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Old Jul 25, 2014, 11:34 AM
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3 different profiles for a slow speed & high payload flying wing

Hello guys,

Further to long hours of an intensive research on the net, I am now focusing on those 3 profiles.

What I want is difficult to achieve as it could be contradictory :
- Chord is 250mm
- calm flights for aerial photography, no 3D flight
- slowest possible speed with high stability
- highest possible lift
- average wind in my area is zero in the morning, 40 km/h in the afternoon, zero in the evening.
The flying wing should be able to fly the whole day.

To me the choice narrows down to MH45 or E329, then CM/alpha makes me think MH45 eventually wins.
Which profile out of those 3 would you choose and why ?
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Old Jul 25, 2014, 03:21 PM
B for Bruce
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The 'Wack, BC, Canada
Joined Oct 2002
11,626 Posts
You've got some contradictory factors in your list.

For slow flying you need a light wing loading. For a fixed, or fairly fixed payload this means a bigger wing than typical for a given overall weight.

But a bigger and lighter model will be kicked around more in any turbulence which leads to more significant image shuddering.

A heavier wing loading will punch through such stuff more easily and be deflected less by any given single event. But now your flying speed, takeoff speed and landing speeds are all higher as well.

So we're left with a compromise situation. And that means you can't nail down any one single answer.

However, there is a solution. Start with the oversize design built strongly and build in ballast boxes at the CG location as well as the wing tips. With these you can play with the wing loading and the inertia moments of the wings to hopefully sneak up on a good compromise.

High stability will come from different factors. First is that you want to fly the photo runs with a wider angle lens if practical. Second is that you want to click in the dual rates for the photo runs and have them set so the throws are minimal. You pretty much want to reduce the throws in dual rate to where you're almost flying "on the trims". This will steady down the model and make your inputs less vomit inducing when seen on the videos.

A beginner often makes the mistake of setting the pitch stability to a high value by putting the CG forward and then using a lot of up trim to fly level. This setup gives a high degree of pitch stability in that the model will try to correct any pitch disturbances very strongly. But this also makes the pitching very choppy. It's actually better from a steadiness standpoint to move the CG back and re-trim the elevator so that the model pulls out of a hands off dive rather slowly. A model set up that way will be more steady in pitch response to disturbances yet still be stable.

Now for some misconceptions. There is no such thing as "highest possible lift". The wing lifts the weight of the model. No more and no less. What we do have is the option for a higher lift coefficient which means we can fly at a slower speed before the stall with one airfoil vs another. So technically the E329 is the winner. But your charts are ignoring the fact that the Reynolds number changes depending on the flying speed. So as the model slows down to where it is using the higher Cl values the Reynolds number also reduces. So for cruising flight you may want to consider the Rn = 200K. But for the higher Cl values you should likely be looking at the Rn=100K or even 80K curves. This is a bit of a guess of course since we don't know exactly how fast or slow the model will be when near the stall until it's built. But you can make some educated guesses so that you're making somewhat more valid studies.

Quote:
The flying wing should be able to fly the whole day.
Pretty much impossible without getting FAR more scientific and looking at using solar cells. Off the shelf motors, batteries and such simply do not have the power density and watts/lb density at this time to allow this with anything closely valid as a "model airplane". If you could pack in an 8 hour duration battery pack your "slow as possible" aspect would be right out the window.
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Old Jul 25, 2014, 03:54 PM
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Quote:
Originally Posted by BMatthews View Post
You've got some contradictory factors in your list.
Yes that is what I wrote.

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Originally Posted by BMatthews View Post
"the whole day long"

Pretty much impossible without getting FAR more scientific and looking at using solar cells.
Funny one. I meant "flying at any time during the day". My bad, my poor standard of English language

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Originally Posted by BMatthews View Post
There is no such thing as "highest possible lift". What we do have is the option for a higher lift coefficient
Yes, that what I mean

Quote:
Originally Posted by BMatthews View Post
So for cruising flight you may want to consider the Rn = 200K.
Absolutely, Because with the chord number I wrote, that means 10m/s
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Old Jul 28, 2014, 03:19 AM
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Ukraine, Crimea, Ordzhonikidze
Joined Jul 2012
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MH45. Big difference makes only Cm.

What about size, weight and A.efficiency of project?

For example: MH45 - different size = different efficiency, drag (thrust), Vz (gliding time).
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Old Jul 28, 2014, 08:57 AM
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Thx Drinkes.

That is for a 2 meter span slow flying wing / bird
Weight approx 1 kg
I do not know what "A.efficiency" means

Do you think MH 45 is a reasonable choice for that one ? I have built it in E329 but definitely not a good choice, although building is so approximative it is hard to blame anything else than my own competences...

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Old Jul 28, 2014, 11:45 AM
Herk
HerkS's Avatar
Virginia USA
Joined Jun 2007
1,652 Posts
Your design objectives - (what you want this aircraft to do) are not compatible with a flying wing.

To get the performance you want, a light weight craft with conventional tail will work better.

If I was to build this aircraft, I would use a Drela airfoil.
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Old Jul 29, 2014, 04:59 AM
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Ukraine, Crimea, Ordzhonikidze
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Or conventional 2 meter craft and ClarkY 10%. profile.
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Old Aug 20, 2014, 11:05 PM
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winnipeg
Joined Jan 2004
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If you could describe all aircraft in one word it would be "compromise".

With a flying wing, you get to a point where a thick high camber wing becomes self defeating because of he necessary reflex necessary to combat the pitching moment. And in this case you are comparing apples to oranges with these two airfoils. Choose airfoils that have the same pitching moment because any performance comparisons become invalid if you do not.
On paper the MH looks good, but try to build that trailing edge accurate! If you don't make that one in a mold forget about a true surface.
If you insist on using a flying wing, try using something around 10 percent thick with a wide speed range. This will go a long way in giving you a versatile performing airframe.
If you are serious on getting the best airfoil for your needs, get an airfoil program to be able to compare them. I use Profili, it has a handy feature called "show airfoils similar to the selected one".
I have used it to design my own flying wing airfoils, and the performance is very close to predictions. (it uses the xfoil kernel)
Best of luck.
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Old Aug 21, 2014, 11:26 AM
I bail out, anywhere, anytime
Taurus Flyer's Avatar
The Netherlands, OV, Almelo
Joined Nov 2010
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Quote:
Originally Posted by philgib View Post
Hello guys,

Further to long hours of an intensive research on the net, I am now focusing on those 3 profiles.

What I want is difficult to achieve as it could be contradictory :
- Chord is 250mm
- calm flights for aerial photography, no 3D flight
- slowest possible speed with high stability
- highest possible lift
- average wind in my area is zero in the morning, 40 km/h in the afternoon, zero in the evening.
The flying wing should be able to fly the whole day.

T
Phil, did you discover three different airfoils are often used in one wing?

I thought it was this the thread was about reading: different profiles for a slow speed & high payload flying wing



In the past I did use for a glider Eppler 205, Eppler 203 and Eppler 193. (well known combination)
Center section thickest (in your case for the payload), tip thin.
In a Horten other airfoils are used, a-symmetrical in the center section, symmetrical at the tip. We can write a book how to transfer the a-symm of the center to the symm at the tip. Because what is the airfoil at 50 % for example. Or can we design with three airfoils? Yes, that's also possible.
TF
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Old Aug 21, 2014, 11:05 PM
TheyreComingToTakeMeAway!
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USA, ID, Coeur D'Alene
Joined Dec 2003
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build it light and thin, then add a stabilizing computer with accelerometers so it can keep the wing flying to the lowest possible speed.
a good example is leadfeather's wing here:
Flying wing with gyro stabilization (2 min 37 sec)


in his video it was only on the elevator so that the gyro was not trying to counter every movement, but for video or photography, pitch and roll might be useful.
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Old Dec 08, 2014, 08:40 AM
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Sorry for late answer. Thank you very much guys !

So Taurus, are you saying
- Eppler 205 at the center
- Eppler 203 at 50%
- Eppler 193 at the wingtip ?
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