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My design sports jet - wing incidence and washout

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My design sports jet - wing incidence and washout

Old 11-03-2023, 06:13 AM
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Jeroen.surf.nl
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Default My design sports jet - wing incidence and washout

Hi everyone,

I'm designing a sports-jet from scratch, see pictures. Needs some minor refinements, but this is basically the shape I’m going for.

Before I start working on the internal structures, I want to make sure the flying geometry make sense.
At the moment I'm not sure what wing incidence to use and how much washout at the tip.

For the 1st prototype I was thinking to use zero angle of attack at the root en 1,5 degrees washout at the tip.

The wings are straight, they have no dihedral or anhedral.

Any ideas/advice?
Perhaps someone knows what a very well flying jet such as the sebart Avanti or a tomahawk Futura uses for this?
Thanks in advance



Old 11-03-2023, 07:40 AM
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Malcolm H
 
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With regard to incidence I assume you are talking relative to the thrust line of the pipe/motor? If so, you will require a small amount of positive incidence so that the wing will generate lift without the need for up trim on the tail. This is especially so as your proposed 1.5 degree washout will effectively give you a negative incidence overall. I would start with + 0.25 relative to the pipe axis with perhaps -.25 if anything at the tip.

Also you mention that the wing currently has no dihedral. While sweepback produces a small dihedral effect from your renders you don’t have much. If you build that wing flat you will almost certainly end up with a model which adverse rolls on application of rudder. While this can be offset with rudder to aileron mixing, it would be much better to add a small amount of dihedral to try and minimise any mixing. The problem is how much? I would start by making the top surface of the wing flat, allowing the thickness reduction to introduce a small amount of dihedral but be prepared to add more if the model still adverse rolls.

Unfortunately getting a truly neutral design will always need some experimentation to get right unless you have some prior experience in the field.

Malcolm
Old 11-03-2023, 06:41 PM
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highhorse
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Jerome, I could not agree LESS with the above post. No dihedral means “adverse rolls” (whatever that is) with application of…rudder? No. -.25 degree washout at the tip? No. Dihedral to minimize mixing? No. TOP surface of the wing FLAT? Just hell no.

Do yourself a favor and just ignore everything in that post above. There’s so much wrong with it that it’s worse than useless, it’s actually backwards in several respects.
Old 11-04-2023, 05:01 AM
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invertmast
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Originally Posted by highhorse
Jerome, I could not agree LESS with the above post. No dihedral means “adverse rolls” (whatever that is) with application of…rudder? No. -.25 degree washout at the tip? No. Dihedral to minimize mixing? No. TOP surface of the wing FLAT? Just hell no.

Do yourself a favor and just ignore everything in that post above. There’s so much wrong with it that it’s worse than useless, it’s actually backwards in several respects.

So if its wrong, then what is right? How can you say no about something when you dont even know what it is (adverse yaw).

if you’re going to claim someone is wrong, you ought to atleast explain why, otherwise your post is pointless.
Old 11-04-2023, 05:48 AM
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If the front/ rear views are accurate, then it looks like you have designed the wing with the bottom surface flat. With a tapered wing, that will automatically build in anhedral, as dihedral/ anhedral is measured at the wing section chord line, not the upper or lower surface.

For a sport jet, I would build the wing straight, with no washout. The winglets will help with tip-stall issues, although I have never designed and flown my own sport model to provide that learning experience of what works and what doesn't.


Paul


Last edited by JSF-TC; 11-04-2023 at 08:01 AM.
Old 11-04-2023, 05:49 AM
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Thank you Thomas for your comments. I don’t want to get into a slanging match with Highhorse (appropriate user name!) but will clarify the following for his benefit.

Adverse roll refers to the tendency of an aircraft when rudder is applied, for example in knife edge flight, to roll against the direction of the applied rudder. Adding a very small amount of dihedral counteracts this tendency by introducing a roll couple in the direction of the applied rudder. I’m not going into the explanation of why the above happens but trust me it does. In the early days of pattern, long before computer radios, we would often saw through the centre section of one piece wings and either introduce a wedge to reduce dihedral or close the cut up to increase it. Often this would be repeated several times until the roll with rudder was eliminated. Of course with modern jet models where the wing panels are mounted on tubular spars this is impractical and so I was suggesting building in a slight amount of dihedral and mixing out any remaining couple from there.

With regard to the washout question, personally I wouldn’t build any in. I’m assuming the OP is worried about tip stalling at slow speed? If it was me and I experienced this at altitude, I’d add a small amount of crow instead of washout. I was just trying to stop him building in the large amount of washout he was proposing which wouldn’t have been good from a longitudinal trim point of view.

Incidentally, adverse yaw is a different issue caused by the down going aileron moving into higher pressure than the up going one, causing more drag on that side which yaws the anircraft away from the roll direction. It’s fixed by adjusting aileron differential.

Think it’s probably best I shut up now!

malcolm
Old 11-04-2023, 06:15 AM
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highhorse
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Invertmast, Is it not obvious? I certainly thought so. Before you describe my post as “pointless”, you might look at your own and tell me what leads you to believe that I don’t understand adverse yaw? I am WELL aware of the definition of adverse yaw sir. Can you find even one mention of adverse yaw in the subject post? No sir, because the first mention of yaw of any sort is in your own post, and you’re therefore deducing that I have no understanding of a term previously unmentioned anywhere in this thread :-)

Re-read his post. Carefully. I think that once you compare what he actually wrote (…adverse rolls with application of rudder) vs what you seem to have assumed he wrote, you’ll have spotted the obviously made-up, backwards, and worse than useless nonsense for yourself.
Old 11-04-2023, 06:41 PM
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Someone has obviously never flown a Top Flite Contender! A well known phenomena with that model, application of rudder did indeed induce an opposite (aka adverse) roll.
Top-Flite Contender Post 20
Old 11-04-2023, 08:09 PM
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Originally Posted by DP01
Someone has obviously never flown a Top Flite Contender! A well known phenomena with that model, application of rudder did indeed induce an opposite (aka adverse) roll.
Top-Flite Contender Post 20
Of course I did. That issue had zero to do with the wing and everything to do with the funky rudder layout which induced a roll moment. Don’t confuse the apples with the oranges.

The follow-on post regarding knife edge flight is indeed quite clarifying of the gentleman’s intended point, which I therefore cede.
Old 11-05-2023, 12:52 AM
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Originally Posted by DP01
Someone has obviously never flown a Top Flite Contender! A well known phenomena with that model, application of rudder did indeed induce an opposite (aka adverse) roll.
Top-Flite Contender Post 20
…..or Doug Spreng,s superb design, the Twister. Left rudder, right roll and vv , the large fin and rudder acting as a vertical aileron, overcoming the lateral stability of the flat top wing !
By condemming a proposed flat wing on a swept wing jet, suggests to me that HH does not really understand the aerodynamic issues.
A rough rule of thumb is that 10 degrees of sweep, equals 1 degree of dihedral. Combine that with the small dihedral effect of a tapered wing, the flat top should be fine !
I would , however disagree that washout is not desirable.
I have a PST Revision, 300 flights, a really beautiful and docile flying machine, one of the nicest I have ever flown. The wing has an incidence of about 2 degrees, washout about 1.5.
It is reluctant to spin upright, no problem inverted !
I would be happy to make exact measurements and copy the root and tip aerofoils of the Revision for Jeroen, they work !
Very nice looking jet, go for it !

….as an addendum :
I made a few measurements on the Revision:
Airfoil is symmetrical. T/C ratio is 13%, tip airfoil is same section, no thickening.
Incidence Just over 1.5, washout similar.
Wing upper surface essentially flat, perhaps 1/2 degree dihedral.

Last edited by David Gladwin; 11-05-2023 at 10:57 AM.
Old 11-05-2023, 05:10 AM
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Originally Posted by highhorse
Invertmast, Is it not obvious? I certainly thought so. Before you describe my post as “pointless”, you might look at your own and tell me what leads you to believe that I don’t understand adverse yaw? I am WELL aware of the definition of adverse yaw sir. Can you find even one mention of adverse yaw in the subject post? No sir, because the first mention of yaw of any sort is in your own post, and you’re therefore deducing that I have no understanding of a term previously unmentioned anywhere in this thread :-)

Re-read his post. Carefully. I think that once you compare what he actually wrote (…adverse rolls with application of rudder) vs what you seem to have assumed he wrote, you’ll have spotted the obviously made-up, backwards, and worse than useless nonsense for yourself.

so i mistakenly put adverse yaw, instead of adverse roll. Shoot me, im human. *eyeroll*

it still doesnt change the fact you come in on your high horse and berate someones suggestions with your own opinion and provide no information of why the OP should “just ignore everything in the post above” with counter suggestions

go back to the ama discussion forum with that crap.
Old 11-06-2023, 02:58 AM
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Jeroen.surf.nl
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Hi guys, that's quite a discussion, please be nice to each other

Some good tips and insights in the replies, thanks all for responding.
Thanks David Gladwin for providing actual numbers and measurements! I used the airfoil of an Freewing Avanti V2 as inspiration for the root, and a symmetrical NACA0011 at the wingtips. For the horizontal en vertical stab I used the symmetrical NACA008 airfoil.

I'll update the model so it will have some incidence and some washout and also and build the wing more straight instead of the flat/horizontal bottom that it has now. Let's see how that looks.
The results will be posted in this thread.

Old 11-06-2023, 06:07 AM
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Well, remember the incidence I quoted was for the symmetrical wing section as used on the Revision. If the Avanti section is semi symmetrical you may need to reduce that angle to, say, 1 degree. Perhaps see what incidence is actually on the Avanti and copy !
Anyway, good luck with your project !

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