What can I expect? Gearbox Reduction Question

MrGrey

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Mar 13, 2005
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Munster, IN
I was curious as to what kinda tradeoffs I would get if I went from a 3.48:1 gearbox ratio to a 3.0:1 Since the prop will have to be slightly depitched can I expect any difference in climb or forward speed? I am trying to understand what happens when I speed up the prop and take some pitch out. I would like to see more climb performance. Technically speaking what is the better ratio or does it matter?
 
If you have a 60" prop it the 3 to 1 will be better.you may hace your blades pitched so much that they stall. I would think your climb will be better.
 
i have a 72" prop BTW and that was kinda my thought. Ron and I are gonna switch off gearboxes at Mentone and I believe that he had the 3 to 1 on there and switched it to the 3.48 to 1 when he sold me the engine. After talking to Lockwood they said that would still work but would possibly give me the symptoms I have now. The machine is incredibly fast with the 3.5 to 1 but has little climb with simulated weight of two passengers.

If I depitch the prop to get the max RPM and horsepower for take off I have high EGT numbers reaching near 1150-1200 at 6800 RPM if I really dont load it up with backstick. I even have one sized over jets too.

Now when I pitch to get 6800 straight and level I only get about 6200-6300 RPM on climb out and the engine is not making great power down there. But when I level out and punch it and get 6800 RPM I am going over 100 MPH. Without a windshield that is brutally fast for long periods.

My guess is that if I speed up the prop with the 3 to 1 ratio I can take some pitch out and get better climb rates and afford to lose some forward speed. I would prefer the added push with more weight in the machine for climb-out.

Additionally, when pitched for climb I have to be really careful about descending with power because that unloads the prop and with 3.5 to 1 gears the prop has alot of torque to speed up the engine and lean out the mixture.

Only since it getting hot outside with higher DA's do I notice the serious decline in climb performance. Last weekend when pitched for 6800 straight and level it was darn right dangerously low climb rates with the simulated weight of me and passenger. I clocked it at 50-75 fpm.

I have to get some pitch outta the prop without unloading it to far so that I get the high egts. I really hope that a 3 to 1 like the engine had will be what I need.

heres hoping
 
With what you have said, it makes me wonder if your blade and/or disk loading is higher than it should be with with the simulated passenger.
 
I do not think that 26 footers on a 390 pound machine is outta line ed. ernie recommended them when I bought the machine. I am only 175 pounds and we even had a discussion here on the forum and came to the conclusion that ernie came to
 
Matt, just get us a pair of new gaskets and a quart of 85 /140 weight gear oil and we can experiment and let our calibrated seat of the pants meter figure out what works best.
 
I'm sure you know more about this than I.....but:

390 gyro
175 you
200 passenger
62.2 10 gal fuel

= 827.2 lb

For 8" X 26' rotor:
That is 1.56 lb/sq.ft. of disk loading
With a 3 ft hub bar, that is 53.9 lb/sq.ft. of actual blade loading

If the above weights are correct, these loadings seem rather high, but perhaps that is normal for a two place.
 
that is rather normal for a tandem machine ed.

Ron,

I know we are gonna try to swap the gearboxes but just wanted to see if there is any logic as to the gear swap. I bet your ship will be insane fast with the added pitch to your prop. Like i said before when leveled out my machine is faster than my single place dom with a pod. GPS runs with no wind come out to 100 mph... just can't get any climb w/ the pitch required to get the engine running with the right temps. When pitched for climb (yet high egts) I was getting 500' per min with simulated two passengers. That is very impressive but too hard on the engine considering almost 1200 degree egts
 
Matt go to Warp drives site and check out their prop speed caculator. There you can learn alot about props and what speed to turn it, etc.....
 
Thanks for the heads up Ron. I never heard of the prop speed calculator.
 
Sounds like your temps go up if the engine is not fully loaded. More a carb problem than gearbox. Running lean at anything less than wide open or near wide open throttle.
 
peter,

without proper loading on the engine a lean condition will arise because the engine will turn faster than it is supposed to with the throttle setting under normal load. This is exhibited on a machine when descending with throttle because the prop is being forced to turn faster and the engine rpms increase yet no change in throttle. If you return to level flight after descending the engine regains the load and EGTs will drop because the engine is now turning the proper rpm to throttle relation.

If the gearbox is at the improper ratio and has too much ease to spin the prop at WOT a lean condition will arise. The engine must be properly loaded at WOT and if you reduce the gear ratio it is possible to bring the engine RPM, prop pitch, and throttle setting in to harmony. I have found some great information on the web regarding this exact issue.

Exaserbating the problem is that my 72" prop is VERY light and since it is not heavy would not normally require such a high gear ratio. If I was not running a Kiev and was running a Warp Drive I found out that the gearing would be fit for that prop. The Kiev is very light and efficient. After speaking to Kiev I was informed that my gear ratio is probably too high, but would work much better if I had a faster aircraft. Since we are slow compared to airplanes the lower ratio is recommended. I get superb thrust at high speeds and personally I don't like going 100MPH to achieve full thrust potential from the propeller.
 
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Just be aware of the harmonic issues when using a 3.0 to 1 ratio with a 3 blade prop. That match is not recommended.
There's a good bit of info on the web, and prop sites regarding it.
Dave B
 
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