will a split elevator flutter?
08-15-2003, 01:39 PM
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will a split elevator flutter?
I'm just finishing up a great planes 1/4 scale spacewalker with an os 91 fourstroke. I'm using the dave brown fiberglass pushrods for the elevator and rudder. On the servo end, it's got a single short wire pushrod leading into the fiberglass portion, followed by two short wire pushrods exiting the fuselage at the tail and attaching to the split elevators. When I hooked them up, I noticed quite a bit of differential play in the elevator halves, apparently caused by the tail end of the fiberglass portion of the pushrod moving from side to side. The play only happens if you move one half of the elevator at a time. If you move them together, there is no play at all.
Has anyone had this problem, if it is a problem? And should I go to the trouble to open up the fuselage and install a bracket to support the pushrod?
I noted a post raising the same problem more than a year ago with the stock wood dowel pushrods, but there was only one response and really no follow up. The thread quickly turned to other issues with the spacewalker.
http://www.rcuniverse.com/showthread...walker+flutter]
Has anyone had this problem, if it is a problem? And should I go to the trouble to open up the fuselage and install a bracket to support the pushrod?
I noted a post raising the same problem more than a year ago with the stock wood dowel pushrods, but there was only one response and really no follow up. The thread quickly turned to other issues with the spacewalker.
http://www.rcuniverse.com/showthread...walker+flutter]
08-15-2003, 02:19 PM
#2
Spacewalker elevator
I cannot positively state that "you will not have a problem", but I can honestly say that you probably "shouldn't have a problem".
With that engine and potential speed of the plane, it will only fly so fast. Just make sure that there isn't any excess "slop" in the total linkage, hinges, clevises, servo arms, gear train, etc.
With that engine and potential speed of the plane, it will only fly so fast. Just make sure that there isn't any excess "slop" in the total linkage, hinges, clevises, servo arms, gear train, etc.
08-15-2003, 02:50 PM
#3
will a split elevator flutter?
"will a split elevator flutter? "
If you have too much gap in the hinge line and there is slop in the linkage, yes, it can flutter. It makes no difference if it's a split elevator or not, or any other surface for that matter. If you don't build your surface, hinge and linkage systems properly you open the door for flutter to come waltzing in.
John
If you have too much gap in the hinge line and there is slop in the linkage, yes, it can flutter. It makes no difference if it's a split elevator or not, or any other surface for that matter. If you don't build your surface, hinge and linkage systems properly you open the door for flutter to come waltzing in.
John
08-15-2003, 05:02 PM
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it's only the side to side
Everything is tight except for the lateral movement of the pushrod in the center of the plane when you wiggle one of the elevator halves by hand. There is no hinge gap, the horns are good, the servo arm is good, etc.
Will air movement during flight equalize the pressures on both elevator halves such that this side to side won't be a problem?
Will air movement during flight equalize the pressures on both elevator halves such that this side to side won't be a problem?
08-15-2003, 05:05 PM
#5
will a split elevator flutter?
I doubt very seriously that you will have any problems with a model of this type if you only have a little side to side movement in the pushrod especially if the hinges are tight. A Spacewalker just doesn't fly that fast.
John
John
08-15-2003, 05:24 PM
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If you are worried...
If the worry of flutter has you too concerned, you can add a fix that I do on my planes. The problem is that with many/most of todays ARF's, one of the fixes isn't as easy to do as it is on a kit built plane.
1. IF YOU HAVE ACCESS to the rear internal area of the fuslage, you can make a simple brass sleeve to support the rear portion of the DB rod just an inch or two in from of where the "Y" wire pushrods exit the rod end. I don't remember the exact size of brass tubing I used, but it was a standard size carried in the hoppy shop. Anyway, I cut a short length of the brass tube (about 1 inch long) to make a bushing that will just slide over the rod. I found that after I cut the bushing to length, it helped if I used a small round file to very slightly work over the inside edge of the bushing on each end so it doesn't have an "edge" that can dig into the rod. Then I slide the bushing over the rod BEFORE (IMPORTANT) I assembe the wire ends of the Dave Brown assembly (otherwise the bushing will not be able to be placed on the rod afterwards. Then I go ahead and put the Dave Brown pushrod into the plane and hook it up to the servo and elevators. Once in place, I then slide the brass bushing back until it is just an inch or so ahead of the "Y" end of the pushrod. Make sure to allow enough room for the rod to move fore and aft without the "Y" end coming into contact with the bushing. Then I CAREFULLY glue a wood cross brace (usually 1/4" or a little larger HARD balsa is fine) between the two fuselage sides, and DIRECTLY under/against the bushing. Once the glue for the cross brace is set, I then CAREFULLY glue the bushing to the cross brace (it may help to roughen up the outside of the bushing with coarse sandpaper first) with epoxy. You must be very careful NOT to get any glue on the rod itself, or nothing will move afterwards!
As long as the overall length of the rod is at least 1.5 feet or more long, and you make the bushing no more than about an inch long, there should be just enough play to prevent any binding from the slight side to side movement at the servo end as the servo wheel rotates, but the bushing will hold/center the rear end of the pushrod and eliminate almost all of the differential movement you mention with your split elevator halves.
2. If your plane is an ARF, or you already have sealed up the fuse so access is no longer possible, there is another fix that should at least help. You can take short lengths of plastic "Nyrod" type tubes/guides and side them over each of the wire "Y"'s where they exit the fuselage (you will have to remove your clevises first) and through the exit slots in the fuse sides. Then glue these plastic "guide tubes securely into the fuselage sides.
The trick here is to make the tube/guides just long enough that they leave at least the last couple inches or so of the end of the wire rods exposed so nothing interfers with the clevises, and also that they extend into the fuslage as far as possible without interfering with the main pushrod at the "Y". If they are glued in too far into the fuselage, full travel of the pushrod will be reduced when it runs into the guides. However, if they are not glued in as far along the wire "Y" rods as possible, then more slop will result in the individual elevator movement.
Once the glue sets, the plastic "guide tubes" through the fuselage sides will help reduce the side to side movement of the back end of the main pushrod (that is responsible for your differential movement of the elevators).
Phew...hope this helps!
Lee
1. IF YOU HAVE ACCESS to the rear internal area of the fuslage, you can make a simple brass sleeve to support the rear portion of the DB rod just an inch or two in from of where the "Y" wire pushrods exit the rod end. I don't remember the exact size of brass tubing I used, but it was a standard size carried in the hoppy shop. Anyway, I cut a short length of the brass tube (about 1 inch long) to make a bushing that will just slide over the rod. I found that after I cut the bushing to length, it helped if I used a small round file to very slightly work over the inside edge of the bushing on each end so it doesn't have an "edge" that can dig into the rod. Then I slide the bushing over the rod BEFORE (IMPORTANT) I assembe the wire ends of the Dave Brown assembly (otherwise the bushing will not be able to be placed on the rod afterwards. Then I go ahead and put the Dave Brown pushrod into the plane and hook it up to the servo and elevators. Once in place, I then slide the brass bushing back until it is just an inch or so ahead of the "Y" end of the pushrod. Make sure to allow enough room for the rod to move fore and aft without the "Y" end coming into contact with the bushing. Then I CAREFULLY glue a wood cross brace (usually 1/4" or a little larger HARD balsa is fine) between the two fuselage sides, and DIRECTLY under/against the bushing. Once the glue for the cross brace is set, I then CAREFULLY glue the bushing to the cross brace (it may help to roughen up the outside of the bushing with coarse sandpaper first) with epoxy. You must be very careful NOT to get any glue on the rod itself, or nothing will move afterwards!
As long as the overall length of the rod is at least 1.5 feet or more long, and you make the bushing no more than about an inch long, there should be just enough play to prevent any binding from the slight side to side movement at the servo end as the servo wheel rotates, but the bushing will hold/center the rear end of the pushrod and eliminate almost all of the differential movement you mention with your split elevator halves.
2. If your plane is an ARF, or you already have sealed up the fuse so access is no longer possible, there is another fix that should at least help. You can take short lengths of plastic "Nyrod" type tubes/guides and side them over each of the wire "Y"'s where they exit the fuselage (you will have to remove your clevises first) and through the exit slots in the fuse sides. Then glue these plastic "guide tubes securely into the fuselage sides.
The trick here is to make the tube/guides just long enough that they leave at least the last couple inches or so of the end of the wire rods exposed so nothing interfers with the clevises, and also that they extend into the fuslage as far as possible without interfering with the main pushrod at the "Y". If they are glued in too far into the fuselage, full travel of the pushrod will be reduced when it runs into the guides. However, if they are not glued in as far along the wire "Y" rods as possible, then more slop will result in the individual elevator movement.
Once the glue sets, the plastic "guide tubes" through the fuselage sides will help reduce the side to side movement of the back end of the main pushrod (that is responsible for your differential movement of the elevators).
Phew...hope this helps!
Lee
08-16-2003, 10:10 AM
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I fixed it
I'm sure it would have been fine, but I suppose I was really just worried about flutter. I fixed the sideways movement of the Y rear of the Y pushrod by installing a supporting brace between the formers as far back in the fuselage as I could get and shortly before the pushrod splits and exits.
