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Nov 29, 2016, 07:18 PM
AndyKunz's Avatar
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Discussion

Dihedral Effect


A friend is having "not so much fun" with Internet Exploder, so I'm posting this for him. https://www.rcgroups.com/forums/show...2#post36308346

Anyway, he needs a good explanation of dihedral effect on a model F4D that he wants to build. The plans for the Jetex version show dihedral.

So, would somebody care to explain to him why this is less than ideal, and why he might like having a functional rudder.

Thanks.

Andy
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Nov 30, 2016, 10:19 AM
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builderdude's Avatar
Thank you Andy.
Yes it's me...

I had actually typed out the entire thing and hit the post button, and Exploder said they encountered a problem and had to reload the page...and of course I lost everything and didn't feel like retyping last night... But Andy's got it summed up.
Nov 30, 2016, 07:53 PM
B for Bruce
BMatthews's Avatar
OK, I read the other thread.

Now this may or may not be what Andy was hoping for but given that it's a zoom and fun model with a very low aspect ratio I'm going to say to take the dihedral out all together and use elevons with no rudder. The model will mostly be flying fast at a lower angle of attack where the dihedral effect of the delta wing will, or should be, fairly minimal. And the low aspect ratio of a delta wing should avoid any adverse yaw issues.

That should set the stage for a nice simple and lighter elevons only installation on this fairly small model.

At least as long as you don't slow down too much and try to fly nose high. The way sweep angle acts like dihedral is linked to the lift coefficient and thus the angle of attack. So the faster it flies the lower the Cl and the less dihedral like effect.

Yaw MIGHT be a problem if you try to run down to landing with the nose high and a high angle of attack. At that point you might regret not having a rudder. But for the general sort of zooming around at higher speeds I don't think you'll notice. And as long as you don't try to drag the landings out to a tail first touch down attitude I don't think you'll have an issue.

You'll need to look into what the rocket motor igniters need for power. And it may not always blow the wire core away so I'd suggest a sprung return switch on the Tx so when all hell breaks loose when the motor lights off that you don't need to remember to switch off the igniter circuit before it sucks down the onboard battery pack.
Dec 01, 2016, 04:26 AM
Registered User
Not only should there be NO dihedral, but as in most well-designed fullsize deltas (smtg missed and not understood by the vast majority of hobby modelers) .... deltas are actually best served with some anhedral!




J-20





Hold a straight-edge across the L.E. in this 3-view >
Dec 01, 2016, 09:05 AM
AndyKunz's Avatar
Thread OP
J-20 - isn't that "well copied" vs. "well-designed" ??? Or are you referring to the pre-Chinese version?

Andy
Dec 01, 2016, 10:00 AM
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builderdude's Avatar
Okay I will go for zero dihedral, elevons only, and no rudder. That will keep things a little simpler as well as lighter.

Regarding dihedral on a flying wing, I built a cute little flying wing I called the Baby Gryphon.

https://www.rcgroups.com/forums/show...-RC-Conversion

Now this one I did put dihedral in...but just a little. It had no rudder and it had a small pusher outrunner on it. That plane flew GREAT! It was very stable, hands-off, and reminded me of the way a perfectly dialed-in paper airplane flew. However, it DID have a higher aspect ratio than the Skyray. I didn't miss the rudder though. What was interesting about the Baby Gryphon was that I built it without elevons or mixing. I built it as an experiment to see if I could have split elevators and ailerons, using an old 72 MHz radio, and it worked great. The antenna isn't visible because it is built into the wing, around the circumference.

Earlier in that same thread, I show a picture of an old Super Gryphon that I had modified for rudder. I always thought it was a mistake to put a rudder in that planform. But from what I'm reading here, maybe not. The Gryphon is first and foremost, a slope soarer. Therefore, maneuverability is key. I didn't think the rudder would do anything, but maybe it does. The Baby Gryphon had a lifting airfoil on it with reflex, but the Super Gryphon has a symmetrical airfoil.
What's the thoughts on that?
Dec 01, 2016, 12:04 PM
B for Bruce
BMatthews's Avatar
That SG would have ended up with some reflex just in the trim. So it would not be a totally symmetrical airfoil in the end.

With the BG you built with a touch of dihedral it might well have flown nicely with a rudder and elevators. But either way it's not like dihedral is a requirement if you're using ailerons. That BG you made would have been fine with no dihedral as well. Perhaps it would have needed a touch more pilot intervention to keep the wing level but it's not like it would have been a bad model to fly. With the dihedral it would be somewhat self leveling and straighten the wings out from mild disturbances.

If you were not RC'ing this present Skyhawk project I'd say leave in the dihedral too. But your intended method of flying it would make the dihedral not only pointless but possibly even lead to it flying inverted in enough of a different way than upright that it would be a slight bother. As in with inverted it might have a bit more tendency to want to roll off to either side. If you've flown any models with dihedral inverted for much at all you'll know what I mean. They end up trying to roll this way or that in an inverted turn.
Dec 01, 2016, 02:32 PM
Registered User
both the delta and flying wing (Nurflügel) were invented in Ye Olde Germany

except for an aerobatic design intended to maintain inverted flight, in a delta and Nurflügel dihedral causes instability and reduces efficiency (L/D)


this is an F3B design that was thoroughly tested in minute detail by experts there
hold a straight-edge against the wing to see micro anhedral




and THESE highly evolved & experienced experts use anhedral too




Albatross Encounter - The Biggest Wingspan in the World (2 min 11 sec)


Dynamic Soaring: How the Wandering Albatross Can Fly for Free (2 min 35 sec)
Last edited by xlcrlee; Dec 01, 2016 at 02:43 PM.
Old Dec 01, 2016, 04:35 PM
WankyKuntzz
A moderator felt this post violated the following rule: Trolling (Widespread).
Dec 02, 2016, 06:18 PM
Registered User
just thought I would repeat this to add something a bit more interesting for those who might have been distracted by the amusing troll/rave or whatever above and missed it

Quote:
Originally Posted by xlcrlee
both the delta and flying wing (Nurflügel) were invented in Ye Olde Germany

except for an aerobatic design intended to maintain inverted flight, in a delta and Nurflügel dihedral causes instability and reduces efficiency (L/D)


this is an F3B design that was thoroughly tested in minute detail by experts there
hold a straight-edge against the wing to see micro anhedral




and THESE highly evolved & experienced experts use anhedral too




https://www.youtube.com/watch?v=oVj428oy9zE

https://www.youtube.com/watch?v=uMX2wCJga8g

Albatross Encounter - The Biggest Wingspan in the World (2 min 11 sec)


Dynamic Soaring: How the Wandering Albatross Can Fly for Free (2 min 35 sec)
Dec 07, 2016, 03:52 PM
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aeronaut999's Avatar

highly evolved & experienced experts


Quote:
Originally Posted by xlcrlee


and THESE highly evolved & experienced experts use anhedral too
So do all modern flex-wing hang gliders

In some cases the leading edge is pretty straight as seen from head-on, but the three-dimensional shape of the wing still creates an anhedral geometry in the outboard parts of each wing-- See especially from about 0:13 to 0:20 in this video-- we often can see the top surface of the outer part of the right wing, at the same time as we can see the bottom surface of the outer part of the left wing-- clearly an anhedral geometry is present in these parts of the wing--

Hang Gliding fun (0 min 23 sec)


By the same token there is a dihedral geometry to the inboard part of the wing, but it is closer to the CG and therefore less significant in terms of the roll torque generated by a sideways airflow. So we end up with some anhedral overall-- maybe more than enough to offset the dihedral-like effects of sweep, maybe not, hard to say in any given case without carrying out some careful tests....

But then if you take a step backward in hang glider history by sweeping the leading edges back more, which reduces the aspect ratio and increases the taper, you get to a point where the outboard, anhedral portions are really tiny and the inboard, dihedral portions are really huge, in which case the more billow you build into the sail cut, the more dihedral effect you get overall... the glider in the photo would probably fly well hands-off but look how hard the pilot has to work to make it turn! (See photo)


Steve
Last edited by aeronaut999; Dec 07, 2016 at 10:42 PM.
Aug 20, 2018, 04:15 AM
Registered User

The Dihedral Effect


Fellow modelers. Thought I would weigh in on this after reading so
many rather passionate articles and studying some aircraft lineage from
the days before the need for speed eclipsed all else in airplane design.
Some observations if you please.

1. The B25 Mitchell prototype originally had dihedral only, the switched
to the "gull" type wing. This proved to be a very stable and forgiving
air craft to fly.

2. Many designs with anhedral wings today (DON'T FORGET TO LOOK
AT THE TAIL, IT'S DIHEDRAL) do this to balance roll effect of tail
in a slip (F104).

Now take a look at the good old Piper Cub/Cessna 172. A little dihedral,
with high wing (pendulum) makes for something that can be trimmed
and fly hands off.

Folding wing tips down does create the appearance of anhedral, but
it seems to be in there to minimize the effects of rolling away in the slip.
The wing is dihedral/anhedral.

And it helps yaw stability as well. Evolution put vertical stabilizers on
the wing tips. We put ours on the tail. I wonder.

Bob
Last edited by Deltabob9870; Aug 20, 2018 at 04:20 AM. Reason: Spelling
Aug 20, 2018, 06:46 AM
Registered User
Too much wing dihedral causes roll-yaw coupling, so-called "Dutch Roll"

Too little tail subfin or tail anhedral also causes "Dutch Roll"


https://b-25history.org/history/b25.htm

"Captain Frank Cook flew the plane for the Air Corps to test general handling characteristics. He determined the plane had a "Dutch roll" characteristic that he considered incompatible with bomb runs. The solution to this problem was to negate the dihedral of the outer wing panels. This gave the B-25 it's current "gull wing" configuration"




Note that in swept flying wings, including deltas, the rear wingtips are the TAIL and in albatrosses and gulls they also have a marked yaw vertical-stabilizer effect



further explained in this related post:


From my experience IF there is too much effective dihedral/polyhedral in the WING an easy fix is to add a subfin under the STAB .... or anhedral or anhedralled tips on the stab

For various reasons, not only because the flow at a stab which has negative (or less positive) AoA than the wings, the effect of dihedral in the rear is DE-stabilizing -- OPPOSITE to wing dihedral/polyhedral -- also and creates roll-yaw coupling ("Dutch Roll")



upright V-tail oscillation in turns





equivalent dihedral






effective tail anhedral (NDC FF HLG "SuperGlider")






equivalent anhedral






DE-stabilizing
yaw-induced roll due to EQUIVALENT dihedral of vert stab .... why adding rear sub-area under stab works!


Aug 20, 2018, 09:23 AM
Registered User

Balance


Good to hear from you. Have read you on other threads and love
your pics!

Absolutely agree on sub fin. Want to put mine on the wing tips
(anhedral/dihedral). And really good on deltas who have very large
tails indeed. Tough slow in cross wind. So Delta Ray may want to
have those wing tips DOWN.

Thanks,

Bob
Aug 20, 2018, 10:10 AM
Registered User
Exactly. As soon as i saw the first Delta Ray pics I thought that if I bought one I the first thing I'd do would be to carefully slice off the tips and reverse them L/R so that each tip would point down (less work for the Artificial Stabilization system). These swept wing "Nurflügel"/Flying Wing albatrosses & gulls found that trick out long ago: the albatrosses retract their tails in cruise flight as do the gulls, but the gulls' smaller tail feathers must still be proportionally broader because of Re considerations (when extended) and so seem to not be as retracted, but they are as much as possible, except for landing and sharp maneuvers.
Last edited by xlcrlee; Aug 20, 2018 at 10:44 AM.


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