Balanced control surfaces

[Andy Boylett]

Now I am building a half scale model I have been reading up on control surface balancing for full size planes. Now I understand what it is all about I have some questions for those with the experience in models.

Anyone got any experience or thoughts on this?

[Mark Partington 2989]

Andy,

Balancing of control surfaces and the reasons for flutter can get very involved.

To clear up your terminology firstly;

Static balance is more commonly known as mass balance, and the CG of the surface will always, (from Spitfire to Airliners), balance in front of the hinge line. Modern fully powered surfaces almost always do not get mass balanced as you surmise due to the immense power from the hydraulic actuators, (exception being all-flying surfaces such as tailerons).

Dynamic balance is more commonly known as Aerodynamic balance, and is mainly used to reduce the effort nneded to move/hold the surface in a new position, (it's also a convenient place to add the weight for Mass balancing).

Now flutter - assuming that the servo/actuator/pilot is strong enough to move the surface, the biggest effect on reducing flutter is rigidity of the surface, especially torsional rigidity), i.e. you don't want the surface bending or twisting in flight.

Sorry I can't continue at the moment, got to go to work but I'm quite happy to answer any question you might have if you'd like.


Mark.

[Andy Boylett]

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[Mark Partington 2989]

Thanks Mark,
I am particularly interested in what measures are taken with our model planes re preventing flutter, such as your tip re stiffness of the control surface (thank you).

All of the hinge points on my control surface are at the leading edge of the surfaces so there is no opportunity for mass balancing. This means that I need to get the servo torques large enough to prevent flutter. Twenty five years ago I could calculated this, but now may be a little difficult . Has anyone come across any programs to help with this.

Andy,

The main thing to look for is a slop free linkage, that means from the servo mounting to the horn on the control surface and everything between. IIRC you're building a 1/2 scale Spacewalker I've just checked and it's push-rods for ailerons and cables to the back end, so Ailerons and Elevators - a good quality servo close to the control surface and using 3mm ball link connectors at both ends with 3mm rod inside carbon tube to link them together, (Phil Clark does some 'forked end' servo arms designed to take a 3mm ball link), and a 'twin' horn on the surface. For the Rudder - it's your choice, but I'd be tempted with a cable system running forward to an 'idler' horn with a short connecting rod as above to the servo.

Servo torque calculators - several are available, I'll try and get some links for you.

If you haven't already, I suggest asking Serenity Aviation http://serenityaviation.com/home/index.php?option=com_content&task=view&id=5&Itemid=1 about the hinging on the fullsize.


Mark

[Dave Collis 2296]

Andy.

On my 1/2 S/walker I did static balance the elevators. Was it necessary? I don't know. I use a Hitec 705 metal gear servo on each half of the ele. The 1/3 size does just fine with no static balancing. In all the years I've flown the big one,I've had no issues with regards to the elevators. All it did was add weight a long way behind the C/G

Cheers, Dave.

[Andy Boylett]

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[Mark Partington 2989]

I wondered if anyone had calculated the torque put back onto a servo by a 'flutter' of the control surface.....getting technical........In engineering speak we look to say calculate the resultant load from the aileron if the wing was peturbed by a 30g (or 60g) impulse perpendicular load (turbulance), thus inducing a high frequency vibration (flutter) of the wing/aileron. I know that these loads can be huge and may well be higher than the servo torques required for normal control. If we could calculate the 'flutter' loadings then it would be nice any easy to add the 'flutter' servo torque and the normal control servo torque together and make sure we always have more umph available....thus avoiding ever having a flutter!

The loads on the linkages caused by flutter are not what I'd call huge, rather that they're not constant but a shock loading as the control surface 'vibrates' from one direction to the other usually around the neutral position, (or where there's very small to no load on the linkage to hold the surface in a deflected position). The main damage is caused by the high frequency vibration caused by the surface oscilating about it's hinge, and is primarily caused by the high/low pressure difference at the trailing edge of the surface constantly swapping from top to bottom and back again - as the control surface flexes, the more the cumulative effect, also the faster you go - the worse the effect.

An extreme example is the Rudder on Concorde fluttering and causing it to fracture along the hinge.

There are various techniques to minimilise these effects, such as mass balancing - rigid structure- and others that don't really apply until the high subsonic and above speeds. For our purposes where incorrect mass balancing can destro servos and linkages, the most effective is the rigidity, (and don't forget torsional rigidity), of the control suface structure

[Andy Boylett]

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[ian redshaw]

Sorry to butt in but it is a related topic!!!

I have built the new tailplanes for a scale project that has an unusual balance panel on the rudders. Heres a pic of the full size, plus a sketch of the set up in plan view. The three rudders are closed looped together, completely slop free. On the front cable at the outer ends, where the cables join the rudder forward arms, a pushrod goes out to the balance panels. The theory is that when rudder is applied, the airflow wil try to send the rudders back to neutral. The addition of the balance panels helps joystick (or servo in this case) loads since on rudder application, these panels, being hinged at the rear, are trying to be blown to a greater deflection. Their need isn't essential, but they are on the real one and I want (I want, I want!!!) them on this model. My question is, are the balance panels set parallel, or should they have a tad of toe in or out???

Ian.

[Andy Boylett]

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[ian redshaw]

Thanks Andy, you've explained my thoughts far better than I did mate, backed up with theory to boot, toe out it is then!!! They are completely adjustable, even removable, but should be testable on the ground with all 4 motors revving away, since they are right in the concentrated wash, and I doubt it'll get a right lot more draft in the air!

Ian.