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I have a couple questions on mass balance for elevator: (1) Is mass balance usually required for an elevator? I found that the familiar certified airplanes (such as cessna 152, 172, DA40, etc.) all have mass balances. (2) A mass balance is intended to reduce flutter. Do we need to move the CG of the elevator all the way to hinge line? or just close to hinge line? how close? or what is the criteria of an acceptable mass balance?
the not so simple answer is "it depends"... You will notice some classic aircraft (cubs, Taylorcraft etc) do not have either mass or aerodynamic balances. On my own design however, I was very particular about mass balancing the Elevators, as out of all the control surfaces, the Elevator is the one that will kill you the quickest! The question whether you need it or not depends on a number of factors - 1. what is the speed range of the aircraft? How big and effective are your elevators? how are the elevators actuated? are you likely to be putting large elevator inputs in at high energy levels?
Lower performance aircraft with a narrow speed range are unlikely to hit the flutter range for any length of time, so mass balancing is of less importance. Larger elevators increase the effects of flutter, and tend to be more prone to it and, especially if the chord is quite wide, will need quite a lot of weight or a long moment arm to balance correctly. If the balance is not accurate and still behind the the hinge, it will reduce the flutter effect, but not eliminate it - essentially it makes the elevator behave as if the chord is smaller. the actuation method is quite important - I use a heavy push - pull cable on my elevators, and this has a certain amount of inherant damping effect from the cable friction. Conventional cables may in some circumstances allow a bit of movement and even resonate to amplify flutter tendencies. Pushrods are more solid, but also can have a lot less system friction, so it depends on the installation. Powered flight controls are non - reversible (the movement of the surface doesnt get fed back into the system) so are generally resistant to flutter, but I'm guessing not many homebuilts have fully powered elevators!. I'd say, If in doubt, balance them as close to neutral as you can - it gives piece of mind if nothing else.
Don't forget that the elevator should be balanced after it is painted. Paint, even though it is thin, is applied to a pretty large area. The weight adds up.
If you keep looking at certificated airplanes you will observe that mass balance is very important for ailerons where the two surfaces can work against each other, that some elevators have weight forward of their hinge line, and almost no rudders have weight forward of the hinge line.
The elevator and rudder can have an aerodynamic balance surface (Cessna's and Mooney's) to reduce control forces, especially at speed. My understanding is that a weight forward of the hinge line on an elevator is almost entirely to help with stability and control force rather than flutter. The tail section is much stiffer than the wings and the forces are different.
For what its worth, my Pitts S-2A has no aerodynamic balance surfaces on the rudder or aileron, no balance weights on those surfaces, simple strap hinges holding them on, and I dive to the airframe red line of 202mph regularly. The airplane is certificated for that and the population of those airplanes has never had a problem with tail issues due to flight loads.
Best of luck,
Wes
N78PS
A lot of airplanes do not have any balance on the elevator or any other movable control surface. The designer decides what is needed. Of the surfaces that are balanced, not all are balanced 100%, that is equal wt. on each side of the hinge line. Again, designer decides what is needed. Usually, there is an acceptable balance range that offers quite a bit of flexibility. For example, the instructions for many Cessna models say if you strip and repaint the elevator, any similar finish will not upset the balance. I have seen guys paint heavy finishes over old paint and the elevator still easily balances within range so it's not all that critical.Quote:
Originally Posted by wantobe
Elevator Hinge Line Position
Thanks for the insightful replies. One of the important parameter is the hinge line location (as a percentage of elevator chord). If we can move it backward, it will be easier to balance. What are the considerations in moving it backward? What is a typical value? The only data point I could find on the web claims Cessna 177 has an elevator hinge line at 25% of elevator chord.
Ease of balancing the elevator has little to do with hinge line location. The controls have to be designed to meet the airplane's goal. Elevator hinge location affects stability and control so the location selected by the designer best satisfies the aircraft's design goals. For example, a low stick force gradient on a gen purpose, x-c airplane, would make the airplane tiring because it would be difficult to control. Conversely, if you have an aerobatic airplane and the elevator feels like it is set in cement, it wouldn't be much fun to fly.Quote:
Originally Posted by wantobe
The Cessna 177 Cardinals have stabilators, not elevators. The whole surface moves as one. Big difference.
If I were you I would go look at an airplane, or airplanes, that operate in your speed range and copy it. You really do not have to over analyze the elevator hinge. Many of the older designs, and newer ultralights, put the elevator hinge around the 50% line.
Much more important than the absolute location of the elevator hinge is the relationship of elevator size to the wings, the relative distance between them, and where the CG is.
If you are building something that looks like a Cub, look hard at what Taylor and Piper did. Its worked for a lot of years. All of the fancy stuff that you read and hear are just refinements of the last 3% of the problem solution.
Best of luck,
Wes
N78PS
Same is true for performance. They figured out how to make airplanes go fast a long time ago. Now folks spend a lot of effort and even more money designing with dynamic computer programs and building with exotic materials just so they can have a top speed only ~3% higher than they did 70 yrs ago.Quote:
Originally Posted by WLIU
My Piper Vagabond has a spring which makes it so that, when the aircraft is on the ground, the elevator extends straight out from the stab instead of drooping like most other airplanes on the field. Could that be balancing without changing the shape of the surfaces? I understand the PA-15 was designed very quickly.
Wonder how it would perform if it were ever flown upside down.
Be careful. That spring affects the static and dynamic stability in pitch. It is not there just to hold the elevator up. Some aircraft use a spring for elevator trim. An example is the Aviat Husky, a newer design that includes an elevator trim spring. Some folks hate the way that a spring feels and for some airplanes, adding a spring can change the longitudinal stability.
Best of luck,
Wes
N78Ps