Dihedral question up vs down? - RC Groups
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Nov 02, 2017, 05:43 PM
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psychedvike's Avatar
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Dihedral question up vs down?


My understanding of dihedral is that it adds stability and also adds to a models ability to self level from a roll. It reduces roll rate also. I've designed and built many scratch built planes and have found this to be correct. I'm playing around with building a Quinjet. This is a fictional plane from the Avenger movie series. The issue is that these planes in the movies have the dihedral in the wings tipped downward instead of upward. I my head this seams that it would place the CG higher above the wing and in fact do the opposite making the model less stable. It probably would slow the roll rate. As I look into trying to be true to the overall look of the movie's design I know I will of course need to make some changes to make it fly in the real world. The question is if the center section of a swept wing is flat for 1/3 of the total length. Let's say it's 12". The outer portions of the wing is also about 12" and is tipped downward about 5 or 6 degrees. Will I end up with an unstable plane? I hope this makes clear what I'm talking about. The cord at the root will be about 10" the wing tips will be about 5". The wing is swept back 25 degrees from the center line. It is nearly a flying wing. The fuselage extends forward of the the wing about 4" and aft enough for a pusher prop to clear the trailing edge. Two small vertical stabilizers in the rear. I haven't taken time yet to draw anything up yet. What do you think will it be stable? The sweep in the wing will help I think, but am I barking up the wrong tree?
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Nov 02, 2017, 09:43 PM
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That design is highly unstable .... but could be flown by an expert experienced pilot and/or with AS (Artificial Stabilization).


Quinjet Moments in "The Avengers" (1 min 19 sec)
Nov 02, 2017, 10:35 PM
B for Bruce
BMatthews's Avatar
With the tips full up so the whole wing only has a little anhedral I think it would fly OK. Won't be a great knife edge flier but I doubt that's what you intended

A lot of folks that build Ugly Stiks put in a few degrees of anhedral to improve the stunt flying abilities of what is roughly half way between a high wing and a shoulder wing if the fuselage is the original fairly deep design. And those are not "unstable". But they do require that you be on your toes and willing to give corrections when the model is tipped by a bit of turbulence..

I think that the tips fully up and in line with the inner panels as shown by the rear view at 30 secs into that clip that a model would not be unflyable. At least if you can keep the model specs acceptable for normal model standards for power and wing loading suitable foe the sized of the model.

You may find that in turns you need to hold a slight "outward" pressure to avoid it wanting to tighten up. Or depending on a bunch of other stuff perhaps not even then.

Of course it won't be naturally stable. You'll likely need to make some fairly slight but frequent corrections. Ideally this would not be any worse than one of the neutrally stable 3D flat foamy flippy flyers.

Now if you want to make it so the fans run for VTOL and the nose droops and the tips cant down then you're off on another topic. But as a purely "normal flying" model done in that flatter cruise setting angle of the tips I think you'd be OK.
Nov 02, 2017, 11:05 PM
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Quote:
Originally Posted by psychedvike
My understanding of dihedral is that it adds stability and also adds to a models ability to self level from a roll. It reduces roll rate also. I've designed and built many scratch built planes and have found this to be correct. I'm playing around with building a Quinjet. This is a fictional plane from the Avenger movie series. The issue is that these planes in the movies have the dihedral in the wings tipped downward instead of upward. I my head this seams that it would place the CG higher above the wing and in fact do the opposite making the model less stable. It probably would slow the roll rate. As I look into trying to be true to the overall look of the movie's design I know I will of course need to make some changes to make it fly in the real world. The question is if the center section of a swept wing is flat for 1/3 of the total length. Let's say it's 12". The outer portions of the wing is also about 12" and is tipped downward about 5 or 6 degrees. Will I end up with an unstable plane? I hope this makes clear what I'm talking about. The cord at the root will be about 10" the wing tips will be about 5". The wing is swept back 25 degrees from the center line. It is nearly a flying wing. The fuselage extends forward of the the wing about 4" and aft enough for a pusher prop to clear the trailing edge. Two small vertical stabilizers in the rear. I haven't taken time yet to draw anything up yet. What do you think will it be stable? The sweep in the wing will help I think, but am I barking up the wrong tree?
I looked at some pictures, there seem to be several slightly different sorts.

But the aerodynamics of all of them are appalling

So I think the anhedral will be the least of your problems . It's not normally a problem anyway.

You want something 'different'? Try this, from 'Firefox'. It was supposed to be a future MIG-31, but when it was time to build the real one Gospodin Mikoyan's ideas did not agree with the one in the movie
But the 'Firefox' movie plane looks quite unusual and the aerodynamics make reasonable sense, unlike the Quinjet, which appears to have been inspired by looking at old dumptrucks.



Nov 02, 2017, 11:30 PM
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psychedvike's Avatar
Thanks for the input guys and the pics Xlcrlee. There are actually two versions of the Quinjet in the movies. Here is a pic of the model I an using. I plan on increasing the size of the wing, but I am going to use the toy to build a buck for vacuum forming the fuselage. I will probably do a build log in the foamy forum. I have to work out the proportions. Should be a fun project. I just don't want to sink a lot of time into a plane with no chance flying well.
Nov 02, 2017, 11:59 PM
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Quote:
Originally Posted by psychedvike
Thanks for the input guys and the pics Xlcrlee. There are actually two versions of the Quinjet in the movies. Here is a pic of the model I an using. I plan on increasing the size of the wing, but I am going to use the toy to build a buck for vacuum forming the fuselage. I will probably do a build log in the foamy forum. I have to work out the proportions. Should be a fun project. I just don't want to sink a lot of time into a plane with no chance of flying well.
It'll be fine, model planes are remarkably uncritical.

You are wise not to put too much money/time into it. A friend built a 96 inch wingspan exact scale B-2 bomber with 4x 90mm EDFs, scale 4 wheel main retracts, scale drag rudders, and everything. It flew just like a Zagi and once it was a hundred yards away you couldn't tell the difference
Nov 03, 2017, 01:17 AM
B for Bruce
BMatthews's Avatar
You'll want to model out all those thick areas and other odd things. You'll ruin what lift you can get from the wing area otherwise. Use an airbrush and clever shading to add them in afterwards. Fly first, looks second.....

Quote:
...It flew just like a Zagi and once it was a hundred yards away you couldn't tell the difference
But the low close in passes must have made it worthwhile for him at least.... Besides I can think of a lot worse things than for a model of that sort to fly like a Zagi. I'd actually count that as a huge success!
Nov 03, 2017, 01:46 AM
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Quote:
Originally Posted by BMatthews
You'll want to model out all those thick areas and other odd things. You'll ruin what lift you can get from the wing area otherwise. Use an airbrush and clever shading to add them in afterwards. Fly first, looks second.....



But the low close in passes must have made it worthwhile for him at least.... Besides I can think of a lot worse things than for a model of that sort to fly like a Zagi. I'd actually count that as a huge success!
I'm more of an 'impressionist'. I like scale models and keep the outlines correct.

But you won't see me carefully scribing on the 18,000 !!! flush rivets on a F100

At first he cheated on the B-2. He started with a plexiglass fin on it. He gradually cut it down and eventually removed it completely. It was fine, though directional stability is a bit vague at low speed. You will notice on the real one that that both the drag rudders are always open slightly at low speed even when going straight. I believe this is to provide a degree of 'weathercocking' with regard to the direction of the airflow. I have no idea of its crosswind landing limitations.
Nov 03, 2017, 11:08 AM
An itch?. Scratch build.
eflightray's Avatar
Quote:
Originally Posted by psychedvike
Thanks for the input guys and the pics Xlcrlee. There are actually two versions of the Quinjet in the movies. Here is a pic of the model I an using. I plan on increasing the size of the wing, but I am going to use the toy to build a buck for vacuum forming the fuselage. I will probably do a build log in the foamy forum. I have to work out the proportions. Should be a fun project. I just don't want to sink a lot of time into a plane with no chance flying well.
A Google on - Avengers Quinjet - will find quite a few images, and some videos, including reviews of the Hasbro toy.

There also appears to be a 1/72 scale plastic kit available - http://fantastic-plastic.com/Avenger...atalogPage.htm

As some say, "anything can 'fly' with enough power".

Though personally I prefer, "most models can 'fly', if you build them light enough".

The XB-70 Valkyrie also had wing tips that could be drooped in flight.


Ray.
Nov 03, 2017, 12:01 PM
Registered User
It depends on where the wings and especially where the wingtips are w.r.t. the CL. If the tips are in the rear (delta, canard or swept Nurflügel config) that creates an anhedralled stab which is positively stable. I've gotten more than a little grief about that in this forum but for whatever reason anhedralled stabs and/or large sub-rudders/fins stabilize models.

Use of REAR anhedral in the greatly improved handling of the Gyroflug vs Rutan Vari/LongEZ (verts still vertical or even tilted inward at top under flight load)

https://wikipedia.org/wiki/Gyroflug



The anhedral gives much better tracking in turns and turbulence

Speed Canard Gyroflug @Volkel in de wolken Hamilton airshow 2014 (2 min 14 sec)


Think of the standard dart-like paper glider and high-supersonic aircraft; I forget in which places I read this (more than one place) but very low Re flows have smtg in common with supersonic, the tiny first KFm paper glider being one example.






anhedralled REAR tips




Last edited by xlcrlee; Nov 03, 2017 at 07:54 PM.
Nov 03, 2017, 02:14 PM
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psychedvike's Avatar
Thanks again everyone for the input. Now I have to build it to see how well it flies. I think it might be worth it to build two wings identically with both anhedral and dihedral and see how much difference it makes in the flight characteristics. I have started on the fuselage but just barely. I'll set up the wing core cutter today and knock out two sets of cores. I'll have to start the build log maybe tonight if I can get some good pictures.
Nov 03, 2017, 02:24 PM
B for Bruce
BMatthews's Avatar
.. Or just NO dihedral as in dead flat. Or split the difference and use roughly half the anhedral shown in the video clips. Small amounts of wing dihedral or anhedral are easily seen but don't have a strong effect.
Nov 03, 2017, 03:41 PM
An itch?. Scratch build.
eflightray's Avatar
For most 'unusual' models, a small scale flat sheet profile < chuck glider >is worth trying.
It's also a good way to find a CG starting point.

Ray.
Last edited by eflightray; Nov 03, 2017 at 04:29 PM. Reason: Added the type < >
Nov 03, 2017, 05:22 PM
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How does dihedral reduce the rate of roll?
Nov 03, 2017, 05:28 PM
B for Bruce
BMatthews's Avatar
Quote:
Originally Posted by bogbeagle
How does dihedral reduce the rate of roll?
I've read accounts in forum postings where wings with higher amounts of dihedral that you'd use with rudder control and no ailerons that adding ailerons tends to give a lazy response in roll compared to the same wings and ailerons with less dihedral. Not sure if that's what was meant or not.


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