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Design => Stunt design => Topic started by: Chris Wilson on April 17, 2011, 07:30:48 PM
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Hi all,
What are builders thoughts on engine bearers that only go from the nose ring to the first former?
Are they indeed strong enough to properly transfer the load from the engine to the airframe?
Is it 'really' a weight saving, a ploy for more tank room or simply highlighting the notion that they were too long in the first place?
A lot of replies I will predict revolve around the use of the popular mid sized glow engine but I have seen in the past quite long bearers used to 'spread the love' from diesels and cast iron piston antiques, in fact the Diesel powered Freebird MK 3 had bearers running from the nose ring right down to the tailpost, albeit tapered along the way!
And the arch typical radio engine mount only goes to F1 so perhaps the answer revolves around what it used in conjunction with.
To qualify the question somewhat I must say that I have a friend who has a R+B 75 with said short engine bearers and he has no structural problems at all even with all that massive power trying to convince the nose to part company with the wings.
Thoughts please?
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I reckon that provided the bearers are glued to some good ply doublers which go back beyond the first former, you should be o.k. But that is for normal flying stresses only.
I did build such a model, (picture in recent CLAPTRAP), and it was quite o.k. until it landed nose first, upon which it burst rather like a paper bag!
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Typical RC usage has short engine bearers (or bolts the engine to the firewall), but typical RC usage has fuselage doublers going back to the wings, too. I know that my Nobler brought the engine bearers back to the wings, but had no doublers at all. So I think that both practices are providing strength where you need it, just in different ways. I also suspect that the Control Line way of doing things is lighter, but I couldn't swear to that without some serious consideration.
Tapering the engine bearers starting right behind the first former and going back to where they end sounds like a good idea to me, as does drilling lightening holes in them behind the same former. My knee-jerk reaction to the tapering is that it'll work better, because the formers will tend to flex in the back, instead of snapping, or snapping the fuselage wood. But I don't see that done much -- so I don't know if it's a good idea or not!
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Tapering the engine bearers starting right behind the first former and going back to where they end sounds like a good idea to me, as does drilling lightening holes in them behind the same former. My knee-jerk reaction to the tapering is that it'll work better, because the formers will tend to flex in the back, instead of snapping, or snapping the fuselage wood. But I don't see that done much -- so I don't know if it's a good idea or not!
You want to be careful about "lightening up" anything around the front end, particularly when you are using old 4-2 motors. But it still matters even with modern engines. Even small amounts of excess vibration can have dramatic effects. For just about any engine above a 19, I would use 3/8x1/2 maple all the way to at least the leading edge of the wing. On my airplane I typically use 18" motor mount stock, full dimensions all the way to the wing LE, then taper it down to about 1/8" wide to about the wing high point, and then glue it to the wing sheeting for maximum rigidity. I also use 1/64 ply doublers from the nose to well behind the wing cut-out.
95% of the vaunted "Big Jim" run on the ST60 was, in my opinion, that they built the front end of the airplane sturdy enough to handle the shaking. That was even tougher than mine, at some cost in weight. The engine run was enough better that the weight gain was easily offset.
My airplanes have tended to have generally better engine runs that some others, and I think 99% of the reason is that I build the nose strong - the one thing I actually *did* learn from Big Jim, et al. It definitely matters, even with modern engines, with some interesting examples that I will choose to keep to myself for now.
Brett
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I can attest to what brett said..
I had terrible run of engines not maintaining a setting on one specific series of ship . I was trying an RC soft mount. You could move it with your hadn, I know others have used it ,but It didnt work for me.
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All of my Snaggletooth series Mustangs had short motor mounts exactly as you suggest and compensated for strength by gluing the tank in place. The reason for the short mounts was that I canted the engines to keep the mufflers in and one of the long engine bearers would have interfered with the tank located behind the engine compartment bulkhead.
Snaggletooth is still hanging on my workshop ceiling with 2200 flights, a NATs and Walker Trophy win and 2nd at World. No cracks. I'd show photos of the internal structure but don't know how to do photos on stunthangar.
Al
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Al :
Use Photobucket to Upload - then click the " Generate link codes "
Which will be in the a section on the website called, Message boards + Forums
Click on the it automatically will copy it, then simply paste it into your thread. ( Ctl V )
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Those maple engine bearers come at 12". I normally cut them to 9" or 10". To save weight, I drill holes in them aft of the firewall. Some builders taper the mounts to save weight.
Floyd
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Hey Al, all you have to do is click on "Additional Options" and pick the picture you want. To me it is easier here than SSW as I don't have to shrink the size down so much. Better yet guys, send Al the money and get his disc he has available with his history of his techniques and building posts/articles. H^^
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You reckon this is too short?
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You reckon this is too short?
Not for a VF or 40-65 PA or RO-Jett. I sure wouldn't use it on any 4-2 break engine, a 4-stroke, or a 75/76. Not because it's going to come apart, but because I think it will vibrate too much.
Aside from the plastic bits and the stubby mounts, that looks an awful lot like the inside of my engine compartment.
Brett
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You reckon this is too short?
Hmm, looks like a set of swollen tonsils in someones throat more than a set of engine bearers there mate!
But in all seriousness, that has got to be the absolute minimum I have ever seen in a model.
Thanks.
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I once lived on a lake with a 2 foot thick reinforced concrete seawall. The winter ice destroyed it, almost turning it upside down.
I replaced it with rip-rap (20 lb. bluestones) piled against the shoreline. The ice pushed it around. In the spring I pushed a couple of rocks back on the wall and went home merry.
I once built a Nobler with a firewall R/C engine mount. I crashed it in a wind up accident at the '92 Nats. Knocked out the firewall and fuel tank. The nose, nose block, etc. remained wholly intact. The engine also remained intact except for a destroyed cyl. head!
Oh yea, the firewall mount was HALF the weight of the lightened motor crutch.
Hmmmmmmmm!
W.