B

Bob Axsom

Well Known Member
Something to chew on and if some good inputs come in maybe I'll get a little smarter.

Originally built the RV-6A with O-360-A1A and 72" dia Hartzell prop with 7666 blades. First flew it in 2004 and raced with it until November 1, 2009.

Installed a 72" blended airfoil Hartzell with F7496 blades and picked up 3 kts as reported in Van's testing article in the RVator (sigh) several years ago.

Recently cruising at around 6,000 ft d alt. at 2630 rpm I was seeing 193 kts GS on the GPS so I ran it up to my top 2720 rpm and the speed faded around 3 kts. I played with the mixture to see if I could get it back but I could not. When I pulled back the prop to 2630 again the speed came back to 193 kts. A plane can't fly faster than the prop pitch angle in level flight so to get the rpm the pitch is assumed to have dropped off - thus the slower a/c GS. The efficiency of the prop tips my be less at the higher rpm as well - not calculated yet.

If the diameter were incementally reduced by small amounts it seems to me it could be tuned to achieve a higher aircraft speed at the higher rpm - of course if I had more power the prop could be driven to the higher pitch as is.

Thinking only at this point.

Bob Axsom
 
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I think that every propeller has a sweet spot. 2700 RPM is not the fastest airspeed with the BA prop on the -8 either. There comes a point where the extra HP that comes with RPM is more than offset by blade drag, so you see a net reduction in thrust.
 
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Not necessarily. I had a lot of discussions with Paul about this very same subject when he designed a blade mod for my Hartzell (oops let the cat out of the bag.) Reducing the blade diameter will reduce the critical mach losses but there are more variables to deal with....airfoil CL, tip shape, sweep, etc. Also the RPM/speed coefficient will vary with DALT so try your experiment at 8000 and see what happens.
 
That brings up a question I have had about the difference between aero and auto engines. Most auto engines hit a peak HP at a specific RPM and then often lose power. I have been told that within the acceptable RPM ranges on a LYC 360 more RPM is always more HP. I wonder if this is actually true. I know that those racing often twist the props to 2800 RPM or more. Do most show a gain at the higher RPM? I do gain a couple of knots bringing the prop up to 2700 RPM from my norm of 2500 if looking for top speed at low altitude.

George
 
sailvi767 said:
I have been told that within the acceptable RPM ranges on a LYC 360 more RPM is always more HP. I wonder if this is actually true...
Click to expand...

True... The published "peak" HP for an aircraft engine is based on hitting the limit of propeller RPM, not the peak (max) output the engine is capable of. Spin a 360 Lycoming to 4000 RPM and it will make a lot more power than 200HP.
 
Toobuilder said:
True... The published "peak" HP for an aircraft engine is based on hitting the limit of propeller RPM, not the peak (max) output the engine is capable of. Spin a 360 Lycoming to 4000 RPM and it will make a lot more power than 200HP.
Click to expand...

Which is why the old hangar tale 'a/c engines are designed to run full power, & car engines are not' is, in the immortal words of Harry Morgan, 'Horse Hockey'.

A/c engines are massively de-rated below what they are actually capable of producing, to ensure longevity & reliability.

One reason so many auto 'conversions' fail is trying to exceed the auto engine's factory full power rating & then put it in an a/c.

Charlie
 
Not theoretical anymore

As has been previously noted, there is much more than diameter, tip speed, etc, etc.

One very interesting development that is in the work is the geometric twist of a blade. Now many know about this, but there is some research ongoing to have the twist DYNAMICALLY change inflight. Sounds impossible, but think back to Wright brothers "wing warping" same thing. This is still so nascent that it is going to appear in helicopter main rotor blades first (maybe 10-20 years??). Currently the work is mainly focused on active vibration reduction, but technology leaps forward in surprising ways. Just thought many might find the possibilities interesting! Maybe in 40 years I will be hanging a constant speed prop that does not require oil pressure!
 
There have actually been "flexy" props around. Or at least that's the claim.
OT: Anyone heard of the Felix prop?
 
Prop theory and wonderment aside ...

Van's Aircraft did the basic off-the-shelf performance evaluation and published the results in the RVator several years ago - the best practical technical testing and reporting I have seen on the subject. I bought and read Jack Norris' book on propeller design (well maybe I burned out after rereading the same thing being repeated many many many many times) so the intent of this thread is not to go into prop theory in general.

I am going to try to focus this thread back on diameter reduction of this prop, in this application and the expected effect on speed as well as related subjects such as the proper way to approach reducing the prop diameter. Any experience reporting would be nice to have. I have dealt with prop shops in the Los Angeles area and it is a fastenating specialty.

I received an excellent input off-line that I intend to follow up on.

Bob Axsom
 
Bob- I'm courious of what your manifold pressure was on each rpm setting-is it possible youre using more hp at lower rpms? Youre tip speed is around .78mach(guessing the temp is around 40deg F) so you still have room to turn up the rpm as far as tip speed is concerned.


Recently cruising at around 6,000 ft d alt. at 2630 rpm I was seeing 193 kts GS on the GPS so I ran it up to my top 2720 rpm and the speed faded around 3 kts. I played with the mixture to see if I could get it back but I could not. When I pulled back the prop to 2630 again the speed came back to 193 kts. A plane can't fly faster than the prop pitch angle in level flight so to get the rpm the pitch is assumed to have dropped off - thus the slower a/c GS. The efficiency of the prop tips my be less at the higher rpm as well - not calculated yet.

If the diameter were incementally reduced by small amounts it seems to me it could be tuned to achieve a higher aircraft speed at the higher rpm - of course if I had more power the prop could be driven to the higher pitch as is.

Thinking only at this point.

Bob Axsom[/QUOTE]
 
MAP is not sensitive at all

I can detect no difference in manifold pressure between these two RPM settings. I fly wide open throttle all the time except when landing and the manifold pressure is against the wall reduced by altitude until I retard the throttle. Much of the speed testing I have done at 6,000 ft density altitude has a manifold pressure reading of 24.5.

Bob Axsom
 
Bob, why do you speed test at 6000 anyway? Don't you race at lower altitudes?
 
So, you have increased pitch at 2630rpm and are using the hp more effectively. To go faster you need a fixed pitch prop!
 
LAMPSguy said:
One very interesting development that is in the work is the geometric twist of a blade. Now many know about this, but there is some research ongoing to have the twist DYNAMICALLY change inflight.
Click to expand...

IVO props have been doing this for years. http://www.ivoprop.com/inflightmagnumodel.htm

Also, I seem to remember that Bernie Warnike did something like this with his "Almost Constant Speed" prop, but the change was due to blade flex as it loaded up.

As to the use of a smaller prop to allow the engine to wind up, and make more HP------take a look at the Formula One aircraft, O-200s running at 4,000 RPM, and making a lot of HP. With props a lot smaller than the same engine used on a Cessna 150.
 
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Unless you do something to reduce tip losses, reducing the diameter will have a negative effect on speed. You will be reducing the area of the prop and also you will be bringing the tip losses further inboard.

Before attempting an expensively irreversible mod I suggest you do your homework first with a prop design program. I was fortunate enough to have Paul do this for me. I also have some thrust numbers comparing BA Hartzell airfoils that are very close to the profile Paul designed for me. I don't think you will do much better than what you have. They also have to make some compromises with torsional vibration frequencies and maintainability which can and do have negative effects on performance. Several compromises have to be made.

One area that Paul was very adamant about was a cuff like the Mustangs have to decrease the drag at the root. His design predictions proved that fuselage effects are of no consequence...in fact he had some pretty colorful language about some of Jack's writings on the subject. :)

I have a prototype fiberglass cuff that he laid up for me. The same cuff design was tried on a SX-300 and the gent that tried it picked up a bit over 5kts. I don't know who that is but Paul told me about it.
 
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It was a starting procedure

DanH said:
Bob, why do you speed test at 6000 anyway? Don't you race at lower altitudes?
Click to expand...

I used to fly in the U.S. Air Race cross country air races that assigned a handicap procedure to every airplane based on a triangular pattern flown at 6000 ft. density altitude. It was a fairly detailed procedure that I believe was developed by Bob Cole. It seemed like a good procedure so I adopted it for my testing several years later when I started testing the RV-6A. Then Kevin Horton advised me of the NTPS spreadsheet that would eliminate the underestimate resulting from taking a simple average of the speed of the three legs. I reran the numbers and verified the underestimate potential of the USAR procedure and I added on the NTPS spreadsheet to the end of my process to get the more accurate calculation. As I keep modifying the plane for speed it is important to always apply the same test method, including the test density altitude, so the results are directly comparable. Otherwise I could have used any reasonable altitude.

The race altitude is actually carefully planned according to the course and the winds forecast for race time the night before the race. I have two Excel charts one for wind "visualization" at 1000 ft increments and one for detail planning of each leg. Usually, I fly at least two different altitudes on the various legs corresponding with headwinds and tailwinds. My absolute favorite race is the AirVenture Cup Race from Mitchell, SD to Oshkosh on even years. It is the only straight shot find your way around the clouds etc. without a lot of gates, and special restrictions, etc. race. It is pure speed not some darn zig-zag stunt contest. Pick the right altitude and you have a shot to beat the faster planes - I love it! The two times I have flown it I was at 9,500 and at 11,500 respectively. So the test method is just a reasonable standard that I use that always works with the local ground elevation.

Bob Axsom
 
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Nothing planned yet - Just thinking

rocketbob said:
Unless you do something to reduce tip losses, reducing the diameter will have a negative effect on speed. You will be reducing the area of the prop and also you will be bringing the tip losses further inboard.

Before attempting an expensively irreversible mod I suggest you do your homework first with a prop design program. I was fortunate enough to have Paul do this for me. I also have some thrust numbers comparing BA Hartzell airfoils that are very close to the profile Paul designed for me. I don't think you will do much better than what you have. They also have to make some compromises with torsional vibration frequencies and maintainability which can and do have negative effects on performance. Several compromises have to be made.

One area that Paul was very adamant about was a cuff like the Mustangs have to decrease the drag at the root. His design predictions proved that fuselage effects are of no consequence...in fact he had some pretty colorful language about some of Jack's writings on the subject. :)

I have a prototype fiberglass cuff that he laid up for me. The same cuff design was tried on a SX-300 and the gent that tried it picked up a bit over 5kts. I don't know who that is but Paul told me about it.
Click to expand...

I received messages from Paul about the cuff and the spinner openings as well. These and the diameter reduction are not mutually exclusive. I'm not shutting the door on anything at this point but from experience in my very distant past, I believe fairly clearly that the the length of the prop on my engine and airplane is not optimum - it is too long.

Bob Axsom
 
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"Fuselage Effects"

rocketbob said:
... His design predictions proved that fuselage effects are of no consequence...in fact he had some pretty colorful language about some of Jack's writings on the subject. :)

...
Click to expand...
emphasis added

A little off the subject, but related because a longer prop sticks out farther from the fuselage and thus has more disk area not blowing on the fuselage:

I assume this refers to what Jack Norris called "slowdown" and also, interference (not the same thing at all). I also discussed this with Paul. I also discussed it at great length on many occasions with Jack. To round it out, I also talked to Gary Robinson (think Sherpa). I also looked at quite a few videos on YouTube of smoke streams in wind tunnels. Granted, most of them were cars or motorcycles, but they were streamlined and often more so than our cowls. I also tried to recreate or measure slowdown in a makeshift rig in my hangar with an airspeed indicator and a 5 HP shop vac. I even tried to measure it in front of my SUV.

My bottom line is simply that if it exists, I have yet to see proof of it. Jack claims there is some. Others just shrug. Paul simply said it did not.

Is there a pressure rise where the air hits the front of the plane? Yes, of course. But whether the air slows down or simply veers in direction is a very relevant question. I've gotten to the following question from all this brain-strain: should we, as Jack does, count it as slowdown when the air strikes the prop blade at an angle and thus has a smaller axial vector or should we re-analyze the flow as if the airfoil were of greater chord and thinner thickness? I'm inclined to vote for the second option, but I have received no support that I know of for this idea.

For anyone that is interested, PM or email me if you want to see some of my photographic "data" on this, including my experiments and the YouTube stuff. If anyone can contribute to this part of the discussion, I'd be very grateful!
 
hevansrv7a said:
emphasis added

A little off the subject, but related because a longer prop sticks out farther from the fuselage and thus has more disk area not blowing on the fuselage:

I assume this refers to what Jack Norris called "slowdown" and also, interference (not the same thing at all). I also discussed this with Paul. I also discussed it at great length on many occasions with Jack. To round it out, I also talked to Gary Robinson (think Sherpa). I also looked at quite a few videos on YouTube of smoke streams in wind tunnels. Granted, most of them were cars or motorcycles, but they were streamlined and often more so than our cowls. I also tried to recreate or measure slowdown in a makeshift rig in my hangar with an airspeed indicator and a 5 HP shop vac. I even tried to measure it in front of my SUV.

My bottom line is simply that if it exists, I have yet to see proof of it. Jack claims there is some. Others just shrug. Paul simply said it did not.

Is there a pressure rise where the air hits the front of the plane? Yes, of course. But whether the air slows down or simply veers in direction is a very relevant question. I've gotten to the following question from all this brain-strain: should we, as Jack does, count it as slowdown when the air strikes the prop blade at an angle and thus has a smaller axial vector or should we re-analyze the flow as if the airfoil were of greater chord and thinner thickness? I'm inclined to vote for the second option, but I have received no support that I know of for this idea.

For anyone that is interested, PM or email me if you want to see some of my photographic "data" on this, including my experiments and the YouTube stuff. If anyone can contribute to this part of the discussion, I'd be very grateful!
Click to expand...

I did some simple air flow tests at the intake when attempting to improve Subaru cooling. The inlet lip was tufted and the tufts indicated air would enter the cowl at one point in the inlet opening but exit at another, near the prop hub. It indicated to me there was considerable restriction and general air flow chaos inside the cowl, no wonder cooling was so marginal.

I also wondered about the effect of this reverse air flow forward of the cowl. What was it doing to propeller efficiency? The prop was an MT-7 and it too lost efficiency going from 2600 to 2700 at 8500'.

(obviously this non-scientific experiment was done on the ground, it was the only way I could figure to get a camera in front of the airplane)

Different subject, someone wondered if anyone had used a Felix prop. I have. I had one built for the Subaru in a test of a FP prop and that engine. It was a total failure. Not the prop, but the notion a FP would work with an engine that requires high rpm to produce power. The compromise of power loss on take off so as not to overspeed in flight was too great. I sold the prop to a friend with a 180 HP Lycoming and it was perfect.

Fred Felix has an interesting theory and patent on it. He noticed years ago on a canoe trip that paddles that had a bi-camber feature worked better than those without. So he did some testing of the theory with aircraft blade design and sure enough the bi-camber shape did improve performance and was quieter. As far as I know, he still is building and selling props from his home in Wisconsin.
 
The reason why the inlet area is so turbulent is because of the club shape of the blade roots which when designed this way, does absolutely nothing but degrade performance and cooling efficiency. A cuff can fix that. North American proved that back in WW2. A prop with a proper airfoil down to the spinner can also fix that. Then the inlets can be sized smaller, which will allow for further gains in efficiency.

Back to Bob's original question. I'd like to know why you feel the length of the prop is the issue. Just like the wing of a sailplane, the higher the aspect ratio is the less induced drag will be up until the point at which critical mach is exceeded. Critical mach is also effected by sweep, just as it is on a jet wing. It would be more effective to reshape the tips than it would be to just cut them down shorter. Paul once told me the key to diameter is to make the prop as long as possible where one can keep the tip just under critical mach.
 
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Clarifying Related Issues

Jack Norris's "slowdown" is not about messy airflow around the intakes, although that may be happening. It's about the idea that the air slows down out in front of the cowl. In the original program runs for the design of the Whirlwind RV prop for the -8, the slowdown at the root near the spinner was around 50% and the slowdown out at the tip was around 2 to 3%. Jack uses "source sink simulation" to compute this. Others do, too. There are many disagreements out there as to the best way to compute that source-sink. But, whether the computations are correct is secondary to the basic question to which I don't have the answer: does the air actually slow down or does it merely change direction as it goes around the cowl? In either case, how does the prop design math handle it? Keep in mind that these percentages were computed for the plane of the prop's 1/4 chord point, not back at the cowl (though assumed caused by the cowl).

Once you begin thinking about this, you are thinking about the relationship between prop diameter and cowl dimensions. I favor the view that the longest prop with manageable tip speeds is probably best. That necessarily implies that the area and pitch have to be adjusted within that constraint so that the engine is not overloaded. Although Paul Lipps correctly argued that aspect ratio is not a factor in the calculation of induced drag, it is a factor when you have already decided how long and now have to decide how wide and at what pitch (or CL). You don't design for AR directly, but the highest AR that works for the engine is the best one, all other factors being equalized. All of that is based in part on the idea that prop induced drag is a higher portion of the total drag than in a wing because you are trying to get to the most "lift" for the power.

As for testing airflow around the intakes while on the ground, count me in the thumbs down group because the actual airflow is so different both at the input end and the exhaust end. I fully appreciate how difficult it is to observe it "at speed", but that is what you need to do because that is what you need to know.
 
HOLD IT FOLKS - Very straight information

Hi Bob,

Yes, we have developed a "D" twist for the 7497 blades but not the 7496.
To my knowledge, nobody has flight tested the "D" twist on a RV, only on
some Glasair II's. The "D" twist didn't appear to help them go
noticeably faster but since I didn't do the testing I can't testify to
the uncertainty level in their results.

The "D" twist blade isn't offered through Van's because it is more
expensive (has to come from a bigger and heavier forging with more
machining) and for most RVs it probably wouldn't offer enough additional
performance to justify the cost?

Regarding diameter, I haven't studied anything shorter than 72" on the
RVs. If you can provide a precise flight condition to evaluation, I
will look at it. I need true airspeed, altitude, horsepower and RPM to
analyze and compare shorter diameters. FYI, the 7496 and 7497 blades
were optimized for peak efficiency near 2500 RPM. If you are flying
faster than a typical RV and turning a higher RPM, then it is possible
the additional horsepower of higher RPM is being negated by dropping
propeller efficiency? A shorter diameter may help but we have not
vibration tested/approved anything below 72".

Les
 
"may" help....

Les has the thrust numbers and he could run the calculations to see what the differences are, and can tell you what the speed increase would be from his data. I would be willing to bet a case of beer that the gain would be very slight at 2700 and you would lose thrust at other TASs.
 
After reading this thread I did a bit of testing with a RV6. This is Robbie Attaway[s old aircraft so fairly fast. It has a IO360 with 10 to 1 pistons and a RV200 prop.
I gained about 6 knots from 2500 to 2700 RPM. I indicated about 178 knots at 2500 feet with a OAT of 38 F. At 2700 RPM indicated was 184 knots. This is a 72 inch prop if I recall correctly. Aircraft was solo with half fuel. In the case of my aircraft more RPM was a noticeable increase in speed.

George
 
The increment is a little large

sailvi767 said:
After reading this thread I did a bit of testing with a RV6. This is Robbie Attaway[s old aircraft so fairly fast. It has a IO360 with 10 to 1 pistons and a RV200 prop.
I gained about 6 knots from 2500 to 2700 RPM. I indicated about 178 knots at 2500 feet with a OAT of 38 F. At 2700 RPM indicated was 184 knots. This is a 72 inch prop if I recall correctly. Aircraft was solo with half fuel. In the case of my aircraft more RPM was a noticeable increase in speed.

George
Click to expand...

Your airplane is definitely a fast one. This is good information for a macro view but I think there are some variables that require a lot of work to find the best speed for. It is a tough test exercise.

Bob Axsom
 
Feedback from Les Doud at Hartzell

Bob,

I'm sorry to hear about your wife. My Grandmother battled breast cancer, my
Aunt and one of my sisters are breast cancer survivors and my mother-in-law is
currently battling breast cancer. It is a tormenting disease for sure and does
not discriminate... Hang in there and I'll say a prayer for her.

I ran the 7496, 7497 and 7497D at 6000 ft, 184 KTAS, 140 Hp @ 2700 RPM.

7496 @ 74" 86% 213 lbs of thrust
7496 @ 72" 85.9% 213 lbs of thrust
7496 @ 70" 85.8% 212 lbs of thrust
7496 @ 68" 85.6% 212 lbs of thrust

7497 @ 74" 84% 208 lbs of thrust
7497 @ 72" 83.9% 208 lbs of thrust
7497 @ 70" 84.1% 208 lbs of thrust
7497 @ 68" 84.2% 209 lbs of thrust

7497D @ 74" 84.2% 209 lbs of thrust
7497D @ 72" 84.2% 209 lbs of thrust
7497D @ 70" 84.2% 209 lbs of thrust
7497D @ 68" 84.1% 208 lbs of thrust

As you can see, reducing diameter does not appear to help the analytical
predictions. Additionally there are probably installation effects that favor
the larger diameter blade. I think the trend between these blades would be
similar at lower RPM (and thus Hp) but probably more favorable to the 7496. The
7497 was originally designed for the 200 Hp engines so on the 180 Hp at altitude
it is probably a little too much propeller.

Les
 
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