Hot Wire Wing cutting

 Hot wire cutting of wing out of EPS, XPS, EPP

The details/wisdom of how to cut wing cores out of foam (different variety) is captured below

The Bow

There are many method of making a bow.. the essential points while making one are

1. Should be easy (light) to use.. the feel of  balance  while holding it will ensure good cuts

2. Should be long enough so it is able to cut the required pieces.. You may require more than one bow for your different needs 

3. It is essential that the hot wire is very taunt while cutting... it is good to have some method of  controlling/ tightening the tension applied to the wire. I simply add more twist/turns to the tensioning cord to shorten its length which stretches the lower part of the legs and stretches the cutting wire

4. The cutting wire needs to be hot enough, be thin, and should be able to be stretched taunt .. I use thin ss wire and some times a the thinnest guitar string.

5. Any safe DC/AC power supply @ 30V and 5A should be adequate for most works.. Safety is the key here, and should be able to deliver the power for 20/30 mins without heating up 

6. The power connection to the bow should have a sufficiently long wire to address placement and enable free movement of the bow across the work .. Also it is good to have a ON/OFF switch in the bow which is easy and convenient to operate 

7. It is a big help if there is attachment to rest the bow at one side, so that the legs with the hot wire pivots down due to gravity and follows the top of the template.

Airfoil Templates

I have used templates made out of  thin ply, formica, and Al sheet... I use one template method .. i.e the full airfoil, not two templates per side. I size my templates 5mm longer in cord to compensate for kerf and thin TE

Thin ply is strong (handles the downward force of the wire), but is slightly difficult to cut. Edge finishing is difficult to achieve

Formica is strong , difficult to cut , edge finishing is good

Al sheet is slightly less strong at the LE and TE where it is thin . Cutting is easy and edge finish is good... this is my preferred material.

Keep in mind the mirroring reversal of the RC and TC template printouts  for the left and right wing.

Print and tick the full size airfoil printout onto the template material .. spray adhesive is best, pva and others will also do.. Press firmly with roller and allow to dry. Cut out the template .. for Formica and ply use dremel.. for Al flashing tailoring scissors are ideal. Clearly mark the start and end of the airfoil. Smoothen the top and bottom of the airfoil with a whetting stone or fine sandpaper .. this is important and the edges need to be absolutely smooth with no jagged edges where the wire can be held up. To test, run a thin blade from both sides, it should be able to slide without snagging anywhere.  Also punch small holes for pushing Tpins . 

I use 1.5mm spring wire stuck to the LE and TE to act as wire guides . These are stuck to the cord line with cyno. For AL templates a thin extended strip is cut along with the template and double folded to act as a Lead and Trailing hot wire guide

Process

 Cut out the wing blanks (planform) out of foam. I cut them to the exact planform dimensions..

Print out the airfoil templates. While printing the airfoil templates I add 5mm to the cord to take care of the hotwire kerf specially at the TE. 

In the wing blank  RC and TC ends mark out a datum line such that when the cord line is aligned to it, there is enough material on the top and bottom for the airfoil plus some more

Attach the template to the foam block with spray adhesive.  Make sure that there is no adhesive stuck to the top and bottom edge before it is pasted onto the foam block. The cord line of the template should align with the datum line of the block. Push in t-pins  to secure the template in place. Pay special attention, making sure than none of the holding pins are sticking in a manner to snag the cutting wire.. The template should not move with the wire pushing down on it.

Setup the hotwire bow, making sure that wire is taunt after it has reached cutting temp. Test out on a piece of scrap foam. It should move through the foam with very little push and without sagging.

Position the foam flock and weigh it down firmly, specially across the length of LE and TE. it should not move. Make sure that the bow and wire will not be obstructed during the whole cutting process.

I generally start the cut from the TE. Position the wire on the lead-in support, about 5mm away from the foam block. Switch on, and let the wire heat to the operating temp.. a couple of seconds. 

Start the cut at the same time across whole of the TE. Make sure that the cutting wire is always in touch with the top/bottom of the template, and is at equivalent distance at the root and tip template, and sag is not there. Do not be in a hurry to finish. 

Once close to the point where the wire will exit, be very deliberate to make sure that the wire will exit at the same time across the LE. Just when it it at the tip, deliberately hold it at that place for a second. This will ensure a clean tip and exit.

Repeat the same method on the other side to cut the other surface. Make sure that the LE and TE is weighed down before staring the cut. This will ensure uniform thickness of the surface