# Theoretical question about wing thickness

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by geosnooker2000, Jun 10, 2019.

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1. Jun 19, 2019

### Scheny

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For my project, the biggest problem turned out to be the Reynolds number.

Due to the already tiny wing, the even smaller chords at the tip would stall already at landing speeds. Because of this, the competition is using Hershey bar wings.

Our profile is more than 20% better in L/D at landing speed with even more forgiving stall than NACA 64xxx while performing the same at cruise speed.

Sometimes it makes sense not to concentrate on faster / higher to get the better product.

2. Jun 20, 2019

### pictsidhe

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What are you using to analyse your foils?
I use xflr5, but it isn't too accurate. I am hoping that the error between my outer and root foils is proportional...

3. Jun 20, 2019

### Heliano

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Good point, pictsidhe. But remember that the P-40, as well as many other second world war aircraft, had frise ailerons which were slotted. You can see the details in NACA's Wartime Report L5A29a "Measurement of Aileron Hinge Moments and Aileron Control Characteristics of a P-40f Airplane". Slotted ailerons seem to be a very important benign factor when it comes to stall characteristics. As for the Uirapuru, the ventral fin helped but did not solve the problem 100%. An Uirapuru with ventral fin could not recover from a spin during the production flight test (6-turn spins to both sides during production flight tests were required by Civil Aviation after the crashes). Luckily the factory pilot managed to bail out and open the parachute)
And thank you, pwood66889, for your headsup on the Ercoupe. Very informative.

4. Jun 20, 2019

### simflyer

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18% airfoil, used on CH601, works surprisingly well, but also some other turbulent airfoils as TsAGI R-IIIa-15 are working very well - look for YT videos of Aeroprakt A22 Foxbat. TsAGI R-IIIa-15 is thick enough to build strong and light wing, which perform and save almost STOL chars (could share coords). Other TsAGI airfoils could be interresting too.

5. Jun 20, 2019

### Scheny

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For myself I also use XFLR5, but the profiles have been calculated by a SW from Dr. Eppler and been validated in wind tunnel too. I am just using them and I got some charts showing what differences to expect in real life. The foils have been developed for scale model fliers who ran into low Re problems. They look just like NACA 64xxx at the first glance, but perform wayyyyy better at low speeds. I never have seen them on one of the big sites as airfoiltools.com or incomplete guide to airfoil usage. Maybe the will if I manage to build the Beast One.

Best regards, Andreas

6. Jun 22, 2019

### pwood66889

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You're most welcome, Heliano. Have over 300 hours Pilot in Command time in marque, and love to chat.

7. Jun 22, 2019

### Pops

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Have owned 2 Ercoupes, one with metal wings and rudder pedals and one with fabric wings and no pedals. Have flown 4 Ercoupes. My first airplane was a Ercoupe when a student pilot and my buddy was also a student pilot that owned a Smith Miniplane. ( the only taildragger that I have ever groundlooped). We had the same instructor. He flew a Pitts in competition, so we got all the commercial maneuvers and spins, etc , while student pilots.
We would play and get on each others tail and try to shake the other off. The Ercoupe had an advantage. I could go in a dive, then a chandelle, roll out at min airspeed and then a sharp turn to the outside of the chandelle with full up elevator and high power in the sharp turn hanging it on the prop at low airspeed. The Smith that was behind me had to go past and then I could dive on him and catch up and be on his tail. Also at a full power dive I could always run away from him.
Have tried to spin the Coupe with rudder pedals many, many times. The most I could get was just a spiral dive.
Even with non-coupled rudders, the rudder has very little travel to the inside so you mostly have one rudder with any amount of throw to the outside.
For an Ercoupe, I'll take one with fabric wings and non-coupled rudder pedals. C-85 with the stroker crank, flat windshield and stock panel. Can enlarge the length of the down stops on the main gear for a couple MPH increase. ( Can do a mild slip ). Having the window down and sticking you hand out on the wing helps in the slip
Like 66889, I always liked the Ecroupe airfoil.

Last edited: Jun 22, 2019
8. Jun 22, 2019

### pwood66889

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"Naughty, Naughty..."
The 7-degree wing dihydral adds to that "Characteristically incapable" business. Then there is "Tail volume." There is not that much back there on the coupe.

9. Jun 22, 2019

### Pops

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Don't tell me you have never tried to spin an Ercoupe

10. Jun 23, 2019

### pwood66889

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Never had to, Pops. The CFI's did that for me on the BFRs. Didn't you have to break yours in on the coupe?

11. Jun 23, 2019

### Pops

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BFR's are a recent thing for me That was a long, long time before BFR's. I started my flight instruction in a 1953 Piper Super Cub and bought the Ercoupe while a student pilot. Took the check ride for my private in the Coupe with rudder pedals.
Never had a tailwheel endorcement either. Been flying tailwheels all of my life. Grandfathered in. Same for my old flight instructor, been a flight instructor since 1937 and was grandfathered in when it was required for a flight instructor to have an instrument rating.

Last edited: Jun 23, 2019
Jerry Lytle likes this.
12. Jun 23, 2019

### TFF

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I think it was a good chance you knew how to use your combat Ercoupe better than your opponent. You probably would have been just as deadly swapping planes.

13. Jun 23, 2019

### Pops

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Combat Ercoupe LOL . That is a good one

After the Smith, my buddy went to a Pitts and flying cross country in formation with me, he would be tucked in close, inverted so much I don't know how he stood the blood rush.
He would do snap rolls with the Smith on takeoff when a student pilot. He got the nickname of Suicide at that time. Don't know how,but he is still alive. Some people are just lucky I guess.

Last edited: Jun 23, 2019
14. Jun 23, 2019

### pwood66889

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"BFR's are a recent thing for me..."

"Don't know how, but he is still alive."
Yeah, the "Old Pilots" and "Bold Pilots" thing. Will admit to not being "bold." That's why I like the Ercoupe.

Pops likes this.
15. Jun 24, 2019

### Pops

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If I just had an Ercoupe in all of my life, I would be happy about it. I always said, If you don't like an Ercoupe is because you don't have 25 hrs in one, then you will be an Ercoupe lover.
My last Ercoupe project in 1992. Took first place in the National Ercoupe fly-in a few years latter.
I can't imagine a better airfoil than what is used in the Ercoupe for the purpose it was designed.
I still remember the first Ercoupe that I saw. Taking off at an airport were I was loafing when I was 15 years old.

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16. Jun 24, 2019

### pwood66889

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"If you don't like an Ercoupe (it) is because you don't have 25 hrs in one; then you will be an Ercoupe lover."
I concur over a hundred percent, Pops. Mind if I use this? Remember what the N-number was?
<plug type="Shameless"> They are meeting in Knoxville, Iowa, this week. </plug>
Now back to our regular programming...
Believe I stated the 13% thickness was the best that the head designer of the Ercoupe came up with. Every thing is a compromise.

17. Jun 25, 2019

### pictsidhe

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The Ercoupe is a great example of getting everything to work harmoniously. Individual parts slapped on to something else could work horribly. It's airfoil is hardly universally loved. But it works great in the Ercoupe.

18. Jun 25, 2019

### TFF

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And you get to claim your aircraft was the first with JATO.

19. Jun 25, 2019

### BJC

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Bottom line: Aerodynamically, airfoils are optimum around 12%. Using a thicker section usually is driven by structural considerations.

BJC