07/13/2005 at 2:38 am #40343AnonymousInactive
Chuck emailed me with this question, My following answer may be of interest to others. IM
I’m rebuilding one of my old F1A models that uses 5 mm wing wires. Since I have no metric metal bushings to fit into the fuselage, that starts to create a problem. When I drill a 5 mm hole enlargement into a heavy-walled aluminum tubing, the hole is not coencentric from one end to the other. I’m told that is characteristic of this operation with twist drills sent through a pilot hole. Sometimes I get lucky with brass tubing and lots of lubricating oil to expand the hole from 3/16 to 5 mm (0.188 to 0.197″) without removing any brass (i.e, spinning the brass to a larger diameter, but that is hard on drill bits and leaves me with a bushing with OD that doesn’t match any drill bits that I have.
What do you do?
Chuck07/13/2005 at 2:40 am #42467AnonymousInactive
I / M&K don’t use any bushing. On a single wire wing. I simple drill the hole. I use to use a hard maple block in the fuselage but M&K use a birch like wood.I have not pulled a wire thru a fuselage– knock on wood. I have bought a set of reams which help get a nice fit.
On a 2 wire wing, I rarely get the two holes drilled that correctly position the wing incidence. I then carefully open the front hole. Put transparent tape over the holes and carefully cut out the holes. Wax the wing wires the right fuselage side. Space out the left wing with equal box about .38 inch. Position and secure the wing incidence were you want it. Now build a dam of modeling clay around the wires. Mix up a good laminating epoxy, put in some CF sanding (very fine) and pour into the fuselage / wire gap. After cure, ream the front hole bit by bit until I get the wing wiggler movement needed for smooth information.
I’ve also plugged holes with birch dowels and re-drilled and used the “liquid” shim method described above.
JIM07/13/2005 at 11:54 am #42468AnonymousInactive
Here is the method I use for fitting the wing wire to the nosepod:
For a new nosepod, make sure you drill the hole through a little undersize, best on a drill press, and make sure you get the angle correct for the wing to pod fit.
Then I put in balsa blocks about 1/8″ on both sides of the wing rod that bridge the sides of the nosepod to provide forms for pouring in a slow setting epoxie, high strength type. I rub the wing rod with silicone grease, so it has a thin film, and insert it into the fuselage, so it sticks out about 3/8″ on one side, and hangs way out on the other side.
Mix your epoxie in a container that can also be used to pour into your forms. Mix the epoxie of course to the exact formula given, and cut some carbon fiber and mix it into the epoxie after a few minutes. The mix should then look very black.
Carefully pour the epoxie/carbon mix into the forms covering the wing rod about 1/8″. Let this cure and setup until absolutely hard. Then let it cure longer, to be sure.
Now you have a wing rod bonded into the fuselage. So to remove, apply some force to the rod with a pliers to twist the rod breaking the bond.
The rod can then be forceably removed, pushed out, and you have an absolutely perfect fit, and a very strong rod to fuselage structure.
This sounds like a big job, but it just takes more words to describe how to do it than it actually involves.
I don’t think a hardwood block glued in is adequate given the forces put on the wingrod during bunt launches.07/15/2005 at 2:14 am #42469AnonymousInactive
Great thing about the forum is seeing the differing ideas. In this case, I stated that the wood block in the fuselage is adequate to carry luanch loads– supporting evidence is this is the method that M&K, Stamov and I use and I don’t belive many pull harder.
Lee Hines had a model that used a blob of epoxy / chopped figer class (I believe it was FG because it was white in color). The block did crack causing eradic launches. Dave likely uses different / better epoxy and chopped fiber and the formed epoxy / chopped CF works for him.
Related noted to those that have old M&K models, Several years ago, they started using a rivet at the top and bottom of the alum piece the tow hook mounts to. The lower rivet can be seen. They stated this after a hot contest in Spain were they pulled two hooks off the side of the fuselage during launch. The cause is the room temp epoxy used softens / weakens in +100 F temps. I have drilled tapped countersink 2-56 to serve the same purpose. Good reason not to leave your models in your car at Lost Hills in July.
Amyother ideas comments out there?
JIM07/27/2006 at 12:49 pm #42470AnonymousInactive
You first need to think where the loads the wing wire(s) sees are coming from. The shear and bending forces are due to the tower loads (on the hook) so it helps to have a good, stiff load path between the wire and the hook. I simply use a hardwood block that carries the hook and acts as a ‘bearing’ for the wing wire; I don’t use the side of the fuselage to connect the hook for the reasons expressed earlier. I do bush the wire hole with either brass or a braided CFC tube (as used for joiner tubes). The width of the fuselage makes the wire bearing stress even in wood quite low and the bush increases the effective wire diameter and hence reduces the bearing stresses still further.
More significant loads come from crashes or bad DTs where the wire is bent fore or aft in a symmetric or asymmetric manner. Here I have experienced damage in the past (often in a 2 joiner model where one joiner breakes through into the other’s hole) but it is simple to repair using methods described earlier. If you make the whole area too rigid there is the risk you don’t do local damage to the hole but more significant damage to the fuselage.
In summary :-
Have a good load path between the hook and wire
Bush the wire holes
Use the entire width of the fuselage to take the hook and wire loads
Don’t crash too much
07/27/2006 at 8:27 pm #42471AnonymousInactive
… commenting on CHE’s reply …
I’m having side mounted hook and I’ve used ‘pressurized fiberglass’ plate 1.5 mm thick glued with Devcon Metal Epoxy to reinforce hook mounting holes and to spread the tension. It seems OK.
Pavol08/06/2006 at 4:38 pm #42472JLorbieckiParticipant
Just a comment here on drilling holes, particularly in brass…
The normal twist drill has a positive rake on the face of the drill. Think of this thing as a chisel. When the drill is turned, it attempts to pull itself into the material. This is commonly seen when you hand drill something and it grabs the drill and pulls it into the material. This happens readily in materials such as brass and acrylic.
A common cure for this is to grind a very small flat on the leading edge of the drill. Instead of an angle, you now have a flat edge. It will no longer grab and makes a BIG difference when drilling these materials. It is the only way you can safely drill holes into plastic, plus it keeps it from grabbing and enlarging or elongating the hole in brass.
It takes 20 seconds to grind the drill. Please note that after doing this mod the drill will still work in aluminum but may not be the best in steel. I actually have two sets of drills, with one set up for plastic and one for ferrous materials…
Good luck…And, nice to see Jim again at the NATS!!!
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