Airfoil Shapes?

[thelorax]

Is there a good reference (book,mag, website) for learning what the different shapes/designs of the more common airfoils are and what they're good for (e.g. slow flight, aerobatics)??

Gregg

[FLB]

Hi Thelorax,

I've not so much experience in that field. But before Dave give you the right answer, here is a book on the subject that I've seen sometimes. Try a search on www.amazon.com in books section (search engine is upper left in the page):

Model Aircraft Aerodynamics (by Martin Simons)

The direct link is:
http://www.amazon.com/exec/obidos/search-handle-form/102-8072618-1162548
but it seems not to work directly :(

Regards

FLB

[Don Cooper]

I recently bought the book FLB refers to and I think it is good. You might look at the " Why the Performance Degradation" thread in this forum. In one of the responses, Dave gives a path to a tool for comparing drag polars for a lot of airfoils at slowflyer-type Reynolds Numbers that I have found very useful.

Don

[Dave Robelen]

Hi,
That wind tunnel data program is real handy for evaluating various cases. Very few books have been written on the subject relating to light and/or slow model aerodynamics, mostly because there is very little published information. The book by Simons would be a fine starting point. YOu can also get a lot of good information from the "ask Joe or Don" section of the DJ Aerotech website. Don seems very knowlegeable on these subjects, and he is quite willing to share. Most often you have to get down to specific cases to really get hard information.
Good luck, Dave

[thelorax]

Gentlemen,
Thank you for your posts. The wind tunnel program looks very useful - now I just need to learn more about the results it gives me LOL!

And Dave, I will visit that website when the time comes.

Best,
Gregg

[fly_joy]

Can anyone post a few guidelines for choosing an airfoil? To be honest I don't have the interest (and maybe the aptitude) to read a book on airfoils. Besides, the published work may not be applicable to our small, light stuff.

I have a few basic questions for slowfly planes:

1) I have heard people say the airfoil is not the critical for slow and micro planes. As a matter of fact I have heard of people having a great time flying flat plate contraptions. Any comments?

2) In general, when is an undercambered wing preferred over a flatbottom wing?

I have built some great freeflight designs with flatbottom wings at under 1oz AUW (e.g., Embryo class). If I am going to build micro RC versions of freeflight designs am I better off sticking with the original airfoils or going to an undercambered profile?

I know the question if overly broad but I thought I'd throw it out here anyway.

thanks,

Larry

[Dave Robelen]

Hi Larry,
It sounds like you are shopping for airfoil guidelines for the small models. I too have flown Embryo competitively, so perhaps we can compare cases. The Embryo free flight model is a highly powered light weight model that depends on thermal activity for a really long flight (2 minutes is really long for me). During the burst climb, the flat bottom airfoil probably has a bit less drag than an undercambered section. Other than that, the undercambered airfoils climb higher with less power, and glide longer after wind down.
The idea is that in small models with wing chords less than 6", and flying speeds less than 20 mph, the undercambered sections have a higher maximum lift (slower flight) and less drag over a broad range (less power required). In direct comparisons on one model that weighs 2.5 oz, and has a 4" X 20" wing, the undercambered wing gave a greater speed range, along with a slowere minimum speed.
The only time you can get the most from the flat bottom type sections is when you want to fly fairly fast with a larger model, or really fast with a small model as in sp400 racers.
I hope this is some help, Dave

[GHMBO]

Hi Dave
I have seen models that do not have a constant airfoil. Most have a flat or convex bottom near the wing root and become undercambered or concaved near the tip - usually tapered chord wings. Is this an effort to prevent wingtip stall?
Jim

[Dave Robelen]

Hello GHMBO,
You got it right. By increasing the local lift of the airfoil, the root can be made to stall first. A good example of this in full scale is the DHC Chipmunk. The root airfoil is nearly symetrical, and this blends into a thinner section at the tip that has a slight undercamber. This is an elegant treatment that is not often used in full scale because of the cost of tooling. It is much more common to crank in some washout twist and stick stall strips along the root LE. I don't know which models you saw with this feature, but the first I noticed using the concept were pylon racers back in the 70's.
Regards, Dave

[fly_joy]

As always thanks Dave.

PS In another post you suggested that I modify my flat bottom wing , on my 2.6oz plane, to undercamber. I've done the modification but haven't got a chance to fly it yet (too much snow and wind). It will be interesting to compare flight characteristics.

Larry

[thelorax]

Hah! Thought I dissapeared - well I guess I did. Been super busy with work and will continue through the holidays. Great for the business but not for building planes :(

I'm still keeping tabs.

G.