negative incidence?

[sickwayne]

Hi guys,
I used Ed's experimental airlines techniques and made an MQ-1 type plane this past week and have put a couple sets of wings on at this point to get it to fly well, but I had some issues. I think I have it fixed now, but I can't fly again until the weekend and I am wondering if anyone has made the same mistakes and had the same experience.

I first had a 40" wing span with about 10° of dihedral with wings of about 4.5" root chord and 3" tip chord. It just barely flew and behaved "tail heavy", but more strangely it was completely uncontrollable on its roll axis. It would just snap roll for no reason. I though it was struggling to produce enough lift so I made a 60" wing set with no dihedral (trying to be more accurate) with root chord of about 5" and tip of 3". Same story, worse this time.

What I found was that I had a negative angle of incidence on both wings and slightly uneven from left and right wing. It was probably 6°-7° by the eyeball. Going by the single build picture I took, I made the same mistake on the first set of wings also; slightly less severe though and I did manage to get 4 flights on the first set of wings. Another tendency I noticed on the first set of wings is that in level glide it dropped like a rock (intuitive I know) and also when I made some half decent landings it was pitched up significantly. That all makes sense now, but while flying it felt like I was in a stall so I kept the nose down with the elevator.

So big question, does anyone think the poorly mounted wings explain what happened fully? I just want a warm and fuzzy feeling before I go flying -er crashing again. Currently I have the larger wings set just slightly positive (and as even from side to side as possible).

 

[willsonman]

Negative incidence on the main wing should cause some really tricky handling. Positive incidence was used in WWII era fighters and bombers as a means to create more lift while subsequently sacrificing drag. While not supersonic machines, this was an acceptable aerodynamic change from what they knew was "perfect" in order to minimize materials while also slightly increasing stability for the pilots. If you examine all the FT planes and really any sport scale model the main wing is set at 0 degrees incidence (relative to the datum) on the main wing. Even then... if there is incidence it is positive to increase stability. This is all in relation to the tail moment. depending on your moment you may want a more positive incidence if the moment arm is short. If it is longer then the surface area of the tail should be more efficient at stabilizing the platform.

A good case study here is the SB2C Helldiver. Its a dive bomber and thus accuracy is extremely important in high speed. It is also important at low-speed take off with a full bomb load. The main wing was set at 2 degrees positive incidence while the H-stab was set at 1.5 degrees negative. With the short moment for the tail this was a necessity to create a stable platform for the job it was intended to do. While it had a significant amount of drag from the incidences it was overcome with the large motor and more efficient airfoils selected.

 

[Winglet]

I think you are probably thinking correct. I built a scratch built that ended up with a slight bit of negative incidence once and it flew as you described. I corrected the issue and flew it sucessfully for many hours.

 

[quorneng]

To be exact the wing incidence relative to the fuselage is only of cosmetic interest as it is the incidence relative to the tail plane and the motor thrust line that really counts.
So it is possible to have a plane that flies quite happily with the fuselage in a nose up or nose down attitude but such a situation does make it difficult for an RC pilot on the ground to judge how it is really flying.

Of course ideally you want the fuselage to present the minimum drag attitude to the airflow although normally a couple degrees up or down makes little difference.

My guess is with a negative incidence to the fuselage your plane wanted to fly in a 'nose up' attitude which you tried to correct with down elevator causing it to dive towards the ground.

From a design point of view a 40" wing with a 4.5 to 3" chord is quite narrow so even if built accurately it can be fairly flexible which can produce difficult handling characteristics.

Any chance of a picture of the plane?

 

[sickwayne]

Thanks everyone.
quorneg, yeah those wings seemed narrow and that's why I jumped to thinking it was tip stalling and rolling. Now I think it was pitching up enough for one wing to really start producing more lift while the other wasn't and then rolling on me.

I do have pictures and I'll post them later, maybe tomorrow morning. Thanks guys, I thought I had a good explanation for why it handled so terribly and I needed some assurance.

"Pay attention to the wings" will be the lesson learned here I hope.

 

[sickwayne]

These are of the current setup.



This one shows the original wings next to the fuse., but these were damaged and 86ed.

It still needs landing gear, but it's basically done.

Thanks for the help.