Hi can anyone help with aerodynamic questions?

I built a 3d plane using the root airfoil from the sig somthin' extra (constant cord) It requires alot of up elevator to fly any speed and lots of down when inverted. I balanced the plane on the the original spar location at first, then moved the cg back thinking it was nose heavy but no luck. Then moved it forward and still no luck. The stab area is about 25% of the wing surface area but it also has a shorter moment arm than the SSE.
It has a 51" span weighs 84oz with 739 sq in of wing area and a 52 two stroke.

Any help is appreciated.

If CG changing is not helping... its probably got excessive "decalage" or incorrect thrust line. (or both)

Its also possible you are not going back far enough with the CG... it can get so far back that it seems to be too far when you think about it from the normal CG discussions saying 25% MAC for trainers, 28% to 33% for most aerobatic types. I've just pushed the CG back to 45% on a Goldberg Tiger 60 and that seems to be the limit for that airplane.

Large stab areas tend to allow you to push the CG back further. So... keep pushing the CG back until the elevator starts to get touchy... then go forward 1/8 inch. (do this testing with low fuel in the tank... or you'll fool yourself into setting the CG too far back)

The nose drops when upright, and also when inverted.
That's c.g.
The c.g. on a Fazer/Somethin' Extra can be quite far back.
On the SE, as much as an inch aft of the spar is where most folks put theirs.

Thanks for the replys.

What is excessive decalage? is it the area over the canopy and turtle deck?

It has a tall fuz.

I was sure I had the cg quite far back on the second try because it was twichy at the back and the wings got tippy, cg was 4 1\4 in back on a 15 in root cord but still needed lots of up.

You mention thrust line... Yes this could be wrong, I built it with the engine thrust line, the wing and the horizontal stab on the same line but I did not put any left or down thrust on the engine.

Coming in to land was hairy beacuse the back end is hard to control, too sensitive on the sticks.

I've had a somthin' extra for about a year and I have always found this to need up elevator to fly level apart from when I open the throttle wide. The cg is about 3\4 in after the spar on the SE.

Sounds like you need to move the CG back, AND reduce the elevator throw at the same time. It's normal for the elevator to become more sensitive as the CG goes back, but that doesn't mean it's as back as it should be. 4-1/2" back on a 15" wing is less than 33% of the chord.

Decalage is the angle between the wing and the horizontal stab. But you said you had everything 0-0-0, so that's not the problem. (assuming no warps, and you accurately measured the 0-0-0 alignment of engine-wing-stab).

"decalage" refers to the difference in incidend angles between the upper and lower wing of a biplane.
Inappropriatley used here.
The correct term for a difference in wing and horizontal tail incidences is "longitudinal dihedral".. Think on it, it makes more sense.
Since you have a constant chord, the usual SE problem of a too far forward c.g. should be considered.
The c.g. on the SE plans is at 23% mac.
Most acitive SE's find a 35% c.g. better by far... for your chord, that would be 5-1/4".
And pitch sensitivity is as Montague says, due to excessive elevator throw, especially with your forward c.g..

Oh, so thats what the decalage is.

So if my wing was not perfectly true from leading edge through to trailing edge and had a bit of negative incidence that would require more up elevator, but wouldn't that also mean inverted flight would need less down to fly level. It require slightly more down whilst inverted but not heaps more.

The strange thing is that the amount of elevator throw was quite conservative for a fun fly, with the power on it would still loop nice n tight but if dropped altitude fast without power to line up for an approach I only just had enough to pull the nose up (scary). Then I would power up a little holding the nose up with the elevator and come accross the field for a 3 pointer harrier style landing then it would go all sensitive on me and start porpoising coming on to the strip.

Sorry if my questioning is long.

I hear what you are saying about the cg I will try going further back next flight but I would like to see if there is other possible causes while I'm here.

Your symptons are almost contradictory, so the problem is probably more subtle than mere c.g. or surface deflection; two things can cause a difference in sensitivity are something in the plane moving around, like the receiver battery, or a less-than-stiff control system that is kept from moving a lot when the speed is high due to airloads, but can "over-deflect" at low speeds.
Another is exponential gain on the surface. There's two ways for that to work, and easy to do it wrong.
It sounds like you fly well enough to disentangle the situation, so proceed with a plan, changing one thing at time to pinpoint the source or the cure.

SOunds to me like it's nose heavy and you have too much throw on the elevator

4 1/4 inches on a 15 inch chord is only 4.25/15= 28%. That is FAR from being too far back. In fact it's quite far forward. I think we just found your first answer. 3D acro models often fly with the CG between 30 and 35% back and in some extreme cases even more. That equates to between 4.5 to 5.25 inches back in your case. The porposing you're getting is probably related to the bad balance. You need so much elevator to lever the nose up and hold it that the elevators are probably just giving up. These models need to be set very close to the neutral stability point for the controls to have proper authourity zero and close to zero flying speeds. Often the only thing they have is the prop blast over the surfaces and to be "stable" at that point the model needs to be neutral.

For the elevator sensitivity I would suggest setting your transmitter up for a strong exponential throw value. With the CG's back so far on these funfly models combined with crazy throws the only real hope us mere mortals have to survive is to use LOTS of exponential throw to soften up the controls around the center travel of the sticks.

This tells you that it is nose heavy. Move the CG back to 5" and try it there. The other clue that it's nose heavy is running out of elevator authority at low (landing) speeds. By all means, use exponential if you have it! Try holding the servo arm in place and flexing the elevator by hand. Does the pushrod bend? If it does you need to stiffen/replace it.

Brad

OK, this has made some interesting reading..... Cheers.

Yes I can see the cg is playing a major part in this, it makes sense. As I said the wing was taken from the root cord of the SSE but I made it a constant cord so the original cg position on the spar would be incorrect on the new wing anyway because the SSE wing has a tapered trailing edge. (forgot about that one) So this coupled with the fact that the SSE cg location is on the conservative side has all made for an unbalanced plane.

I have just done a few checks on the plane and radio, I had only 20% expo dialed in on elevator. I generally use 25%-45% so this was a bit low even for me.

I did as suggested and checked for slop in the pushrod and it's Ok, but I did find more flex than is usual actually in the tail plane itself. So this could cause problems with bending under flight pressures. I had made few different designs with the tail plane and settled on this one because it was light as I was assuming it was tail heavy. ( How wrong was I?)

I also calculated the area of the tail and it turns out it's 21% of the wing area, not 25% as I originally thought, it was one of the heavy ones that was 25%.
I don't know where to measure the length of the moment arm from so I have measured from the center of the spar to the tail hinge line and it is 24 1\2 in. Is this too short ?

Even though the wing is constant the aileron is not, it is tapered. So the tip cord is 13 3\4 in as the aileron is 2 1\8 at the tip and 3 1\4 at the root. Sorry I forgot to mention this.

For your wing, considerning the aileron taper, the mac is 14-1/4", and 35% is at 5"..

Hmmmm........
It might just be an idea to put the old "heavy" stab/elev back on..........that should push your CG back. Tail moment is calculated from the MAC of the wing to the MAC of the stab/elev - ask too tall how he figures this.

Brad

The moment arm length is considered as the 25% MAC of the wing to the 25% MAC of the stab. So your spar to hinge measurement is in the ballpark. For a 50 inch span that's pretty nice. Not overlong but not short by any means. The 21% stab is probably fine, flex and all. At the speeds funfly models fly at I doubt you're getting any serious flex that will compromise the usual 45 degree stab travel too much. But if you have to add tail weight rather than take off nose weight you may as well put the stronger bigger tail on if it's not that much trouble.

I' m going to replace the tail with the heavier one and move that cg back some more and retry. I have put some reference lines on the wing in 1/4 in increments going back to 5 1\4 in from leading edge so I can quickly experiment at the field.

Can you help with other aspects of it's flight performance?

As I said it has a high fuz to enable slow Knife edge and it is certainly capable as I managed to knife edge past me at walking pace in a brisk wind and it also does a good knife edge harrier.
The problem I have is that it pulls to the canopy quite a bit and I'm guessing this is because the stab is on the same line as the wing and needs to be raised a little but most annoying is that it wants to keep rolling to inverted.
So what I mean is as I roll to the left and apply right rudder then I have to fight with the ailerons to stop it rolling inverted but cannot just hold a bit of right aileron either as it will easily roll out right side up if I I go further than the 90 degrees.

The rudder is larger at the bottom than the top but has a counter balance and there is very little fin area. The bottom of the plane is straight throughout it's length and the rudder is level with the bottom. I was wondering, as I roll left and holding right rudder the area at bottom is wider so therefore lift the bottom of the plane and the counter balance also pulling the top of the fin down. Is this relavant or am I way off bat here? I have seen other planes with large rudders this shape but I may of over done it.

I trust by now you have the cg moved rearward and that has made it better. However remember that an aircraft with a symmetrical airfoil with a zero zero incidence setup will always require up elevator to fly level and down elevator to fly level inverted. Otherwise the wing will have no angle of attack and will develop no lift. The amount of elevator will vary depending on wing loading but will always be necessary.

That just isn't true. Been there, done that - balanced a plane that was in-trim hands-off upright AND inverted. It all depends on the CG location and a stab size that allows a CG location at or behind the AC of the wing. Upright the rearward CG pulls the tail down until the stab reacts and balances the plane at a positive angle of attack. Now, flip the plane over, and the rearward CG pulls the tail down until the stab reacts and balances the plane at a positive angle of attack. See pic:

I probably shouldn’t have underlined “always�. I was focused on real airplanes and forgot that R/C hot dogs often fly models that are neutral or even unstable longitudinally. You are correct in that if the cg is moved aft to the neutral point, the airplane will simply go where it’s pointed either upright or inverted. To fly level, there still must be a positive deck angle for the wing to develop enough lift to balance the weight. In the case of a neutrally stable model, this attitude need only be set momentarily and it will remain there hands off unless upset by a gust or something. It’s not quite accurate to say that the airplane is “in-trim� since a neutrally stable machine in effect doesn’t have a trim speed. That’s why it behaves like it does.

I will revise my statement. “If an aircraft with a symmetrical airfoil with a zero zero incidence setup has positive longitudinal stability, it will always require up elevator to fly level and down elevator to fly level inverted.�

I was reading over this post and it occured to me that I have never seen a general formula that relates this information. Is there one or is this just what you have had good results with?

I am looking for a formula that relates rear moment arm to horizontal stabilizer area and wing area.

While I am at it, is there a webstite or anything of the like that ever had formulas like this one? I have seen websites describe how to make a simple trainer and the like, they just used the chord of the wing times X to pick a tail moment. I am looking for something a little different.

Reason I ask is I want to design a plane with servo's in the tail, so this would add tail weight, so making a tail moment shorter would decrease this effect, at the same time it would change the effectiveness of the horizontal stabilizer.

Steve

The tail wags the plane...
Decide on the degree of responsiveness/ stability you want. This sizes the tail.
Then ballast or move equipment around to get the c.g. where you want it.
With pull-pull and CF pushrods, servos mounted up front are as good or better than those close to the tail, especially when considering inertias.

I like it... The tail wags the plane

I actually was considering the inertia of the extra mass in the tail and liked the idea. I am hoping it will help my flying style with the mass spread around the airplane fuselage and the aileron servos in the wings.

Thanks for the quick reply, I ended up finding what I was looking for just a few posts below mine.

Steve

I think you are nose heavy from your symptoms... If not did you check your stab to wing incidence? See below... Could also be your thrust incidence... I rebalanced my Big Stick 40 with 2 1/2 oz of tail wieght to make it more aerobatic and track straighter through rolls and it now requires little to no down elevator when I roll inverted...

Check out this page. Lots of good info here along with the formula for the Horizontal Tail Volume Coefficient and how to relate it to the CG. The higher the TVC the more damping the model will have. Also a higher TVC puts the neutral point further back so your CG should follow.

It's all down near the bottom of the page but the first part explains where it's all going so be sure to read it from the top.

http://www.bumpygreen.co.uk/Articles/cofg.php

Flew it again today after some mods I have made.

Even though I had mixed results I did have a lot of fun with it and really enjoyed myself.

I put the other stab on which has a slightly higher area but also moved it back 3 in so the moment arm is closer to the SSE. I also raised the stab by 3\8 in to help stop the pitch to the canopy in knife edge. But the most significant change was moving the cg back, and I didn't mess about I went straight in with 5in back from leading edge. I also moved the engine up so now the crank is 1 1\2 in above the wing center line.

I trimmed the elevator out to fly level at about two thirds throttle then brought it back in to see if the elevator was still flat, and it was, more or less. It does not need excessive up to fly level at lower speeds now but still requires a fair bit of down when inverted especially when flying slow and I nearly run out of elevator when diving inverted with the power off. I' ll have to check to see if I have the same down travel as I do up.

I made the elevator travel lower as suggested and increased the expo. I think it flies as good, if not better than the SSE but I also feel I have made it more stable which is not what I intended to do. It is very similar to the SSE in the air. I tried to build a plane that would do what the SSE would not do well, Like Knife edge and prop hang easier. Well it does both of those but I feel I could improve it still further.

In knife edge it now pitches to the wheels instead of the canopy, I don't think the 3\8 in raise in the stab has done this alone, it is probably a combination of increasing the moment arm and raising the engine. (what do you think)

I raised the engine because I thought it would help having more of the prop wash over the top of the wing to improve the slow fly. It lands like a heli every time.... very pleased with that aspect.

It still has this roll to inverted when holding in lots of rudder for knife edge, so the next flight I cut off the counter balance and stuck it to the top of the fin and it improved this problem a great deal and now only seems to roll when I use all of the rudder travel. Does anyone know what I did here?, if I could find the theory I may be able to eliminate it all together.

It is not really twitchy at the back end so I thought about moving the cg back still further or am I asking for trouble.

Please feel free to chime in here, all comments greatly appreciated. Thanks for all the suggestions so far they have proved very helpful.