Rotor Damping

[troed@aon.at]

Now this for the first time is a real scientific approach to the discussion...

Millions of thanks to You Greg ! You are really special.

Could You translate Your measurements and calcs into some nice graphics for easier understanding ?

Angelo

[gyrogreg]

Originally Posted by Bob Gregory

I figure the horizontal sides to be 4.375 sq ft or 8.75 sq ft without the tip plates.

The CG to quarter chord is 78 inches.

Rotor Thrust Vector sheet to follow.

Bob, thanks for the spread sheet - I'm still trying to understand what it is telling us. Can you summarize any conclusions this leads you to.

One question though - looks like this sheet requires a measured prop thrustline offset - 6.44 in that I had estimated above? That number actually needs to be verified, I'm not sure all my assumptions to derive that number are accurate. For instance, I assumed prop thrust of 500 Lbs - the typical static prop thrust on the ground. At some higher airspeeds, that might not be the real prop thrust - the 500 Lbs static may include some stalled prop blade area, and the reduced prop blade AOAs at higher airspeed will probably reduce the prop blade thrusts at higher airspeeds. Anyone know if the prop thrust at 60 mph can be expected to be higher or lower than static on the ground? Typically we woulkd expect prop thrust to be less and less at higher and higher airspeeds - at least after portions of the blade become unstalled at the lower airspeeds.

How did you compute the "effective" quarter chord location - take into account the varying chord effect, or just the mid point of the 1/4 chord line? Probably not a lot of difference between the two though.

(If anyone has any trouble opening Bob's file attachment above - when I saved it it is a .php file? Just change the extention from .php to xls - then it will open with Excel.)

- Thanks, Greg

[Bob Gregory]

Determine quarter chord.

[gyrogreg]

Originally Posted by Bob Gregory

Determine quarter chord.

Bob, probably plenty close enough - better than my original number! I'm not sure this does account for the higher component of the lift at the inner - longer chord - sections of the tapered "wing". There are probably standard formulas to determine the "effective" 1/4 chord for swept and tapered wings. I think your computation accounts for the sweep, but maybe not the taper? I'll bet the difrference is not ore than 1 inch though. As a learning experience, anyone have thoughts or formulas for this?

- Thanks, Greg

[mceagle]

For instance, I assumed prop thrust of 500 Lbs - the typical static prop thrust on the ground. At some higher airspeeds, that might not be the real prop thrust - the 500 Lbs static may include some stalled prop blade area, and the reduced prop blade AOAs at higher airspeed will probably reduce the prop blade thrusts at higher airspeeds. Anyone know if the prop thrust at 60 mph can be expected to be higher or lower than static on the ground? Typically we woulkd expect prop thrust to be less and less at higher and higher airspeeds - at least after portions of the blade become unstalled at the lower airspeeds.

In-air prop thrust is something that is very difficult to determine.

At the lower end, some combinations will produce maximum thrust at zero mph while others will not reach their maximum until a large percentage of the blade is un-stalled, typically somewhere between 10 and 30 mph. In the latter case it is usually the higher horsepower engines running high pitch angles.

At the higher airspeed end of the scale there is no doubt that at some forward speed the prop thrust would be zero (it might require a steep descent to achieve this). It can even reach a negative figure (drag) if the gyro overruns propeller speed. Given the rotational speed and the pitch angle it is not too hard to work out at what airspeed the thrust equals zero.

Whether the thrust line between this maximum and minimum would be a straight line or not, I do not know.

To determine the in-air thrust at different airspeeds would ideally require the use of a large wind tunnel, but even then this would only be accurate for the given engine/prop combination. Realistically it would be out of the question and impractical to do this sort of testing.
A more practical alternative would be to do thrust testing on an open flat-bed trailer or truck at differing airspeeds. Even this would require some time, dedication and a long airstrip, but could afford some useful data.

Knowing thrust over the normal flight range could go a long way towards understanding the requirements and effectivness of a horizontal stabilizer.

[birdy]

Bloodyell, i picked the rong time to take me holiday.
Im go'n to have square eyes wen i get home waden through all this.