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Help! Weird Nose Drop/Tuck at speed

#1
Hello Everyone! I just want to start off by saying that the Flite Test community is awesome.

I am having this new nose drop problem with increased airspeed after I added a nice rounded leading edge to my under camber Jedelsky airfoil on my slow...ish low wing stunt trainer (it only does loops and hammer heads, stuff like that)

For context, this was my second scratch build and it first flew in summer 2019. As designed, it had a 42 inch wingspan, no taper, and has 20 degrees of total dihedral with ten on each wing. It flew great, but I wanted to experiment a bit. So about a month ago I added 5 inches to the root and 4 inches to the tip, making the cord around 15 inches total (taper to 14). I didn't not extend the fuselage, and it flew with a new nasty nose heavy stall, albeit at a slower speed. My planes never stalled this hard, they kind of mush into it, where you can rudder into a spin or play with the throttle if you are going in to land.
The top speed was still there, however. After moving my battery back to get the CG to 40%, i ran out of space and just extended the fuselage 3 inches. Threw her up and she was fine, although the stall was still kind of terrible. I noticed that the early morning dew had made my leading edge sharper than it needed to be so I took the paper off and with the help of some foam formers and packing tape, made a nice circular leading edge. It is not perfect, but it works, no nasty stall.
Here is where the problem begins. On the next test flight, it flew tail heavy so i moved the battery up and got it to 35%, right up against the wing spar. All the while, if i cruise and get hit by a bit of wind, she tucks or tries to dive. If I punch out...at a crazy 30mph, she tucks or dives. With the same CG, tail coupling, tail size, and down thrust, but different leading edge, the thing flew like it was on rails. Now, At launch at low speeds its great, I can screw up a hand launch, helicopter up, pitch 90 degrees and fly straight or land easily...but still, it just wants to nose over at high speed. Last night increased the incidence and that didn't to much besides take some up trim out of it.

So here is what im thinking

- The rounded leading edge made it tuck?

What the heck is wrong with my trainer fighter? Im stumped, Ive searched and searched and i have found nothing. Please explain to me (or link me an article/Journal/Free book PDF) so I can understand why this happens and avoid this problem in the future.

Thank you for reading my essay!
 
Last edited:

Hai-Lee

Old and Bold RC PILOT
#2
When you altered the profile of the LE you effectively altered the airflow significantly. Where the air is parted to either flow under or over wing wing was atered and so was the cord line of the wing. Effectively I suspect that you lowered the wing incidence accidentally.

As the bird travels faster the Tail until (still at the original incidence angle), tries to assume a truly horizontal attitude but the wing at the same time is pushed to where its lift is reduced. Overall the effect is a plane that wants to nose over into a dive at high speed.

Try increasing the wing incidence slightly or reduce the tail incidence angle.

Just my thoughts!

Have fun!
 
#3
When you altered the profile of the LE you effectively altered the airflow significantly. Where the air is parted to either flow under or over wing wing was atered and so was the cord line of the wing. Effectively I suspect that you lowered the wing incidence accidentally.

As the bird travels faster the Tail until (still at the original incidence angle), tries to assume a truly horizontal attitude but the wing at the same time is pushed to where its lift is reduced. Overall the effect is a plane that wants to nose over into a dive at high speed.

Try increasing the wing incidence slightly or reduce the tail incidence angle.

Just my thoughts!

Have fun!
Ohh, I see how that could be it. Thank you for your response, Ill try and implement those changes!