Choosing airfoils based on calculated polars

[yclui]

Hello,

I have just download the Profili and after getting acquainted with the functions, the first thing I did was to compare the polars of HN354 and S4083 at Re = 216K which is typical for 60” slope racers in moderate lift conditions.

These are the polars of HN354 and S4083 plotted by Profili. HN354 is the airfoil used in my 60” slope racer http://www.euro-sailplanes.co.uk/uk/html/sport-hlg/mini-nyx.htm and the S4083 is used in my 60” HLG http://www.euro-sailplanes.co.uk/uk/html/sport-hlg/highlight.htm . The polars indicates that HN354 performs poorer than S4083 at high Cl and I wonder why it is still chosen for my slope racer as one of the major requirements for racers is that it won’t loose speed in tight turn.

I am trying to learn the basics of model design and if someone can kindly shed some light on this, I will appreciate it.

Thanks,

Y C Lui

[Sparky Paul]

You have to determine the Cl you will be using in the turn, not just the performance at extremes...
Look also at the RG-15, another good racing section..

[markdrela]

Your wing has an aspect ratio of about 10, which limits the CL at which you want to do the turn. The problem is induced drag. For instance, if you do the turn at S4083's limit of CL=1.2, you will have CDi=0.046, which is more than triple the profile drag. You don't want to go there. Something like CL=0.7-0.8 will give a less lossy turn. In the straighaways you'll probably be flying at CL=0.1 or less. In this case the HN354 is better in the turns AND the straightaways, especially if you use snap-flap which will shift up the HN's drag bucket up by CL=+0.1 or more.

The bottom line is that you have to figure out what CL's you'll be operating at before you start comparing airfoils.

[markdrela]

Originally posted by markdrela
Your wing has an aspect ratio of 9.4, which limits the CL at which you want to do the turn. The problem is induced drag. For instance, if you do the turn at S4083's limit of CL=1.2, you will have CDi=0.048, which is more than triple the profile drag. You don't want to go there. Something like CL=0.7-0.8 will give a less lossy turn. In the straighaways you'll probably be flying at CL=0.1 or less. In this case the HN354 is better in the turns AND the straightaways, especially if you use snap-flap which will shift up the HN's drag bucket up by CL=+0.1 or more.

The bottom line is that you have to figure out what CL's you'll be operating at before you start comparing airfoils.

[yclui]

Thanks a lot !

[nuevo]

Originally posted by markdrela

The bottom line is that you have to figure out what CL's you'll be operating at before you start comparing airfoils.

Mark,

So how do I figure out what Cl's I'll be operating at?

Say for something like your Aegea 130" glider wing, and I want to compare airfoils.

Here's some specs:

Span 130"
Area 1062 in^2

Let's assume the plane comes out at 60 oz weight.

If I do my math right, I come out with a loading of 8.1 oz/sq ft, and average chord of 8.17", and a reduced Re of ~ 90,000.

Assuming I'm right so far (maybe a stretch), how can we compute what Cl is the plane likely to operate at under various conditions?

max L/D?
min-sink?
Launch?
Cruise?

Can I figure out all of these without knowing the airfoil?

If we can compute these Cl's, I think we can compute velocity, and thus Re's for each condition using these equations.

Cl = 2 x Weight / (rho x Velocity^2 x Area )
Re = rho x Velocity x chord / mu

Now, given Re's and Cl's for each operating condition, I presume I can compare airfoils.

Jon

[Ollie]

See:
http://my.athenet.net/~atkron95/pcsoar.htm
PC Soar is now freeware that will calculate and compare the L/D and Sink rate of up to four models at a time and plot the results over a range of airspeeds. These results are calculated from the wind tunnel test of airfoils.

Lowest sink rate is usually associated with a coefficient of lift near the maximum (just before stall). High speeds are usually associated with coefficients of lift near 0.1. If you draw a tangent from the origin to the Cl vs.Cd polar plot it will touch the curve at the coefficient of lift for the best L/D.