# Airfoil selection for an aircraft out there?

### Help Support Homebuilt Aircraft & Kit Plane Forum:

#### PiperCruisin

##### Well-Known Member
HBA Supporter
That is a high Reynolds number for most E-AB airplanes with flaps deployed.
Yes, the Re would be a bit lower (Bigfoils uses 6m as a default), but would not change what I was trying to show. The point I am making is at stall, the unflapped portion of the wing is at a MUCH lower AoA, and therefore lower local Cl than the flapped portion for the purpose of estimating wing Clmax with flaps.

The use of GA37A-215 is simply to use as an example.

#### rv7charlie

##### Well-Known Member
the surfaces will be glass smooth.
Where I live, in all but a couple of months of the year, this will last (for each flight) until the plane taxis under its own power. It will almost never apply when the plane is returning to land. 'Real world' stuff...

On the flaperon vs flap/aileron question: There's a lot of anecdotal evidence of flaperon equipped planes running out of roll control when flown near stall. Based on what I've heard from friends who have experience with some of the well known flaperon a/c, most owners never experience it because they never fly the planes anywhere near the claimed stall speed; when landing they're basically flying them onto the ground.

#### Lendo

##### Well-Known Member
How experienced is your Aero Engineer, I know some who know EVERYTHING except how to design an Aircraft. I think someone already said the 63215 is a problem with stall.
2° Dihedral, maybe look at 4° to5° for stability. I personally would love Zero° Dihedral, but you will only see High wing Aircraft with 1 to 2 °.
Building in Twist is just adding in inefficiencies.
Some people just have to learn the hard way.
George

#### Vigilant1

##### Well-Known Member
Building in Twist is just adding in inefficiencies.
But if a wing is straight and tapered, the two most common ways of getting the root to stall first are:
- Washout (twist)
- Varying the airfoil thickness from root to tip

#### TFF

##### Well-Known Member
The top of the line unlimited Aerobatic planes have zero dihedral. Nothing special. Dihedral or washout is not an inefficiency if it tailors the flight character you are after. Some people don’t want to go upside down. Lots of dihedral and washout will help with that. That’s efficient. Thinking there is true perfection just means you will be walking, because no matter how good you get, it’s not perfect.

#### WARPilot

##### Well-Known Member
HBA Supporter
There is a guy in London who has the only flying WAR FW in country. I’ve posted his numbers. Note the weight o his airplane is over 800#!
Also posted is the WAR spec sheet from their info pac.

#### Attachments

• 35.2 KB Views: 30
• 34.5 KB Views: 28
• 58 KB Views: 25

#### WARPilot

##### Well-Known Member
HBA Supporter
How experienced is your Aero Engineer, I know some who know EVERYTHING except how to design an Aircraft. I think someone already said the 63215 is a problem with stall.
2° Dihedral, maybe look at 4° to5° for stability. I personally would love Zero° Dihedral, but you will only see High wing Aircraft with 1 to 2 °.
Building in Twist is just adding in inefficiencies.
Some people just have to learn the hard way.
George
I do t know him to well yet. He is designing for my composites friend and we were talking about wing sections.
I think people have said the 23015 has rather abrupt stalls too. We did talk a lot about the Riblett.
I just want to pick a good safe performer airfoil. Descent stall characteristics and good cruise speed.

#### TFF

##### Well-Known Member
I think abrupt stall is a relative thing. It’s a stall. You too close to the edge, and it will show you the stall, but it’s not an evil stall. A bonanza is not going to stall like a Taylorcraft. If the % is kept thick along the span, it won’t have the same bite of the real plane and have the correct airfoil. I would probably have 12% at the root and 15% at the tip. Opposite of the real plane but you have no need to go over 300 mph either.

#### BigL

##### Member
There is still a bit more to add to this thread. ‘Twist’ is worth a visit because a lot of Aero Engineers are taught from a NACA test of tapered wings, that tapered wings stall first at the tips. And under the circumstances where the test 'wings' not only tapered (chord wise) but also reduced in thickness to 9% at the tips - the results were understandably bad. One example had a 20% root reducing down to 9%! Not only does a wing's Coefficient of Lift (Cl) reduce with reduced Reynolds Numbers but its Cl also suffers with reduced thickness - accompanied with reduction of the Stall Angle of Attack (AoA). That leads to the tip stalling before the rest of the wing. To combat that, Twist is added but the efficiency of the wing goes down. In the FW190 project, that is exactly what you don't want.

If those NACA tests had kept the section thickness the same from root to tip - the results would have been entirely different; resembling the constant chord with stall starting at the root. A note of interest - it is felt that a General Aviation airfoil thickness should never go below 10%. That is with the benefit of hindsight.

The benefit of the tapered chord with a constant thickness is more efficiency than any wing using camber change to mimic twist. Some of the docile handling Pipers come to mind.

The Riblett airfoils are different from the NACA sections from which they were developed. Keep in mind the stall characteristics of the Riblett are 'softer'.

Last edited:

#### BigL

##### Member
An interesting example of how using computer generated airfoil data can vary from Wind Tunnel results was kindly supplied in the form of the BigFoil.com generated analysis for the Riblett airfoil with 35% of Chord flap. I tried to find one for the NACA 63-215 from BigFoil.com for 20% of chord to compare with the NACA Wind Tunnel data for 60-degrees of Flap, but the program was already having trouble with the 25% of Chord and just 25-degrees of Flap. I will try to attach a photo from the Theory of Wing Sections for the NACA airfoil used as an example in an earlier post. From it we can compare the computer generated characteristic with that of the test results. Both examples are for a Reynolds Number of 6-Million. The 63-215 is similar to the Riblett but none of the computer generated results come close to the NACA tested Flap Stall Angle of Attack of 10 degrees at 60 degrees of Flap.

My special thanks to PiperCruisin for bringing this to light.

Another example of wild results generated by computer analysis such as the JavaFoil presented in BigFoil.com is for the USA 35B. If we used this program to choose an airfoil for the WAR FW190 we would choose the USA 35B airfoil with a claimed Coefficient of Lift (Cl) of about 1.8. In real Life it returned a Cl of only 1.2. We could have become very misled by the program. That being stated, sometimes the program responds well to the airfoil shape and gives a close result. I am referring to the Cl vs Alpha diagram for the Riblett GA 37A215.

BigFoil is commended for gathering so many results together in one place and especially where presentation of the Wind Tunnel results was included along with the computer generated data. Although the wind tunnel Cl vs Alpha for the NACA 63215 is not labeled correctly.

Computer generated data can be useful for comparison purposes. If we compare the Riblett to the NACA the difficulty the program had with supplying a full set of returns for the NACA with a 25% of Chord flap shows that the two airfoils behave differently. That carries some merit.

#### Attachments

• 59.9 KB Views: 9
Last edited:

#### BJC

##### Well-Known Member
HBA Supporter
A note of interest - it is felt that a General Aviation airfoil thickness should never go below 10%. That is with the benefit of hindsight.
IIRC, a NACA report indicated that for typical light aircraft Reynolds, a 12% thickness was close to optimum. The Brokaw Bullet, if I accurately remember what Doc said, has a 9% section, the thinnest that I know of for a sport aircraft. EDIT: For a sport aircraft with a cantilever wing.
I tried to find one for the NACA 63-215
I don’t know of a sport aircraft that uses that airfoil. Can anyone identify one?

BJC

Last edited:

BJC

HBA Supporter
Thanks, Mark.

BJC

#### BigL

##### Member
Uses NACA 63-215 :-
AviaBellanca Skyrocket II
Aviation Industries of Iran AVA-202
Bagalini Bagalini
Israel Aircraft Industries Arava
Mooney 20
Seabird SB-4 Sentinel
Tri-R Technologies KIS TR-1
Viper Aircraft ViperJet

That is almost all from The Incomplete Guide to Airfoil Usage.

#### BigL

##### Member
The 12% airfoil being optimal with respect to Maximum Lift / Minimum Drag. Two different ends of the spectrum. But it would be the perfect compromise.

As for the thinnest wing section. I shudder to add this one to the list. Again 9% for the 4309 but I think the 4309 (Top) / 0006 (bottom) was a little thicker (11%) but the W-10 co-ordinates suggest that the W-10 was thinner (8%):-

Aircraft - Root - Tip
Wittman W-8 Tailwind (early) - NACA 4309 - NACA 4309
Wittman W-8 Tailwind (later) - NACA 4309/0006 - NACA 4309/0006

Last edited:

#### Pops

##### Well-Known Member
HBA Supporter
Log Member
The AviaBellanca Skyrockett II was built local. My old flight instructor/ friend and I helped with a little work putting it together when taken to the airport for the test flying. I have some pictures I posted on this site when I joined when we had a thread about it. Don't remember the year when I joined.
When built, all the factory molds for it was built at the same time. '

Last edited:

#### BigL

##### Member
Going back to Chris's earlier post with performance specs for the WAR FW 190, the recommendation could change a little. Averaging out the top speed, climb rates indicating prop selection and factoring in the Verner Radial engine that may pose a problem with Cooling Drag, a safer route to go would be the (if you want) Riblett GA 37A-315.

Still believing that a weight of 924 lbs can be achieved (noting the aerobatic specs for G Loads requiring a certain level of structure to maintain strength) but airspeed may realistically be in the 160 mph area; a Lift Coefficient at that speed and weight would be 0.21 which fits the GA 37A-315 almost perfectly, or should we put it the other way round.

The maximum Coefficient of Lift (Cl) would be 1.49. Really just a small gain but the airfoil's Drag Bucket extends up to a (comparatively) higher Cl of 0.9 (from 0.83). Yet still reaches down to 0.03. That is usefully less than needed.

The no-Flap stall speed is around 56 mph.

Last edited:

#### BJC

##### Well-Known Member
HBA Supporter
As for the thinnest wing section.
The Tailwind wing is a thin section. I edited my comment about the Brokow Bullet to specify that I was referring to cantilever wings.

BJC

#### WARPilot

##### Well-Known Member
HBA Supporter
Going back to Chris's earlier post with performance specs for the WAR FW 190, the recommendation could change a little. Averaging out the top speed, climb rates indicating prop selection and factoring in the Verner Radial engine that may pose a problem with Cooling Drag, a safer route to go would be the (if you want) Riblett GA 37A-315.

Still believing that a weight of 924 lbs can be achieved (noting the aerobatic specs for G Loads requiring a certain level of structure to maintain strength) but airspeed may realistically be in the 160 mph area; a Lift Coefficient at that speed and weight would be 0.21 which fits the GA 37A-315 almost perfectly, or should we put it the other way round.

The maximum Coefficient of Lift (Cl) would be 1.49. Really just a small gain but the airfoil's Drag Bucket extends up to a (comparatively) higher Cl of 0.9 (from 0.83). Yet still reaches down to 0.03. That is usefully less than needed.

The no-Flap stall speed is around 56 mph.