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Undercambered wings and no rudder authority

#1
So I have been trying to build a extra slow, extra light plane. It's a 3 channel with very heavily under cambered wings and a respectable amount of dihedral. It glides and climbs like a dream at a jogging pace but rudder input does almost ZERO. Its a fairly substantial control surface but still nothing. What could be amiss?
 
#4
Here's the tail in proportion to the wing. Originally it was a V-tail but also had no rudder authority. The front section took some damage and is obviously absent. Also here is the underside of the wing.
IMG_20200506_234149.jpg

IMG_20200506_233243.jpg

IMG_20200506_233133.jpg
 

Attachments

Ryan O.

Well-known member
#5
Here's the tail in proportion to the wing. Originally it was a V-tail but also had no rudder authority. The front section took some damage and is obviously absent. Also here is the underside of the wing. View attachment 168238
View attachment 168240
View attachment 168242
I don't know, maybe something is gettkng blanked out, but that seems unlikely. To test that try building it with an inverted tail, that way it is getting clean airflow, and any disturbance can be minimized.
 
#6
My only thought is that the wings are creating a massive amount of drag. Enough that the rudder is rendered useless. Maybe I'll try rebuilding with a oversized rudder or V-tail and see what happens.
 

Hai-Lee

Old and Bold RC PILOT
#8
My estimate is that the model has insufficient rudder authority because it is flying too slow for the rudder to generate sufficient force to overcome the dihedral.

You could reduce the dihedral or make the vertical fin ALL moving as in there is no fixed vertical fin, only rudder.

In addition if the rudder was greater in cord the rudder would really start to perform.

You cannot use a rudder design that you would use on a high speed model on a slow speed model and expect it to perform the same.

Just my thoughts!

Have fun!
 

Tench745

Well-known member
#10
You have an awful lot of ribs under the wing to form that undercamber. I wonder if, among the other things mentioned, they might be acting like a kind of v-stab and countering any yaw the rudder tries to induce.
 

Sero

Well-known member
#11
My estimate is that the model has insufficient rudder authority because it is flying too slow for the rudder to generate sufficient force to overcome the dihedral.

You could reduce the dihedral or make the vertical fin ALL moving as in there is no fixed vertical fin, only rudder.

In addition if the rudder was greater in cord the rudder would really start to perform.

You cannot use a rudder design that you would use on a high speed model on a slow speed model and expect it to perform the same.

Just my thoughts!

Have fun!
Thats a good point about the dihedral ( and other good points as usual(y))

The rudder looks a bit small to me. In addition the hinge line of the rudder may be slanted. A raked back rudder will not have the same authority as one that is straight. Either way, it would be easy to add an extension to the existing rudder.
I agree it does look small. Temporarily adding an extension to see if it helps is a good idea. If it does help, you can redesign for a permanent solution.

You have an awful lot of ribs under the wing to form that undercamber. I wonder if, among the other things mentioned, they might be acting like a kind of v-stab and countering any yaw the rudder tries to induce.
That had initially crossed my mind but since the ribs are in line with CG/CP it shouldn't affect yaw.
 

Keno

Well-known member
#13
Ok all that has been said. All above have solutions and needs to thought about. A good friend of mine, long gone now passed, wanted the same thing as you, a real slow flyer. He really like the "Telmaster" which is a great flyer from Hobby lobby. He thought if it flew so well he would just add more wing. With the addition we and he could not control its direction, rudder plus aileron but it still wanted to fly in own direction. Very difficult to control. He took it home and removed the added wing portions and after the fix it was as again a Telmaster. lesson learned.
 

Hondo76251

Well-known member
#14
If you want to fly slow you need to manipulate a larger percentage of the available control surfaces to achieve the desired motion.

Look how much rudder I have to get for these little chuck gliders to have yaw authority:

20200507_214421.jpg

And remember, a smaller surface moving farther is less effective than a larger surface moving a little...
 

leaded50

Well-known member
#15
Very generally, you want a horizontal stabilizer/ elevator area 15-20% of your main wing, and a vertical stabilizer/rudder area 33% of your main wing.
 

quorneng

Well-known member
#16
Jetstreamer
I am not at all surprised you have limited rudder authority with such a wing. Those huge ribs holding the under camber will virtually eliminate the banking effect on the wing resulting from yaw. It won't bank so the turn will be very slow unless you have a huge fin & rudder.
For very slow speed flight you do better to use differential lift across the wing to create the bank. A plane will turn if banked and without a rudder input.
My suggestion for very slow speed flight is to have a fixed fin and use your rudder servo to work ailerons or even warp the wings. ;)