# Slow delta wings?

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#### BDD

##### Well-Known Member
GTEX-09

What was the cause of the instability which caused the crash? C.G. location, airfoil selection, some wing or elevon configuration issue, etc?

#### gtex09

##### Active Member
Hi BDD,
Well, I guess that was because I made too much changes in the plan shape between the 05 and 06 version....
Actually, if you look in detail to v06, you will see that the wing size is really much more smaller than the v05, for te same length of airplane.
It was much more sensible to the CG location than the previous version.
That's the reason why I came back to the same plan form for the v07 version.....
GTx

#### deskpilot

##### Well-Known Member
Hi guys, been a while since I last posted. Here are a few images of my Eagle-Ray. Still a long, long way to go. The green represents F/glass although the turtle-deck side walls and all under side surfaces might be fabric as there are no con-cave curves included. The frame is riveted alloy, either square or rectangular.

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#### steveair2

##### Well-Known Member
Log Member
That's really cool Doug. Hope to see more progress.

#### ThadBeier

##### Well-Known Member
deskflyer, this is a remarkable progression of design. I am a huge fan of low-aspect ratio airplanes -- I think they have a lot to recommend them. They can be light, very strong, simple to built, have a lot of interior volume, and aren't too fast. The Arup planes were an early example, and yours might be the most recent one. Another example of a low-aspect ratio plane, not completely unlike yours, is the Pelican -- see pelican

I agree with your analysis that your wing area will have to be rather large to get the stall speeds that you want. The Vermeer wing mentioned upthread stalls at way too high a speed, but it's tiny -- 10 ft long and about 15 ft span. (3.3m by 4.5m) Making it a bit bigger is a good idea. His design was a true delta (except that it isn't swept enough, I suppose) and he needed the dihedral to eliminate dutch roll. Your wing has much less sweep, and probably won't have the odd rudder response that the Vermeer wing has.

Good luck!

Log Member

#### deskpilot

##### Well-Known Member
Thanks guys. Haven't done much lately as I got side tracked with the Ring Tail project, another 'out there' design. Maybe I'll build something one day instead of just dreaming.

#### ThadBeier

##### Well-Known Member
I've been thinking about your design for a while, and have come up with something similar for similar reasons. I am struck by how similar this is to the RANS S-11 Pursuit -- check it out!

#### deskpilot

##### Well-Known Member
I've been thinking about your design for a while, and have come up with something similar for similar reasons. I am struck by how similar this is to the RANS S-11 Pursuit -- check it out!
A few years back, I saw a picture of the S-11 but it was parked behind another aircraft, and wasn't referred to in the write up. Since then I've tried to find out what it was and yesterday, I finally found it. A very nice looking plane and it's a great pity it didn't make it into production. I'm sure it would have been a winner in only they could have solved the problem that caused the second aircraft to crash (seniors moment)

#### orion

##### Well-Known Member
The aircraft was plagued by control and stability problems due to the wide fuselage. This is what I try to warn folks about when they endeavor to design aircraft with strakes and wider bodies. The geometry can set up various forms of vortex flow, which tends to be unpredictable and very destabilizing due to non-linear behavior. Secondary issues also include pressure shifts not coordinated with wing characteristics and blanketing effects.

#### Topaz

##### Super Moderator
Staff member
Log Member
Everyone was excited about the S-11 when the proto started making the airshow and magazine circuit rounds. The big question was, "When are you going to release kits!??!?" The company played it cagey, saying "this is only a concept demonstrator" - for a transport airplane, of all things. Later it was revealed that the airplane was a real handful to fly, for the reasons Orion states above.

#### Lucrum

##### Well-Known Member
Log Member
I don't even pretend to be an Aerodynamicist but just eye balling it I don't care for how the forward fuselage is blended on the S-11.

#### ThadBeier

##### Well-Known Member
Everyone was excited about the S-11 when the proto started making the airshow and magazine circuit rounds. The big question was, "When are you going to release kits!??!?" The company played it cagey, saying "this is only a concept demonstrator" - for a transport airplane, of all things. Later it was revealed that the airplane was a real handful to fly, for the reasons Orion states above.
Back in the day, there were not the computer power or the publicly-available software to do simulations of these vortex flows. Would it not be possible today to design a plane that could reliably take advantage of them, rather than have them make the plane difficult or dangerous to fly? And I know, I've seen the abomination of the vortex-breaker on the F/A-18, that was made necessary by exactly these same kinds of missed-predictions (even with the resources of Northrup to do the simulation and testing.)

#### orion

##### Well-Known Member
Possible? Maybe. Affordable? Not to the small airplane company. But the biggest problem here is the lack of applicable design experience more so than simulation software or other technologies. The S-11 was a radical departure from their regular product line and done by a seemingly unqualified designer to take on something that unique.

#### TFF

##### Well-Known Member
If I remember the first S11 crashed after takeoff of an engine seizure almost killing the the head honcho, and the second plane crashed after 250 hours when a regular employee decided to test the slow flight and spun it in. I know a third was built but with such bad karma it was probably time to quit.

#### delta

##### Well-Known Member
Hi Doug

I'm going to second the motion that your CG will be to far forward in your current configuration. I believe it's marked close to right in your drawing, but in full scale I don't think the aft portion of your airplane will offset the weight of the engine and pilot. Build some rubber band thrown flying models with paper, balsa, and then a powered RC to fine tune your CG. You might consider a mid engine, a push pull twin, or a really light pusher to keep your cranked arrow configuration. I think you will find that it will fly nice, land on a dime, but be anything but slow.
If the tractor configuration is more important to you than the cranked arrow, then you can have an unstraked leading edge. You will probably wind up with more than a 60* sweep. I've gone from 45* to more than 90* with RC and the more sweep the further aft the CG has to be.

Later...

Rick

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