CG Position for Delta Wing

[jez_uk1]

Hi Guys

I've made a small Delta winged aircraft with an SC .25 engine on the front. It is attached to a dolly which will be left behind once it gets airborne.

I'm almost ready for the first flight but I thought it would be a good idea to get some advice on the correct CG position. I've attached a couple of images that I've created to represent the Aerodynamic Centre (AC) in relation to the actual zero-fuel CG position.

The first image shows the AC point without considering the fuselage as part of the wing area. The second image includes the fuselage as part of the wing area. In both cases the CG is slightly forward of the AC point (by roughly 10% in the first instance).

Do you think this is far enough forward and is there anything else I could measure? Any comments appreciated!

[macboffin]

Include fuselage as part of wing area ; not too important in this case since nose is quite short, would make a lot of difference for something "Concorde" shaped. The second position looks spot on. Allow for more movement of elevater than aileron control.Do you have rudders, or only fins?
Macboffin

[Tom Harper]

jez,

Stay forward of the half chord point. I agree with macboffin that the number 2 point is the best for a start.

Tom

[Ollie]

Include the wing chords from to leading edge to the trailing edge of elevator and aileron. So that the MAC is measured from leading edge to elevator or/and aileron trailing edge. The wing includes the part inside the fuselage. The root chord of the wing is the centerline of the whole aircraft.

The neutral point in this delta aircraft is very near the 25% of the MAC of the wing. The aircraft CG position has a range (no fuel to full tank) from too much pitch stability (large static margin) to little pitch stability ( very small static margin). The positive static margin means the CG is ahead or the neutral point. Normally the static margin range is about +3% to +7% of the MAC. Put the fuel tank position on the ideal CG position and keep the fuel from sloshing in the tank.

[MarkusN]

I don't know too much about deltas, just that they are different.

Is the AC also located at one quarter mean chord usually?

[jez_uk1]

Thanks for the replies, very useful. I'll let you know how it goes!

[JetPlaneFlyer]

I don't know too much about deltas, just that they are different.

Is the AC also located at one quarter mean chord usually?
Yes, there is nothing really 'magical' about deltas... Deltas are just a wing whose sweep and taper combine to produces the classic delta triangle shape. They have their AC at about 25% MAC, just like any other subsonic wing.

Steve

[yoyoML]

Yes, there is nothing really 'magical' about deltas... Deltas are just a wing whose sweep and taper combine to produces the classic delta triangle shape. They have their AC at about 25% MAC, just like any other subsonic wing.

Steve

Just coincidentally, that AC is also at 50% root chord on a delta.

[MarkusN]

Yes, there is nothing really 'magical' about deltas... Deltas are just a wing whose sweep and taper combine to produces the classic delta triangle shape. They have their AC at about 25% MAC, just like any other subsonic wing.
Dunno about that. A delta of considerable sweep flies with the flow detached (at least at high angles of attack). Lift comes from the vortex separating at the leading edge and rotating over (rather than around) the wing. I can imagine that the rules of aerodynamics that "nail" the AC to 1/4 on a conventional wing do not apply there.

Not saying that they don't, just that they might.

[edit] Checked some sources. They all agree that AC is at one quarter mean chord for deltas as well.

[Tom Harper]

The difference is:

On a rectangular wing 1/4 of the area is forward of the quarter chord point

On a triangular wing 1/4 of the area is ahead of the half chord point

[JetPlaneFlyer]

Dunno about that. A delta of considerable sweep flies with the flow detached (at least at high angles of attack). Lift comes from the vortex separating at the leading edge and rotating over (rather than around) the wing.

Mark, Is this really unique to Deltas? i dont think so... You would surely get a similar effect with any wing with a highly swept LE, so is it not a 'swept wing phenomenon' rather than a 'delta wing phenomenon'?

Steve

[nmasters]

You would surely get a similar effect with any wing with a highly swept LE...

Steve
True but the LEV doesn't stay permanently attached at LE sweep angles less than 55 degrees. At that sweep angle an AR greater than ~1.5 will lead to pretty bad stall characteristics unless you use gobs of washout which limits the top speed. There have been some (actually I can only think of one offhand (http://www.nurflugel.com/Nurflugel/Horten_Nurflugels/ho_xiii/body_ho_xiii.html) ). The combination of high sweep and long wings is very susceptible to aeroelasticity and flutter. The delta planform greatly simplifies the structure, eliminates aeroelasticity and gives you a lot of usable volume on the center line.

BTW the usable CG range of deltas is between 25 and 15% of the average geometric chord.

--Norm

[MarkusN]

Mark, Is this really unique to Deltas? i dont think so... You would surely get a similar effect with any wing with a highly swept LE, so is it not a 'swept wing phenomenon' rather than a 'delta wing phenomenon'?
Highly swept wing with low aspect ratio, I think. Of full sized planes I can think only of the EE Lighting and the swing wing designs that would fall in that category. It's usually easier to go delta than to tackle the torsional problems of a 'straight' wing of such high sweep.

[edit]I just realized that Norm has given a far more accurate and exhaustive answer.

[nmasters]

Norm has given a far more accurate and exhaustive answer.

Well, that's a first. :) Usually I don't use enough words and end up just confusing people

[JetPlaneFlyer]

Highly swept wing with low aspect ratio, I think. Of full sized planes I can think only of the EE Lighting and the swing wing designs that would fall in that category. It's usually easier to go delta than to tackle the torsional problems of a 'straight' wing of such high sweep.
.
I fully accept that Deltas have big structural advantages over some swept wing configurations; but this was not the point that was originally contasted... What i said was there was "Nothing magical about Deltas" and I stick by this... there is no aerodynamic phanominom that is unique to the delta configuration...
BTW.. I'm not 'anti-delta'... I'm just trying to point out that they obay the same aerodynamic rules as other wing, they are not aerodynamically unique.

Steve

[MarkusN]

I'm just trying to point out that they obay the same aerodynamic rules as other wing, they are not aerodynamically unique
That's where I partially disagree. It turns out that the different principle of generating lift leads to the same physical rule about AC location; but I'd say thats coincidence. And as you probably know things change at supersonic speeds. (AC moves to 50% chord.)

Thing is that the airflow over a delta is very much three dimensional, and that many methods used to calculate high aspect ratio wings fail on a delta. E.g. there is no way to determine stall angle from wing section data, to to pick the aspect where the two concepts differ the most.

[JetPlaneFlyer]

Maybe we will just have to partially disagree Mark :)

However picking up on your last post I certainly never meant to imply that a low aspect ratio delta could be compared to a high aspect ratio straight wing; that would be a pretty dumb comparison...
What I was trying to say was that a delta is not unique and the fact the wing forms a triangle is not, in it's own right, aerodynamically significant. A Delta wings flying characteristics would be largely shared by 'non triangular' wings of similar sweep and aspect ratio. The EE Lightning is a very good example. It flies pretty much like a delta, same vortex induced lift etc. Many may say the Lightning wing is just a delta with a 'chunk' chopped out; however it's equally true that a delta is just a highly swept wing with 'chunk' added in. The point is 'chunk' or no 'chunk' the two wings behave in a similar manner. The flying characteristics are due to the sweep angle and aspect ratio rather than the triangular shape.

Steve

[Brandano]

AFAIK all low AR wings show pretty much the same behavior, and a very large stall AOA. The Delta layout works slightly better than the square or round designs because it shapes the "vortex cones" the same way LERXes do on modern planes, and reduces to a minimum the wingtip losses. That said, I agree with Steve that the Delta wing can't be considered an "out of the norm" wing without then discriminating each single wing shape in turn. It produces lift by the same principles that all other wings use. What is then the definition of a "special case" wing? :)

[MarkusN]

What I was trying to say was that a delta is not unique and the fact the wing forms a triangle is not, in it's own right, aerodynamically significant. A Delta wings flying characteristics would be largely shared by 'non triangular' wings of similar sweep and aspect ratio. The EE Lightning is a very good example. It flies pretty much like a delta, same vortex induced lift etc. Many may say the Lightning wing is just a delta with a 'chunk' chopped out; however it's equally true that a delta is just a highly swept wing with 'chunk' added in. The point is 'chunk' or no 'chunk' the two wings behave in a similar manner. The flying characteristics are due to the sweep angle and aspect ratio rather than the triangular shape.That I agree with completely. But I still consider it quite astonishing that a wing taking lift from more or less two dimensional airflow around the wing section follows the same rules concerning AC as one that basically generates lift from the wing tip / LE vortex. The mechanics of the two phenomena are quite dissimilar, yet the result is the same.


AFAIK all low AR wings show pretty much the same behaviorTrue, but the reasons are different. Straight, low AR wings stall late because of the large induced AOA. The delta (or highly swept wing) maintains lift despite airflow having separated.
The LERX achieves the same result.