Constant Speed Prop vs. Controllable Pitch

[MCA]

Originally Posted by gmcjetpilot

Of course 72" is better than 74" for high speed cruise.

Why is a smaller prop faster than a larger prop??

[gmcjetpilot]

Originally Posted by Bryan Wood

Bob,
They claim 15-20 knots on a Lancair and seemed to think that on an RV it should be good for at least 10 knots. I was also told to assume the climb would go down 300 or so fpm. They offered to do this to my new Hartzell, or to get a set of Baron blades that were no longer serviceable for a certified plane and work them up for $700 each. Regards,

I have to express doubt that the prop will give an increase of 10 kts. That is a HUGE increase in thrust. The Hartzell BA prop is optimized for the RV airframe, right. I also know prop efficiency improvements are measured in fractions of percent. Its hard to get, in my opinion from what I know (not much ).

10 kts is hard to believe. Now if a prop is way off, sure you can gain, but the F7666 blade and the more optimal (for the RV airframe) BA prop probably can not be improved that much, for a given HP. Now if you lowered airframe drag or increased engine HP radically than yes it would no longer be ideal. However I think Bob's plane is pretty stock. Meaning the wings not clipped, canopy lowered, cowl inlets reduced.

[Bob Axsom]

50 years ago when my attention was focused on models the props for control line speed classes were optimized for that competition were small diameter high pitch products. Free Flight on the other hand depended on maximum climb performance (20 sec. engine run, 5 minute max, three times then an unlimited flight if you got three maxs) and the event optimized props for the same engines were much larger in diameter and much smaller in pitch. The engine can get loaded with either daimeter or pitch and there is "best" combination to optimize performance for cruise speed or climb. The 72" compared to the 74" would be better for maximum speed if it allows the engine to produce the maximum power before diameter loading starts bogging down the engine. Another factor is the tip speed of the propeller. At supersonic speed the performance decays and at the same RPM the 74" tip speed is higher.

Bob Axsom

[Kevin Horton]

Originally Posted by MCA

Why is a smaller prop faster than a larger prop??

Whether the aircraft with the smaller prop is faster or slower than the one with the larger prop will depend on the details of the particular engine, prop and airframe. In the case of the older Hartzells (this may, or may not, be applicable to the new BA Hartzells), the higher Mach number at the blade tip reduces the prop efficiency of the larger diameter prop at max speed and max rpm. The larger prop is more efficient than the smaller one at lower speeds (take-off, climb) and at lower rpm in cruise (e.g. 2300 rpm).

If you have a very small diameter prop, there are a couple of issues:

1. You need high pitch to absorb the engine power, and this means the speed of the prop blast has to be very high. This high speed prop blast creates a large amount of drag when it blows against the cowling and forward fuselage.

2. Prop efficiency (and thus the thrust produced) is greatest when you have a very large amount of air moving at a slow speed through the prop (think helicopter rotor). A small diameter prop moves a small amount of air at high speed, and thus has low efficiency.

If you incrementally increase the diameter of our very small diameter prop, the thrust produced will increase, until eventually you get the diameter large enough that the Mach number at the blade tip gets too high. The air has to accelerate as it goes over the prop airfoil, and parts of the airflow will become supersonic even when the blade tip Mach is less than 1. This reduces the prop efficiency, meaning the thrust is lower, and the speed is lower.

[Bryan Wood]

Originally Posted by gmcjetpilot

I have to express doubt that the prop will give an increase of 10 kts.

Me too! The Lancair guy swore by it though. Maybe somebody should call Sullivan Propeller at Hayward and get the straight scoop as they tell it and explain it to us. I want to say the persons name was Brian, but it has been a few years since calling him. I do recall him saying that the change in twist from the C to the D produces around 100 or so additional pounds of thrust on a stock Hartzell.

[Bob Axsom]

I got up early (for a retired guy) this morning to test the top speed of my airplane in its current cooling mod state. Instead of one run I made two, one at 2450 RPM and one at 2700 PPM. The surface winds were calm but there was some smooth air flow aloft. The OAT at 6,000 ft pressure altitude was 21C so the test was flown at 4,200 ft pressure altitude (approximately 200 ft lower than the corrected Altimeter display). The altimeter setting on ATIS at Drake field was 29.97 but the test was flown using 29.92. The OAT at 4,200 ft was 26C. I think the results substantiate what George has stated here.

2450 RPM (heading - speed in knots)
360 deg. - 174, 175, 174, 175, 174
120 deg. - 170, 170, 169, 169, 169
240 deg. - 162, 162, 162, 162, 163

Average Speed = 168.7

2700 RPM
360 deg. - 177, 177, 177, 177, 178
120 deg. - 172, 172, 172, 172, 173
240 deg. - 165, 166, 165, 165, 166

Average Speed = 171.6

MAP = 25.5
Oil Temp = 200
Oil Pressure = 80
Mixture = 100 ROP
CHT (1,2,3,4) = 324, 379, 377, 343

Bob Axsom

[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.

I sent an e-mail to Sullivan Propeller at sullyprop@aol.com to follow up on Brian Woods input to this thread. I'll report back what I learn.

Bob Axsom

[Bob Axsom]

I was going to cut and paste a paragraph from the "prop" section of the website listed above (Randy Lervold's - he sold the RV-8 and is building an RV-3B) but it is copy righted on the home page so I'll have to pass on that idea but early on he states that the Hartzell prop sold by Van's is optimized for speeds lower than the RVs fly. Like I said a very interesting 11 page write-up just on the prop and his eventual finding of a design/propeller he thinks is better.

I haven't heard from Sullivan Propeller and probably won't for a while. Winter is coming and I have to shift my focus to designing and building a manual wood splitter. I refuse to pay well over a $1,000 when I have a perfectly good bottle jack left over from building my RV-6A and a splitting wedge. It's amazing how creative you can get after building an airplane. The problem is it will probably cost me $2,000 and 1 manyear of work.

Bob Axsom

[Bob Axsom]

Bob:

You can contact Brian Sullivan via telephone to discuss your specific
needs at the numbers below. If you get our voicemail have faith and
leave a message he will return the call. Fridays and Mondays are
particularly busy with our regularly scheduled work. I have passed your
email on to him to get him up to speed and ready to chat with you.
Have a great day we look forward to talking to you! And thanks for
contacting us!

Sullivan Propeller Specialists
20995 Skywest Drive
Hayward, CA 94541
tel: 510.782.0920, 800.782.0920

[Bob Axsom]

I did not talk to Brian Sullivan himself but the person I talked to was well informed. They do not have an off the shelf propeller mod for any airplane. They do as Brian stated in an earlier post, they carefully layout and twist the propeller and cut down the diameter in an evolutionalry manner to achieve maximum speed. It is expensive and you do have to take your airplane to Hayward Airport in California's Bay Area to have them modify you prop. What they do is something that any competent prop specialist can do. It is expensive in that they cut of the tips of your prop, test fly it, record the speed, cut off more of the tips, test fly it, record the speed, cut of more of the tips, record the speed, etc. until the maximum speed drops. Then they take a new set of blades and reconfigure them to the configuration that gave the maximum speed and the old blades are what ever the scrap value is I guess. They do not gaurantee any specific speed gain although some of their customers claim as high as 20 knot gains. The mod is tailored to the specific airplane with a lot of careful work and if the weather is good it can be completed in a week. This is getting really special and seriously into the world of propellers. Interesting.

Bob Axsom