Hi All
Quick question.
Why do we setup Anhedral in the stabs of our current pattern designs? I think it's to eliminate some type of coupling between control surfaces. What I have found is that mass production ARF's have a hard time getting the geometry correct causing bigger problems than any coupling benefit. Am I the only one seeing this or just unlucky or is it just a non factor? There is no doubt that Anhedral makes the set up much more difficult.
On a different subject I am fing that in order to eliminate the rudder-elevator mix in knife edge I am running a 25% MAC C/G. This is pretty far forward, is this what most of you are doing? Or are you settling up for a more rearward C/G 30%-35% and utilizing some rudder to elevator mixing?
Dick

The reason for anhedral in a stab is to decrease the Dihedral effect (tendency for the aircraft to roll back to level). That comes from my years at Texas A&M Aerospace department.

I've "heard" that anhedral in the stab on models stabilizes inverted flight, which stands to reason in engineering.

If you're talking about the DJV, 25% MAC is where I finally started removing the rudder-elevator mix on mine too but called for a little more wing incidence at that CG. I found I was very satisfied at 27-28% MAC with this plane without having to change the incidence. If you like 25%, try increasing your wing incidence and I think you will find you get happier.

Dick,

Do you mean anhedral? I don't know what Au-Hydral is.

As far as using a cg set-up at 25% MAC, that's what Bryan Hebert uses in his triangulation trimming technique. Actual practice is somewhere between this far forward to somewhere aft. Current pattern models will fly very well with very aft CG location (>35% MAC) but they will likely need some mix. There is no set hard rule on where you place the CG; to some extent it's personal preference. A little e-mix is no big deal ( a few % ); a lot of e-mix is a problem for many.

Remember that all the great trimming in the world is always trumpted by great flying skill

Scott
Thanks for the reply. I am finding that with the contra drive the C/G has become even more critical as it definitely makes the rudder coupling worse if not setup correctly.

Matt
I understand all that. My question was are most flyers compromising the actual no mix C/G or not. As stated above the Contra Drive is having an impact on that and any stab/elevator geometry issues are greatly amplified due to the decreased yaw stability of the drive. I am getting close on the final setup but needed to understand better the relationships of anhedral and C/G. I am beginning to learn that both of those issues are much more critical with the Contra Drive. The aircraft is better than anything else I have ever put my hands on and improving with almost every change I make.

Thanks for the input.

Dick

An anhedral stab will cause a relative pitch up in knife edge. Relative, of course to the pitching you get with a flat stab in the same position. Of course, lowering the stab position on the fuselage will do the same thing.
A low stab position is not very attractive. Keeping the position higher and adding some anhedral is more attractive. Also if you already have the fuselage mold done and you end up pitching to the belly in knife edge, adding anhedral to the stab is an easy fix.

I don't think anyone is adding anhedral to the stab to fix wing dihedral problems in models. It's easier to change the dihedral. In the case of the F-4, there were probably a lot of other difficulties preventing lowering the wing dihedral. I expect they were not too concerned about knife edge coupling.

John

They did the anhedral in the F-4 to get the stab out of the wing wake thus make it more effective especially at higher angles of attack.

At any rate Dick, try leaving your CG at 25% (because this is where mixing starts to be removed from the DJV) and increasing your wing incidence. I really think you'll find some merit to this. This also follows Bryan Hebert's practices.

Scott
1. Just to make sure we are on the same page you actually calculated MAC and set based on that, not 25% of wing root. They are definitely not the same.
2. What was the problem after going to 25%MAC?
3. At what degree incendence did you go to on wing. I am already at .5+!

John
Thanks for the explaination.

MAC was calculated per the website I issued in the Kfactor (if you need it, I'll send it to you via e-mail). It is not 25% of the wing root. There was no problem at 25% but the plane felt more "comfortable" to me at 27-28% for the spins and snaps. As Mr. Kimbro said, there is a lot of truth to the CG being a matter of personal preference with today's 2M ships. We discussed this many times you and I at contests and even back then, we were playing with different %'s. I got all the way to 25% and Iwas able to start removing some of the mix. What Ifound was that I personally flew the plane better with the CG just slightly aft of that. I wound up just a tick over .5+ per Hebert after discussing with him but wound up going back to 0 when I went to 28% MAC. You may just need a little more positive incidence with the contra. I also think that with that adjustment, you'll see less mixing as well. I think the slower the DJV flies, the more + incidence you will need to carry. Ionly say this due to trying all sorts of props from the 20x15 all the way through 22x12 which obviously varied the flight speed envelope drastically. The slower it flew, the more it seemed to need as witnessed by Bryan Hebert. I spent 4 days with him and this is what Ilearned.

I remember hearing an FAI pilot talk about how anhedral helped 'clean the plane up' in looping maneuvers, exits I think. Don't remember or know any more than that though.

Dick,
  I know NOTHING about the contra drive that Mr. Brenner made other than what you told me and what I saw at the Nats but...  did you put the 0.5 degree down thrust and remove the right thrust completely?<br type="_moz"/>

Many years ago.... In a galaxy far far away... Oh wait wrong forum

Seriously. I don't remember if it was somebody like Don Lowe or who but they "said" they took the anhedral out of their stab and lowered the stab by the median amount of the anhedral, the plane flew the same. Simply put. If the tips of the stab drooped by 1 inch, stab was made flat and then lowered in the fuse by 1/2 inch.

Supposedly action this had the same effect as adding anhedral. It was said the vertical location of the stab was the biggest factor in the aerodynamics of things.

Now I may be wrong as this was in reference to what we call "classic" pattern today. Might be a whole new ball game with the modern ships.

Ken

Scott
Yes, I have had a Combo C/G MAC program for years.
Yes, you can read about it and see Pic's on the Contra thread on electric forum.
Dick

In practice, you can move the stab vertically +/- 2", at least, and the result will be nil.

However, you can change the shape of your rudder and affect coupling a great deal.

You may also add a dorsal strake aft of the wing TE and affect coupling considerably.

Aerodynamics are the same whether one flies classic or current stuff. Physics don't change to suit the model. However, the current larger and slower models show the effects far quicker than classics flying at ballistic speed.

Matt
You really hit the nail on the head with that statement.
That is exactly what I am dealing with and the reason I started this thread!
Thanks
Dick

I stand corrected Like I said it was long time ago......

Your assessment does make more scene though. If only I had gone to college and majored in aeronautical science Instead I was born with a wrench in my hand. All I can do is fix 'em LOL

Know what you mean....Not everyone has that talent. Turning a wrench is a daunting task to some, maybe many.

One other thing to keep in mind....I can think of only two or three aerodynamicists in this game. Not many. What we discuss, sometimes passionately, regarding Pattern model trim is not a subject a degree in aerodynamics will solve. Sure, it helps to have engineering knowledge but even that is not mandatory. We all have learned by reading everyone else's scrible and trying things.

The only things that helps are common sense, understanding that, in spite of the fact that one can't see it, air produces a terrific amout of force as it spirals past the airframe, the guts to experiment and being truthful to yourself of the result.

That's the tough one right there. That is why I gave up helicopters for the most part. I got truthful with my self.

Anhedral of tha stab has the same effect than placing it lower or higher: deals with coupling while in knife edge position. Better said: correctly used, helps to cancel the mix needed to correct the yaw+roll coupling.
In technical terms; helps to input a "0" (or close to it) to the coupling factor between roll and pitch coefficent.