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How to figure Thrust to weight ratio?

Old 04-14-2004, 11:40 AM
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rod2_hi
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Default How to figure Thrust to weight ratio?

I have been using the " P-Calc " on the Diversity Model Aircraft site-

http://brantuas.com/ezcalc/dma1.asp

It is a really great tool. I fly at about 6,500 ft. asl. I know from my nitro planes that I need more power than at sea level. Does anyone out there know what a good rule of thumb is for guessing a ballpark 'thrust to all-up-weight' ratio? Like all up weight of 1 lb. should have equal thrust? or more/less thrust than weight. I have a Rascal C with a Speed 400 and gear reduction, 9/6 prop, 8 cell nmh 1100 mah. and it took me about 100' to get it off the ground yesterday and it wouldn't get up much past stall speed. I need to work on my system to get it to fly here. I just ordered 9/7.5 and 9/9 props and I hope they will help get a little more speed. Any help would be appreciated.

rod
Old 04-14-2004, 12:10 PM
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Greg Covey
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Default RE: How to figure Thrust to weight ratio?

Rod,

The easiest way is to measure current. At 6500' above sea level, the thinner air produces less thrust which in turn draws less current. By measuring the current level, you can adjust prop size, gear ratio, or cell count to get back to the limits of the power system.

Typically, at your level, you can increase your prop size by 1". If this causes ground clearance issues, you can reduce the gear ratio or add a couple extra cells without over-taxing the motor.

Good luck!
Old 04-15-2004, 12:04 AM
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bojangle
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Default RE: How to figure Thrust to weight ratio?

Rod
We fly at 6500' also. Our planes range from 16oz AUW to 24 oz. We tried several ratios/props and arrived at the following:

Sport plane, (Mini Telemaster), about 18 oz. speed 400 7.2volt, 2.33 ratio, props 8x6 and 8x8.

"Sparky" type, about 24 oz. speed 400 3.0 ratio, prop 10x7.

I have a scratch built sport, similar to Telemaster, the climb rate is fantastic, and very aerobatic. We tried the 1100 batteries, and gave them up for the Global 8cell, 800 mah. Why? The 800 only weighs 4 oz, and we get 10 to 15 minute flights. With the 1100, weighing almost 8 oz, the planes were struggling for altitude and the flight times actually went down to 8 minutes. I bought 4 of the 800 batteries, Tower has them for $28.

Since we have a grass field, we hand launch, so I don't know about ROG with this setup. With a brushed motor, if you have a 1.1 thrust to weight ratio, that's about the best you can expect.

Here is a picture of "Midge", I have over 50 flights on her, she's a blast to fly.

BO
Old 04-15-2004, 08:49 AM
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rod2_hi
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Default RE: How to figure Thrust to weight ratio?

Thanks for the replies, Greg and Bo.

My Rascal C weighs in at 28.5 oz. auw. I did the ROG because I didn't know if it would have enough power to stay aloft if I hand launched and I didn't want to have a crash waiting to happen. I figured if it couldn't get off the ground It wouldn't hand launch. I would prefer to hand launch when everything is up to snuff because the areas I fly from are dirt or gravel covered roads/parking lots and they do a number on the tail skid. Switching to the Global sounds like my next step, it would get me down to about 24 oz. I will check out a 9/9 prop first then maybe go to the 10/7 if it doesn't help.

rod
Old 04-15-2004, 08:19 PM
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bojangle
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Default RE: How to figure Thrust to weight ratio?

I am assuming you are using a 3.0 ratio. The 9x9 prop and 10x7 prop will give you about the same static thrust. The 9x9 will unload more giving you a little less in flight thrust with a higher pitchspeed (46 mph), the 10x7 will maintain more in flight thrust (unloads less), but the pitchspeed would go down to about 34 mph. It's a trade off of power or speed.

When I hand launch Midge, I keep my thumb off the elevator for as long as I can, let her build up airspeed. She sinks a little at first, but then starts to climb. Don't be alarmed by this sinking, just be patient and you will be fine.

Good luck
BO
Old 05-12-2004, 06:03 PM
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budrodis
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Default RE: How to figure Thrust to weight ratio?

I heard for electric 3d you need about 10 watts per ounce
Old 05-12-2004, 07:26 PM
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Dr Kiwi
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Default RE: How to figure Thrust to weight ratio?

Hi budrodis - you might need 160W/lb for supreme aerobatics, but a trainer/slow flyer like a Rascal can get away with 40W/lb. The rule of thumb for these aircraft is 1:2 (thrust:weight).

Cheers, Phil

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