Airfoil selection for an aircraft out there?

[WARPilot]

If you are going composite it’s a new design with the exact same shape as a WAR. 500 lb would be impressive. The firewall forward is going to be 250 lb engine, prop, cowl, and other stuff. 250 lb airframe will be impressive, especially with retractable gear. At least for the shape and surface. I’m sure you can loose some weight, substantial I’m sure. Half weight is pretty ambitious. Easier to build once the moulds are done, sure; but building the moulds is part of the job; I bet it’s not zero hours. 500 plus 1000 making moulds probably another 500 for systems. Canopy, flight control runs, fuel lines, landing gear, wiring is not going to be free time. If you are turning it into a kit, I bet plenty will be able to capitalize on your hard work laying everything out, but the first one will not be that quick.

True, It would be a kit. The mold is the hard part but my friend does it all the time. He also analyzes the stresses and builds the laminate schedule. We would use carbon fiber, he is a dealer of this material, engines I’m considering are Rotax, Verner, D-Motor. All these are under 180# less prop. There is also a Higgs engine that may work. Probably just design for a MGL or GRT all in one flight instruments for simplicity and light weight.

 

[mcrae0104]

There must be better airfoils out there.

Better at what? A problem must be defined before it can be solved.

 

[WARPilot]

Better at what? A problem must be defined before it can be solved.

I was thinking there might be obvious better airfoils for higher cruise speeds and better stall/lower speed. Losing 200# will help the stall.

 

[WARPilot]

Sample single seat aircraft fuselage molded. Very similar in size to the WAR

 

[mcrae0104]

I was thinking there might be obvious better airfoils for higher cruise speeds and better stall/lower speed. Losing 200# will help the stall.

Sure, a higher Cl and lower Cd would be better, but generally, they are at odds. Therefore, "better" depends on the degree to which one values one more than the other, which boils down to one's mission. I had a professor in school who often said, "You pay your money, you take your chances." (Dr. Cate, you taught religion & philosophy, and you'll probably never read this forum, but you deserve credit for the broader concept.)

TANSTAAFL. You just have to decide what you want for lunch.

 

[jimrittinghouse]

Have you read the book GA Airfoils by Harry Riblett?

 

[crusty old aviator]

Essentially it’s the same airfoil as most RVs and Taylorcraft. It’s going to be hard to beat. For just slow, the USA35B will be hard to beat but 230xx is better overall. Weight loss is your real work.

Those airfoils are what finally split CG Taylor and Bill Piper apart. CG wanted to change from the USA35 to the 23012 and Bill said no, he didn’t want to invest in more tooling and a wing redesign, and the Cub flew, so that was good enough. So now you know why a Taylorcraft will get you there and you’re forever going nowhere in a Cub, unless you have more than 85 hp.

 

[WARPilot]

Have you read the book GA Airfoils by Harry Riblett?

No…..is it written for lay readers? I have some technical training but complex differential equations are long gone!

 

[wsimpso1]

I was just thinking there had to be a really good, modern airfoil that would improve slow and fast speed performance.

Define better. Airfoils are kind of simple things. They take incoming airflow and divert it downward. The change in airflow momentum in the downward direction IS the lift. The change in airflow momentum in the fore-aft direction IS the drag. Airfoils out there do this pretty much the same in all. Oh, you can do worse than the mathematically derived functions, but no one is doing better...

Once you have planform settled, the relationship between AOA and lift and drag is basically fixed. In replica fighters, the aspect ratio and planform is pretty much fixed, and you scale it up or down to achieve your desired landing speed with your flap choice.

The things we can do with airfoil selection are:

• Pick the foils for higher cruise speed or for lower landing speed - We can go full one way or full the other or some blend, but we can not get max of both. This is pretty much a matter of deciding what your desired blend of the two is and picking foils that suit your mission. Most of us know that foil selection can only reduce landing speed by tiny amounts, but we can increase cruise speed some with the right foil. The biggest actors in both of these is wing area, not foil selection;
• Pick foils that are good without flaps or work well with flaps, as you will build. Most foils like flaps just fine for reducing landing speeds;
• Pick foils that we can execute reliably in our shops;
• Pick laminar flow foils if we can execute them well enough in our shops for reduced cruise drag. There is little point in a laminar flow foil without sufficient smoothness to maintain laminar flow;
• Pick foils that make for good stall behavior. Some things that were popular in WWII birds, like tapering to lower percent thickness as you go out the span, tends to make for nasty stalls. We do not have to. Holding % thickness throughout seems to behave better as does a modestly drooped leading edge. See Harry Riblett's book.

Billski

 

[wsimpso1]

No…..is it written for lay readers? I have some technical training but complex differential equations are long gone!

No diffyq in Riblett. He played with camber curves to get just a tiny bit of leading edge droop and then applied standard thickness distributions over them, then ran in models. Produced a whole catalog. I do know of an aerodynamicist who said "Harry's a nut case, but his airfoils are good". His book is mostly about his foils, selecting the right one per mission profile, tapering the wing in a way that leaves good stall behaviour, wing tip design, oh, and Harry's view on government in airplane design... Folks using his foils his way have been getting good stall behaviour and good performance.

 

[Vigilant1]

The things we can do with airfoil selection are:

All great points. At the risk of being overly conservative, I'd add this:
- It is worth strongly considering airfoils that are already performing well on design similar to your own (i.e similar Re number at stall and cruise etc). Some airfoils that look good on paper didn't do well in real life and some very good aerodynamicists have produced planes with bad stall behavior that required a lot of re-work, fences, strips, VGs, etc to make things work. Perhaps the new desktop digital wind tunnels are accurate enough to prevent surprises, but there's some comfort if hundreds of existing airplanes are flying well with the same airfoil, washout, etc you are planning to use.
If I were designing a plane and choosing an airfoil, it's a bit like choosing a horse to go on a long trip. I wouldn't want a high-strung, twitchy, tempermental thoroughbred. I want a reliable, strong daily workhorse with no bad habits. I want to hit a solid double not swing for the fences. (Insert your own tired trope here).

 

[PiperCruisin]

Holding % thickness throughout seems to behave better as does a modestly drooped leading edge. See Harry Riblett's book.

I wanted to second Billski's point here. Just to be clear, if you are using a 23015 at the root, continue with it to the tip. If you taper and go to a 23012 at the tip, the tip has a lower Clmax (thinner and lower Re because the chord is shorter) which then requires more twist or stall strips to avoid a nasty stall behavior.

I think the key is smooth construction and for the lower stall speed very effective flaps. The Dyn Aero achieves this by using higher aspect ratio and a double slotted flap (it's pretty cool). See here http://contrails.free.fr/temp/Pilot100Yrs-06-164.pdf Slotted/fowler-like might be sufficient, don't know.

Also BigFoil is a great tool for searching and comparing airfoils.

 

[Pops]

Take a look at Bob Barrows Bearhawks using the Ribblet airfoils. Low stall and faster cruise than similar aircraft. The Patrol with fly slow with the Super Cub but a lot faster.

 

[PiperCruisin]

Here is a sample comparison from bigfoil.com:

 

[Riggerrob]

...

Interference drag is usually minimized with the fuselage vertical walled and straight section through the wing. Think Mustang, NOT Spitfire. Once you accept fuselage taper through the wing, you are committing to expanding root fillets and still big drag from separation. If you can keep the FW look without tapering through the wing, you will be faster. ...
Big project. I want to see it fly.

Billski

Keeping fuselage walls parallel - to the trailing edge - will also improve shoulder room in the cockpit. I tried sitting in a WAR Sea Fury, but concluded that my 6 foot tall, 190 pound self was too tight a fit for a pilot emergency parachute with the stock seat. Even with minimal clothing it would have been difficult to bail out. A few extra inches of shoulder room would be a large improvement.

 

[Riggerrob]

I think I may need to compromise on both ends. Maybe a little faster stall for a little faster cruise. Hp will be in the 110-130hp range.

Remember that many builders will be low-time private pilots or rusty from flying too few hours while building. Ergo slow landing speed and good low-speed handling are important.
Definitely don't copy the original Focke-Wulf 190 wing as it had a nasty stall before flaps were lowered.

Does anyone have a pilot report on a stock WAR FW.190?
We are especially curious about handling during landing.

 

[WARPilot]

Keeping fuselage walls parallel - to the trailing edge - will also improve shoulder room in the cockpit. I tried sitting in a WAR Sea Fury, but concluded that my 6 foot tall, 190 pound self was too tight a fit for a pilot emergency parachute with the stock seat. Even with minimal clothing it would have been difficult to bail out. A few extra inches of shoulder room would be a large improvement.

We are thinking of adding a ballistic recovery for that reason. Also, a redesign could incorporate a seat that accommodates a softie parachute and is slightly adjustable.

 

[WARPilot]

Remember that many builders will be low-time private pilots or rusty from flying too few hours while building. Ergo slow landing speed and good low-speed handling are important.
Definitely don't copy the original Focke-Wulf 190 wing as it had a nasty stall before flaps were lowered.

Does anyone have a pilot report on a stock WAR FW.190?
We are especially curious about handling during landing.

Looks like the 23xxx series is the good choice unless I can find something that is significantly better in either slow speed or high speed regime.

 

[Lendo]

I would suggest you'd need more wing area and Span and a lot of Flap area to get those performance numbers. The Actual shape of the Aircraft doesn't inspire low drag and maybe good for landing speed but not good for high speed which would require a Laminar Airfoil, I would accept Billski's suggestion of a Riblett Airfoil which are very similar to the NASA 6 series Airfoils and look to reducing Drag wherever you can.
For low weight I would consider using Carbon composite.
George

 

[wsimpso1]

Here is a sample comparison from bigfoil.com:

So, who is flying the S8036 and how does it perform in the real world?