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Martin353
05-26-2012, 01:06 AM
Is it possible to work out the extra lift gained (or lost) from different sized blades if running at the same hs? i.e what % extra lift would you get from changing from a 550 size to a 600 size blade.


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Martin353
05-26-2012, 01:17 PM
I'm kinda guessing that be a no then lol


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TowPilot
05-26-2012, 01:50 PM
You could use a small bucket with weights that you add until it can't be lifted.

Change blades, go through the process again to see the difference of vertical lift.

This doesn't account for how a blade will perform during forward flight during maneuvers though.

npomeroy
05-26-2012, 05:36 PM
I'm kinda guessing that be a no then lol

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Hang in there. I don't think many are viewing this forum yet.
There will be a simple calculation for the theoretical lift increase, and I think it will be significant as much of the lift comes from the outer end ofthe blade.

TowPilot
05-26-2012, 06:47 PM
And there is going to be a cost, and increased drag for increased lift. The calculation will also probably show, the additional power needed, to generate increased lift. Just a guess.

extrapilot
05-26-2012, 10:06 PM
There is no simple calculation; most of the parameters that define lift are undefined here. Easiest option is to throw the machine on a scale, and run it up in negative pitch 550 vs 600 blades, and see the delta.

Just based on blade theory, by my calcs, the 53” dia rotor will generate approx 55% more thrust than the 47” dia rotor at a given non-stalled AOI/RPM. Best to measure- the calcs dont account for important variables.

npomeroy
05-26-2012, 10:36 PM
There is no simple calculation; most of the parameters that define lift are undefined here. Easiest option is to throw the machine on a scale, and run it up in negative pitch 550 vs 600 blades, and see the delta.

Just based on blade theory, by my calcs, the 53 dia rotor will generate approx 55% more thrust than the 47 dia rotor at a given non-stalled AOI/RPM. Best to measure- the calcs dont account for important variables.

I hoped you would contribute to this one extrapilot.
Do you not agree that in the way the OP phrased the question there is an assumption of "all other things being equal" e.g. HS and blade design; so shouldn't these cancel out and allow a genuinely useful calculation?

Martin353
05-27-2012, 05:13 AM
Hi all.

Thanks for the replies. The question was asked assuming everything else remained the same. I.e same pinion, motor, batts etc and just changing the blades.
Some interesting points made tho. Where do u get thus info from? I've looked on google but can't find anything.


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YoungBull
05-27-2012, 09:17 AM
Bear in mind I've had a few drinks and a noob. Extrapilot sounds ontrack.

Could you (ignoring all other parameters, motor, headspeed) posibly measure by blade pitch?
eg: throttle hold at 2000rpm head speed. Set max throw at +1, then +2, then +3 ect ect, until lift has occured -place tape on skids that break at 1kg of force - test with kitchen scales

550 blade surface area(a) / 600 blade surface area (b)
Pitch req for a and b. eg: a = 3deg, b = 2deg
diff of 1 deg ( remebering head speed and motor not inc)
The increased surface area of b req 1 deg less pitch = 50% lift increase.

Almost impossible calc as any % will change through full collective range, fwd flight, ect.
Or maybe this link could help
http://www.ehow.com/how_7680704_calculate-lift-rotor-blades.html

jmmccain
05-29-2012, 03:47 PM
Simply? Yes. Accurately? Don't know.

In this thread (http://www.helifreak.com/showthread.php?t=219914) I posted a number of equations for lift, drag, torque, and power. They are all nothing more than very simple integrals. I know the three for drag, torque, and power work quite well for both models and full scale. The one for lift probably works well too, but I don't know what range of coefficient to plug into it (lack of data).