| Author | Message | ||
     Kevin Dennes (Kdennes) |
Greetings all. I am aware of some serious discussion a couple of months ago about how important it is to have the control surfaces balanced correctly. My questions is in relation to how you builders actually do the balancing. I have been told that lead shot is mixed with resin and poured in through a hole in the side of the balance horn. Would someone mind adding a bit more detail for me please. I have just had the control surfaces painted and am now ready to do the balancing. Regards. Kevin (from Downunder) | ||
| amoghadd |
Kevin: I used lead shots to balance the elevators. For the procedure I used , please refer to "General Express Aircraft Discussion" dated June 23, 2001. It all worked out fine. Send me an email if you want more specific details. Ali Moghaddas | ||
| Kevin Dennes (Kdennes) |
Ali. What subject heading is your procedure listed under in "General Express Aircraft Discussion"? sorry to give you the extra work. I appreciate your help. Regards. Kevin (from Downunder) | ||
| amoghadd |
It is in the "General Question" section, Archive through Sept 16, 2001. Here is the text: This paragraph details the problem I had with my elevator counter weight. I purchased an Express project(with 2000 tail conversion) from Mike AuClair in April 2001. I hauled the project from Arizona back to Los Angeles. When I was unloading the elevators, I noticed that the counter weight in one of the elevators has come loose, and it was moving quite a bit in the cavity. I found out the counter weight is the diver's lead weight type. I removed a portion of the inboard/ edge rib in the forward area, removed the loosen block weight. Then I used a flexible curve(a type of French cure for those who are familiar with drafting tools), duplicated the curvature of the cavity, made a rib with 3/8" blue foam, covered with BID on both side. After the rib was dried, I place the rib as a dam in the cavity, added BID in all 4 corners, waited for about two hours to get the epoxy cured, mixed lead shots (the type used in shot guns, mixed with epoxy & mill fiber(measured the weight), poured into the cavity, & let it cure for a couple of days. I balanced the elevator by gradually drilling out(removing) the extra lead using 3/4 " drill bit. I used the spray type urethane foam in the unfilled area of the cavity, then, used a file to flatten the extra foam, and finally, added three layers of BID on the edge rib to enclose. | ||
| CBROS Unregistered guest |
For the group: We have been having some heated discussions regarding the need to balance the rudder on an EXPRESS 2000. Local "opinion" tends toward 110% balance on an unpainted (primered) surface. Discussion with other "local" EXPRESS builders has been inconclusive. The factory says that rudder balancing was not done until the introduction of the 300hp examples. 1.Are there any 2000 models with IO-540 engines out there flying without a balanced rudder? 2.Approximately how much counterbalance weight was required? What's your opinion? | ||
| wayne Unregistered guest |
I was told to balance mine. I did it after it was painted and it took ALOT, maybe 10#. I had made a sealed box in the aero balance section before I bonded on the second side. then poured lead shot in though a hole, then EPOXY resin. For some reason when I used vinalester resin it would not set up mixed with lead shot. I still wonder why a vertical control surface needs to be balanced. | ||
| Jim Butler Unregistered guest |
Any shape, vertical or otherwise, needs to know which direction it is supposed to fly. If it doesn't, it can start trying to swap ends, resulting in a catostrophic failure. The reason for this is because the center of gravity must be ahead of the center of aerodynamic pressure. The center of aerodynamic pressure is the point looking at the side of an object where the load due to aerodynamic forces can be assumed to be concentrated. A shape will always try to fly with the center of gravity leading. This is why an arrow has the heavy end on the tip, and the feathers on the back. If the center of gravity is too close to the center of pressure, the shape will be vary unstable in the air. You can simulate this with a post card. Take a post card and drop it. You can see it flutter down. Now put a heavy paper clip on the leading end and drop it again. It will fly straight down without fluttering. When flutter starts in an airfoil, whether it be a rudder, wing, aileron, etc. it is not gentle and is more like an explosion. I would highly recommend all control surfaces be balanced, after painting. I even balanced my wheel pants. Jim Butler | ||
| Ted Gaston New member Username: Ted_gaston Post Number: 3 Registered: 09-2003 |
Here is an easy way to estimate the total weight you will need to balance any control surface. First, lay the surface horizontaly, supported at three points, two of which are the top and bottom hinge. A bolt in the hinge resting on the edge of a piece of angle works fine for this. If you screw the angle to your bench with sufficient hang over so the counterbalance horn will clear the bench, you can use this fixture for the final tuning of balance. The third point of support is your grahm scale place as close to the trailing edge as possible. By weighing at the trailing edge, you now know the moment aft of the hingeline, (weight x arm = moment). Now just measure to the center of the counterbalance horn for arm forward of hingeline, divide the moment from the previous step by the arm forward. The result will be a very close approximation of the weight required to balance your surface. Since the balance horn is not a rectangle, yhis method will have you a bit light so add about 10% to the final number. you will still need to fine tune the balance but this will save you quite a bit of drilling. A side note, lead shot is about 30% lighter for a given vollume since the spherical form results in voids between the shot. You should use pieces of solid lead with shot filling in around the edges for the weight material. And always use epoxy to glue your lead in place since the vinyl ester products will shrink and suck in the sides of the counterbalance horn leaving an unsightly depression. Ted | ||
| Reinhard Metz New member Username: Reinhard_metz Post Number: 22 Registered: 03-2003 |
Back to the need for balance - here's one more vote for "absolutely, definitely"! There was a really comprehensive article on flutter just recently in the December issue of Sport Aviation. The way I understand it, a great deal of it (and there are many more complexities involving e.g. stiffness) is a matter of where the center of gravity of the control surface is with respect to the hinge line. The reason is 'how does the structure respond to a sudden deflection perpendicular to the hinge line'. If the center of gravity is not on the hinge line, the control surface will deflect, due to inertia. This deflection can then in turn lead to compounding forces and deflections, which can become an oscillation without bound - flutter. If the center of gravity is on the hinge line, then any sudden lateral inputs will lead to no deflection of the control surface. Imagine, for example, moving up or down the fulcrum under a balanced item - it will just move up and down the same, not rotate about the fulcrum. In addition, there is the matter of static v.s. dynamic balance. For a rudder balanced with a horn, such as ours, only static balance is achieved since the weight is not distributed. The result is that there will be internal tortional forces that the rudder must sustain in dynamic situations. As long as the rudder is stiff enough, that is typically ok. The fact that the rudder is vertical v.s. horizontal, say for an aileron, means only that it has no static perpendicular force on the hinge, since the hinge is parallel (relatively) to gravity. That does not exempt the rudder from dynamic lateral inputs from wind gusts, tail wagging, etc., which is why it too must be balanced. In fact, I have seen planes in the Boeing factory all chromate green, except for the rudder, which will already be in the customer's colors, because even the paint needs to be balanced for. By the way, I cast my counterweights in sand molds, made from plaster molds, in turn made from the control surface horns. That gets you a smaller, denser piece, and no filler issues. | ||
| Jerry Sjostrand New member Username: Jerry Post Number: 42 Registered: 08-1999 |
A comment in addition to Reinhard's (which is very informative and correct). Be sure to keep the weight as far forward as possible as it greatly effects the amount of lead you use therefore causing the control surface to be ligther. I have seen some very bad examples of large weights too close to the center line (hinge line). The last elevators I balanced were done by allowing the pre-cast lead (not shot) piece to actually extend and become the end of the balance horn, then glassed over it to finish it. A pound or two less weight.!! Drill out excess lead after painting to final balance. | ||
| Brynjar Thordarson New member Username: Benny Post Number: 23 Registered: 10-2000 |
Hello all CT specialists! Should the counterweights on the elevators and rudder be slightly heavier than the surface? As an A&P I've never heard that, but I know the importance of balancing. Question: Are the bottom surfaces on the ailerons and flaps flat or do they have a slight curvature? I need to know asap, as I'm closing them out on the weekend. Rgds. Benny. | ||
| Reinhard Metz New member Username: Reinhard_metz Post Number: 23 Registered: 03-2003 |
The counterweights should slightly overbalance to start with, so that when they are finished and painted, as Jerry mentioned, some of the lead can be drilled out, bringing them to exact balance. Another reason not to use lead shot is that it usually is not pure lead, and therefore is of lower density, and at a given moment arm is less effective per volume, further fueling the problem to avoid that Jerry was describing. Lighter contol surfaces have their resonances at higher frequencies, and therefore are less prone to flutter. The bottoms of the control surfaces are flat from front to back at any given spot, but technically should have a slight warp from inboard to outboard. Many Expresses (including mine) have been built mistakenly without the warp - i.e. flat, because it was not mentioned in the instructions until you did the rudder, which has to have the warp since it is thicker at the bottom than at the top. It would look funny if done flat! The flaps and ailerons are the same - they should have a warp that interpolates the change in thickness inboard to outboard (so that the trailing edge runs paralle to the front of the flap center line). The net result total difference is about 3/8ths of an inch difference in inboard height of the flap trailing edge if you make them flat. | ||
| David Frederiksen New member Username: Frederik Post Number: 1 Registered: 01-2004 |
I am building an Express 2000 in Central Australia.I have been watching this excellent site for months now. This is my contribution. Years ago I experienced rudder flutter in a DH114 Riley Heron while on descent at 180 kts into Ayers Rock. It was so voilent I could not stop it with rudder input. I turned the rudder trim and it stopped. The rudder was 4 lbs. out of balance and had been since manufacture. I will be balancing the primary controls very carefully |