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  #16  
Old 07-07-2006, 10:39 AM
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Quote:
Originally Posted by neverquit
I didn't know a GPS can give you air speed. Are you implying your canard will fall before your pitot tells you you're flying too slow?
yes, agree with the airspeed, but for stalling, mmmmm, with a load of ice it changes the plane's flying surfaces, new stalling characteristics.

On the habit of turning on the pitot heat when entering clouds, my feelings - KEEP IT. It is like turning on carb heat in the desert, stupid, but, if the habit is not in you and you happen to be in atlanta one day - you may remember too late why the plane is losing power on short final.

Carb heat and pitot heat are just plane good habits
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  #17  
Old 07-07-2006, 10:53 AM
Wayne Hicks Wayne Hicks is offline
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Oh come on now. GPS speed over ground is certainly not airspeed. But any pilot who is supposed to be aware of windspeed and direction ought to be capable of doing the math in her head to be "good enough", "in the ball park".
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  #18  
Old 07-07-2006, 11:40 AM
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I built a heated pitot just like what is described on the berkut13 site. I have photos of the machining and the proposed mounting at: http://winnfamily.smugmug.com/gallery/946708




However, I performed an initial test with the pitot tube mounted in a block of a phenolic like material (I think it was called G38). This material has a higher temperature tolerance than phenolic. However, the cessna heater element put out so much heat that it caused the material to delaminate and burn slightly. This is on the ground, BEFORE I floxed it into place on the aircraft. It reached 350+ degrees F. I am re-thinking my need for pitot heat, or considering installing it with a timer that cycles it on and off to reduce the temperature to a reasonable temperature.
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  #19  
Old 07-07-2006, 12:02 PM
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Great stuff Swinn, wonder if the original builder had this problem?
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  #20  
Old 07-07-2006, 01:54 PM
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Quote:
Great stuff Swinn, wonder if the original builder had this problem?
Yes he did. JR tried a couple things to keep the temp down. Check out his website.

Any electronics gurus out there? How about some kind of resistor or something to get the amps down?
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  #21  
Old 07-07-2006, 02:28 PM
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Quote:
Originally Posted by swinn
I built a heated pitot just like what is described on the berkut13 site.
I did the exact same thing. What did you do for corrosion protection?

Also what O.D. G38 did you use? I was planning on using 1-1/8" O.D. Garolite from McMaster Carr (P/N 8640K221.) The temperature range on that is only 250degF... The the grade 'L' phenolic called out in the Berkut plans is only good to 257degF however.

I would imagine that the stagnant test you did on the ground would be worst case. One would think that the system would dissipate heat much better with airflow across the exposed aluminum.

Regards.
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  #22  
Old 07-07-2006, 02:52 PM
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I haven't completed the pitot tube yet, so I haven't done anything for corrosion protection. I plan to polish the aluminum, and just keep it polished. The pitot tube is easily removeable with a single retaining screw holding it in place, so part of the annual will be to remove pitot tube and polish. If it becomes a hassle, I'll have it anodized.

Actually, I think that the stagnant test is worse than it would be in flight. The problem is that once I mount it in the nose, it will be insulated by the fuselage. I tried putting a portion of the tube in a block of foam and the foam melted nicely. I am sure it would blister the paint on the nose as well.

The G38 I used is 1.5" OD. I had to polish the .750 aluminum tube to get it to fit inside the G38, I did this by chucking it up on my lathe and using sandpaper to hand polish it, up to 600 grit to make it smooth.

A resistor wouldn't work well to reduce the temps. The Amps are pretty high and it would just move the heat to the resistor instead of the pitot tube. A better solution would be to imbed a temp sensor in the tip of the tube, and create a FET based switching power supply for the heating element that controls the temp. You might be able to do something with a 555 timer chip as an oscillator driving a FET. Use a resistive temp sensor as part of RC network that controls the timer. I was thinking about using an 8 pin microcontroller to do the job.

I understand at least one company developed and marketed a thermostatically heated system with less than desireable results (I believe it still iced up). That particular heated pitot is off the market now.

At least with using the circuit 'full power' you can create a Concorde type effect. Just turn on your pitot heat and watch the nose droop down as the foam and epoxy melt.

An off the wall idea, maybe the pitot heat could do double duty as a cabin heater. Just put fins on the outside of the pitot tube and direct air over them and into the cabin.
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  #23  
Old 07-07-2006, 02:55 PM
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One other thing, I found that the temp on the OD surface of the G38 came within about 40 degrees of the aluminum temp after a 30 minute heat soak. I expected the G38 to be a very poor conductor of heat and a fairly good insulator, but it actually soaked through quite well.
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  #24  
Old 07-07-2006, 05:53 PM
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Can you get the heating element out there away from the phenolic? Say, a longer tube?
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  #25  
Old 07-07-2006, 08:44 PM
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So it is the heating element from a cessna. Am i to assume that it is reaching its design temp and that better isolation is needed?

Or could it be that because it is in a different holder - it is creating more heat.
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  #26  
Old 07-08-2006, 12:47 AM
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The heating element is the cessna element. THe test was done with the heating element in the aluminum tube. The aluminum tube was mounted in the G38, and the heating element was clear of the G38 by about 1/2". The heat conducted through the aluminum tube into the G38. Perhaps a two piece tube with a stainless steel barrel in the G38 and an aluminum tube for the portion that protrudes from the aircraft and encloses the heating element. This should reduce the heat conduction down the tube and into the G38 and airframe. I haven't really come up with a real solution yet for it myself.
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  #27  
Old 07-08-2006, 03:03 PM
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Lightbulb fins to the left...

Make a heat sink, sure you're wasting electiricity, but you'll need it later..

You don't want ice on the inside, make some fins on the outside, the heat will be dissipated before it can do damage, although it may be a moot point as most people don't do prolonged pitot tube tests on the ground (JonC can pipe in anytime), the air blowing at 70 mph + can do wonders at carrying heat away.

Or, it maybe ok anyway, use a probe to test it during flight. On the ground it should get pretty hot. Part of using pitot heat is knowing when to turn it off.
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  #28  
Old 07-09-2006, 10:09 PM
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Lots of plastic airplanes have pitot heat - Cirrus, Lancair, etc. What are they doing to protect the airframe from the heat???
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  #29  
Old 07-09-2006, 10:43 PM
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Quote:
Originally Posted by Clutch Cargo
You don't want ice on the inside, make some fins on the outside, the heat will be dissipated before it can do damage, although it may be a moot point as most people don't do prolonged pitot tube tests on the ground (JonC can pipe in anytime), the air blowing at 70 mph + can do wonders at carrying heat away.
You covered it quite well. They are like electric prop-deice, you dont run it,except for a quick test, without airflow... lest you want to pay for a new heating element and the shop labor rates to replace it.... but hey, keeps maint. shops in buisness.... so if you have a certified plane, run it all the time!
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  #30  
Old 07-09-2006, 11:35 PM
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thanks jonc
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