Montague

Do you think CG placement affects the high speed turning radius of a plane?

I'm talking RC Combat here, where we run full throttle, full elevator, and get as close to snapping out of turns as possible. So, low speed isn't an issue, we're talking max-G loading turns here.

I know that CG location makes a huge differnece with how much elevator throw you can (or have to) use to snap out of the turn, but does the turning radius actually get smaller with a more aft CG?

My experience seems to indicate that if the CG is too far forward, you can't pull the nose though, but if it's too far back, you snap out earlier, so you need to be in a fairly small range. It also seems to vary a lot with the airfoil, some airfoils seem to be much more "picky" about the CG location in a turn than others.

However, part of me thinks that my "expereince" is really off a bit, and what I'm really noticeing is the interaction between the elevator throw and the CG, not the CG by itself. That if the elevator throw was properly adjusted for any given CG, that the resulting turning radius would be exactly the same. (A rearwards CG will be quicker at getting in to and out of the turn, while a more forward CG will take a while to change AOA from level flight to just below critical AOA).

I say this because given a set weight and airspeed, turning radius should be determined only by the critical alpha of the wing, right? At critical alpha, you're produceing as much lift as the wing can possibly generate (and lots of drag too), which means you're turning as tight as you can. increase alpha beyond that and you stall. I don't see how CG would affect the critical alpha or the lift generated at that AOA? Am I missing something?

rmh

Close enough
If you want to REALLY turn tighter - reduce wing loading
Hopefully without increasing wing area --

Tony_M

Aft CG is going to give you tighter radius turn and the need for less elevater to maintain control, but, will make it more sensitive and pitch stability.

The reason you'll get tighter turns is because the elevater is producing less down force, or negative lift. This downforce ultimately detracts from the lift created by the wing, the more lift you generate (pos' and neg'), the more drag you create. Unstable military jets can have such an aft CG position that the elevater is actually producing positive lift, only because computers are doing the humanly impossible job of maintaining pitch stability.

Swept wing aircraft can have unusual characteristics in high speed/high G turns where the tips (behind the CG) stall first but the roots (forward of CG) remain in flight. This effectively moves the centre of pressure forward of the CG (same effect as moving the CG too far back) and the aircraft becomes pitch unstable, usually resulting in an uncontrolled pitch up, which in a high speed turn can rip yer wings off.

I'd consider going to a canard format if you want to squeeze every ounce of positive lift from every square inch of lifting surface.

Tall Paul

The aft c.g. as Tony says lets more of the lift be used as lift.
For your situation, probably finding a better airfoil would be the way to go, and keep the c.g. aft.
A hgher alpha before stall.
Possibly a 3D airfoil, fat forward, moderate leading edge radius.
For combat, a canard.. no, I don't think so.
All the other problems canards bring with them, and have the control surface doing the impacting on the enemy...

Montague

Yeah, a canard would be a stripped elevator servo waiting to happen. But flying wings are fairly common, as they are even more efficient. (Though I have to admit, I've wondered what effect some thrust vanes just behind the prop would have, but I digress)

Ok, the effects of the stab make sense to explain what I thought I experienced in practice. Thanks for sorting that out, I don't know why it didn't occur to me. (fwiw, I tend to move my CG as far back as I can handle, which is pretty far back, if nothing else, the quick "twitch" is a useful trait at times).

What about a flying wing? Same effect? No stab, but you still have a balance of forces?

As for airfoils, yes, I do use a "differnet" airfoil, in fact, I came up wit it on my own (a lot of messing around in Profili looking at L/D graphs at the higher alpha vaules. I wanted an airfoil with a good L/D at very high and very low alpha, but I didn't care much about the middle range. Kind of the opposite of the typical "drag bucket" effect would be nice.)

I don't think a funfly airfoil would work very well in combat. For one thing, the drag is way to high at high AOA, so while you can turn, you can't hold that turn forever and you can't go fast though the turn. Also, the drag in level flight is too high, so you'd be slower. Thicker wings do tend to be stronger, but they are also heavier (we're talking solid foam wings here, and fairly dense foam, not that white stuff). Also, I personally don't use a symetric airfoil, as my tactics don't use a lot of negative maneuvers (not that I don't do ANY, just that I trade outside turning for better inside turning), and an airfoil with some camber will produce more lift for less drag than a symetrical one. There are guys that do use symetric airfoils, and do a lot of outside looping in their tactics, but it's not my flying style. I do, however, use a fairly blunt LE. I also tried turbulators a bit ago. Results were inconclusive, so I stopped doing it as it increased build time too much.

Dick, Yeah, it's all about wing loading. Most competitive combat ships have wingloadings around or well below 12oz/sqft. 600+ sqin of wingarea, some guys are over 700, on 2.5-3.5lb planes. Combat plane design frequently comes down to trading armor for performance. Add strength and durability, but give up turning. Differnet guys make the tradeoff at different points.

Montague

Here's my airfoil, fwiw. In practice, the actual LE is usually sanded a hair more blunt than the profile. I didn't notice any reduction in level flight speed when I did that, so I've kept doing it.

Tall Paul

That's an ideal candidate for a modest amount of camber change!
A little amount of down trailing edge with up elevator, and reverse for down elevator.
Either hard-set or mixed.
Semi's have higher stall Cls than symms.. exploit it with the flap effect.

Montague

Paul,
You're saying to mix in a little flap with elevator? The planes are very short coupled, and while I haven't tried it myself, most guys who have tried elevator-> flaperon mixes have given them up, since the down flap usually results in a nose-down pitch. With a longer tail moment, it might work though. But short tails and big surfaces make for lighter, more durable structure and a quicker feel on elevator.

davidfee

Perhaps they used too much flap in the mix... it doesn't take much additional camber to make a big difference in the max Cl a wing can generate. Play with it in Profili/XFoil. Try just a couple degrees of flap...

-David

Montague

Oh, I forgot to add, I can't do this anyway. I only have 1 aileron servo, and I don't have the weight budget to add a second servo (plus the cost). I'll see about trying it later, in the next batch of wings though.

David, Good point. I suspect people were trying too much flap.

The planes are usually trimmed with a fair bit of aileron reflex, about 1/8" measured at the hinge line on the top surface of the wing (1.5" ailerons). It makes a big differnece in the pitch stability. When I change the reflex, even a couple of aileron clevis turns, I do see a pitch change. increasing reflex even a couple of turns results in needing down-trim to stay level.

Tall Paul

Kirk, I'm thinking you're at the point where you're just going to have outfly the other guy!
Too many reasons to keep what you got.
If it ain't broke, don't fix it. !!

Flypaper 2

When I used to fly controline combat, way back when, We used to give the flaps 1/3 of the elevator throw to do those right angle turns. Called them maneuvering flaps. The Mitsubishi Zero used them. Called butterfly flaps, although they weren't connected to the elevator.

Montague

Paul,
Well, yeah, that's really what it comes down to (and I do pretty well in that area, though I'm still too erratic). But getting the most out of the gear helps . And what I'm really doing on the forums here is making sure that what I think I see in the air is actually what is going on. (I have one more oddball question for another thread shortly )

I'm not making any major changes to my design at the moment, I only got the bugs worked out of my current wing mid-way through last season in B class, but the SSC version really kicked tail at Muncie and Dixie. My planes are competitive with what's out there, but they can also be improved a bit, I think. I just want to make sure I focus on things that will actually help.

FHHuber

for close coupled planes... sometimes the pitch effect of the flaps acting on the WING can overpower the elevator. When the flaps go down, the nose goes down and the elevator doesn;t have the authority to overcome the flaps.

(we had a discusion about this with a Slow Poke owner that set up flaperons...)

The answer?... deflect the flaps the SAME direction as the elevator so that you have ELEVONS on the wing, amplifying the elevator's pitch action for some AMAZINGLY tight loops.

Its a question of what is the ratio of the wing chor to the separation of the wing TE from the elevator hinge line... and then the aspect ratio of the wing is a bit of a factor. And then the chord of the aileron as a percentge of wing chord. (all kinds of variables as to if this works...)

The Slow Poke (and Lil' Poke.. its Park Flyer "little brother") can have pitch controled by the ELEVON action of the flaperons with more authority than by the elevator. These things will pitch with the flap function and the elevator just CAN'T overpower the flaps in pitch control. These models approach being "stabilized flying wings"

I have seen combat models that were stabilized flying wings using this to very good effect. (One was a R A cores Gremlin with a small horizontal added with the horizontal stab LE actually over the elevons. the stab/elevator had 5% as much area as the main wing and was split 50-50 fixed and moveable.) the small elevator and stab just AIDED the elevons in making the thing do tighter turns... and reduced the reflex needed on the elevons for level flight (higher top speed and less trim needed with speed change)

geauxtigers

Hey Kirk,

We both seem to be about the only combat dudes hanging out in Aerodynamics.
I have noticed similar things that you stated on CG and elevator throw. I tend to go with a more rear CG and reduce my elevator throws so that I can go into a full stick deflection loop without it snapping out. One of my litmus tests for a good CG is to be able to glide in dead stick without having to hold too much elevator.

Oh, yeah, I have just come up with a new favorite wing / airfoil design. I'll try to post some pictures of my templates and that will explain it all.