Gear drives effect on power/weight ratio

[Shakey Jake]

I haven't seen a discussion anywhere on any of the forums on this particular subject, though I'm sure it has been talked to death somewhere. I'm curious as to the experiences any of you may have had with gear drives VS power to weight ratio. For example I'm building a powered glider (2M) and all up weight is 3lb for a wing loading of 9oz Max ready to fly. (This is no fire ball, just a floater) Now say I have a 480 can- motor that I would like to use on this plane. Running 9 cells at around 10A gives a little over 90W or a power loading of 30W/LB. If I go to a gear drive of 3:1 ratio and run a larger prop creating the same 10A load, how would you calculate the power/weight ratio allowing for the effect of the gear box? Do I now have a power loading of 3X30=90W/lb? I hardly think so. This thing would be a rocket!!
Maybe you would 1/2 the gear ratio for 1.5X30=45W/lb.

Anyone???

Thanks

Shakey

[megawatt]

The easiest way to compare the effects of various power systems and motor gearbox combinations, is to get a copy of motocalc or e-calc and start playing. ..................Jeff

[Steve Fehr]

You can use P-Calc online here:
http://custompaintjob.com/ezcalc/dma.asp

Basically, the higher the gear ratio, the more thrust, but pitch speed will be reduced. For what you want to do (efficiently gain altitude), a high gear ratio is probably very desirable. Find what others have been using on similar sailplanes, and start from there I keep a spreadsheet with the results from a myraid of powerplants in the range I'm looking for, for my planes.

[poobs]

I'm not sure I understand your question.

The power to weight ratio will not change but the gearing will allow more of the power to be used to moving the airplane.

I agree that the programs are great fpr mucking arond to see what will take place with various props gears etc.

[Shakey Jake]

Thanks for the replies. Steve, I tried P-calc. It works nicely.
I guess I should have been more explicit about what I'm looking for. In electronics we used to call it a Rule of Thumb. Read (Educated Guess). So, If you go from a 30W direct drive to a 3:1 gear drive what would the effective (not actual) power to weight ratio be?
I tried out the P-calc program and adjusting the prop size for the 3:1 drive and best in flight thrust at roughly the same current draw I come up with (according to my convoluted thinking) an effective increase in power of 55% or about 46W/lb for this plane. So, my new ,though admittedly not very scientific, Rule of Thumb is that for a 3:1 gear ratio you may expect an approximate increase in power to weight of 1.5. My second guesstimate was possibly correct and the given scenario may well fly.
Is my hypothesis a little shakey here???

Shakey

[chrisc]

How accurate is P-Cal? I have tried it with a few motor set up and the resultant is higher than motor-cal.
Which software can I rely on?
How can I use it for DUCTED FAN computation?

Thanks

[Steve Fehr]

The W:oz ratio kinda breaks down when you start talking about a gearbox. The actual power (W) stays pretty much the same, no matter what gearbox you use. The thrust can go way up, but the other variables come down to meet it, and so total power required to move stays the same. Sorry, there's really no rule of thumb, other than to try to get the amperage & rpm for the highest efficiency possible for the motor. That's why we let motocalc do all the hard work for us :)


Chris- I don't know how accurate P-Calc really is, but they're all based on empirical tests and extrapolated data, so should be fairly good as far as a ballpark figure goes. Nothing beats good ole real-world testing though! No two planes are going to be exactly the same, and things like real-world thrust are notoriously difficult to measure because props are often stalled at rest and will measure much lower than in flight, which is even harder to measure. I only did one test so far- in one instance, P-Calc estimated 12oz of thrust on my stock FMA Razor w/NiMH but I only measured 9oz on the scale. However, the flight duration is right on the button, so the current calcs are right.

P-Calc is kinda limited in scope; motocalc is a lot more sophisticated if you need more data.