Rebalancing the rudder

[ArtVandelay]

I was going to install Orion LED tail light, but my mechanic says they would need to pull the rudder off to rebalance it. Anybody gone through this, it weighs 0.28lbs?

[carusoam]

What does the equipment coming off weigh?

Best regards,

-a-

[ArtVandelay]

I didn't get exact weight, was told "not much"....the maintenance manual makes removal sound easy, only 3 steps, I'm guessing it's not so easy

[carusoam]

If the new and the old have the same weight and cg...there isn't much to rebalance.

If it is on the moving rudder part...

It is on a very sensitive part of the control system where flutter would be a big concern.

1/3 of a pound sounds pretty heavy for the LED part. The bulb that is coming out doesn't weigh an ounce... Glass, socket, power supply, and structure are included with that...

For something that important, I would want to have a real knowledgable expert making me comfortable with the change.

Fortunately that is why we have certified mechanics...

Best regards,

-a-

[cliffy]

Movable flight surfaces are very sensitive to weight as far as their specified balance goes. The actual removal and replacement of your rudder is not too difficult or time consuming. There is a certain part of the Maintenance Manual for your airplane that shows the specific procedure for checking its balance. It must be followed to the letter. This is something you do not want to short cut.

I had a talk with Bill Wheat this summer and he noted that when he did flight testing for flutter, when he hit a spot where flutter started it was sudden and dramatic.

On some airplanes even one extra coat of paint can put the surface out of balance.

A quarter pound of weight at the far end of the rudder arm is a LOT of weight out there. The difference between what is removed and what is installed will determine if you can stay in balance. It must be checked and not just calculated.

[DonMuncy]

I don't have a lot of knowledge in this area, but it seems to me that a change in weight on the empennage or the bottom of the rudder would require a look at the weight and balance of the plane but would have no effect on flutter. I know that the elevator can flutter, but can the rudder flutter?

[MB65E]

Anything can flutter... Just depends on the airspeed.

Overbalanced conditions tend to limit flutter, under balanced condition can induce it.

The range can be found in the maintenance manual.

-Matt

[carusoam]

Let's go with...

Flutter can effect any control surface.

When it is connected by things with limited physical strength like aluminum sheet metal, we have limits to how much flutter is allowed.

The failure mode of flutter ends with tearing bits off the plane.

Losing or partially losing a rudder would be completely bad.

This is not intended to be a negative comment towards the OP for making a good / modern change.

But, it is a pat on the back for going through the effort of doing it right. Following the expensive advice of a person that is trained and has expertise in the area.

If aerodynamics weren't important, we would be celebrating Davinci as the father of flight? Or somebody like that...

Best regards,

-a-

[ArtVandelay]

Anything can flutter... Just depends on the airspeed.

Overbalanced conditions tend to limit flutter, under balanced condition can induce it.

What exactly do you mean by under balanced or over balanced?

I thought the idea was to balance the rudder on its hinges?

[N601RX]

They are not balanced until they are neutral. They should be under balanced.  The exact amount can be found in the service manual

[MB65E]

Sorry TJ, I was not referring to anything spacific about the M20 just some aerodynamic stuff...

I corrected my post to reflect the maintenance manual. Overbalance condition is when the Cg of the flight control is in front of the balance point. Under balanced is when the flight control CG is aft of the balance point. There is a range in most cases. Under balanced condition can be less safe than an overbalanced condition as it can leas to flutter if too far aft. Some Maintenance manuals will also show a total max weight of a flight control. Adding weight to the trailing edge will move the CG aft on the flight control.

-Matt

[tony]

are you talking about the white tail light on the empennage?  I don't remember a light on the rudder...

[MB65E]

Later M20's had the longer rudder that was full span and included an AFT nav light, sometimes strobe.

-Matt

[ArtVandelay]

are you talking about the white tail light on the empennage? I don't remember a light on the rudder...

Yes, the bottom of the rudder, there is no place to put the tail light anywhere else, the rudder is full length.

[tony]

and your mechanic wants you to get the rudder rebalanced if you replace that old grimes light with a Whelen Orion 500 LED light?  Well I guess technically he's correct but I think he's being a little conservative, I think I might call Whelen and see what they think.  They should know the weight difference and you can begin the discussion with your Mechanic.  I see the Orion 500 light, per Whelen's sales brochure, weights 0.5 pounds; I couldn't find what the grimes assembly weights but as Anthony said I would guess a couple of ounces.  Or you could just replace the bulb and be done. 

[cliffy]

Folks,

Flutter and flutter testing is a BIG deal!

Here's a video showing flutter on a Twin Comanche.

Any flight surface can flutter if not balanced correctly (balanced means the correct CG vs. the hinge line) 

This is why there are specialists in flutter dynamics in aviation. Mooney uses a flutter specialist on their airplanes. They start with ground flutter tests with oscillating weights to find the resonant frequencies and go from there. 

Flutter margins is why Mooney will not allow any repairs to a flight control surface other than complete factory designed parts being replaced. NO PATCHES. 

Flutter issues are why the control surfaces have to be rebalanced after ANY repaint of them. 

Even replacing a tail light bulb with another "FAA Approved" bulb (yes I said it has to be an FAA Approved bulb, not the original incandescent bulb but an LED one as LED bulbs are not standard parts by definition and need specific FAA approval for installation) needs a rebalance. 

The faster you go toward red line the closer you are to flutter. That is the purpose of the flutter margins. If the plane inadvertently exceeds Vmo you wouldn't want it to flutter apart. 

When doing flight testing for flutter, airspeeds are gradually increased to see if flutter is anywhere near. 

The mechanic is absolutely correct in requiring a rebalance check after replacing the entire tail light assembly.  No honest mechanic would sign off the change without a rebalance UNLESS there was approved documentation from the manufacture specifically saying that a rebalance was not needed (as in an STC). 

[Andy95W]

Dang, Cliffy. You make me glad I fly a Mooney.

[tony]

Cliffy, thanks for the awesome video. That's pretty dramatic.

However if I remember correctly from the last test program I worked on, flutter was cause by an instability in the control system, resonance in the structure, a resonance in the control surface, or structural coupling. Not a control surface imbalance. A control surface imbalance resulted in heavy stick forces which could result in less than favorable flying qualities. By adding a couple of ounces to a rudder, I really don't think will have that dramatic of an effect. However, without a correlated model to run a simulation on (which I doubt exists) or flight test, none of us will know for sure.

[cliffy]

From what I understand ( I'm not an aero engineer) you are correct in that flight control surfaces all have a certain resonant frequency that is found by doing ground flutter testing. In addition any "flutter encountered with a corresponding "looseness" in the control surface linkage will exasperate the flutter as there is no dampening by the mechanism at the inception of flutter. The original flutter though ( I feel) does have to originate with the surface itself. The point of the surface CG (with respect to the hinge line) has a determinate effect on the  resonant frequency and severity of the magnitude.

A postulation- If balance (CG position) was not very important why is there such an import placed on surface balance on every airplane made?  Why would it be required to be checked after every paint job? 

Bonanzas are particularly effected by a very tight  balance tolerance as even one coat of paint extra while painting will throw them beyond limits.  

Control surface "imbalance" that effects the heavy stick forces may be caused by "aerodynamic" imbalance. Not enough area ahead of the hinge line to off set the amount of area behind the hinge line for control force or "aerodynamic" balance.  Early airplanes had no surface area ahead of the hinge line and as speeds increased it was found that by extending "movable" surface area ahead of the hinge line a resultant force counter to the "heavy" stick could be achieved. Too much forward area and stick forces become non-existant- a very dangerous world indeed. 

 

Here is an abstract of a paper written for a conference on mass, vibration and flutter studies in 1999-

 

Mass properties' engineers often are concentrating on creating and maintaining the most accurate and current weight database possible on our aircraft or system. They tend to lose sight of why accurate weight data is important. While having accurate weight data is intrinsically valuable, it can have a major impact on design decisions and even on the viability of a design. Some systems are more sensitive to changes in weight and center of gravity than others. One of the critical elements in the design of aircraft is "flutter modes". Every moving body has vibration frequencies. It is important to avoid coupling frequency modes in the airplane components. This can have catastrophic results, including loss of property and lives. Excessive vibration can result in metal fatigue, even without catastrophic failure. It is important that accurate weight and balance data be available to design engineers early in a program. Many problems can be avoided if a potential flutter situation is identified early in a program. An important way the mass properties' engineer can assist in preventing flutter is by working closely with loads and flutter engineers to optimize the control surface balance. Parametric studies are run by loads and flutter engineers to determine the optimal balance weight and location to achieve no coupling of vibration modes. The mass properties' engineer needs to assure that the mass properties' data provided for future analyses reflects this optimal value and the system balance about its hinge line is within the limits established.

 

As can be seen, "surface balance" is concomitant with and paramount in application to avoid flutter in the surface.