have any way of knowing the raise in induction temps when you are at an economy cruise setting. A carb temp gauge or a downstream temp probe in the induction track?

George

 

[Flybuddy2]

Colder, denser air increases engine power. The reduced fuel burn via carb ht is solely a result of (another method) of reducing power. A prop spinning at 2750 in cruise is inefficient and not properly loaded for normal cruise power.

 

[elippse]

Paul,


Great info. Could you tell us more about your new propeller. A picture or two perhaps(or link to one). Does the planform resemble the prop you designed for Tony Higa with the more swept tips?

Regarding carb heat rpm control, do you have any way of knowing the raise in induction temps when you are at an economy cruise setting. A carb temp gauge or a downstream temp probe in the induction track?

George

Hi, George! Yes, it looks exactly like Tony's except three-blades rather than two. I don't get as much heating with my set-up since I get heated air from the inner fins of the front two cylinders rather than an exhaust muff and I think my valve in the inlet is leaking somewhat. I've seen 18C carb temp when OAT is 4C, so I'm getting about a 25F rise which would decrease power 2.5%.

 

[PCHunt]

....... I've seen 18C carb temp when OAT is 4C, so I'm getting about a 25F rise which would decrease power 2.5%.

Why would it make a difference what method is used to reduce the power by 2.5%? A 2.5% reduction is a 2.5% reduction, I would think, whether it was obtained by adding carb heat, reducing RPM (CS), or throttling back a bit.

 

[elippse]

Colder, denser air increases engine power. The reduced fuel burn via carb ht is solely a result of (another method) of reducing power. A prop spinning at 2750 in cruise is inefficient and not properly loaded for normal cruise power.

Yes, the density goes up 1%/5.2F decrease, but the engine efficiency goes down 1/2%/5.2F decrease, so that the overall effect is that the engine power increases 1%/10.4F decrease. If you'd like, I can cite this effect in C.F. Taylor. I'm not sure what rpm factor that you used to make the statement about a propeller not being efficient at 2750 rpm in cruise; can you cite your source?
So far I've designed props for 3750 rpm for biplane racing, 4500 rpm for IF1 racing, 7250 rpm for one UAV, and two different UAV designs at 6500 rpm. The one thing that has always stood out with my designs is that they have almost no noise, so that power isn't being converted into noise. This has been demostrated in an anechoic chamber test of one of my UAV designs which has demonstrated remarkably low noise!. This is very important with UAVs as it helps them to avoid detection by the bad guys!
In my propeller design equations the only thing that has anything to do with propeller efficiency is lift distribution, parasite drag, and mass flow. If you know of something that I am missing, I request that you let me know as I am always trying to improve my product.

 

[Flybuddy2]

Yes, the density goes up 1%/5.2F decrease, but the engine efficiency goes down 1/2%/5.2F decrease, so that the overall effect is that the engine power increases 1%/10.4F decrease. If you'd like, I can cite this effect in C.F. Taylor. I'm not sure what rpm factor that you used to make the statement about a propeller not being efficient at 2750 rpm in cruise; can you cite your source?
So far I've designed props for 3750 rpm for biplane racing, 4500 rpm for IF1 racing, 7250 rpm for one UAV, and two different UAV designs at 6500 rpm. The one thing that has always stood out with my designs is that they have almost no noise, so that power isn't being converted into noise. This has been demostrated in an anechoic chamber test of one of my UAV designs which has demonstrated remarkably low noise!. This is very important with UAVs as it helps them to avoid detection by the bad guys!
In my propeller design equations the only thing that has anything to do with propeller efficiency is lift distribution, parasite drag, and mass flow. If you know of something that I am missing, I request that you let me know as I am always trying to improve my product.

Bit fuzzy on some of this. How does reducing incoming air density via carb heat increase engine power? How are you dealing with mach tip speeds at the extreme rpms mentioned (gear reduction or very short diameter?).
To answer your question on 2750 rpm cruise being inefficent. The OPs engine is rated at 2700 rpm. The engine is not properly loaded running at the edge of it's rpm envelope while reducing power to gain back some semblance of fuel economy. It's more "efficient", meaning easier on the engine to get this fuel burn in a range toward which the engine was designed, say 23 " 2300 rpm. Per Lycoming, you would have to reduce power to 19" @ 2700 rpm to get the same fuel burn at 23 squared. It's been proven that oversquare operations are the most fuel efficient, it stands to reason that grossly undersquare would be inefficient.

 

[elippse]

Why would it make a difference what method is used to reduce the power by 2.5%? A 2.5% reduction is a 2.5% reduction, I would think, whether it was obtained by adding carb heat, reducing RPM (CS), or throttling back a bit.

Any time the throttle plate is not wide open there will be a pressure drop that the engine must use power to overcome known as pumping loss. That's why if you reduce power by the use of carb heat, which also makes the engine more efficient, you can operate at WOT. The advantage of a CS is that you can reduce power by decreasing rpm through use of a higher pitch with WOT. These engines have a very flat torque curve, almost constant torque around the rated rpm, which is why the engine power is related linearly with rpm. More rpm, more power, and more friction loss; less rpm, less power, and less friction loss.
Rubbing friction is known as coulomb loss, and the friction FORCE is almost constant, so the friction POWER is force X rpm; the higher the rpm, the greater the friction power. Friction power goes up with rpm as does engine power; they are locked together. It's all a trade off! That's why Reno racers run their IO-360s at 3200 rpm-3750 rpm and O-200s at 4100rpm-4500rpm.
But the propeller efficency can still be high at high rpm if properly designed!

 

[woodmanrog]

Fixed VS Constant Speed

I love this arguement and had to put in my 2 cents worth. I have owned 3 RV's with FP props. I converted the first two to Catto, 2 blade props and left the pitch as Craig Catto designed them for the engines. With the leading edge tape installed, rain became a no factor and the props performed beautifully. Total cost of each prop around $1600. My latest plane came with a Sensinich 2 blade prop with 85" of pitch for cruise. If the plane hadn't come with a metal prop, I would have installed another Catto. Less noise, smoother, and slows down (less enertia) much better when power is pulled. I routinely go on trips with guys who have CS props and find that I keep up with them with no problem. They climb out quicker but when they are at 5000 feet I am at 3000 and I still have them in sight. I keep climbing and they pull back to cruise and eventually I catch up. Here's the bottom line: RV's climb out so well that the CS vs FP really doesn't hold much water. (Nobody has this arguement when they fly 172's that only climb out at 500 fpm on a good day). How much fuel could you buy for the 5K to 10K it costs for a CS? I still cruise at 155 knots at 8-9 GPH. I will always opt for saving the money and having less maintenance.
Woodman

 

[L.Adamson]

I will always opt for saving the money and having less maintenance.

If that was my full intentions.............I wouldn't be keeping an airplane!

L.Adamson ---- RV6A Hartzell C/S

 

[woodmanrog]

About my quote

I guess I should have added or stated, "also less to go wrong". People are so touchy. Not all of us are so wealthy that costs are not a concern. If that were true, a lot of us would buy rather than spend all that time building.
Woodman

 

[L.Adamson]

I guess I should have added or stated, "also less to go wrong". People are so touchy. Not all of us are so wealthy that costs are not a concern. If that were true, a lot of us would buy rather than spend all that time building.
Woodman

I'm not wealthy at all. The last few years haven't been the greatest. But I do live in mountain country, and the C/S prop was number one on my list. The ratio of C/S versus F/P on RV's around here, is very noticeable. Someday..........I'll finally move past the "six pac" and get a TV screen panel like the rich guys...

P.S. --- I'm not touchy. I just like the C/S versus F/P argument.

L.Adamson --- RV6A

 

[Flybuddy2]

I love this arguement and had to put in my 2 cents worth. I have owned 3 RV's with FP props. I converted the first two to Catto, 2 blade props and left the pitch as Craig Catto designed them for the engines. With the leading edge tape installed, rain became a no factor and the props performed beautifully. Total cost of each prop around $1600. My latest plane came with a Sensinich 2 blade prop with 85" of pitch for cruise. If the plane hadn't come with a metal prop, I would have installed another Catto. Less noise, smoother, and slows down (less enertia) much better when power is pulled. I routinely go on trips with guys who have CS props and find that I keep up with them with no problem. They climb out quicker but when they are at 5000 feet I am at 3000 and I still have them in sight. I keep climbing and they pull back to cruise and eventually I catch up. Here's the bottom line: RV's climb out so well that the CS vs FP really doesn't hold much water. (Nobody has this arguement when they fly 172's that only climb out at 500 fpm on a good day). How much fuel could you buy for the 5K to 10K it costs for a CS? I still cruise at 155 knots at 8-9 GPH. I will always opt for saving the money and having less maintenance.
Woodman

That's a VERY sensible argument. I love my Catto and the only point I was trying to make was mostly one of range loss. On the few long trips I do it would be nice to have similar speeds with about a 2 gph savings which would give me about another 120+ nm range. HOWEVER, this is not a bad tradeoff overall. If you do the math and say a CS costs 4K more than an FP. At 2 gph saving (if you could get that much) and $5 a gallon fuel, it would take 400 hours to make up the difference. Some C/S props have recommended O/H at 500 hours. Another 2k would = 400 gallons of fuel.

 

[kauaikarl]

Wood prop

I've got a new, never used wood prop built by Bernie Warneke. The numbers on it are 68x76 and it was designed to go on a 180 hp RV6. I've got it listed for sale in the Classifieds. It's a phenomenal prop.

 

[handkas]

But an RV will fly superbly with a F/P prop. I've flown my -6 out of Leadville, CO. with a "tired" 150 hp and a F/P wood prop. No problem at all. I also fly out of my 1500' home strip in Texas summers with no problems.

Mel, were there any trees or other obstacles at the thereshold of your strip?
I plan to buy O-360 with 85" sensenich for my rv-7. I just wonder if I will be able to take off form a 600m (~2000ft) grass strip with 20m (65ft) trees on one side.

 

[SteinAir]

Mel, were there any trees or other obstacles at the thereshold of your strip?
I plan to buy O-360 with 85" sensenich for my rv-7. I just wonder if I will be able to take off form a 600m (~2000ft) grass strip with 20m (65ft) trees on one side.

That should be no problem. We operated our black RV6 with that same setup for many years off of a similar strip with similar trees....but that's no guarantee. Surprisingly, most of these RV's will do more than what those of us guiding them are capable of.

Cheers,
Stein