I am trying to do research on gear ratio/prop size combinations I could use with the intended Hirth 65hp engine, to achieve maximal thrust for each available hp the engine and gearbox would provide without having an excessive prop tip speed.

The new RR will have significantly higher prop clearance, so I was considering putting a bigger 2 blade up front, but nothing bigger than 72". I have found some tip speed calculators online but have received varying opinions on, whether tip speed should be just below .92 Mach or just below .88 Mach, or even well below .88 Mach. My existing Ridge Runner, with the 503 at 6500 rpm and a 2.58:1 gearbox with a 68" Ivo, tip speed is .65 Mach, well below anything that has been suggested to me to be optimal use of the Rotax's hp. However, it performs remarkably well and I'm very happy with it, so I really don't want to change anything there.

For the new RR, I have calculated that a 72" prop with the Hirth at 6300 rpm and using a 2.29:1 gear ratio would provide a tip speed of .75 Mach.

Slightly higher, would be a 72" prop with the Hirth at 6300 rpm and using a 2.16:1 gear ratio would provide a tip speed of .79 Mach.

I'm looking to match a prop diameter to the engine and gear ratio that will provide the maximal, best take-off thrust..without getting into excessive near-Mach inefficiency and noise issues. Then, after I'm at cruise, I would use the GTA in-flight adjustable mechanism to pitch it for cruise and reduce the rpms.

Any of you guys have any experience or knowledge on tip speed issues and matching props with gearbox ratios for maximal thrust at take-off? Is there any definitive source for information on this?

Thanks. RR

I have a basic thrust calculator you could use but I would have to email it to you.

On many Supercubs guys are running 90" props @ 2500rpm static and even though this puts the tips well over the speed of sound it still provides the best static thrust, and a whopping 90mph cruse speed....


Prop DIA 72
Prop Pitch 30
RPM 2750
Baro 29.92
CF Value 0.77

Speed 78.1
HP 98.63
Load 806215680.0
Static Thrust 442.9
Propeller Tip Speed 589.0
Air Speed 706.9
% Mach 93%


Prop DIA 70
Prop Pitch 34
RPM 2750
Baro 29.92
CF Value 0.77

Speed 88.5
HP 99.87
Load 816340000.0
Static Thrust 395.7
Propeller Tip Speed 572.7
Air Speed 687.2
% Mach 90%


Prop DIA 68
Prop Pitch 38
RPM 2750
Baro 29.92
CF Value 0.77

Speed 99.0
HP 99.40
Load 812492288.0
Static Thrust 352.4
Propeller Tip Speed 556.3
Air Speed 667.6
% Mach 88%


Prop DIA 76
Prop Pitch 28.7
RPM 2600
Baro 29.92
CF Value 0.77

Speed 70.7
HP 98.99
Load 957494451.2
Static Thrust 491.4
Propeller Tip Speed 587.9
Air Speed 705.4
% Mach 93%

Here is another program to calculate tip speeds... It won't gather trust numbers as that is based on blade profile and a ton of other 'black' magic wonders.

http://www.pponk.com/HTML%20PAGES/propcalc.html

Just for reference I spin my V-8 4400 rpms on take off.. redrive ratio is 1.43-1 The prop is turning 3076. It is a Ivo 76" diameter Magnum series... Not too noisy at all..

Here is a link to a low pass at full bore.... Motor is ALOT louder then the prop.

http://www.youtube.com/watch?v=AOF6eT6FRmY

Ben.

Generally speaking, for higher drag airplanes like Cubs, your RR, etc the longer the prop, the better the performance. If it were me, I'd select as long a prop as is reasonable for your plane, then gear it to spin up into the low 0.9 mach range for the temprature and humidity you typicallly fly in.

I'm turning an 84-inch Hartzell at 2700 RPM on the Bearhawk for comparison. I't can get loud, but it climbs very well.

I'm on the same 84" prop and I want to try a 90".
That will take me from about 650lbs of static thrust to around 820, an very nice gain...

It looks like many of the SuperCub planes are running larger props for better thrust at take off, but then they ease off the rpms at cruise, thereby eliminating the compressibility problem at or near Mach tip speeds.

My plane statics at 2450-2500 depending on conditions with a 84/42 prop but in cruise I could easily get 2800-2900 if I tried.
Cruise is a tad less than 50% throttle position for 2500rpm, but she climbs.....

I've been messing around with that tip speed calculator spreadsheet program Mongo emailed me, and also the calculator over at Pponk's site and Culver too. Based on just about every tip speed calculator I've tried, my existing RR's tip speed appears to be quite low, yet I'm getting I think very good performance out of it at take-off (less than 100' in a hot, SE Asian tropical environment). So I was curious about Mongo's comment of how the SuperCub guys are running at or over the speed of sound to get the best static thrust. This is very interesting, but it seems to contradict other views of performance loss when tip speed is at or over the speed of sound.

Does anyone know of anyone reputed to be a pro at this topic that I could consult? A referral of someone who has some kind of professional background with this topic would be greatly appreciated.

Thanx
RR

whoops wrong thread. How do I delete this?

From another thread this is a good propeller mach calculator.

http://pponk.com/HTML%20PAGES/propcalc.html

According to that pPonk calculater, 90" is ok to 2550 rpm at 40F

According to that pPonk calculater, 90" is ok to 2550 rpm at 40F

Jeremy, I have to believe the mach value is only a part of the issue. Probably not going to achieve the mach value if the tips are digging in the dirt.

TD

Actually Tom, I was not thinking for the Maules, but responding to Mongo's original post about Cubs.
The 86" we currently have available is still quiet on the 2400rpm 235hp engines.
We never did the certification on the 2575 rpm B4B5 engine which would pass, or the 2700 rpm 260hp V4A5.
The 86" does not pass ground clearance requirements on 8.50 tyres but does on the next size up Goodyear blimps.
With the ABW long gear when ready, clearance will be no problem.