Aft tailed designs can have their minimum trim drag with a positive stability margin. Having the lowest trim drag and a stable configuration at the same time is a great advantage.

http://aero.stanford.edu/Reports/MultOp/multop.html

A stab span of about 20% of the wing span, with an aspect ratio half that of the wing, and a small positive static margin will minimize drag with an aft tail.

Kevin

Originally Posted by ShoeDLG

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.

Yes you can -
However to have a plane which is maneuverable yet has a good sense of dierction- a setup with some trim drag is much easier to handle. I don't know what types model you prefer to fly but a setup which has a locked in feel always has some trim drag

Bruce,

Minimum trim drag does not usually occur with zero load on the stabilizer. For most aft-tailed configurations, the overall aircraft drag is actually lower with a slight down load on the stab. This occurs because the stab is in the downwash field of the wing. The interaction stab in the tilted flow field means the airplane makes less drag if it the stab lifting down slightly - the stab negative lift vector is rotated forward by the downwash.

An electronic stability system doesn't have to have trim drag in the conventional sense. Let's take a plank wing design for example. If the CG is behind the wing AC (usually taken as the 1/4c), then there is fixed nose up moment at 1G unaccelerated flight. To balance this nose up moment, the electronics must control the wing Cm to provide a nose down moment. The wing nose down moment is dependant on the dynamic pressure and the wing Cm (M = Cm*c*q; q = 1/2 * ro*V^2).

When flying fast, the wing only has to have a very small negative Cm because the dynamic pressure is high (the V squared term), so the flap may actually have to be reflexed on a cambered airfoil. When flying slowly (low dynamic pressure), the wing has to develop a large Cm. The flap will have to be deflected downward, making a highly cambered airfoil, which is what you want for low speed flight.

Of course this set-up is pitch unstable with out the electronics to artificially provide stability.

If properly designed, the minimum airfoil drag for a given airspeed can occur with the average camber setting required for moment balance. This means the trim drag is basically zero, and the airfoil camber is optimized for the airplane speed. This can be done without any CG shifting.

Conventional sailplanes use airfoil camber changing to optimize the airfoil for the flight speed as well. They use their powerful stabilizer to balance the wing moment changes with minimum drag.

Kevin

Originally Posted by BMatthews

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.



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.

Kevin,

I agree (and Ilan's chart is a lot to digest!). All I was trying to suggest is that trim drag does not inherently improve longitudinal flying qualities.

It's interesting from the chart that for an aft-tail design with a span ratio of about 3 to 1, the static margin for minimum drag (about 8% or so) also happens to be a good choice for longitudinal flying qualities!

Shoe,

No argument with your points.

Pretty hard to beat that aft tail for performance, stability (dynamic and static), and handling qualities!

Kevin

Ummmmm... I was still stuck on the plank style flying wing thing. Did I miss the segue over to tails?

you were not supposed to notice --
I am still waiting for the tailless thingy (plank), which has no trim drag.

It just a hypothetical design richard I'm just trying to figure things out

Originally Posted by b36roxs

It just a hypothetical design richard I'm just trying to figure things out

I don't know how much of the Gizmo thread you read,but the foil shapes for the centre section were designed by Peter Wick.A pm to either Peter or the guy who built it may help?Just a thought.

Originally Posted by b36roxs

It just a hypothetical design richard I'm just trying to figure things out

Consider - that for a wing to operate - SOMETHING must hold it at the desired angle as it progresses thru the air.
In the case of the "flying wing"- which has no other device than it's own shape - the shape must obviously be such that it will hold this desired angle

that's trim /trim drag- - can't avoid it
calculate all you like - it has to be .

Originally Posted by richard hanson

In the case of the "flying wing"- which has no other device than it's own shape - the shape must obviously be such that it will hold this desired angle

Or the pendulum effect. Paragliders and some early hang gliders have very "bad" airfoils for flying wings - fully undercambered with no reflex. But the very low CG stabilizes them. Of course, if one goes this route one will be increasing drag anyway due to the added fuselage below the wing.

Also, I don't get why people started arguing about the impossibility of eliminating trim drag. Of course it's impossible. But that is not the topic of this thread. It's about REDUCING trim drag. Specifically for flying wings. And it's definitely possible with the right airfoil design and careful CG positioning (and perhaps some active stabilization to make the wing flyable at a CG that has minimum trim drag). Compared to a competition thermal duration wing, sure, you can't improve much because they've already been optimized to have minimum drag (in fact, doesn't their existence prove that it can be done?). Compared to a Zagi or Assassin you can certainly make significant improvements.

as speed changes -so must trim ----and trim drag is the inevitable passenger

How about -

DARPA Oblique Flying Wing (0 min 26 sec)

I saw a squadron of those the other day----

I have seen an RC oblique flying wing fly. It used AOA feedback for longitudinal stability (and several servos per elevon to ensure enough bandwidth). Its maiden flight was quite smooth (sweep was increased beyond 45 degrees). All without any trim drag (well under certain conditions anyway).

http://www.b2streamlines.com/Morris.html