Read Martin Simmons article in QFI August/September 2001 about polars and then read my part about airfoil history again before you continue.  

Go to this homepage for a great explanation of why airfoils and weight are important factors when flying gliders:

http://home.att.net/~jdburch/polar.htm

Now put down you empty glass and head to the flying field on an average windy flying day – ups, did you forget to read Martins article? Do it.

When flying with thermal gliders we need thermals. Have you ever wondered where we get them? The sun heats up the earth. The now warm earth heats up the air above it. If the earth at one place gets hotter than in another spot, the air above it will also get hotter than air in other places. Hot air balloon skippers use this - the hotter air will start to expand and rise. Its’ still only a balloon in the making - we don’t have a thermal yet. This rising bubble of hot air will stick its top up into air that is blowing faster; a thing called the wind gradient courses this increased wind speed.

On this particular average windy day try to stand upright, notice the wind speed at head height, then kneel and notice the wind speed and at last lay down (if it’s not to wet) and notice the almost lack of wind speed. If you climb even higher than you started, you’ll notice that the wind speed picks up as you go higher. This is another important part of the creation of thermals; we now have hot air and the wind gradient. As our hot air bubble grows bigger it sticks its top higher and higher in the surrounding moving air, which have increasing wind speed as you get further away from the ground, the risk of the warm top being blown of gets greater, and eventually a bubble of hot air gets blown away. This bubble pushes away colder air above it as it rises and pulls air from the sides and upwards and suddenly we have ourselves a thermal. Hot air is being sucked in from the sides and pulled upwards while steadily growing, and if you are lucky or clever enough to guide you glider to this rising air and turn it around in a closed circle at the right moment, you’ll suddenly see your model gaining height while drifting and circling slowly downwind. This is free energy – no taxes yet. Height is potential energy that can be converted into kinetic energy = movement, speed, gliding, penetrating, searching! Or in other words when you gain height you can glide all over the place almost forever. It is said that the best thermals are created when the wind speed is about 2-5 m/s. But don’t think that there is no thermals just because the wind is stronger or weaker, there is plenty. It’s the same with women. Even though there was/is only a few Marilyn Monroe’s around you are still flabbergasted when seeing a woman in a skirt – not at any particular wind speed and maybe even more when it’s windy? And remember that the thermals have no engine and no rudder so they can only drift with the wind (are thermals female?). Drifting with the wind is also what you are going to do, while circling and climbing and singing “What a wonderful world “ with the voice of Satchmo.  

What has this got to do with choosing airfoils? Read Martin Simons article about polars once again before you turn to my humble wisdom.

If the average wind speed is 4 m/s and you circle and rise in the thermal for 2 minutes, you actually drift 2 x 60 seconds x 4 m/s = 480 meters behind where you found the thermal while you gained height. And so what? You have to get back – penetrate against the wind unless you want a good exercise, walking downwind to retrieve you glider from the trees on the other side of the river. Some guys go all the way. They deliberately don’t put radios in their gliders to get the most fun out of aero modelling – and they fly +180 seconds. Try to calculate how far downwind that brings them! To get your radio controlled glider home you have to glide quite a bit faster than the 4 m/s - maybe 8-10 m/s depending on the air you fly through (this is airspeed), and you’ll have to glider pretty flat at this airspeed to get home with height to spare and time to find the next thermal. In this example I only had a wind speed of 4 m/s, that is a very nice summer day. Quite often we fly at higher wind speeds and hence needs even more penetrating ability. If the wind is 8-10 m/s you’ll have to glide flat with a speed of at least 15 m/s, maybe even 18-20 m/s if you are hardcore and know where you are heading (i.e. know that a new thermal has been indicated by birds or insects and you have seen it). Flying at a speed of 16 m/s with a glider, which has a wing loading of 31 grams pr. square decimetres will make the airfoil fly at a CL-value of about 0,2. 16 m/s is fast enough for most thermal flying, so the airfoil you choose ought to be able to penetrate pretty flat at this speed and Cl-value. This means that the airfoil ought to have pretty low drag at a CL value as low as 0,2 (at least with negative flaps). You don’t have and never will get this penetrating ability and high-speed flat glide with an Aquila airfoil! This was without ballast. If you ballast you’ll fly 16 m/s at a higher Cl, think about that for a moment. If you raise the wing loading to 47 grams pr. square decimetres you’ll fly 20 m/s at a Cl-value of 0,2 and the 16 m/s will be reached at a Cl-value of 0,3. This is what I think Adrian experiences when he flies his High Five with 2,8 kilo empty weight with a SD 7032 airfoil – from Adrian’s articles in QFI I’m guestimating that the wing area is around 70 sqdm and therefore the wing loading must be about 40 grams pr sqdm. He is flying heavy which makes the model fly fast at a higher Cl-value – SD 7032 has low drag at Cl 0,4-0,8 – presto, Adrian can penetrate with a high cambered airfoil. And since the SD 7032 is able to work at high Cl-values the High Five is still able to fly fairly slow even though it on the heavy side.

Now read Martin Simons article in QFI # 52 again – he tells about this in a very understandable way. Then consult this homepage again for visualising the therory and return to this page after a short while.

Please notice that lowering you flying speed from 7,6 to 6,5 m/s when flying at the low wingloading makes the airfoil change from Cl 0,9 to 1,2! With most airfoils this’ll also give you huge amounts of extra drag for only a little decrease in airspeed. The lesson is not to fly too slowly with any airfoils!

  Some of you may still think that it doesn’t matter which airfoil you use. Some may think that it isn’t worth the extra work building something with a not-flat underside. The fact that I have written this article ought to prove that I disagree. If you build two identical models, one with MH 32 and one with HD 48a (almost the same thickness and camber), I would guess that the difference would be hardly noticeable. Both gliders will launch, climb and penetrate equally (remember that thermals drift with the wind?). But in that critical situation when you have followed the thermal just a bit further downwind than you are used to, you’ll be aware of the difference between choosing aerodynamic efficiency and building efficiency. A glider designed for building efficiency and with the Aquila airfoil will only slowly get closer to you while rapidly sinking and sinking and bury its nose deep in the newly ploughed field with lots of cleaning for you to do. A glider build with aerodynamic efficiency and penetration in mind, will get your glider back to you with delightfully whistling or maybe even non-whistling dynamic. Ready for another ride in one of mother natures great elevators. Choosing building efficiency will often give you downwind walks, shorter flights and dirty planes and Wellingtons, because you can’t follow thermals for very long and if you do, you can’t get home.

That the best pilots in the world at F3J contest doesn’t choose airfoils like the Aquila shouldn’t be a surprise to you by now. But do they choose a specific airfoil? No sir.

At the F3J WC in England in 1998 the top 12 pilots choose airfoils with camber between 1,7% and 3,0% on models with spans between 2,8 and 3,3 meters and with empty weights between 1900 and 2600 grams.

At the F3J WC in Corfu, Greece in 2000 the top pilots choose airfoils with camber between 2,0% and 3,0% on models with spans between 3,0 and 3,3 meters with empty weights between 1620 and 2350 grams.