I remember BVM had instructions on how to finish trailing edges with epoxy/microballons.

I remember the process vaguely... some pictures of the process would help.

Thanks

Not sure of that method but have used the thin carbon strip method on built up wings...

If you glue a thin piece of carbon strip between two pieces of balsa or trailing edge material, you can sand the balsa down until it exposes the carbon strip at the trailing edge, doing this both sides will give you a trailing edge which is as thin as the carbon used and durable too. Other options are to use poly ply (BVM product) or thin FR4 / epoxy glass board which is the same as poly ply to achieve the same results. Only issue with carbon is it can stain the balsa when sanded, but if its being covered or painted it makes no difference.

marcs

Thanks Marc for the quick reply.

However, I'm looking for a "after the fact" method using epoxy/microballons, as the surface is already built.

I have been told the razor sharp frailing edges set you up for flutter. My favorite kit manufacturer specifically says to blunt the t/e.

I just finished using the BVM method for filling all of the gaps on my Scorpion with epoxy/micro balloons. I am not sure how BVM does this for a trailing edge but it should be as simple as laying one side against a flat surface and filling the trailing edge area with micro balloons. I typically use a 6" rule and lay it along the surface of the part as a guide and then wipe the excess off. Be sure to use some wax, or maybe Vaseline on the flat surface to allow it to release.

I've been under the same impression. Though I've flown pylon aircraft with razor sharp te's (thin carbon layered between two pieces of balsa) and never had flutter. Never seen over 160mph though. And why bother, just asking for hanger rash repairs lol!

Good question.. i've got the answer

first off, you need to determine how long your trailing edge will need to be to get to the razor sharp point. take 2 rulers, place them edge down on the trailing edge and then measure from the wing TE to the point where the rulers cross.

using this dimension, make a strip of G-10 this width and the length of the wing-span.

Then on the top.bottom of the wing, take some masking tape and tape up the the trailing edge of the wing.

On the other side, take some wide masking tape and extend it past the trailing edge of the wing atleast 1" past the "razor sharp" edge of the New TE. Take your G-10 strip and place it on this piece of tape with the edge butted up against the wing TE.


Fold the over-hanging piece of tape up 90* to help keep the trailing edge straight.
Now mix a mixture of epoxy and micro-balloons and put it over the piece of G-10 to fill the cap between the G-10, tape and wing TE. Once cured, sand to shape.


You should then have a razor sharp trailing edge:

What I've always heard was that you needed to either taper the surface to a point (razor sharp isn't a requirement, just an even taper to a reasonably thin edge), or leave the edge blunt. Rounding the edges is what I've always heard/read tends to produce or exacerbate flutter.

Bob

sharp edges like that are not good, its actually better to have curved edges.

I don't think so...

http://members.cox.net/moormanrc1/Flutter.htm

http://www.sedation4.com/rudder.html



Bob

BVM has been in this hobby a LONG time and was a pylon racer before getting into the jet scene. I HIGHLY doubt he would recommend it on his models plans and in their manuals if "razor sharp" trailing edges could potentially cause flutter and the destruction of one of his models.

Also Go look at any full-scale airplane, I have Never seen one that has "rounded" trailing edges.

I know on a F-16, the training edges are very sharp in relation to there flight control surface size. The trailing edge of most flight controls are in the 1/8" range or thinner. Seen many people receive a few stitches from walking into a flaperon, it is just high enough to be out of your peripheral vision for an average height person.

This seems to work for Mach 1.6...

This is a figure taken from "The effect of trailing end geometry on the vibration of a circular cantilevered rod in nominally axial flow" by M.W. Wambsganssa and J.A. Jendrzejczyka.

What is shows is that for a tapered trailing edge, the angle between the two sides of the taper needs to be about 30 degrees or less for there to be a low level of induced vibrations (that can lead to larger vibrations known as flutter). A taper with an angle of 60 degrees is actually over 3 times worse than a simple flat (blunt) end and a rounded end is over 2 times worse than a blunt end. A sharp taper is not only fast, it is much less likely to flutter (which are both related, because vibrations means that there is turbulence - which is drag - which is wasted energy).

Bob

You either want the trailing edges sharp, or blunt. The pitching moments get unstable with round TE's and flutter is more possible as well as poor trimming and tracking qualities. Sharp is great, it reduces drag. Blunt is second best - small drag penalty but much easier to make consistent and hangar rash much improved. In the air I'll take sharp, and on the ground I'll take blunt. I need a shape shifting TE.

All my nicely molded pylon racer planes for electric have nearly razor sharp TE's. Those things are glass slippers - the meaning of efficient and fast.

I like the diagram Bob included! It's an easy way to show what people have "observed" over time but in an objective, measured way. I can use this for scratch builds, THANKS Bob!

no this is not right. blunt is better than rounded, or sharp TE that is poorly implemented (which is jut a rounded TE in the end)

Thomas, I have seen your trailing edges on your UV thread. You did a great job!

However, in my case, i'm not looking to extend the surface as the stabs are glued in the fuse already. I'm only looking to strengthen and "correct" the trailing edges. I'm considering just brushing the epoxy/baloons mix and sand.

BTW, are you going to continue the UV thread? I've just ordered BVM EVF 2 10s to convert my UV to electric. It has been setup for BVM .91 DF but the NIB powertrain has never been installed (the DF engine and shroud will be up for sale). This is a ongoing project for me, and I was wondering if you mind me posting once in a while my progress on your "old" thread.

Cheers,
V.

Vasek,
Unfortunately I won't be continuing the thread, as the airplane has found new-ownership. The new owner may continue it though. Please do post up in the thread, as i'm sure those who were following my progress will be interested to see a UV conversion completed. I wish I hadn't of sold the viper now, as it would of fit in the back of my truck w/o having to carry all the stuff the turbine stuff needs.. oh well, maybe i'll pickup an electra later this year.

Thomas, that's too bad you sold the UV. It should fit with the wings on in my Santa-Fe

It is possible i might have to cut-off a piece of the trailing edge on the rudder. In that case I could use the same technique as you did. What thickness of polyply did you use?

Cheers,
V.