Constant Speed Prop vs. Controllable Pitch

[Stephen Lindberg]

Interesting posts. Two thoughts come to mind. First, the centrifugal loading on the propeller hub increasing with the square of the propeller rpm. An increase of rpm from 2700 to 2800 would increase the hub loading by about 8%. I wonder, how strong are the hub and blade grips? Second, experimenting with blade twist and length puts the propeller/engine combination into areas of untested and unknown resonance, ie: potentially distructive vibration that is not detectable by the pilot. It might be prudent to install large cables that wrap around the engine and attach to the airframe in the event that a piece of propeller blade breaks off and the engine tries to jump off of the airplane. Aerobats and Reno racers do this, I think.

[smokyray]

Bob,
Having run five different props on my RV4 over the last 13 years and 1400 hours, I have a couple of observations. First, it's not about how fast you can go for me, it's how slow (short strips, dogfighting, etc..) you can go. I have flown all the models of the RV now, with and without C/S. My favorite is still the RV4 with the HR2 a close second. The absolute fastest RV I have ever flown is my Harmon Rocket but it has 300 ponies out front which isn't a fair comparison. C/S prop airplanes are heavy nosed if you like over the top acro but are nice for slowing down in the pattern, short takeoff rolls, cross country etc. My Rocket can get into my 1900' strip with no worries, mainly due to the braking effect and power in the huge Hartzell paddle out front. The best of both worlds would be a composite C/S like the Whirlwind, Aerocomposites or MT. Any C/S is $$$ though.
So, what do I like best? A light nose and simplicity. My RV4 has a FP MT 2 blade after having run a Catto, Sterba, Warnke and Gary Hertzler racing prop selling the previous to pay for the subsequent. With my FP MT and a 160+HP 0-320, I cruise easily at 165 Knots true at 8500' at 8.5 gph and average 400' takeoff rolls at SL with 1500 fpm climb rates. You gotta like that!
It all boils down to what YOU like and can afford. If a C/S is in your budget, buy one! It adds value and overall utility of the airplane but it also adds weight and complexity.
If you like simplicity, acro, bang for the buck and experimentation get a FP composite like the Catto, Hertzler or MT. Then go buy a years worth of gas with what $$$ you have leftover. FP props and Radio Shack avionics are also nice conversation pieces at fly-in's now as EVERY-body seems to have the bucks for EFIS and C/S props. Not so 15 years ago..of course we used to drill all the holes too...

Seeya!

Rob Ray

[gmcjetpilot]

Originally Posted by smokyray

Bob,
C/S prop airplanes are heavy nosed if you like over the top acro but are nice for slowing down in the pattern, short takeoff rolls, cross country etc. My Rocket can get into my 1900' strip with no worries, mainly due to the braking effect and power in the huge Hartzell paddle out front. The best of both worlds would be a composite C/S like the Whirlwind, Aerocomposites or MT. Any C/S is $$$ though.

If you like simplicity, acro, bang for the buck and experimentation get a FP composite like the Catto, Hertzler or MT. Seeya! Rob Ray

I don't think he was asking if FP or C/S is better, however I think you are wrong on two points.


Rob I have to disagree, fwd CG is often a good thing. First some RV's with FP props are tail heavy, like the RV-7, which is designed for metal C/S prop. With my Hartzell and O360 I am still a little heavy in the tail. If I had a wood prop I could not carry any bags. Of course more prop weight means fwd CG, but that is a good thing in some cases.

My RV-4, I flew over 1000 hours, had a C/S extended hub Hartzell on a O320. It put me right near the fwd CG solo, which was fine. I just needed full nose up trim on approach. However when I traveled with my girlfriend (120 lbs) and all kinds of bags and camping gear (all aft CG), I was glad to have the nose weight. With a light prop I would be way aft CG.

Aerobatics with a C/S is a dream. To say a FP is better for aerobatics is not correct IMHO. When have you seen any Prop acro pilot or serious acro plane with a FP? Set the C/S prop and loop and forget engine controls, as you roll and loop without the RPM's going overspeed and throttle jockeying.

As far as the props you mention they cost way more, cost more to repair and are less efficent than a metal Hartzell or Sensenich. That is just the way I see it. Cheers

[Bob Axsom]

After getting all of these learned inputs and talking to Sulivan Propellers at Hayward, California my comprehension of speed optimization with the prop has raised to a new level. As was said the torque load of the prop and the torque output of the engine are equal in a steady state running condition. In a constant speed prop installation there is one variable propeller parameter and that is the pitch. The pilot has no way to directly change the pitch which was my original concern. However, if you reduce the diameter the load on the engine is reduced and the pitch must be increased to maintain the same RPM. Up to a point, the aircraft speed will increase with each reduction in diameter. For a given prop/engine/airplane combination there is one diameter that will give maximum aircraft speed. This can be found experimentally as done by Sulivan Propeller but the cost is to find the optimum you have to go beyond the peak and that means to get the peak aircraft speed you have to buy replacement blades and cut them down to the diameter that produced the maximum aircraft speed.

Bob Axsom

[randylervold]

Originally Posted by Bob Axsom

I found an interesting 11 page writeup by an RV-8 owner at http://www.romeolima.com/RV8/Prop.htm. Lots of test data.

Bob Axsom

Hi Bob. Yes, I did quite a bit of testing with props, very interesting stuff. I also collaborated with Van's (since I live near by) on some testing and in fact in the two Van's RVator articles they indicate that some of their test data was my data that they had extrapolated, and therein can lay a problem. Since the Hartzell BA was just becoming available when I did my testing we did not have an opportunity to test the BA directly against the WW 200RV on the same aircraft. Van's comparative data for those two props is therefore extrapolated and that can of course introduce errors. My sense, after flying my RV-8 next to BA equipped birds, is that the 200RV is still the more efficient prop, but certainly not by a wide margin. I sure like the lighter weight, lower noise, and lack of any rpm restrictions. But of course this must be offset by greater cost and a much shorter service/inspection interval.

The WW 200RV "OptiQ" airfoil really is a breakthrough. Jack Norris (noted aerodynamic engineer) worked with Jim Rust (former owner of WW) on the planform and airfoil and it does represent new thinking not heretofore incorporated in a GA prop. Further, it was designed specifically for the RV airframe (the RV-8 actually) with its specific frontal area and speed envelope.

What most people don't realize is that Jim didn't like the fact that the WW 150 gave up a little speed being a 3 blade design so once my test data was in on the 200RV he took that "OptiQ" blade design, scaled it down to a 68" diameter, and retrofitted the 150 making it into the 151 in an effort to get that speed back. No one has done any back to back testing on the 150 vs. the 151 that I'm aware of but based on the performance of the 200RV it can't help but be a bit faster. Being a 2 blade design the 200RV will still be the faster of the two, but some guys just like the 3-blade aesthetic and/or need a lighter prop. The 200RV weighs 49 lbs while the 151 weighs 29 lbs due to it's much smaller hub (which therefore requires a higher pressure prop governor). I literally built my RV-3B around the 151 because of the light weight and the fact that the 200RV isn't made to fit an O-320. Personally, I don't think 3-blade props look any better, and they can make taking your cowl off/on slightly more hassle, but 29 lbs is truly remarkable, and with the "OptiQ" airfoil it should be a good performer.

In the final analysis each builder much weigh which factors are most important to him/her and select a prop accordingly. Yes, one can rationalize the relatively small differences in top speed on these props as "not a big difference", but when you pull up next to your buddy and have a 2-5 mph advantage the difference becomes "priceless" as you slowly walk away. C'mon, fess up, tell me you don't know that feeling! ;-)

[erich weaver]

Randy wrote:

"...the 200RV is still the more efficient prop, but certainly not by a wide margin. I sure like the lighter weight, lower noise, and lack of any rpm restrictions."

I was always perplexed by Whirlwind advertising no rpm restrictions, but then "recommending" that certain RPM ranges be avoided. My understanding is that they are simply copying the restricted ranges straight from Hartzell's research, which seems a bit silly since the Whirlwind prop is certainly not a copy of the Hartzell. Anybody have any further info on this?

I did buy the Whirlwind. No flight time yet, but it sure does look nice.

Erich Weaver

[gmcjetpilot]

Originally Posted by Bob Axsom

The pilot has no way to directly change the pitch which was my original concern. However, if you reduce the diameter the load on the engine is reduced and the pitch must be increased to maintain the same RPM. Up to a point, the aircraft speed will increase with each reduction in diameter. For a given prop/engine/airplane combination there is one diameter that will give maximum aircraft speed. This can be found experimentally as done by Sulivan Propeller but the cost is to find the optimum you have to go beyond the peak and that means to get the peak aircraft speed you have to buy replacement blades and cut them down to the diameter that produced the maximum aircraft speed. Bob Axsom

Yes that is true, dia is critical. In general for RV's with Hartzell's 72" are more efficient (faster) than a 74" for high speed cruise. Going less than 72" is academic because most Hartzells have a min 72" dia limitation. Hartzell does make some 68" dia props for the Lancair's. So Hartzell is very aware of twist and dia in optimizing for the airframe. However when we talk about optimization we are talking about fractions of a percent.

Look at the old standard, the C2YK/F7666-4 used for RV's for years. This was a common prop hub/blade used on early Mooneys and Piper Arrows, which have similar cruise speeds as a RV, so it was a good match. Now enter the Blended airfoil PROP, C2YR-1BF/F7496-2 or -4. This is a gain of about 3.5 mph over the older prop. That is fantastic. Here is Vans test data (click to enlarge):



So from 205.4 to 208.9 mph, about 1.8%, is phenomenal. The BA prop should be called the RV-prop, because that is exactly what it was optimized for, RV's with nominal 180 hp engines and cruise speeds in the low 200's mph at typical cross country altitudes (8,500 feet).

The point is anyone claiming 10 mph increase for their prop are either comparing it to a prop that's a poor match for the airframe, either because it is just a bad match or the airframe/engine is highly modified or is exaggerating.

Hartzells are about as good as it gets in efficiency. You have to be very radical in HP or drag reduction (clipped wings, chopped canopy and so on).

When you have a company like Hartzell, with their background, set out to make a prop spacifically for a particular airframe its going to be a good match. Now Hartzell no doubt made some compromises to give good overall performance, with emphasis on normal cruise at normal altitudes with a normal engine. Altitude BTW makes a difference in optimal prop design. So even with a given airframe there are many factors to consider. No prop is optimal at all conditions, however C/S props are have a way broader sweet spot than a FP which is truly a one trick pony, all other operations are significant compromise. Still a C/s prop can't change its twist, airfoil shape or diameter in flight. So if you choose to make some radical change in your C/S prop for say WOT, max speed at sea level, you will take a hit in all other flight regimes. Its all about trade-offs.

Bob unless you are going to Reno and had a radical plane, prop changes will not make much difference. Now as I said, keep the prop leading edge and back smooth as glass and waxed. This will gain a MPH or two.

Bob you plane is fairly stock, but if you start to significantly reduce airframe drag, radically decrease drag or increase HP, than the prop you have would start to became even less ideal. Obviously the new BA prop is a good start. I am still running the old C2YK/F7666, but the BA prop is what I'd like to upgrade to.

I am not sure why you don't make your cowl into a Holy cowl, or a cowl with small, wide spaced, round inlets, with a smooth sealed connection to a plenum, aka Sam James cowl? That is worth 8 mph, plus/minus a few. The cowl change would give you most bang for your buck. You can MAKE your own cowl from your existing one and your own plenum with your existing baffles. Cost will be small, time and effort who knows. A 100 hours? Of course fiberglass is involved to glass in new round inlets. (I can help size them for you.) The plenum can be made with sheet metal top and some fiberglass transitions (diffusers) to lead into the plenum box.

Just suggesting the cowl as the best mod you could make, and I would not fret on your prop too much, it will only gain you so much speed. A BA prop will be good for 3.5 mph over what you have; it cost less than $6,000 minus what you can get for your own prop. The cowl may be a few $100.

As far as Randy's comments, he did great work. Van's tables uses some of Randy's data, but as you can see the 200RV prop is slower than the BA prop and not much better than the good old HC-C2YK/F7666. However again to make my main point, difference in prop performance is very small between the top contenders. The MT's of course are slower simply because their wood core material requires a thicker airfoil. Many MT props marketed for RV's are sold very slow planes. One size does not fit all. A prop must be optimize for a spacific airframe and engine to get the most out of it. The 200RV and Hartzell BA are about as good as it gets, with the old HC-C2YK/F7666, like you and I have, is still pretty good after a few decades.

Just keep your prop clean, smooth, waxed and make sure the leading edge profile is perfect. What perfect means is a bit of a secret, however as the leading edges wear as they get dressed out from nicks, it looses its airfoil shape. When I say secret, some have different profiles they file into the prop. Warning! If you don't know what you are doing I'd not whittle on your prop, get someone to do it. You only have so much meat to remove, take great care to remove minimal material, no sharp edges and blend ratios of 10 to 1. Prop design is ART and Science.

[cobra]

Im trying to find data that illustrates prop efficiency differences at different prop speeds. Is there any such animal floating around?

The confusion: The texts Ive seen suggest a prop tip speed around 0.9 mach is supposed to provide the most efficient performance. The ad hoc data reported here seem to suggest that slower tip speeds are more common. Given average sized RV propellers (68-70" diameter) and normal engine speeds (I'm assuming <2700 rpm), the tip speeds calculate out to much lower mach numbers than should be ideal.

Ive read that it is a good idea to include a speed vector with tip speed equasion to account for for the forward velocity of the aircraft. That might account for some of the discrepency. Any comments?

[woodmanrog]

I love reading these articles. I have owned 2 RV6's and flown 5 or 6 more with all varieties of props. Here's my 2 cents worth. Both of my personal RV's came with fixed pitch wooden props. One Sterba and the other was a Sensinich cruise prop. Both were said to be max cruise props. I replaced both of these with a fixed pitch composite prop by Craig Catto. In both cases, I gained 8-10 knots in cruise level flight (gps over the ground) and 200 fpm in climb. Several friends of mine who fly constant speeds were really amazed at the performance. Of course I couldn't match thier climb speeds but I was only off by about 400 fpm. The big differences came in the amount of vibration and noise that the metal constant speeds produced and the added cost of maintenance and weight. Being a KISS kind of guy I will always opt for the fixed pitch. My first RV 6 with a worn out 0320 150HP cruised at 190 mph at 2500 rpm. (Read my article on speed mods on this site.) I was clocked at 164 knots average speed over a 100 mile triangular course at the Sun 100 air race. By the way, the props cost $1200.00 complete with leading edge tape for rain.

[Bob Axsom]

Thanks George as usual a thoughtful post.

I have sent Sulivan Propeller an e-mail to inquire about cost but I couched my request in such reserved terms that they no doubt consider me a waste of time in a busy work-a-day world. I have the money set aside to upgrade my instrument panel with a GNS430 with the forthcoming WAAS upgrade but speed and travel are all that matter to me (until I get on a tough approach with minimal equipment). I would (secretly have already) seriously consider dumping the gadget for a pure maximum performance speed increase. I do not care a whit about fun flying, etc. I would secretly like to ease the throttle and prop full forward and out run every RV in the sky - period. When I fly, I want to be going somewhere, fast!

My RV-6A is not stock for speed - it is worse because, just the opposite of a clipped wing, I have a 1.5 foot increase in wingspan which is a compromise for "bolt-on" range provided by 17 gallons of extra fuel.

Given that speed disadvantage (I like range as much as speed) I am progressively modifying the airflow inside the cowl with the intent to reduce the air mass flow to the level necessary to cool the engine without excess. In other words reduce cooling drag.

When I fly, the plane is waxed (prop included and it is in pristine condition) and it is as slick as it can be. However, when I go to 2,700 RPM, WOT, leaned for max speed it just feels like it is not flying as fast as it should. I do not want to go into propeller theory but actual propeller performance in my specific application. I am not shy about it, I want maximum performance in terms of speed and I know instinctively that I am not getting it. Hartzell and any other manufacturer can only provide their generic product. The Blended Airfoil design is just an improved generic product. There are compromises in the propeller system design to provide good performance for all of the applications that the general pilot population wants to put them to. I want to go as fast as my RV-6A will go in a straight line and I seriously believe now that an expert could tune my propeller system to achieve my desire by reducing the diameter to the exact dimension necessary to cause an energy absorbing pitch change to the maximum angle before inefficiencies cause a thrust reduction. I am not opposed to shaving margins and sacrificing other performance parameters to gain speed in a calculated and serious manner. Focus is very important here - either I want all out speed with range or I want a general application airplane that compromises excellence in all areas. I want the former. I think Sullivan Propeller could help but I do not have enough information to be sure so I think and plot and look at cooling drag - for now.

As for the James Cowl the finished products look nice but I do not believe I have all I can get out of what I have now. To make a "me too" jump to the James Cowl would have a serious impact on my experimental efforts to improve performance. I cannot have a breakthrough if I choose to copy the current popular lead and I need the stimulation provided by creative desire.

Ah well, this is a lot of words to say I haven't given up yet. Thanks for trying to help me avoid mistakes.

Bob Axsom