Prop Modification

[Bob Axsom]

"The 7496 improves in all areas, and will run off and hide from the 7666 all other factors being equal."

This quote from Bill Wrightman's post really hits me where I live. Over the past couple of years I have been cleaning up drag areas to compete in the AirVenture Cup Race and the SARL races (Sport Air Race League - http://www.sportairrace.org) but the blended airfoil prop guys are "running off and hiding from me" with my 7666-4 blades. Now, being a racer at heart, I would like something more than a "me too" solution but that is the best I have seen so far - a very expensive solution. Catching John Huft in his single wide RV-8 with with well over 200 hp from his specially modified engine, extreme attention to detail on drag reduction AND the Hartzell blended airfoil prop, in my RV-6A is an elusive dream but every reduction in MPH difference in our speeds at the end of a race is very sweet. It has been stated by various sources that twisting my prop would (not could) improve the speed of my plane. This would be a difficult implementation and a significant risk but the idea will not not leave my mind.

Bob Axsom

[Bob Axsom]

(http://www.mstewart.net/downloads/si1435.pdf) This provides instructions for converting back and forth between fixed pich and constant speed propellers. I want to get that in the post before I forget it. I am still in the first quarter of Jack Norris' book "Propellers..." ($14.95 +2 or 3 dollars for shipping, http://www.propellersexplained.com) and there is still a veil of uncertainty but not as much as there was a few weeks ago. I cannot speak for anyone other than myself because all interests are individually prioritized. I am looking for speed and I am willing to compromize the other flying concerns (max climb rate, fuel economy, endurance, TBO-within reasonable limits, etc.). In my 22 years of owning a fixed pitch prop airplane I lusted for a C/S so when I built my RV-6A I bought the one that seemed to be the very best one available. As I have flown and raced the plane over the past four years I at first suspected and now I know the performance of the Hartzell 7666-4 is not providing the best speed my plane can achieve and all commercially available props fall into the same basket. I have found for reliable sources that the extreme performers (John Huft excepted) have had the twist on their props changed. The solution to my search for improvement has lead me to believe the following:

- The Hartzell 7666-4 is worse than the blended airfoil 7496 bladed 72" prop by approximately 3 mph as reported by Van.
- The efficiency of the new 7497 bladed Hartzell blended airfoil is not as efficient as the 7496 B/A prop.
- Having the 7666-4 blades retwisted and trimed for maximum speed on a specific airplane is dangerous but effective.
- A fixed pitch custom prop customized for racing on a particular airplane is the best solution.

More to study, more to learn and hopefully a good course of action for more speed in the future.

Bob Axsom

[RVjim]

[quote=Bob Axsom;188013(Stuff Cut)

- The Hartzell 7666-4 is worse than the blended airfoil 7496 bladed 72" prop by approximately 3 mph as reported by Van.
- The efficiency of the new 7497 bladed Hartzell blended airfoil is not as efficient as the 7496 B/A prop.
- Having the 7666-4 blades retwisted and trimed for maximum speed on a specific airplane is dangerous but effective.
- A fixed pitch custom prop customized for racing on a particular airplane is the best solution.

More to study, more to learn and hopefully a good course of action for more speed in the future.

Bob Axsom[/quote]

You could add another option:
A custom made constant speed propeller for racing on a particular airplane.

One of my customers owns the Pirahna. This is a one of a kind aircraft. It is powered by a Lyc. 360 180 hp engine and had a custom aluminum 2 blade propeller. And it was just a little faster than his son's HR2.
I had MT Propeller design him a 3 blade CS propeller that met his criteria. The Pirahna is still a little faster than his son's HR2.
This was not an effort to provide him with a racing propeller. We did not want to compromise his performance in any other area. Take off and climb performance was also important. Mainly because of the 120 mph stall speed of the aircraft.

Jim Ayers
Custom Aircraft Propeller - www.ca-propeller.com
A division of Less Drag products, Inc. - A MT Propeller distributor

[Bob Axsom]

What kind of improvement can you give over Hartzell with 7666-4 blades. So far I see the Hartzell blended airfoil props (7496 and 7497) and Paul Pipps' Ellipse prop giving about 3 mph increase over my 204.6 at 6,000ft density altitude at 2730 rpm. What is your best suggestion and cost? I need to find 22 mph to be competitive with John Huft.

Bob Axsom

[RVjim]

Originally Posted by Bob Axsom

What kind of improvement can you give over Hartzell with 7666-4 blades. So far I see the Hartzell blended airfoil props (7496 and 7497) and Paul Pipps' Ellipse prop giving about 3 mph increase over my 204.6 at 6,000ft density altitude at 2730 rpm. What is your best suggestion and cost? I need to find 22 mph to be competitive with John Huft.

Bob Axsom

Hi Bob,

From my conversations with Tracy Saylor about his RV-6, he got to 230 mph tias before he found he needed to modify his propeller.

A friend of mine with a RV-6A 180 hp Lyc. 360 and Hartzell 7666-4 prop increased his top speed by 2 mph at 7,500' density altitude by reducing the engine RPM from 2,700 to 2,600 RPM. The reduced RPM increased his airspeed from 201 to 203 mph tias. At your slightly lower altitude, your peak airspeed may occur between 2600 and 2650 RPM. Just my guess.

Unfortunately there is more to clean up on the RV-6A than a RV-6.

My thoughts: Reduce the drag until there's nothing left to do.
Then get a custom CS propeller to improve on the speed you can actually achieve.

The Pirahna is flying in the 250 mph range with the 180 hp Lyc. 360 engine and MT propeller. That is the design speed for his MT Propeller.

Regards,
Jim Ayers

[Kevin Horton]

Originally Posted by RVjim

Hi Bob,

From my conversations with Tracy Saylor about his RV-6, he got to 230 mph tias before he found he needed to modify his propeller.

What is "tias"? Is that true airspeed, or indicated airspeed, or indicated airspeed corrected for instrument error, or indicated airspeed corrected for instrument error and position error (which is actually calibrated airspeed) or "true" airspeed calculated from an IAS reading? Or something else altogether?

[tin man]

Originally Posted by Bob Axsom

What kind of improvement can you give over Hartzell with 7666-4 blades. So far I see the Hartzell blended airfoil props (7496 and 7497) and Paul Pipps' Ellipse prop giving about 3 mph increase over my 204.6 at 6,000ft density altitude at 2730 rpm. What is your best suggestion and cost? I need to find 22 mph to be competitive with John Huft.

Bob Axsom

Bob
I've been following your post becuase you do a lot of racing but you should really stop trying to benifit from others experimentation and refinement and do some stuff on your own. If others can re-invent the wheel so can you.
Anyone can assemble a fast airplane using all the aftermarket race mods, takes a real experimenter to improve the breed on his own. Try it, you'll like it. Maybe some day someone will want to buy a Bob Axsom part.
Tom
RV3

[Ironflight]

Tom,

I have enjoyed a number of Bob's threads on doing his own experiments. You might try:

http://www.vansairforce.com/communit...ad.php?t=23260

or:

http://www.vansairforce.com/communit...ad.php?t=22611

or:

http://www.vansairforce.com/communit...ad.php?t=18858

or:

http://www.vansairforce.com/communit...ead.php?t=6888

I have a lot of respect for Bob as an engineer/experimenter. I'd let him borrow a few ideas of my own, if I had any!

[RVjim]

Originally Posted by Kevin Horton

What is "tias"? Is that true airspeed, or indicated airspeed, or indicated airspeed corrected for instrument error, or indicated airspeed corrected for instrument error and position error (which is actually calibrated airspeed) or "true" airspeed calculated from an IAS reading? Or something else altogether?

"true" airspeed calculated from an IAS reading.

No calibration or correction.

Very useful in obtaining comparable data on one aircraft.
Totally worthless in comparing data between two different aircraft.

(Comparable data - Data obtained that is repeatable and can be compared.
Repeatable data - Data that can be obtained on two or more different flights and provides the same data.)

Regards,
Jim Ayers