Hi,

I've been reading up a little on the Reynolds Number Rn, and I don't really understand how to use it. Would one of you geniuses please fill me in a little? I know how to calculate it, but what do you do with it? I know it will change with airspeed, but what makes a good Rn vs a bad Rn? I'll probably have more questions later.

Thanks in advance,
Soj

There's no good versus bad Reynolds number or Rn. Rather it's just another tool to help predict how an airfoil or other shape will perform.

If you look at the airfoil lift to drag coefficient curves you'll see that they start to suffer when the Rn gets too low. This happens when your glider starts to try to fly at low airspeeds near the stall and it has a narrow chord. With most power models the Rn isn't really a factor that needs much attention since the models fly fast enough to be above the critical low speed zone for the most part.

This Rn issue is also why you see both two meter gliders and unlimited ships sharing much the same wing chord. It was determined some time ago that since the Rn is related to speed and chord (all else being equal) that the chord is the issue rather than wing area or aspect ratio. This is why you can often find unlimited ships and the smaller two meter models flying with nearly the same wing chord. This is generally 8 to 10 inches.

Now when you get down to hand or discus launched gliders there's some compromises to be made so the airfoil is then chosen such that the lower Rn performance is suitable to the model. For REALLY low Rn's that we free flighters are used to there's some guidlines that let us select or design our own airfoils for such models.

Ahh, thanks.[8D]

Soj