jarvis johnson

My Feedback: (15)

Hey Guys
Recently I seen a few Pattern and 3D planes , with Wings and Stab fences, do they really improve the tracking and stability of a pattern plane ? I'm thinking of putting a set on my Vanquish 2 meter, is it worth all the trouble of making them. If so, what should I make them out of ?


Thanks

rcpattern

My Feedback: (45)

Jarvis,

That stab fences definitely help. I did not like the wing fences on my Wind S Pro, and after discussing with some engineering types at work and other places, the reason the wing fences aren't effective as they are in front of the center of pressure, so they can actually destabalize the plane in yaw. Others may argue this, but I've had some really smart people explain this phenomenon, and I could feel it. I know people that are running contras though, that they do seem to work, and work well. The stab fences definitely add some yaw stability on some planes. I think a lot of it depends on the design though.

Arch

DaveL322

Jarvis,

The answer is....it depends.

Increasing the amount of side area aft of the CG (such as stab SFGs) will shift the yaw characteristics of the plane to being more stable, more damped, and more likely to yaw into a crosswind (especially in turbulent air), and the amount of rudder input require to generate a given amount of yaw will increase (and drag will increase as well when in the yawed state).

Increasing the amount of side area forward of (or at) the CG will decrease the yaw stability, make the plane more likely to drift with a crosswind, and reduce the amount of rudder input required to generate a given amount of yaw. Of course adding this type of side area will effectively yield more KE power.

I have seen examples where the plane was minimally damped in yaw, and prone to skidding in yaw with any amount of rudder input. In this instance, adding SFGs to the stab both increased yaw damping and increased KE power as the fuse was likely to skid in yaw.

If you don't like the amount of KE power or yaw stability (or the balance between the two), SFGs on the wings / stabs are an effective way to change the balance.

Doug Cronkhite

My Feedback: (34)

I think the biggest reason to add stab fences is if your airplane is lacking yaw stability. As canopies get larger, and move further forward, you need to increase the yaw dampening to account for the increased area ahead of the center of pressure. It doesn't take much especially if you're adding those fences out in clear air. If you're running a contra, you'll likely need even less.

serious power

Hi Doug,
Why do you suggest that less ,I assume fence, is likely to be needed with a contra.
We have been experiencing the opposite. At least some have , ie us contra users.
When a contra is fitted to most existing models a loss of yaw stability is noted.

Brian

Doug Cronkhite

My Feedback: (34)

Because you then have the entire vertical stab working essentially in 'clean' airflow. I don't think it's so much a loss of yaw stability with a contra as there is a lack of a 'breakout' force in yaw. It just takes less force applied in yaw to upset the airplane. The best solution may be to use small fences at a small angle to recreate the breakout force. The same sort of thing is created by the fluted trailing edge seen on airplanes like Seba's Wind and Mythos airplanes.

It's entirely possible this is just a sematic difference though as they both result in the same 'feel' of the airplane in yaw.

serious power

Hi Doug,
Interesting, and again I am in agreement.
With the contra the 'battle' of opposing opposing forces (right thrust v p-factor/gyro,,precession & torque) is gone as are the compromises that result from sorting that lot out.
What is there then is best described as being more sensitive not as unstable.
Thanks.

Brian

MTK

Brian,

I flew Dave L's contar equipped Bravo last year and absolutely loved the way that plane felt. I told Dave it is one ofthe best if not the best model I have ever flown.

It has "fences" on the stab and wing and a ventralstrake that starts at around the cg and goes to the tail post. Although the model has about 50% the area of the fin-rudder included in the "fences" and ventral fin, it felt just sensitive enough in yaw; ie-well damped but with enough authority to react NOW! Nothing goofy on Dave's rudder either

I think that the contra equippedvertical stabsneed to be considerably larger than the current crop. Maybe as large as +25%, but maintaining rudders fairly close to today's sizes. Maybe one way is to add ventral area while maintaining the dorsal side as is today

Jetdesign

My Feedback: (8)

One thing I never hear mentioned is what happens if the wing/stab fences are not perfectly installed? In other words, what would be the impact of someone installing crooked or warped fences?

I hesitate to add fences to my models for two reasons: 1) I don't fly the airplane to its limits as it is (meaning I will benefit more from learning how to fly the plane than adding 'fixes') and 2) I don't know that I would be able to tell if there was a slight negative effect of something not working the way it should, and it hurting me more than helping me.

I think most modern pattern planes can be setup for impeccable tracking and stability out of the box, without fences. That is kind of the purpose of these planes, and if a plane isn't tracking right or stable enough, there's probably something else wrong. OR, you are at the top of your class and can actually understand the benefits of setting your plane up a certain way that reduces tracking/stability and gain it back with fences.

serious power

Hi Matt,
Dave is someone for whom I have a lot of regard.
I believe that all pattern models will/do fly better if contra driven provided the yaw 'sensitivity' is damped correctly/adequately.

I like my goofy rudder flair.
With this in place the rudder is still more effective than on the same model without a contra but the model is stable in yaw / tracks well.
So for 'normal' flying less rudder throw is required.
This may go some way to offset any extra drag the flair may produce.
I believe this 'extra' drag to be very little in the greater scheme of things while giving the desired benefit.
The flair is very likely to have the additional benefit of making the rudder more effective at larger deflection angles.
This will/does help with all those manoeuvres that use larger rudder deflections.

Brian

Rendegade

You know that back in the 60's when spoilers were first introduced by Jim Hall and Chapparal (look up the 2E) Eric Broadley from Lola was heard saying: "If Hall had the suspension right he wouldn't need that wing".


I'll let you mull that one over.