How much dihedral for rudder only?

[zbrubaker]

I'm working on a design for a parkflyer sized cessna 337. If I want this to be R/E/T for controls, is there a rule of thumb for how much dihedral to build into the wings?

TIA...Zeke

[Ollie]

The trade off is between scale like looks on the one hand and good rudder response and spiral stability on the other hand. If scale like looks are a high priority I would use 8 to 10 degrees of dihedral. If snappy rudder response is a high priority then about 12 to 14 degrees dihedral would be better. Dihedral on the root end of the wings does little good. I would keep the inner panels flat and put dihedral out beyond the tail boom junction of the wings.

[zbrubaker]

Originally posted by Ollie
I would keep the inner panels flat and put dihedral out beyond the tail boom junction of the wings.

This is exactly how I have it designed. Right now I have 5 degrees of dihedral on each side. I just want it to track straight without requireing a lot of rudder work to keep it straight.

[Ollie]

With only five degrees of dihedral per side, you may find it difficult to recover from tight highly banked turns. It shouldn't be too much of a problem if you stick to shallow angles of bank and wide turns.

[vintage1]

I have found the hard way that 5 degrees is minimum for high wing, 7 degrees for low wing. 10 dgerees makes an easy to fly plane, but does look a tad silly sometimes.

Parasol and sweepback both reduce need for dihedral.

[zbrubaker]

Originally posted by vintage1
10 dgerees makes an easy to fly plane, but does look a tad silly sometimes.

That's my concern...I'm going to try 7degrees to see what it looks like.

[BMatthews]

If it's going to be a slow flying model similar to things like the Lite Stik then you'll find it's often better to have a bit more dihedral for snappier response. I'd go with at least 7 degrees right off the bat for this reason. If it's a fast model then 5 to 7 should do fine.

Using upward bevelled wing tips can also have a bit of a tip dihedral effect. These are most often seen on flat bottomed wings where a stretched version of the rib is used to form a beveled angle up from the bottom surface and such that the top covering of the tip is level with the rest of the upper surface. If you do this make the tips similar to the rib in shape but about 4 times the height so that the bevel is a long low rise with enough area to act on the air quite well. CAD makes this sort of thing easy obviously.

[Sail 'n Soar]

I have found the hard way that 5 degrees is minimum for high wing, 7 degrees for low wing. 10 dgerees makes an easy to fly plane, but does look a tad silly sometimes. I've seen published dihedral suggestions matching Vintage1's hard-learned lessons and have been quite pleased with my own RET models when built within those bounds. However, the relative control responsiveness will depend on your airfoil choice and lift coefficient at cruise. The higher the CL at cruise (high camber foil), the more dihedral will be needed. Conversely, you can go with less dihedral by going with a very low to no camberl foil. You can also go with less dihedral if you go to lower aspect ratiios. My first RC, a rudder-only high mid-wing model, was very responsive with only 4 degrees dihedral. Of course, the aspect ratio was only 4.1 and the airfoil was almost symmetrical (1% camber).

[poulsbobill]

I have this plane silverlit plane ( http://static.rcgroups.com/forums/attachments/1/0/3/0/2/0/a1029311-11-SW1.jpg) a 2 channel that has a lot dihedral but still has nasty right hand turns into a death spiral. What do i change/bend?

Thanks

Bill

[BMatthews]

I have this plane silverlit plane ( http://static.rcgroups.com/forums/attachments/1/0/3/0/2/0/a1029311-11-SW1.jpg) a 2 channel that has a lot dihedral but still has nasty right hand turns into a death spiral. What do i change/bend?

Thanks

Bill

Hard to say without eyeballing it for alignment. But I can say for sure that the really small stuff like this is VERY sensitive to warps and side to side differences that you can barely make out with the naked eye. Use some rulers and other tricks to check that the wing isn't warped, or at least it's warped the same on both sides, and that the airfoil is cambered the same on both sides and that the fin is lined up right on the centerline and that the wings are not skewed in their mount. FInally you may just have too much right thrust.

[vintage1]

I have this plane silverlit plane ( http://static.rcgroups.com/forums/attachments/1/0/3/0/2/0/a1029311-11-SW1.jpg) a 2 channel that has a lot dihedral but still has nasty right hand turns into a death spiral. What do i change/bend?

Thanks

Bill

Death spiral means CG is too far back or it needs more fin...if it flies ok up and down wise, add a depron fin extension.

Oherwise add noseweight.

[poulsbobill]

Hard to say without eyeballing it for alignment. But I can say for sure that the really small stuff like this is VERY sensitive to warps and side to side differences that you can barely make out with the naked eye. Use some rulers and other tricks to check that the wing isn't warped, or at least it's warped the same on both sides, and that the airfoil is cambered the same on both sides and that the fin is lined up right on the centerline and that the wings are not skewed in their mount. FInally you may just have too much right thrust.
bmathews and vintage1

More fin? Make the unmovable part of the rudder bigger? I have read an article in quietflyer that talks about this.

Thanks

Bill

[vintage1]

certainly worth a try. Some people just tape a bit of clear plastic on. Its teh work of 5 mnutes to try it out..

[poulsbobill]

ok another question. If it is death spiraling to the right in a right turn and ok in a left turn ...could i make the dihedral bigger (larger angle)? on the offending side or smaller or is this not a solution?

Bill

Cant fly now as winds are too strong.

[Sail 'n Soar]

ok another question. If it is death spiraling to the right in a right turn and ok in a left turn ...could i make the dihedral bigger (larger angle)? on the offending side or smaller or is this not a solution?

Bill

Cant fly now as winds are too strong.

Either the problem is a basic instability problem accented by the motor torque or you have something aerodynamically different between the left and the right, e.g., effective washout in the left wing but not the right or wash in the right, but not the left. Do you have the same death spiral problem when you turn while in a glide? If the problem isn't a motor torque accented problem, then your goal is to make the left and right dihedral, camber, washout, etc., the same. Asside from the basic symmetry problem, as was mentioned earlier, a too far aft CG will contribute to stability problems as well. Where is your CG. Once you've convinced yourself that the rudder is aligned along the fuselage centerline, the wings are at right angles to the centerline, and the left and right sides are symmetrical, I'd try taping on a little ballast to move the CG forward a few percent to see what affect that would have. Like adding fin area, it's an easy thing to try.

[vintage1]

ok another question. If it is death spiraling to the right in a right turn and ok in a left turn ...could i make the dihedral bigger (larger angle)? on the offending side or smaller or is this not a solution?

Bill

Cant fly now as winds are too strong.

That's a warped airframe. PROBABLY tip stalling on the right wing..so its got maybe positive incidence there..sight along the wings to check..even a little makes a huge difference.

You will find the plane always wants to turm left if that's what's wrong. Till you force it into a right hander and then it snaps and goes down..