3 ch. Dihedral?

[OhioMike]

Is there a rule of thumb or TLAR guesstimate for how much dihedral should be in a given wing? High wing/non-aileron? I understand that low wing/non-aileron planes may require more than high winger/non-aileron as well?

[Ollie]

There are two main considerations in designing dihedral for a nonaileron configuration. They are stability and control.

Dihedral results in a yaw to roll coupling that allows a plane to turn efficiently. When the plane yaws, the angle of attack of one side of a dihedraled wing increases and the otherside decreases. This results in more lift on one side and less lift on the other side. The plane rolls in response. When the plane banks the lift force is no longer vertical and the horizontal component of the lift force pulls the model in the direction of the turn. The more dihedral the less yaw it takes to produce a given roll response. The less yaw necessary, the less drag that will be produced by the yawed fuselage. The less yaw necessary, the less rudder deflection required and the less drag from the rudder. The greater the aspect ratio of the wing and the greater the rolling moment of inertia, the more control power is needed to produce a given roll rate. Generally speaking, the higher the aspect ratio of the wing the greater the dihedral needed to produce a given rate of roll. Gliders with high aspect ratio and a need to limit drag use lots more dihedral than low aspect ratio trainers with lots of thrust available to overcome drag.

Dihedral results in spiral stability. The more dihedral the greater the bank angle can be without the plane starting to spiral dive. Opposite rudder must be applied to halt the progression of the spiral dive. If, instead, up elevator is applied to try to keep the nose up the bank angle increases and the dive becomes steeper. Up to the critical angle of bank associated with the degree of dihedral the plane is spirally stable. If the controls are neutralized the plane will level itself automatically. The down side of dihedral is inverted flight which is more difficult to maintain the greater the dihedral. Trainers need generous dihedral to limit the work load on the student pilot.

To summarize: The considerations in selecting a dihedral angle are related to the tolerance for drag increase in turning, the desired spiral stability, the importance of handling qualities in inverted flight, the control power of the rudder and, the aspect ratio.

My advise is to pick a model that has the qualities that you desire and copy its dihedral without departing much from its aspect ratio, wing placement, tail area and tail moment arm. If you are clever enough you can design a wing mounting that allows you to adjust the dihedral between flights and experiment to see what dihedral results in the best compromises to meet your objectives.

Oh, and more dihedral makes cross wind takeoff and landing more difficult.

[BMatthews]

Another thing you can do if the wing is a basic rectangular form is to use upward beveled tips. This is where the top surface extends out past the last rib in an unbroken line and the bottom surface comes up to meet it over about 2 to 3 wing thicknesses to keep the angle within reason. The angled tip seems to add some "polyhedral" effect. I used this form of tip, basically the rib upper curve streched out, on a little rudder only model and it can do barrel rolls from level flight there is so much yaw-roll coupling. The dihedral in this case is also in the ball park but I've flown other models with similar dihedral but "normal" tips and they didn't perform as this one does. I can't prove that this tip form is that effective without making another wing but so far I'm not going to say it's a bad thing either.

I've attached a pic to show what I mean.

[Dick Huang]

OhioMIke,
For a high wing model flying on rudder,elevator and throttle use 9 degrees per panel of dihedral. You can see this with the included Playboy picture.
Dick Huang:)

[BMatthews]

A lot of the determination of how much dihedral to use is related to is also based on the size of the model and the speed it will be flying at. In Dick's case with the old timer flying at a fairly low airspeed the 9 degrees offers a nice snappy response without being TOO sensitive. I had a SAM OT Miss Tiny and first flew it with a 6x3 because I didn't know any better. The speed, size of rudder and throw coupled with close to the same 9 degrees of dihedral made it a true handful. But after the engine quite and the glide cut in the controls were BEAUTIFULL. The flying speed and dihedral were just not matched correctly. My little rudder only model that the wingtip shot is from has about 1/2 the dihedral as Dick's and with the Black Widow at full snarl I can barrel roll it from level flight with full rudder. And with a bit of extra speed from a spiral dive it actually FLICKS around so fast at full rudder that it looks like an aileron equipped 3D model. If I'd put in MORE dihedral it would just be too squirrley.

Olllie's description fits right into this scenario. Two different models with two different types of flight and planform. Each is right but they are different because their shapes and flying styles are different. He's also right in advising to look at other models that look the same and fly the same as what you are interested in and plagarize like heck... :D

[Grejen]

Hmm.. I keep hearing that for rudder only to have good control some dihedral is required. I have a GWS Beaver. With the stock wing there was quite a bit of dihedral. I wanted to get a more scale look so I created a more scale wing. Same area, but higher aspect ratio (8) and almost no dihedral. It turns just great. It now definitly likes more airspeed and you have to give opposite rudder a bit to roll back but it does still roll fine.

I have also built a small foam Se5 bipe with scale dihedral (around 2 or 3 degrees) and rudder only. It does have a huge rudder and pretty short fuse. Total area is more than the Beaver but effective aspect ratio (5) is much lower. Its is extremly responsive. It'll snap roll but its definitly not axial.

Would a totaly flat wing airframe not roll at all on rudder only?

[Grejen]

He's also right in advising to look at other models that look the same and fly the same as what you are interested in and plagarize like heck...

I wonder what Mr. Rutan was looking at when he drew the White Knight? :D :cool:

[vintage1]

my rule of thumb is 5 dg high wing, 7 deg low. minimum.

less than hat and you may get into un recoverable banks.

each plane is different f course, but that heas served me fairly weell.

[Grejen]

Hey Vintage1. It's your rule of thumb (and others reinforcement of it) that's confusing me. My Beaver has about 2 dg and the SE5 about 3. Now I'll have to go dig up a protracter and check (or do some trigonometry). I've seen other Beavers with almost flat wings and they too turn and recover just fine. Perhaps it's the large tail surface area helping out.

Question is - Can a totaly flat wing be banked with just rudder? I may have to wait till my Ryan NYP is compete to find out.

[Sparky Paul]

...

Question is - Can a totaly flat wing be banked with just rudder? I may have to wait till my Ryan NYP is compete to find out.
.
Simple answer, yes.
BUT!!!
With my Eindecker, (mid-wing) turning with the rudder is practical only for low bank angles.
It can recover from these.
Slightly steeper banks, and the elevator is needed to hold the nose up.
Steep banks with the rudder and the plane slips uncontrollably. Ailerons (which it also has) are the only salvation there, the rudder doesn't work to recover.

[HELModels]

my rule of thumb is 5 dg high wing, 7 deg low. minimum.

less than hat and you may get into un recoverable banks.

each plane is different f course, but that heas served me fairly weell.

If only I had this thread available before I flight tested the Storch. It suffered from violent spins and dives "coupled" with pilot confusion. I thught it had 5 degrees at least, but the tips were square - lazy builder. Her wreckage begs a rebuild.

[Grejen]

, and the elevator is needed to hold the nose up.
OK .. Can a Rudder/Elevator model have a flat wing? It seems that the answer is yes, but it may be prone to unrecoverable spins if banked too far. This is not the case with my Beaver but then it's wing is not quite flat.