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Old Feb 11, 2013, 09:41 PM
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Reduced trim drag on a flying wing?

Hi all!

I was day dreaming earlier and I was thinking about flying wings and the amount of up trim/reflex needed to fly correctly. Normal aircraft require downforce on the tail to lift the nose up and canards lift the nose up but have close to the same drag could there be some half-way point? My reasoning is that if there was a way to achieve lift infront of the CG like a canard without the seperate surface, it would have less overall drag than a seperate stabilizer and wing arrangement (conventional tail/ canard/or delta). So I drew this up in my sketch pad and I'm not quite sure if it would work how I imagine. What do you think? why/why not?
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Old Feb 11, 2013, 11:20 PM
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Originally Posted by b36roxs View Post
Hi all!

I was day dreaming earlier and I was thinking about flying wings and the amount of up trim/reflex needed to fly correctly. Normal aircraft require downforce on the tail to lift the nose up and canards lift the nose up but have close to the same drag could there be some half-way point? My reasoning is that if there was a way to achieve lift infront of the CG like a canard without the seperate surface, it would have less overall drag than a seperate stabilizer and wing arrangement (conventional tail/ canard/or delta). So I drew this up in my sketch pad and I'm not quite sure if it would work how I imagine. What do you think? why/why not?
Why not just move the CG back if you want more lift in front of the CG?

Many aft tailed airplanes have stabs that lift, or have near zero load either way. A download on the stab is not a requirement in any way. The wing lift on most aft tailed designs is ahead of the CG.

The CG must be ahead of the aircraft neutral point for pitch stability. That is true for any flying wing shape, tailed or canard airplane you care to draw.

Reducing the trim drag on flying wings could be done by having the CG behind the aircraft neutral point. The airplane would then be pitch unstable. It would need electronic stabilization of the angle of attack to be flyable.

Aft tailed designs fortunately have minimum trim drag with a slight down load on the stabilizer, so that is the way they are typically designed. Canards have higher trim drag for any pitch stable CG location, along with other drawbacks. Flying wings are very short coupled and tend to have sweep, poor lift distributions, and/or compromised airfoils for pitch stability. Aerodynamic pitch stability in flying wings always has a relatively high drag penalty. A neutrally stable flying wing can have low trim drag, but likely requires wing sweep to provide some dynamic damping to keep the pitch rate to something a human can keep up with.

Biber's Multibumm is probably the lowest trim drag flying wing that can be flown without electronic stabilization (neutrally stable):

http://www.rcgroups.com/forums/showthread.php?t=534401

Electronic stabilization of pitch unstable planks has been done (CG behind the aircraft neutral point).

Kevin
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Old Feb 13, 2013, 06:09 AM
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As Kevin points out, there are two basic things you need to do to make a flying wing fly.

1. Provide sufficent longitudinal stability. This means positioning the CG relative to the Neutral Point such that a pilot can maintain control in pitch. Further forward makes for a more stable airplane, further back makes for a less stable airplane (too far either way generally makes for unfavorable flying qualities). Pilots can control wings that are unstable in pitch (to a certain point), but it demands significant attention. Feedback control of elevons can be used to make an airplane with an unstable CG postion appear to be pitch stable, but there are limits here as well (limits determined by the bandwidth of the AOA sensing system and elevon actuators) .

2. Trim for steady flight. With the CG positioned for sufficient stability, the aerodynamic moments about the CG have to sum to zero in steady flight. For an unswept flying wing to be statically stable, the CG generally has to be in front of the 25% chord line. With the CG forward of the 25% chord line, you will generally need reflex in order to achieve trimmed flight (zero pitching moment about the CG).

With twisted swept flying wings you can achieve basic and additional lift distributions that allow for longitudinal stability and pitch trim without any reflex. This is one of the rare instances where it makes sense to (moderately) sweep a low-speed wing.

Your design could possibly meet both the stability and trim requirements through twist (without any reflex) because of the sweep.
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Old Feb 13, 2013, 11:07 AM
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How com-- people think trim drag is a bad thing?
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Old Feb 13, 2013, 12:56 PM
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How com-- people think trim drag is a bad thing?
Because it degrades performance.
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Old Feb 13, 2013, 01:32 PM
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http://www.rcgroups.com/forums/showthread.php?t=1781085
Relevant?
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Old Feb 13, 2013, 03:28 PM
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Because it degrades performance.
eliminate all trim drag - - now how does it fly?
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Old Feb 13, 2013, 06:04 PM
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Stupot ya just like that but with a bit more forward area, I feel like there could be some alternative to using electronic/digital stabilization (more weight, complexity). On a flying wing I think alot of performace could be gained from eliminating or reducing reflex/trim drag especially during low speed flight (higher lift coefficient?)
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Old Feb 13, 2013, 06:31 PM
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sure with a computer on board you can do wonders - I DO have models with 3 axis stabilization and I also have model which can be flown by hand with cg way aft the cp- -but only at very low speeds
In the real world - the trim drag is your friend - it keeps it all pointed the right direction
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Old Feb 14, 2013, 12:01 AM
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With active stabilization you can fly with a rear enough CG as to need down elevator trim. Big birds like vultures, storks and eagles do this all the time on landing approach. They shift their CG back by sweeping their wings forward making them tail heavy so that they can fly with their tails angled down. This allows them to use their "elevator" as a flap in order to slow down for a landing.

Several RC models have been tried with very neutral CG and actively stabilized. Some by gyros some by using small weathervanes to sense the actual angle of attack. Most have been successful as projects but have not gone on to be tested in competitions etc. I guess active gyros may run afoul of competition rules. So I guess we may never know if they perform better or worse then regular flying wings (or even regular gliders for that matter).
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Old Feb 14, 2013, 12:30 AM
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Originally Posted by richard hanson View Post
eliminate all trim drag - - now how does it fly?
Just fine. There are plenty of stable configurations with a tail that pushes neither up nor down in steady flight.
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Old Feb 14, 2013, 08:31 AM
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OK how many of these setups have you flown?
trim drag is a std part of most flyable models .
I have flown the exceptions - but they are exceptions .
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Old Feb 14, 2013, 10:43 AM
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Quote:
Originally Posted by b36roxs View Post
Stupot ya just like that but with a bit more forward area, I feel like there could be some alternative to using electronic/digital stabilization (more weight, complexity). On a flying wing I think alot of performace could be gained from eliminating or reducing reflex/trim drag especially during low speed flight (higher lift coefficient?)
You might put in active stabilization but that might not eliminate the trim drag. If the model wants to pitch and you need to move anything to counteract this then the drag of those displaced surfaces is still trim drag. The route to minimizing the trim drag is to use an airfoil with a slight but still positive pitching moment. Then set up this airfoil with full span elevons. Finally set the CG at the point where the model isn't quite stable OR unstable in pitch and use the active stabilization to continuously trim the full span elevons and possibly a small movable mass to alter the CG as a trim function to provide the missing degree of stability. Such a setup would be very nicely slippery.

Now it may seem like an Active Stability (AS) setup and a stronger lifting airfoil that has a negative pitching moment would be the way to go, right? But consider that when you're pushing your higher max lift airfoil at lower Cl's that you get a lot of force trying to pitch the wing down. To counteract this pitching moment the AS has to either reflex the elevons, and thus you change to a new airfoil, or it has to shift the CG back to where the wing remains flying at the correct angle of attack. This might seem like a great idea. But there is still going to be trim drag associated with supporting the CG at this strongly aft point to counteract the force trying to pitch the wing nose down. The energy needed to hold the wing from doing this has to come from someplace. And that place is the trim drag.

So we come back to the idea that the best airfoil and CG location is the one which is just barely neutrally stable. Such a system will require the least amount of AS. Mostly just to make the model fly more easily out at greater distances.

Quote:
Originally Posted by ShoeDLG View Post
Just fine. There are plenty of stable configurations with a tail that pushes neither up nor down in steady flight.
Shoe's got you on this one Richard. Years back I had the old DOS based David Fraser sailplane program. One of the columns of data was the Cl for the stabilizer. I was surprised at how easily a model with a "normal" planform and "normal" CG location would result in the stabilizer operating at near zero lift during level gliding flight. Based on that I'm pretty sure that there's a surprising number of regular sport models flying around with zero to near zero tail load for at least part of their straight and level flying.
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Old Feb 14, 2013, 11:36 AM
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I have a bunch of models with very nuetral setups BUT they are not easy to fly - for the typical powered flight Sunday flier -
Sailplanes are just one facet of model flying and as you note can be setup to fly using a very neutral arrangement .
A powered aerobatic design can also use a very neutral setup -but it's all a trade off -
BTW I have a new RTF setup coming which has a 3 axis stability system - it is called the VisionAir - - from Horizon- these setups are simply astounding at how well they correct for minor deviations yet allow full maneuvering.
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Old Feb 14, 2013, 12:17 PM
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I think you're confusing neutral stability with zero trim drag. You can have a longitudinally stable airplane that requires no up/down force on the horizontall tail.
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