Fidgit contest plane pics
05-01-2005, 05:21 PM
#26
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RE: Fidgit contest plane pics
I have been following your project and have been really impressed with your design. I'm hoping you can get it cruising! Sorry to hear about your control probs but if you dont mind I have 2 cents I'd like to toss in.
After putting some thought into it I'm thinking the cause of the pitch problem is connected with the airfoil. Most flat bottomed and semi-symmetrical airfoils have a negetive pitching moment as they generate lift. That is, they all want to nose down with the more lift they generate. The horizontal stab's job is to counteract this.
The Scimitar gets around this by the use of elevons which effectively change the shape of the airfoil when they move. Giving the up elevon command gives the wing an inverted camber shape and gives it a positive pitching force.
I dont know what airfoil you are using but it sounds like it is probably generating a negative pitch. Your pivoting wing setup changes only the angle of attack but not the airfoil shape. I'm guessing that despite changing the AOA they still are generating a negative pitching moment and that is why your nose is dropping.
I dont see any way to remedy the issue unless you add a horiz stab. It would still be a cool setup even if you could just use the pivoting wing for roll control. Keep working on it, it's way too gorgeous to trash!
After putting some thought into it I'm thinking the cause of the pitch problem is connected with the airfoil. Most flat bottomed and semi-symmetrical airfoils have a negetive pitching moment as they generate lift. That is, they all want to nose down with the more lift they generate. The horizontal stab's job is to counteract this.
The Scimitar gets around this by the use of elevons which effectively change the shape of the airfoil when they move. Giving the up elevon command gives the wing an inverted camber shape and gives it a positive pitching force.
I dont know what airfoil you are using but it sounds like it is probably generating a negative pitch. Your pivoting wing setup changes only the angle of attack but not the airfoil shape. I'm guessing that despite changing the AOA they still are generating a negative pitching moment and that is why your nose is dropping.
I dont see any way to remedy the issue unless you add a horiz stab. It would still be a cool setup even if you could just use the pivoting wing for roll control. Keep working on it, it's way too gorgeous to trash!
05-01-2005, 07:06 PM
#27
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RE: Fidgit contest plane pics
ORIGINAL: BMatthews
Poodle, your latest pics made me go back and re-read the early posts. I had not noticed that you intend this as a flying wing style of model. But there's a problem. The airfoils you are using are not plank style positive pitching moment airfoils. So basically this isn't going to work other than for the time it takes to arrive at the crash point. The upper wingerons will provide pitch and roll but you need a stabilizer. You don't need elevators. Just the fixed stab will do the job.
If you want to try it as is I suggest a field of very tall grass or similar soft stalk crop. And get ready to duck if need be.
Poodle, your latest pics made me go back and re-read the early posts. I had not noticed that you intend this as a flying wing style of model. But there's a problem. The airfoils you are using are not plank style positive pitching moment airfoils. So basically this isn't going to work other than for the time it takes to arrive at the crash point. The upper wingerons will provide pitch and roll but you need a stabilizer. You don't need elevators. Just the fixed stab will do the job.
If you want to try it as is I suggest a field of very tall grass or similar soft stalk crop. And get ready to duck if need be.
You NEED a stab. There's nothing for the wingeron control to work against in pitch. If you used elevons instead of wingerons then it would have worked but since you didn't you NEED a stab. All you've got right now is the one wing working against the other. If the wingerons were more to the rear with a marked negative stagger then I think that would work as well. But as it is all you've got is the lift of one wing fighting the other and the drag induced by this battle is probably fighting the desired pitch response.
So either put a stab on it or rebuild/repair it with a strong negative stagger. The leading edge of the wingerons being above about the 75% point of the lower wing should do the trick. If you go with the strong negative stagger try the CG at the 20% point of the projected top view of the model.
dieFluggeister, I didn't see your post until I'd posted mine above.... we're on the same wavelength I see. The Scimitar also used a proper positive pitching moment airfoil and that's another reason it works.
05-01-2005, 07:55 PM
#28
RE: Fidgit contest plane pics
I think if he could get that wing working against enough prop blast it could take off and be controllable [responsive] under power, and it seems like if it had any air speed at all it would be working against the lower wing. I have seen a small model equipped with ailerons that were stationed above the wing work nicely for roll control. For pitch control the POODLEs' upper wing just needs more exposure to oncoming air and probably more stagger for more pitch control leverage. I really think the sheet glider concept model needs to be slapped together and wind tunnel tested in the back of a pickup. Shoot a string through it right at the CG and tie it off at both ends, now the model is free to rotate in all axises for some painless testing. Bring along some scrap building materials and tools, and brainstorm away with it until you find a combination that works
!
!
05-01-2005, 10:27 PM
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RE: Fidgit contest plane pics
Sorry but it's got plenty of exposure to the oncoming air. What it lacks is the moment arm to lever itself against in order to rotate the model. A wing generates its lift at the 25% point more or less. If you have conventional elevons then it can change its pitching moment but if you have wingerons you're only chaning the angle of attack and the pitching moment stays the same so the wing is trying to lift but it's located very close to directly over the lower wing so there's no moment arm to lever against. Hence the lack of pitch rotation. If it had a stabilizer then there would be something to work with as the decalage would be changing and that would provide the pitching moment for the overall model. Similarly staggering the wingerons back would make the upper wing act sort of like elevons for the lower wing so there could be a pitching moment. But with the wings more or less on top of each other there is just no pitching moment Instead the wings are both trying to lift in the same location and just end up fighting each other.
