Having just read the discussion on adjustable leadouts, and line tension, I have a question. What part does engine offset play in all this? Do you set it up with 0 degrees offset or a couple of degrees? The reason I am asking is some of the things I have read in magazines "seem" to contradict what I have learned on my models. Also want to thank ya'll for these discussions. I read them all and learn more here in ten minutes than in a pile of old magazines. Everbody is nice, even when they disagree. Makes it nice on us new guys. Would also like to ask Mr. Neumann if it would be alright to copy some of this to show my flying partner who has no computer. Always nice to ask first.
Engine Offset?
Stuka Stunt Main Forum · 10 of 10 known posts recovered
LAST EDITED ON Jan-11-01 AT 03:20 AM (EST)
Personally, I am shooting for ZERO offset if at all possible on all models, but the smallest and slowest sometimes don't generate sufficient line tension for my comfort. Little planes seem to need it the most. High performance stuff needs it the least (you should just try feeling how hard a two pound Rat Racer going only 130 mph, with zero offset, pulls! Enough that pilots weighing less than 175 may be in trouble.)
I've been known to use an offset wedge on a newly completed plane that I've got my doubts about, at first, until I finish a couple of test hops. I really dislike washers as a method for adding a little offset. They tend to work their way into the wood rather faster than a flat-mounted engine, requiring constant attention to remove the loosening effects.
(I already like to cut away any outboard plywood doublers under the mount lugs and blind nuts, because that crushes too fast. Many of my own scratchbuilt profiles totally omit an outboard doubler, using a single reinforced inboard doubler plus tripler, instead.)
Kiwi
Kiwi --
Maybe I just need another cup of coffee. I can't get a good picture of the way you do doublers. Seems like omitting them would expose the even softer balsa. Obviously I am not getting the picture. And where does the tripler go? Do you use 'glass or other types of reinforcement in conjunction? Do you think soft cheeks on the inboard side covering the blind nuts helps?
I'll reread your entry this eveniing after work when I will hopefully be right much sharper.
Larry F
LAST EDITED ON Jan-11-01 AT 10:20 AM (EST)
I think Kiwi is saying that he cuts away the plywood so that the nuts/washers rest on hard maple rather than birch plywood. Fancher's Doctor article makes a good case for the thick soft tripler-that is, half inch balsa on top of the plywood doubler. I am trying gluing in metal tubes that carry the crush loads of the motor mount bolts. Then using a large washer under each nut on the inboard side, with elastic stop nuts.
We always put some kind of load distributing surface under the motor mounts. The engine mount will dig into even hard Maple when you tighten it down.
You can use aluminum pads for this, but a lightweight, cheap, and easy solution is to use phenolic circuit board material (available at Radio Shack and other fine electronics supply houses). Make these pads about twice as long as the motor mounts themselves, and you can even screw them down to the mount with a screw on each side. Otherwise they can be simply glued into place and the mounting bolts will hold them secure. A single 1/16 inch thickness is good enough on profile planes, but we use these on all of our full fuselage ships as well and use a double (1/8 inch) thickness on these. This brings the engine 1/8 inch off the motor mounts, and the location of the motor mounts is adjusted accordingly to keep the thrust line where desired. This, then, has the added benefit, when using a removeable and adjustable height tank, of having room for up to 1/8 inches of adjustment towards the mount in addition to the allowed 1/8 inch in the other direction (we give it about 1/4 inch room for adjustment total).
On a profile, don't use Lite (crush) plywood for the doublers. 1/16 inch Birch plywood is just as light and just as strong as 1/8 inch Lite Ply and will not crush nearly as much. Put a layer of this on each side--and use a balsa tripler on the inside if you wish.
Oh, yes, on your original question, if everything is built properly and trimmed properly, engine offset should not be needed. In fact, it can get in the way, add unneeded additional yaw, and cause more problems than good. Go with straight down the line, and you will be happy. One word of caution: You do not want inthrust (the engine aimed in). Bad news! We will, sometimes, on a full fuselage ship aim for 1/2 (or, maybe even 1) degree of out thrust "just to be sure". If we miss and angle it in from what we were intending, it is still straight ahead. If, on the other hand, you aimed for 1 degree of out thrust and then ended up with 2, we start getting into some of those problems I was mentioning. It is better to measure twice and cut once. When we get to the alignment stages--including motor mount drilling--we spend an extraordinary amount of time measuring and making sure. It is hard to correct later on after you discover the "oops"!
On your question of having permission to print this information, distribute it, whatever, certainly. Go ahead. This is a public forum and the information being posted here is for the good of the hobby. If somebody doesn't want their information read by others, I don't think they would be posting it.
Leonard Neumann
I Absolutly positively agree with exactly 100% of what Len says about engine offset. You have to remember back in the early classic and old time days, they didn't use adjustable leadouts, so to ensure that they wouldn't have a plane chasing them around used a good bit of engine offset, which in fact caused some other aerodynamic problems.
RCJ
I'm all for 0 offset. I once accidentally built a 1/2 A stunt airplane with about 3 degrees of inthrust. I was able to compensate with the adjustable leadouts and it turned out fine. I'm in agreement with Kiwi on not likeing plywood under the engine. It's OK if you have a larger metal plate under the mounts to spread the compression more. I also like cutting away the plywood to let the nuts set against the bearers. On the airplanes I'm building at the moment I'm using 1/8 liteply on each side, because the fuselage needs to be that thich. The liteply is cut out on both sides to clear the engine mounts and the mounting nuts so they will go directly on the hard maple bearers. I've gone more and more to using locknuts rather than blind mounting nuts. With the lock nuts you tighten them down, run the engine once, tighten them again and forget it. With the blind nuts, I use blue locktite.
Jim Thomerson
>Kiwi --
>
>Maybe I just need another cup of coffee. I can't get
>a good picture of the way you do doublers. Seems
>like omitting ("one of")them would expose the even
>softer balsa.
>
>Maybe I just need another cup of coffee. I can't get
>a good picture of the way you do doublers. Seems
>like omitting ("one of")them would expose the even
>softer balsa.
Over the last 15-20 years, several profile planes we built have had inset tank compartments between the hardwood bearers. To do that required hardwood spreaders the width of the engine which are either screwed to the bearers or attached with dowels. The bearers on such planes are longer than merely back to the wing's LE, and sometimes of staggered lengths to avoid stress riser type problems.
Some of those had an outboard doubler starting behind the engine atop the spreader if we weren't allowed a tripler (maximum width of a profile fuselage may be defined by event rules). Between a doubler on the inboard side and a tripler of 1/2" balsa (or a nice cheek cowl), we might add triangular, plywood LE to fuse gussets or a layer of class cloth under the tripler. The gusset isn't exactly "between" since part of it sticks out of the tripler - the two equal sides are about 1 1/4" long, and it's 1/8 thick.
> ... And where does the tripler go? Do you use 'glass or
>other types ofreinforcement in conjunction? Do you think
>soft cheeks on the inboard side covering the blind nuts
>helps?
>
Yes. Quite worthwhile.
>I'll reread your entry this eveniing after work when I
>will hopefully be right much sharper.
>
Shoot, I hope this addition has made my thoughts easy enough to understand that even if you don't manage to get back here before tomorrow morning's early hour wooziness is back again, you'll get a good mental picture next time.
>
Kiwi
I have used aluminum pads next to the maple mounts on both of my profile ships. Nobody told me too, I just thought it was a good idea. We have machined offset pads to put under the engine but later went to flat plate. I did not like the washer under the engine idea from the first time I ever saw it. Working with pumps for the last twenty years has taught me to bolt them down tight and straight and they dont vibrate near as much. Makes them last much longer also. Vibration is a killer of machinery. Ya gotta make it all go hummmmmmmmmmmmmm. Thanks for the alternate material ideas and permission to copy the forum. Working on my first built up fusalage at this point in my career,an Oriental. Kind of like the man falling off the Empire State Building, "So far no complaints".
Kiwi --
Thanks. It is a LOT easier to understand! I had this picture of the motor mounts "floating" out there with just a thin glue line holding them to the balsa fuselage. Old prejudices die hard. We always thought that doublers needed to be a "sandwich" to hold the motor mounts in place and tie them to the fuselage. From your comments and others, I now believe that there are other and better ways to build a good nose structure.
I suspect many of us who read all the threads related to this will try some changes in their next plane.
Larry F