Thursday, October 25, 2012

(Wing) mold making for rapid prototyping

I am progressing forwards from concentrating on aerodynamics to also fabrication and optimizing the fabrication process. Been doing hand layups for a quite long time now, but I need to start doing shape accurate parts. Testing aerodynamics requires very high accuracy.

I have been doing several molds lately. One was almost successful, but it was a lot of work and it still had non-sharp edges.

So what I am trying to do is a wing mold with CNC fabrication. I have a CNC mill that can carve blue styrofoam, wood, MDF, Corecell etc. and is large enough for producing a half (lower or upper) of a wing section (with the limitation of the length, have to glue a big mold from ~1.2 meter pieces together).

I would need a rapid prototyping technique to produce shape accurate molds with glossy or at least almost glossy surface. So that manual work would be minimal. This is especially pronounced in case of making wing molds.

The problem I am facing is this:
- if I carve the plug to blue styrofoam, and then paint it and polish it, there is the downside:
- the styrofoam can be only painted with a paint that does not have solvent in it, and the only paint that will fill the surface is solvent free epoxy primer. The problem with that is that it also makes sharp edges round, and especially in a small scale (RC scale) the roundness becomes way too big to be acceptable. The edges where the lower and upper half meet, should be also absolutely accurate and sharp. Doesn't happen with this technique.

Has any reader used molding epoxy? I saw some picture of a mold being filled with a molding epoxy and then milled with CNC again to shiny surface directly (?). Would that be viable option for my use? As it would be for rapid prototyping and for fabricating many wings (and not just one pair), it should be somewhat reasonably cost effective. Making the mold from wood is not completely inexpensive either - requires a very thick perfect wood block (or MDF block). I could not afford consider replacing the styrofoam with a huge solid mold plastic block (that is used in industry for prototyping shapes with CNC), because the same volume is much more expensive, would be possibly fine for a CNC model of a small device, but for making a mold for large wing the cost hikes out of the roof very quickly. Styrofoam is cheap and very easy for the machine to carve, but that's the best part of it, otherwise it is really poor material.

Any first hand experiences on this?

Tuesday, October 23, 2012

Propeller placement article (external link)

There are many considerations where to put a propellers in a small aircraft or RPV. The common place to put them is at the nose. The biggest reason and driver for this placement is that it is advantageous for CG location. However, from aerodynamic standpoint that is not very optimal. There was discussion at HBA Forums about propeller placement and this document was linked (it studies difference of prop placed in pusher and tractor configuration):

Optimal place is behind the wing, a bit above the wing centerline (only small part of the prop circle goes below the wing). This placement has the typical CG challenges with it. And it will require either pylon on the wing, or a pylon on the fuselage (assuming a single fuselage). There is then the question about the effect of the body to the prop located near the fuselage behind the trailing edge of the wing. There might be unfavorable flow due to the effects of the wing-fuselage joint that this study did not take in account.

According to the article, it was possible to increase quite significantly the Clmax of the wing with the rear placement of the propeller due to the suction effect to the wing. This leads to interesting thought about a line-thruster - multiple small electric motors turning multiple relatively small props behind the trailing edge of the wing, providing suction to the whole wing surface, or at least large part of it. Interesting question then would be that would a varying thrust angle be beneficial, should the pylons be actually mounted on the flaps? Downside of this is that this may lead to flap mechanism that is not very lightweight as the flaps have to take all the torque and push from the motors. Normal flap mechanisms would not like that.

Any comments on this?