Sneasle

Agreed, No matter what you change, only change one thing at a time. If you change something and it gets better, then you know your own the right track instead of having to try and guess between the 3 changes you made...

rollo

I am just glad that I didn't have to be in this plane on the last two test flights.

I think the logical path to success is to shim it first then if I need to raise the thrust line do that. Examining the pictures from the manual and how my plane is set up I might be low but not by much. The engine is not where the stick mounts to that is pretty high; however, the engine hangs below the stick.

Sneasle

hopefully it won't be a problem.. but keep en eye on how it flies. Thrust lines can have weird effects on planes if they are off. Being too low is less of a deal then being to the left or right.. but still keep an eye on it.

SMUGator

It may be my beer goggles, but I'm not following your thinking. I think we are confusing two topics.

1) Is the engine mounted on the aircraft centerline?
2) Is the thrust angle parallel to the centerline?

I think you are off on 2) and that the engine thrust angle is is pointing downward relative to the centerline of the aircraft. A shim should fix that and no other adjustment should be needed if that is the problem. Let me know if that makes sense.

Tom

mjfrederick

If you put a shim in to point the engine up, it will exacerbate the problem. Right now you already have negative wing incidence in relation to the thrust line. Shimming the engine as you have shown will increase the negative incidence, requiring an even more nose-high attitude to maintain level flight. The reason you're getting porpoising right now is because that nose-high attitude, while allowing the wing to produce lift, is stalling the tail, causing it to drop. Then you have to add down elevator to unstall the tail, then the wing stalls, causing you to pull up, stalling the tail. Like I said, you need to adjust the incidence of the wing. Your engine is already mounted at an angle that will make the fuselage look level during level flight, the problem is that the wing is not in a position to produce lift at that attitude. Remember, the ONLY way for a symmetrical airfoil to produce lift is for it to have a positive incidence in relation to the thrust line. You can test the incidence relative to a level fuselage, but that really has absoluteley no bearing on whether or not the airplane will fly.

Sneasle

SMU, The thrust line on the Funtanna is not on the center line of the plane, it is a bit higher up, about 1cm below the line where the turtle deck joins... maybe a tad higher.

When I was mounting my engine, I took the cowl and inserted the engine shaft through the cowl and then added on a spinner to keep the engine from falling through the cowl. Then I put the cowl on the plan and took a felt pen and drew the outline of where the engine was in relation to the firewall. It is a lot higher then you would think.

The wing incidence looked ok to me, but It could very easily be off considering you re drilled all of those holes.

rollo

The shim is to actually make the thrust line level and not up. Looking at the second set of pictures [link=http://www.rcuniverse.com/forum/fb.asp?m=6070018](previous post)[/link]you can clearly see the engine pointing downward in the first and level in the second where I added a folded piece of paper to eye ball it. The first picture I posted was not lined up properly and the plane was not level on the bench so it give the appearance of negative wing incidence. Additionally, the plane flies ok at low speeds it is when you throttle it up that the crazy characteristist start. As one test a pilot inverted the plane and increased throttle the plane immediatly did an outside loop all by itself.

Sneasle: I actually went back when we had the placement of the engine discussion the first time and raised the position of the engine so it is higher up then center of the plane. [link=http://www.rcuniverse.com/forum/fb.asp?m=5791124](previous post)[/link]

Sneasle

Ya, I remember you foign that now. I think originally you had it centered on the firewall.

Alright, hopefully it is high enough. I think that that... could... be part of the problem, but hopefully the shim helps some.

mjfrederick

I feel the first picture showed the problem better. But since you feel the second 2 are, I'll use those to show you what I'm talking about. Keep in mind, it DOES NOT matter if the fuselage is level or not... the fuselage doesn't have to produce lift. What I did was take the 2 photos you posted and I drew a line through the thrust line and a line through the wing to show its angle relative to the thrust line. In order for your wing to produce lift, the lines should intersect somewhere in front of the wing, not behind it. As you can see, the line on your airplane intersects behind the wing, and it gets even worse after you put your shim in.

SMUGator

I can see why the negative wing incidence could cause some of the diving and porposiing behavior. I seem to remember that most of the fuse in that area was trashed. Chris, how comfortable do you feel that you got the wing mounts built to spec?

Tom

Sneasle

I added some to the thrust line images.

The red line is where the thrust line on this plane should be. I think you can achieve this by moving the engine around on the mount a bit. It will probably require some new holes, so you might need a new mount.

rollo

Sneasle, so how does your wings line up in comparison to the thrust line? could you shoot a side angle like I did as a comparison? I would be interested in seeing if the wing incidence is off.

SMUGator

I thought about the thrust line question some more and have a problem with the notion that a symetrical wing needs positive incidence relative to the engine thrust line to create lift. If that were true, as soon as you rolled the airplane to inverted, you would have negative incidence, so in theory the plane should want to stall and fall out of the sky. I would bet that most aerobatic planes have a wing to thrustline incidence of 0 degrees since they are generally indifferent as to whether they are upright or inverted.

Tom

Sneasle

I agree SMU, if you look at my image, the red line is the thrust line. If you use that as the thrust line, then it is perpendicular to the incidence line of the wing, which is where it should be. His wing looks fine, I think he just needs to adjust the engine in the mount some.


I have 4 tests this week so I will be very buys. If I have some spare time I will grab the camera and try and take a pic for you.

rollo

Good luck Sneasle on those test. If you have done the visual comparison and you're seeing no difference I can live with that no rush to get out the camera.

Sneasle

Thanks.

What I think I will do for you is lay my fuse on the ground and take two straight edges and lay them out on the thrust line and on the center line of the wing tube holes and the hold down pin holes... that should give you a pretty good idea.

mjfrederick

SMU... I fly nothing but competitive aerobatics, and I talk about this stuff very often with one of the top pattern airplane designers in the nation. The ONLY way a symmetrical airfoil can produce lift is by having a positive incidence (actually once the airplane is flying you can think of it as angle of attack). Otherwise the airplane will have to maintain a nose-high attitude in order to mainatin level flight. Think about Bernoulli's principle and you'll understand why. If a truly symmetrical airfoil is used, the pressures on the top and bottom of the wing will be equal until the angle of attack is increased, and no lift will be produced. In order to increase angle of attack without making the airplane look funny when flying through the air a slight (extremely slight) amount of positive incidence is required. This can be tested on the funtana in question if it has the ability to program in flaperons. Try flying the airplane with about 2-3 degree of down flaps to see if it improves the performance of the aircraft. As for flying inverted... well, there's a reason down elevator is required to maintain level flight while inverted. Many people thought for a while that a 0 incidence, 0 downthrust, 0 right thrust setup would be ideal for pattern, but then everyone ended up having to change the shapes of the fuselages to make them look right when flying, or they would just look like they were flying around with their noses sticking up a little bit.

SMUGator

That makes sense. But to be clear, you do agree that the downthrust that was clearly built in is a bad thing. Correct?

Sneasle

well, with the way he was drawing his lines, the only way to get them to cross in front of the wing would be to 1., move the wing incidence 2., straighten the engine out on the mount 3., put in a massive amount of down thrust.

SMUGator

This thread caused me to be curious enough to sort through my dusty bin of plans and pull out the drawings for the GP Super Sportster which is the closest thing to the Funtana in my hangar. The airfoil is fully symmetrical, and the plans call for Thrust - 0, Wing - 0, stab - 0. It tracks like an arrow both upright and inverted and has no nose up tendencies in either attitude, FWIW.

Tom

mjfrederick

Now when I say cross in front of the wing, I mean waaay out in front... The positive incidence needs to be so slight that the lines would probably intersect somewhere in the next room. The downthrust that was built in isn't necessarily bad. Leaving it in would reduce the change in incidence that will eventually be required to make that airplane fly right. As far as the Super Sportster goes... I flew one for a few years, I don't remember anything about that airplane that tracked like an arrow. And the dihedral in the wing makes it too stable for precise aerobatics, especially inverted flight. It's a good sport plane, but that's about it. I also seem to remember there being down thrust. I'll have to check the plans the next time I'm at my parents' house.

SMUGator

You're correct on the dihedral on the Sportster. It probably has about 3/4" at the tip rib, so it isn't going to fly hands-off inverted forever, but it doesn't require a lot of horsing around to keep it that way. I'd agree that it's not going to ever be a competition pattern ship. But given that "good sport plane" is what rollo is trying to achieve with the Funtana, it's as good a test subject as any. I also agree that negative incidence is a generally bad thing. If there is any built in, then is should be adjusted.

Tom

rollo

I missed the wing incidence class in school so I am going to just adjust the engine for now and hope for the best. At this point if it even flies ok I will be happy. If it doesn't fly then I have some good electronics to put into my combat aircraft I am building. Tomorrow will be the next test for the 'Mighty Funtana'

rollo

The 'mighty funtana' flew and now we have too much up thrust. The wind was beating all of the planes around but a one flight clearly showed that the thrust had a little bit too much up. Since I used washers to make the correction removing one was simple; however, the wind was causing havoc all day so consensious was to hold off wait for a better flying day. At this point I am comfortable saying that the plane should do well. Ahhh the joys of crashed and rebuilt planes.

Sneasle

Very true. I can't wait to finish mine.


Either of you ever messed with CF pushrods? I plan on using them on this to keep my tail weight down (since I am using a smaller engine then you... weight in general really). I was planning on expoxy/CAing one end into a tight fitting brass threaded end that would then attach to a clevis to the servo horn. The other end would have a short piece of music wire (the Z-bend for the surface horn) glued to it (was thinking about wrapping this end with thread, then hitting it with CA, and then some heat shrink).

Any thoughts on that?