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Discussion Thread: The design of a tailless flying wing

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Aerowerx

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As I understand it, scaling up is a completely different problem from scaling down, as most powered models are over powered. Which I think would mean that the engine by itself does a lot of the lifting.
 

Topaz

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As I understand it, scaling up is a completely different problem from scaling down, as most powered models are over powered. Which I think would mean that the engine by itself does a lot of the lifting.
Models typically have a much higher thrust-to-weight or horsepower-to-weight ratio than full-scale aircraft. That factors into the same performance equations we use for "real" aircraft. A C-150 would perform a lot better with three times the power for the same weight, too, and the engine isn't doing the lifting. Not directly, anyway.
 

pictsidhe

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Re: Visualizing vortex and downwash

Just had a thought....take something like the fact mobile, except smooth it out, and extend the wings outward with sweep.
You're not the first. I liked the idea of the facetmobile, but it needed more span and a higher AR for my far103 plans. I'm looking at a faceted lifting body, with foldable wings sticking out. I'm not planning to smooth the centre. Wings I'm currently thinking d box then ribs/fabric. On my smartphone too, it sucks!
 

Aerowerx

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I don't know why I hadn't thought of this before now, as it seems rather obvious. I have certainly known the theory.

As you may have read in my other posts I have been working on a tailless aircraft design. So far the most successful ones have had a linear taper wing with a pod or fuselage. But I think a quasi-BWB* would be better, and certainly looks neat.:)

The problem is the center chord has to be 9 or 10 feet at a minimum, for a single seat. More for a tandem seat.

I have been using the PRANDTL-D as a starting point. If I keep a linear taper, it actually has quite a bit of latitude for the wing shape, aspect ratio, etc. But when I stretch the center chord, it destroys the nice bell-shaped lift distribution.

What I have discovered, however, is if you stretch the root chord you can lower the twist angle in proportion. For example, the PRANDTL-D has an incidence at the root of about 9 degrees. If the reference wing has a 4 foot root chord, and you stretch it to 12 feet, lower the twist to 3 degrees. This ends up with the same amount of lift, since CL is linear (for the most part) with the AoA.

This can be continued further out on the wing, until you have sufficient room to stuff in a cockpit, engine, fuel, baggage, etc.

It may also be possible to adjust the AoA in this method for optimum CL/CD.

I have just started looking into this, but will report back with further results.


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*Quasi-BWB: looks something like a BWB except there is a cockpit canopy and engine pod sticking out of the top.
 

Aerowerx

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Thanks, SJ. I will be sure to keep everyone informed.

What I meant was to do the "adjustments" out to 1/3rd or 1/4th of the semispan as needed, then continue with the linear taper and PRANDTL-D twist.

I have been getting some CL/CD values of over 30, by the way.
 

pictsidhe

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I've been thinking along similar lines. I'm currently looking at very thick airfoils for the body part, there are some boundary suction airfoils up to around 40% thickness. This is a possibility as a bwb is very low drag and a ducted fan won't be too inefficient at the power and size I'll need. Look for suction airfoil papers by Light hill and Glauert. It seems the British were quite interested in this in the 40s and 50s. On the US, Goldschmied had a boundary suction airship. A more modern investigation is here: https://www.google.com/url?sa=t&source=web&rct=j&url=http://www.lr.tudelft.nl/fileadmin/Faculteit/LR/Organisatie/Afdelingen_en_Leerstoelen/Afdeling_AEWE/Aerodynamics/Contributor_Area/Secretary/Publications/publications/Walvd_100824_Thesis.pdf&ved=0ahUKEwiHo-7Ss9vPAhVMxoMKHQ4BBqUQFggeMAA&usg=AFQjCNGjPTJ6gCfiBSdWhClUzyEojh3_bA&sig2=VXrJKjd5rKI08graCiHD4A
 

Aerowerx

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The problem is that you need low pitching moment for a tailless flying wing, and all of them I have seen are 12% or less thick, unless you can come up with your own design.

What would the drag be on a 40% thick airfoil? As far as the boundary suction idea, that sounds rather complicated to me, and what about the extra weight?
 

pictsidhe

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The very thick airfoils are draggy without suction, with suction, less drag than a thin non laminar section. The laminar suction airfoils make everything else look like bricks, but I won't be building to laminar smoothness myself. Some of the Glauert laminar suction airfoils were designed for Cm =0. Ive not found enough convincing data to be sure of the idea myself yet. With a ducted fan, it's a matter of having the air intake in the right place. I wouldn't suck the whole wing for my project, maybe the centre 1/3, which is half the lift. I'm constrained by a low power off stall speed, so I have an extra design ball to juggle. Tailless wants lots of wing area unless I can use some sort of high lift trickery. That won't hurt cruise too much, but makes construction harder and probably has a surface accuracy penalty. I'm expecting to need to do some truck 'windtunnel' experiments.
 

Aerowerx

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My latest iteration....
Capture.jpg

You can see the extended center section chord. This has a tremendous beneficial effect on the longitudinal stability. I guess it is acting like a horizontal stabilizer.

In this picture I have added a canopy and engine pod. This is the first iteration of the canopy/pod, but you can get the idea. As you can see, there is more work to be done.

The picture shows the air stream coming off the structure. Lots of turbulence behind the engine pod==lots of drag! With the wing by itself, everything is nice and smooth. As I have said before in other posts (not necessarily in this thread), XFLR5 is not a CAD program, and doing structure design, even in outline form like this, is clumsy at the best.
 

StarJar

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Looks interesting.
Maybe with the small sweep ° you might find some of the Marske airfoils beneficial?
BTW are you still getting lateral stability with no vertical tail? If you say so, I believe it, and that would be an amazing feature.
 

Aerowerx

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...are you still getting lateral stability with no vertical tail? If you say so, I believe it, and that would be an amazing feature.
Don't confuse stability with damping.

A properly designed flying wing with BSLD and proverse yaw is stable. That is, it will return to its original orientation after a disturbance. The question is, how long does it take?

Using MIL-F-8785-C as a guide, yes I am getting decent parameters with the above configuration. At least with the wing by itself. The XFLR5 stability analysis does not like something about the pod and canopy in the current configuration. Will have to work on it.
 

henryk

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My latest iteration....
View attachment 55467

You can see the extended center section chord. This has a tremendous beneficial effect on the longitudinal stability. I guess it is acting like a horizontal stabilizer.

In this picture I have added a canopy and engine pod. This is the first iteration of the canopy/pod, but you can get the idea. As you can see, there is more work to be done.

The picture shows the air stream coming off the structure. Lots of turbulence behind the engine pod==lots of drag! With the wing by itself, everything is nice and smooth. As I have said before in other posts (not necessarily in this thread), XFLR5 is not a CAD program, and doing structure design, even in outline form like this, is clumsy at the best.
"turbulence behind the engine pod==lots of drag!

=long shaft can help !
 

Norman

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Just remember that that formula is 60 years old. A lot has been learned since then. Horten used an 18 or 19% thick airfoil section at the root of many of his planes but LD of his sections was not very impressive compared to later designs. Here's an XFLR5 comparison of a 19% thick airfoil I produced with that spreadsheet some time ago with one of my whales and a version of the NACA 23112 that I blew up to 19%*. I hope the graph is still legible after the site compression gets through with it. I'm also attaching the coords of my whale. As usual you'll have to change the extension to .DAT

Oops forgot to change the airfoil to be analyzed to the new one after I thickened it so the curve for the 23112 is for the 12% version not the 19% version. The thickened 231 is much closer to the 19% Horten.
 

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StarJar

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That's a good point Norm.
Also, I think the MH-78 was designed to be used in Horten type layouts and is also an excellent 'update', if you will.mh78ko1.gif

If your sweep is less than about 15-20°, the Marske airfoils have excellent Cl max and L/D, and have more positive moment.
The Marske airfoils range from about 10-13% in thickness, and can get modified up to about 14%, I think.
 
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