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  1. #1
    Pull Up Now!'s Avatar
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    DANGER ALERT Dubro Cat. No. 913 "HD Adjustable Control Horn (.91 & above)

    I'll try to be briefwhile alerting modelers to the dangers of improper use of giant scale Dubro control horns, catalog #913.

    I believe my application of this product was one of the two known causes of the crash, on first flight, of my 1/4 scale Brisigella Steen Skybolt. Pictures included. Dubro's control horn is designed with two "ball joint" washers so that the assembly can be tightened against non-parallel surfaces such as would be found on a thick aileron. My mistake was using the same hardware on a thin, slab elevator. The integrity of #913 DEPENDS on the screw engaging a LONG, close fit hole thru a THICK surface. The long hole prevents the screw from pivoting on the double ball joint washers. If used on a thin slab elevator, the horn will slip under load, causing a loss of surface travel.

    The second cause was the loss of a hinge pin. See the picture belowthe elevator is shown as it was pulled out of the woods. About halfway thru the first flight, I had to adjust more up trim unexpectedly. Then, in the final moment before the crash, I experienced a more severe and fatal loss of "up" elevator. The plane arc'ed into the forest. Here's a link to the fatal flight. http://www.youtube.com/watch?v=dat2mjpOjkM

    I don't know if the Dubro slipped first, or the hinge pin was the first event. But the combination of the two proved too much to overcome. I rebult the Skybolt, using the same hardware on the elevator. But I turned the ball joint washers around and let the "ball" dig into the balsa surface. The standoff and washer, both of which contain a concave mate to the ball, now mated to a flat surface. Problem solved. Note that Dubro does not show this hardware installed on a flat surface at all. That was my doing. But they don't say NOT to do that either. Maybe they should. There's a picture of their installation diagram below, too.

    The provide pictures show the missing hinge pin, the crash site, partial rebuild, the completed rebuild, and Dubro's diagram. And here's a link to the sucessful 2nd flight, after the rebuild. http://www.youtube.com/watch?v=Kyq_hZul-4Q That took about 6 weeks. Power is a CRRC Pro 50cc with a 22x6 laminated XOAR prop. Please don't give me c**p about the bouncy first landing attempt! Plane gain about 3/4 lb in epoxy, glass, paint, an extra elevator servo, and some metal aileron tabs. Total weight is 19.5lbs.
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  2. #2

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    RE: DANGER ALERT Dubro Cat. No. 913

    It is likely that unless you provide a hardwood insert on the stab for the horn... it is going to crash again.

  3. #3

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    RE: DANGER ALERT Dubro Cat. No. 913

    Let me get this straight.......Are you warning against the use of Dubro hardware, or the application of said hardware, OR are you saying that it is your fault that you installed the hardware wrong??
    RC MANIAC-PRO BRO 1900-AMA# 95841

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    RE: DANGER ALERT Dubro Cat. No. 913

    He makes it clear that his application was the error but he wrongly diagnoses the error. The error is that hardwood or its equivalent are needed to support the use of the horn.

    The equivalent would be to drill the hole and then saturate the wood on both sides and into the hole with thin CA. This would require using an L-bend awl in the hole and lifting the covering away from the wood and flowing thin CA around the area, doing so on both sides.

    A good ARF will provide a hardwood insert but sometimes the builder chooses another area for the horn. If the arf does not have the hardwood insert, then the equivalent must be provided as outlined.


  5. #5
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    RE: DANGER ALERT Dubro Cat. No. 913

    I never said don't use that Dubro hardware. I'm saying that if you use it in the way I described, you're exposed to elevator degradation. Also, let me address the hardwood mounting underlayment. If one is using wood screws to hold a control horn to the surface, hardwood should be present (or soak with CA, as AA5 said). But many control horns come with both the horn, AND a backing plate. These work fine with balsa underneath, but I'd sure limit that to small/medium sized planes. HOWEVER, I think it's important to point out that with the Dubro #913, using hardwood underneath won't keep that pivoting from happening as I described, especially if the control surface's thru hole is a little oversize. Many people wouldn't size the thru hole all that carefully. So there will be some slop in the hole, resulting in enough clearance to allow that slippage. Sure, a tight fit hole in hardwood would be better, and would limit the screw's side play, but I think people would have to be aware of this issue beforehand. Otherwise, it wouldn't occur to people to size the hole so tight.

  6. #6

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    RE: DANGER ALERT Dubro Cat. No. 913


    ORIGINAL: Pull Up Now!

    ... Otherwise, it wouldn't occur to people to size the hole so tight.

    Dubro say to drill a 5/32 inch hole. Is that the hole size you drilled? Also a hardwood insert would would minimize if not eliminate the problem as it allows the hardware to be sufficiently tightened and would not compress to permit slipping. JMHO.

    Sincerely, Richard

  7. #7
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    RE: DANGER ALERT Dubro Cat. No. 913


    ORIGINAL: Pull Up Now!

    I never said don't use that Dubro hardware. I'm saying that if you use it in the way I described, you're exposed to elevator degradation. Also, let me address the hardwood mounting underlayment. If one is using wood screws to hold a control horn to the surface, hardwood should be present (or soak with CA, as AA5 said). But many control horns come with both the horn, AND a backing plate. These work fine with balsa underneath, but I'd sure limit that to small/medium sized planes. HOWEVER, I think it's important to point out that with the Dubro #913, using hardwood underneath won't keep that pivoting from happening as I described, especially if the control surface's thru hole is a little oversize. Many people wouldn't size the thru hole all that carefully. So there will be some slop in the hole, resulting in enough clearance to allow that slippage. Sure, a tight fit hole in hardwood would be better, and would limit the screw's side play, but I think people would have to be aware of this issue beforehand. Otherwise, it wouldn't occur to people to size the hole so tight.
    Any time you use a screw type of control horn it MUST be run through some type of hardwood! We have been using 10-32 screws for control horns on 40% IMAC aircraft for years. The difference is that they are run through a 3/4" birch dowel that gets installed in the surface. Usually a foam surface and the dowel is in contact with the upper skin, lower skin and leading edge material. The screw horn is threaded into the dowl and epoxied in place. For a 50cc airplane with flat balsa surfaces on would want to use a horn with at least one square inch of mounting surface. Classic case of the wrong thing was being used. IMO the hinge came apart first ( The pin NEEDS to be sunk into the surface material after the 90 degree bend ) the resulting flutter was a contributor to the horn failure. The horn failure will happen again due to the lack of surface area spreding the load and being installed through balsa.

  8. #8

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    RE: DANGER ALERT Dubro Cat. No. 913

    Next time......just say you made a mistake that caused a crash........own it Dude!!![:@]
    RC MANIAC-PRO BRO 1900-AMA# 95841

  9. #9
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    RE: DANGER ALERT Dubro Cat. No. 913


    ORIGINAL: spaceworm


    ORIGINAL: Pull Up Now!

    ... Otherwise, it wouldn't occur to people to size the hole so tight.

    Dubro say to drill a 5/32 inch hole. Is that the hole size you drilled? Also a hardwood insert would would minimize if not eliminate the problem as it allows the hardware to be sufficiently tightened and would not compress to permit slipping. JMHO.

    Sincerely, Richard
    Yep, the mistake is definitely mine. I built to the plans, which said nothing about hardwood in that spot. Live and learn. I already think I owned it, RCManiac.

    Richard, I don't know what size I drilled the thru hole. I'd have to take the plane apart to see. I might do that. My whole point is to warn people that this pivoting can happen if one doesn't do everything perfectly. Thanks for your respectful comments. By the way, hardwood will compress too, over time, and the screws will lose torque. It'll be next year though, after sitting long enough to make you think everything's ok. The only real solution here is to follow the instructions on hole size, and use this hardware only on a surface thick enough to provide sufficient wheel base on the screw. Then, if torque fails, the screw won't pivot. For what it's worth, I think people should know this.

  10. #10
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    RE: DANGER ALERT Dubro Cat. No. 913


    [/quote]

    By the way, hardwood will compress too, over time, and the screws will lose torque. It'll be next year though, after sitting long enough to make you think everything's ok.
    [/quote]


    Using end grain wood will help lesson the compression

    Ken
    Sent from my Dry-Erase-Board

  11. #11
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    RE: DANGER ALERT Dubro Cat. No. 913


    ORIGINAL: kenh3497

    By the way, hardwood will compress too, over time, and the screws will lose torque. It'll be next year though, after sitting long enough to make you think everything's ok.
    [/quote]


    Using end grain wood will help lesson the compression

    Ken
    [/quote]
    I like this as a general purpose suggestion a lot! But sand the surface well, otherwise the compression washer will be sitting on tiny points surrounded by air, defeating the advantage.

  12. #12
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    RE: DANGER ALERT Dubro Cat. No. 913

    I don't know nothin about nothin, but I have been using those dubro control horns quite a bit lately. Like Speed, I always use a dowel thru the control surface. Only thing is, on that elevator I would use a 1/2" hardwood dowel. Drill a 1/2" hole all the way thru the elevator then glue in a dowel. Then drill the dowel for your control horn. In my opinion, even if you put a plywood plate on the top and bottom of the cotrol surface, the balsa in the middle is still going to compress, which will eventually lead to a lose control horn

    Pat

  13. #13
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    RE: DANGER ALERT Dubro Cat. No. 913

    Pat, thanks for you comments. The compressibility of the wood is definitely an issue. But I think the thinness of the control surface is being underestimated as a failure accellerant. Just drill the hole a little oversize and you're exposed to failure, hardwood or not. In engineering circles, this is called "wheelbase". The longer the hole (thicker the control surface), the more resistant there is to this pivot movement. Why rely on compression alone for control horn integrity? Eventually, over time, if not sooner, the compression will be degraded enough to allow those ball joints to start slipping. Plainly said, I just would not use these horns on a thin surface without at least substituting flat washers for those ball washers. And yes, use hardwood too.

  14. #14

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    RE: DANGER ALERT Dubro Cat. No. 913

    I make control horns from aluminum angle stock. The support footprint can be whatever you want it to be then.

  15. #15
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    RE: DANGER ALERT Dubro Cat. No. 913

    Yep, that's a good idea. In fact, there are many better control horns out there either homemade or store-bought. Regarding the footprint, that is the insidious thing about these Dubros.
    The footprint could be one square inch and they could still pivot regardless.

  16. #16
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    RE: DANGER ALERT Dubro Cat. No. 913

    Dubro makes the same control horn with a base that does not swivel. They are not the best looking, but work fine for flat control surfaces. I still use some kind of hardwood with these.

    For a kit you are, and to a kit you shall return.

  17. #17
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    RE: DANGER ALERT Dubro Cat. No. 913

    I have quite a few of these Dubro control horns left over. I'm currently completing a 27% Cap232, 54cc gas conversion. The hardware is expensive, and I don't want to waste them. So I used the Dubros in hardwood on the rudder and elevator. These are THIN control surfaces, just like the Skybolt. I simply sanded off the semi-sphere from one side of the washer thus preventing any pivoting. The ailerons are thick, so there will be no pivoting there anyway.


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