# candidate for magic airfoil section?

### Help Support HomeBuiltAirplanes.com:

#### topspeed100

##### Banned
Funny FX-73 ( reflexed ) is closest to the foil that I drew intuitively for the small plane I have on avatar.

or FX 84 http://www.worldofkrauss.com/foils/765

Thickest point is not 50% but less tha 40%..this could be maneuverable in short plane ?

-------------

Could GOE 425 like foil be better in aerobatics ? Better spin recovery ?

http://www.worldofkrauss.com/foils/245

-------------

Any comment on foils like JPM3-07 where the thickness and camber are the same point ( 42.1 % of the chord ). Easy pitch moment ?

Last edited:

#### Mac790

##### Well-Known Member
Top keep in mind that that high cl, means usually high cm, which means higher trim drag, it's not easy to use those foils in planes, special with short tailcone.

Another tool to play with. Some results are really strange, hmm
Kevin's Online Panel Code - Version 2.0

Seb

#### Topaz

##### Super Moderator
Staff member
Log Member
...If you take a look at those attachment you will notice difference between XFLR5 and DesignFoil, I'm wondering which one is closer to the true results. I've read somewhere that DesignFoil is better for drag bucket estimation, it generally also has lower Cl for particular foils than XFLR5, and even Orion wrote that Xfoil isn't very good at predicting Cl max. http://www.homebuiltairplanes.com/forums/aircraft-design-aerodynamics-new-technology/4154-maximum-lift-values.html
Hi Seb,

Just want to throw in the point that simulating the performance of an airfoil out to the "nth" degree is largely a fruitless exercise. Remember that the construction tolerances on almost any wing we might build as homebuilders is roughly of the same magnitude as the errors in the various codes such as XFoil, which is why those codes weren't developed to a higher degree of accuracy. There simply is little point in determining the exact performance of a given airfoil, when what will be on your airplane will not be that airfoil, to that same degree of "exactness". None of our aircraft are so performance-critical that we need to nail performance numbers exactly.

At some point you have to say "good enough" and accept that the real article will differ in performance from your calculations to some slight degree. When someone says XFoil is no longer accurate enough, they've probably passed that point, IMHO.

#### HumanPoweredDesigner

##### Well-Known Member

Airfoil Investigation Database - Showing SIKORSKY DBLN-526

It is 26% thick with L/D = 53 at a high Cl. Only problem is I don't know how you'd attach ailerons or flaps. The Cl max is over 2. The lack of a low drag bucket is probably not good for a speedster, though.

Do you believe that wind tunnel data? Even more interesting is with the same front half and a pointier back, the performance drops dramatically:

http://www.worldofkrauss.com/foils/show_compare/?id[]=1244&id[]=1950&chord=6.5

I suspect this airfoil gets laminar flow over most of its surface, which makes up for the blunt trailing edge. Funny looking airfoil though.

Here is a 22% thick one that has less high speed drag and more aileron potential:
http://www.worldofkrauss.com/foils/show_compare/?id[]=1244&id[]=1566&chord=6.5

Another tool to play with. Some results are really strange, hmm
Kevin's Online Panel Code - Version 2.0

Seb
I use that site extensively for estimating the Cm at non 25% cord locations. But the Coefficient of drag is so unbelievably low, unless that is hydraulically smooth and Re well above a million.

Last edited:

#### orion

##### Well-Known Member
A two dimensional l/d of 53 at high cl is downright awful

#### HumanPoweredDesigner

##### Well-Known Member
A two dimensional l/d of 53 at high cl is downright awful
That is at Re = 100,000. It is probably much better above 500,000. The clark Y smoothed has a 2D Cl of 54 in the naca wind tunnel at Re = 100,000.

#### Norman

##### Well-Known Member
Thanks Norman, sorry for troubles, it seems that result which you got for higher Re are very similar to those I got with XFLR5 [ /QUOTE]
That's probably because both programs use Xfoil for 2D analysis:lex::computer:

#### Mac790

##### Well-Known Member
That's probably because both programs use Xfoil for 2D analysis:lex::computer:
Norman,

Yes I knew that, I was expecting that your version is improved because it's payable and mine is free, but it seems that there is no difference between them. I was looking also at Airfoil, but I found a suggestion about using xfoil instead, because it's better, hmmm, there are some differences between mine charts and Harry's. It seems that also John Roncz was offering a free software, I found this (I like this guy style)
John Roncz said:
FINAL NOTES

#### Mac790

##### Well-Known Member
Jarno, you are right this HQ35 is very thin, leading edge it's extremely thin, but on the other hand it doesn't look bad at charts, of course without proper coordinates it's hard to compare, that chart which I did with original foil.

But I must say that it doesn't matter , I found something much more interesting, it has slightly higher Cd but overall it's a real killer , you probably heard about it before it's AH 93-K-132/15, I know you probably say, I've never heard about a glider with that foil... of course but keep in mind that it's a relatively new airfoil, and take into account that all modern sailplanes (besides Diana) use Boermans airfoils so who is gonna to try something else (besides one crazy Dutch and one mad Pole:roll, besides I found something http://www.icas-proceedings.net/ICAS2008/PAPERS/319.PDF

NACA 6-digit airfoils and the more recent NASA Natural Laminar Flow (NLF) sections show relatively low drag when compared to widely used turbulent airfoils. However, it appears to be difficult to combine a large laminar bucket with low drag in the laminar region with the NACA and NLF airfoils.

After a consultation with L.M.M. Boermans (a lecturer in aerodynamics at the TU Delft), the decision was made to use a laminar airfoil developed by the University of Stuttgart, the AH 93-K- 131/15. It is a 13% thick airfoil, with maximal t/c at 50% chord, and a design lift coefficient of 0.2 at Re = 2.5e6.

This airfoil exhibits a large laminar bucket combined with a low drag count. Compared to NACA 6-digit and NLF airfoils at similar Reynolds number, the AH airfoil shows at least 1 drag count less at the same Cl, combined with a more extensive laminar bucket.

A GREEN ALTERNATIVE FOR PILOT TRAINING reaches its maximum lift coefficient in fully turbulent flow ensuring the aircraft will not stall catastrophically if the wing surface happens to become contaminated by rain or bugs. This, combined with very gradual stall characteristics, made it a good airfoil choice for a trainer aircraft. The original airfoil camber is tailored using XFoil to reach a cruise lift coefficient of 0.38 while remaining at the lowest drag point.
Of course he recommended it for different purposes, I did comparison between AH 93-K-131/15 (which he recommended) and AH 93-K-132/15, and it seems that 132 is even better.

Take a look at those charts: Re 5x10^6, Mach 0.28, NCrit 9

black AH-93K-132/15 (mod)
violet AH-93K-132/15 (mod)
light green NACA 671-215
navy blue NLS 414F
orange I tried KL-002, but as you can see with little success (without proper coordinates).

Second chart (pix1) for Cl/Cd at cruise.

I was going also to include FX 79 in those charts, but it's a piece of %&^% compared to those.

Norman,

I think I'll stay with XFLR5, I think I solved all problems, the key is to use at least 290 panels, previously I was using around 100, NCrit 9 instead of 12, and calculate every 0.5 ideally 0.1 AOA, setting like that works fine even with NACA 66-serries foils .

Seb

#### Attachments

• 49.7 KB Views: 297
• 58.5 KB Views: 285
Last edited:

#### Navy guy

##### New Member
I am still looking for that magic airfoil that has low pitching motion, high lift, a slow stall and easy to build. So far i have tried GEO387 and GEO384 airfoils with a full length Junkers flap of an eleven inch cord. Lots of rear airfoil chamber when the flap is deployed (too much pitching motion I should think) which would likely result in a first flight crash. Not good.

#### flat6

##### Well-Known Member
Of course he recommended it for different purposes, I did comparison between AH 93-K-131/15 (which he recommended) and AH 93-K-132/15, and it seems that 132 is even better.

Seb
this is very interesting. where can we get data for this airfoil?

#### Mac790

##### Well-Known Member
You can find it here Airfoil Investigation Database - Showing AH 93-K-132/15 or here http://www.ae.illinois.edu/m-selig/ads/coord_database.html#A to get decent performance in cruise, you have two choices, first lower camber to around 2.3 (if I remember correctly), second use flaps at around -3 deg.

You will also find coordinates and graphs in Stuttgarter Profilkatalog II: Niedriggeschwindigkeitsprofile, you can order copy here http://www.iag.uni-stuttgart.de/laminarwindkanal/profcat1.html

Seb

Last edited: