Canopy Research

Essentially took the last couple of days off because I have been waiting for parts from Van's. I have done a bit of research on working with the plexiglass canopy. It IS most likely the MOST expensive part of the airframe. Here is what I have learned to date (and, I will most likely add items to this listing as I learn more.)

Only thing I did yesterday on airframe parts was to clear coat the tunnel covers and seatbacks. Also, YESTERDAY, received the upholstery and seatbelts from Flightline Interiors. Looks real nice!

Item #1 - Heat the work area to 80-90 degrees. The canopy will be much more forgiving if its ambient temperature is high. May be uncomfortable for you, but, the canopy loves it.

Item #2 - Put a ceramic space heater in the fuselage to further help warming the plastic. Use a thermostat on the heater to maintain the fuselage temperature at 90 degrees. Thermostats are available from Sears and others for the bullet type heaters. Got mine from Sears. Do NOT get the temperature too high to warp the bubble.

Item #3 - NEVER, and I repeat NEVER enlarge a hole in the plexiglass using a normal drill bit or a plexi drill bit. If you do it with a plexi or normal drill bit the plexiglass will get sucked up the bit as it penetrates and then the plexiglass will snap down. A good scenario to create a NICE crack. A SINGLE fluted UNIBIT is preferred. Go very slow with the UNIBIT in a battery power hand drill with little pressure. (As added insurance the drill clutch can be set on its lowest setting). The craftsmen (multiple RV offenders) have told me during an EAA Chapter 75 meeting to drill dry (a lubricant is not required). (The RV-12 plans tell you to enlarge the #40 holes with a #27 plexi bit....Not a good idea!!!)

The craftsmen at the EAA meeting also indicated that another way to drill the canopy is using a hand drill such as this:

Click here for an example of a Hand Drill

With the Unibit in the hand drill they tell me that you have very good control. You can feel when the bit is just about to break thru and back up a bit. Then you can complete the hole from the same or opposite side. The Unibit can then be used to camfer the hole from both sides.

Item #4 - Ensure there is enough play between the screw outer diameter and the inner diameter of the screw hole that you drill in the plexiglass canopy to account for expansion and contraction of the plexiglass.

Some additional information on expansion and contraction.

From the VAF forums:
If the holes drilled in the canopy are for flat head screws then the holes in the canopy are for clearance only. When the screws are thightened down the conical taper on the screw engages the conical shape of the countersunk hole. The interface will center the screw in the hole. Therefore the hole through the canopy plexi is purely for clearance. A number 27 drill is 0.144" diameter, a #6 screw is .138" diameter. That is only .006" clearance. Not very much if you ask me. There is nothing wrong with using a #21 (.159" 5/32" diameter) drill for the clearance holes in the canopy plexiglas.

From http://www.ceebaileys.com/install.htm:
Drilling windshields
Whenever screws or bolts are used through holes drilled in the Plexiglass as for the securing of an aircraft windshield or window, consideration should be given to the expansion or contraction of the Plexiglass as effected by changes in temperature. Plexiglass expands or contracts with temperature at the rate of 1/16 inch per foot per 100 degrees F. temperature change. For example a line of holes at the top of the windshield 36 inches across using a 1/8 diameter. screw would require that the holes be 5/16 inch in diameter to take care of a temperature spread of 100 degree F. When holes are drilled with insufficient clearance excessive strain is placed on the windshield and may cause the holes to crack out at high or low temperatures. Excessive strain on Plexiglass windshields shortens their service life and invite cracking and crazing.

I am going to have to ask engineering at Van's and see if a larger hole (5/32") is acceptable.

Item #5 - Follow the plans precisely when drilling and clecoing the holes thru the plexiglass. You do not want to stress the plexiglass in one direction or the other.

Item #6 - Think this is a good suggestion, based on my research. After final drilling all of the holes in the plexiglass, VERY VERY LIGHTLY touch the holes on both sides thru the plexiglass with a permagrit countersink bit with the correct pilot (available from Avery). Will eliminate any stress risers or microcracks where the bit entered or exited. The craftsmen at the EAA meeting also indicated that the Unibit shoulder or a hand deburring bit will also work here. When using the Unibit to smooth or camfer the edge of the hole, use the hand drill.

The craftsmen at the EAA meeting have also indicated that it is a good idea to place a thin plastic washer between the metal canopy frame and the plexiglass. This is NOT in the RV-12 plans. I am going to have to ask engineering at Van's and see if this is acceptable.

Item #7 - When drilling thru plexiglass always back it up with a wood block and also place pressure on the plexiglass where the bit enters so the plexiglass is not sucked up with the bit as the bit penetrates.

Item #8 - Set any pulled rivets thru plexiglass using a hand puller and NOT a pneumatic one.

PLEASE e-mail me with any other suggestions! marty.santic at gmail.com

Time Yesterday - 2 hours (Clearcoating)