REFLECTOR: Insulation?

Tue Jun 24 13:27:30 CDT 2008

I have used this technique for over 400 hours with no problem.  I made a
4" by 6" rectangle of aluminum and place some red silicone baffle
sheeting between it and the gear leg.  I attached it with a screw clamp
around the leg.  Easy to remove and very strong.  Blocks radiant heat
and the silicon blocks transmission to the leg if the plate gets warm.
I bent it to direct more air to the rotor and flared it behind to suck
air away from the rotor bewind.

Rene'

-----Original Message-----
From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of Jim Agnew
Sent: Sunday, June 22, 2008 6:44 PM
To: Velocity Aircraft Owners and Builders list
Subject: Re: REFLECTOR: Insulation?

I would like to add my evaluation of the problem and a fix that has
worked for Sam DaSilva and myself for many years.  I have attached a
photo to assist in you understanding the picture.

1 - The heat shield is 0.040 stainless steel a little wider than the
gear leg and higher than the brake rotor. Since SS is not a very good
heat conductor it is standing on top of the axle mounting block and
attached to the gear with two # 8 SS sheet metal screws spaced out with
two small SS washers on each screw to allow a small slot for cooling air
between the gear leg and the plate.  The top of the plate is also spaced
out with SS washers and is fixed with the 1/4" SS bolt that goes thru
the gear leg and the leg reinforcing plate. Small dabs of silicone glue
keep the plate from vibrating.

2 - The gear leg reinforcing plate is 1/4" 6061-T6 shaped to the gear
leg bend and bedded in micro-glass.  This plate provides stiffness to
prevent, as much as possible, the gear leg from bending if over heated.
It also acts as a backing plate for the Axel attach bolts (put steel
washers on these bolts). The plate also provides some heat sink effect
for the axle bolts.

 Your mileage may vary, however shielding that can also act as a heat
sink as well is better.

 Jim

James F. Agnew
Jim_Agnew_2 at Yahoo.Com
Tampa, FL
Velocity 173 Elite Aircraft Completed & Flying 

----- Original Message ----
From: "nmflyer1 at aol.com" <nmflyer1 at aol.com>
To: reflector at tvbf.org
Sent: Sunday, June 22, 2008 6:37:46 PM
Subject: Re: REFLECTOR: Insulation?

Hey Al, 

Always enjoy reading your input. Do  you think that a layer of stainless
insultape material over the Bakelite stuff would add any reasonabel
level of radiation away from the leg? I figured its biggest advantage
was that it would insulate the gear leg from the heat transferred to the
axle pad. Now I see that it might not be as big of a deal as I thought. 

Thanks 

 Kurt 

-----Original Message-----
From: Al Gietzen <ALVentures at cox.net>
To: 'Velocity Aircraft Owners and Builders list' <reflector at tvbf.org>
Sent: Sun, 22 Jun 2008 3:48 pm
Subject: Re: REFLECTOR: Insulation?

Let me put my 'heat transfer engineer' hat on; and give you my take on
the heating/protecting of the gear leg issue. (you knew I would; didn't
you).  It is obviously a complex configuration with different potential
paths, so this is not based on any detailed analysis. 

First of all, the large majority of heat generated by the friction goes
into the rotor.  Some will be transferred to the caliper, but - the
brake pad is an insulator, the contact area to the puck is not large,
the fluid is a relatively poor conductor, the mass of the caliper is
relatively large; etc. So the caliper will heat up, but not nearly as
much as the rotor.

How does the heat get from the rotor to the gear leg? Three basic path;
radiation directly across the gap, conduction through the air across the
gap, and conduction from the rotor to the wheel - to the axle, to the
leg.  Let's look at these in reverse order:

Conduction from the rotor to the wheel is effective but somewhat choked
by the limited area of contact where the bolts are; it then has to heat
up the wheel, then transfer to the bearings, through the balls (limited
contact area), to the axle and on to the gear leg.  A very torturous
path - so my thinking is not much goes this way.  And if it did, the
first thing we would see would be damage to the tire at the bead,
because that would be the first place to get hot.  I haven't heard of
heat damage to the tires.

Conduction through the air from caliper to leg: as already pointed out,
air is a very good insulator, and there is nearly an inch of intervening
air.  And this air is not stagnant, it is moving - a lot in the case of
RG.  Not likely a significant transfer path.

That leaves radiation.  Radiant transfer goes as T (absolute) to the
fourth power, times the emissivity (yes, and don't forget
Stephan-Boltzmann and their constant).  I'd guess that if you did a hard
stop at night and looked at the rotors; they'd be glowing - so probably
over 1400F.  The emissivity of the rotor is relatively low on the shiny
part (maybe .15) and higher on the rest (maybe .4 - .5) and the
absorptivity of the leg is high.  So; based on some actual calculations,
the amount of heat going into the leg opposite the rotor may be 75-100
watts.  I'd say that much heat going into a small area of the leg over
the period of time for a stop, and then some more braking for turning,
and maybe dragging . . is definitely significant.

So, here's the deal, too.  Whether it becomes a problem or not is
critically dependant on how hot the rotor gets; if the temp is just 200
degrees higher one time, the amount of radiation heat transfer goes up
by 1.5 times.  So the biggest factor is how you use your brakes.

Supposing that the above evaluation is somewhere close to reality, then
the biggest gain can be made by protecting against radiation heat
transfer.  By far the best way to reduce it is to interpose a layer of
shiny material (something with low emissivity). One baffle of shiny
metal, particularly if it is cooled by the air, can reduce the radiant
transfer many fold.  Clean SS may have an emissivity of 0.05. Similar
for aluminum. The foil coated insulating wrap can also be effective.

Personally I'm not that impressed with the 1/8 of high density
'bakelite-type' material. It's emissivity/absoptivity is likely
relatively high, and clamped against the leg, it will transfer a
significant part of what it absorps.  And it's not the greatest
conduction insulator.  No doubt it will reduce the heat transfer so
that's a good thing.  

Probably in most cases, no protection is needed - as long as you are
careful about minimum use of the brakes. But doing something to limit
the radiation part of the equation can surely be helpful.

OK, this got long, and I won't even guarantee that I've got it right:-).

Al

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