jetster81

I have an FB Hawk 120 and am curious as to whether to balance the all moving tailplane, I have made a simple jig up and at the moment it would need about 105 grammes in the front of the tailplane to balance, any suggestions?

Dave Wilshere

Depends how many servos, using two don't balance.

GrayUK

I have been using all moving tail planes for more years than I care to mention.
I have never, mass balanced them, use a good slop free mechanical link and a servo 'man' enough for the job..
Check pivot point, on tail plane, as a rough guild I pivot with 66% area behind the pivot.
The 33% in front cancels out 33% behind, then the servo only has to move 33% of the area. (its slightly more complex than that but 'roughly its a good guide)
I have seen many kits where the area balance is wrong, this produces a large load on the servo.
An aerodynamically balance tail plane should produce very little additional load on the servo over and above normal elevator loads.

Paul G

In the Haze

As above, built a few Hawks and the weight needed to balance them would have made a serious difference to the CG, so just made sure all linkages and servos were top notch and never had a problem.

I could refer you to a thread that brought out the best in everyone, including hanging stabs out of car windows doing excessive speeds etc,,,,,,,,,,,,,,,,,but I won't.

essyou35

My Feedback: (11)

It depends if they are tail heavy or nose heavy.

Mig 29, stabs semi balanced. Without it, the servos (400+ oz) would buzz loudly holding them flat, turn off the power and the stabs fall fast.


That's the only reason I did it. It took nearly 4oz in each stab.

HarryC

My Feedback: (1)

That is highly aerodynamically unstable and will produce a significant load on the servo. The aerodynamic centre is at 25% MAC (plus or minus up to 2% depending on section but most are clustered at or very close to 25%). If the tailplane is symmetrical then if the pivot is forward of that is stable in that it wants to blow back towards level, but if the pivot is behind 25% it is unstable and will try to move even further away from whatever position it is put in. So as soon as it moves away from an AoA of zero, the servo isn't trying to move it, it is trying to hold it back from flipping to a vertical position.

Dave Wilshere

Jetster81
What I said on the phone ;-)
D

dbsonic

My Feedback: (3)

Oli used to have a great article on this. Some degree of mass balancing is done to lessen the impact of moment loads due to vertical accelerations. This includes vertical gusts, hard landings and depending on the roads and how you constrain the surfaces, transport to the field(which can bang the crap out of your servos at the limit inducing wear). Since there are several benefits, I mass balance stabs to some degree now.

GrayUK

I think you miss read my post.

HarryC

My Feedback: (1)

What did I misread Paul?

stevekott

I have a JL F-18C, YA F-18C Single, SM F-18F, SM F-5 and a FB F-4E. All have Flying Stabilizers with one servo on each side. None of them are balanced and they all fly great .. no flutter.

That being said, slop will kill you. IMHO a little bit of free motion is much more crucial in starting an oscillation.

I think of a pendulum. If you put a little extra weight on one side of the pendulum it just moves to one side the point where it oscillates. If you clamp out the free motion the pendulum really can't swing back and forth at all.

Of course you are dealing with aerodynamic forces acting like the gravitational force. It can be much stronger and more violent. What you want to avoid is a standing wave where the elastic wave goes from one position and the deflects back over to the other position with the aerodynamic energy amplifying the wave. You are more likely to have problems when you have very little torsional load in one direction. By that I mean straight and level flight rather than climbing or diving. Think of a flag fluttering in the breeze verses a sail on a sailboat with side forces on it.

The only flutter I ever had was on a Eurosport that had some sloppy linkage on the rudder. At 150 mph it would start fluttering and the whole Veritcal Stabilizer would start to shake violently.

I put a stiffer double sided servo linkage in place and it solved the flutter problem immediately.

Happy Flying .. Good Luck!

GrayUK

all my designs use that priciple...never ballanced...never had flutter. .blowback...or any other issues. .worked for me for 40 years.

erbroens

My r/c experience in F-15/F-16/F-18´s ... never balanced their tailplanes.

Also some pics for you take a conclusion..

ravill

My Feedback: (11)

Nah....

dbsonic

My Feedback: (3)

neat pics but complete mass balancing is not recommended as it helps to counter the vertical gust perturbation (i.e generating a proper AOA response to the gust). So you want it slightly rearward. Again Oli's paper on this explained it much better than me, an aero engineer wannabe.

HarryC

My Feedback: (1)

Paul, it works if by luck you have a servo powerful enough to hold against the aerodynamic force. But that's a method that will go badly wrong one day and should never be advocated as something other people should do. Pivot at 25% MAC for minimum aerodynamic and hence minimum servo force requirement.

jetster81

Thanks for your input gentlemen, got the old grey cells going. I have just measured my tailplane at the root and it is 23cm, the pivot bar is situated at exactly 11cm from the leading edge. This is how it was supplied which makes it just forward of 50%. I will be using good quality titanium 35kg servos that in test are Very strong and have an excellent hold capacity.

HarryC

My Feedback: (1)

That's not a valid measure of anything. The Hawk has a swept and tapered tail so 25% MAC is somewhere far behind 25% of the root and may well be at the location of that pivot.

jetster81

Sorry, my English sense of humour getting the better of me. Just done a quick calc and the MAC shows to be virtually at 30% from the rear of the root whilst the pivot arm is still at a tad behind 50% similar to scale.

Chipper Griewahn

My Feedback: (4)

What do the instructions say??

essyou35

My Feedback: (11)

Uhm, this has nothing to do with the balance of the stabs. They manually put them that way to reduce the load on the hydraulics. Otherwise gravity keeps the system pressurized.

jetster81

There are no instructions with this kit

icepilot

Here is a fun experiment that a friend of mine did here in Norway - he may have broken a few traffic rules, but it kinda illustrates the point quite well!
I must admit that I had never balanced my stabs before, but now I have..
https://vimeo.com/64549836

lightningmcnulty

My Feedback: (11)

I have little knowledge either way on this subject but I would think the aerodynamics at the side of a car just above the mirror are very different to that of the side of a slick jet.

Maybe in a different environment the result would be the same but I can't take this test seriously.

dbsonic

My Feedback: (3)

Tor, very interesting test. Thanks. Glad I mass balanced mine already. The initial reason I did this to the SM F16 was because i noticed one elevator was heavier than the other. So part of the reason was to equalize both sides and then attempt to balance for the reasons stated.