how hot does a pitot tube need to get?

Some of you may remember my earlier post regarding the use of an F-16 pitot tube on my RV8A. Some asked me to keep them informed on how my efforts work out. For those, I have an update. I also have a question.

I hooked up the pitot tube to 12volts today. It only drew 1 amp and 10 watts. The tube heated up. It was noticably warm, but not too hot to hold comfortably.

I think upped the voltage to 26volts. The tube drew 1.5 amps and became too hot to comfortably hold.

My question is: How hot does a heated pitot tube need to get to perform its mission and to be acceptable under FAA regs (not that I would need to comply with them, but it seems a reasonable goal to shoot for).

Any ideas about how to up the voltage without a lot of cost, weight, and noise, would be appreciated.

Thanks,

Tony Johnson

Re: how hot -- pitot

that's a good question, Tony.

i recently purchased a heated pitot tube from a salvage yard that came
from a cessna 172. it was advertised to be a "12 volt" unit. when i
received the unit i hooked it up to 12 volts, and it drew ~1.5 amps.
got pretty warm, and i was thinking maybe it would work, but wondered
why all the pitot heat breakers in planes i checked were either 10 or
15 amps. finally it dawned on me that i must have received a 28 volt
pitot tube. i called the guy that sold it to me, and he sent me another
one. this one draws 4.5 amps, and gets way too hot to hold.

my guess is that when you have 180+mph wind carrying the heat away,
the "pretty warm" is insufficient to melt or prevent pitot ice. i'm pretty
satisfied with my 50+ watts of heating (versus the 18 watts i had initially.)

i suspect there is some way to calculate how much heat one needs, but
that's beyond me, without a little more help.

john

I cannot speak to the science of it, only the experience. When I was flying T-37s (28V system), the pitot heat would get very hot. We would turn it on, warn the crew chief before doing so and he would tap it just until it warmed up and then we'd turn it back off. If we didn't turn it off right when we landed, we had to warn him again when we taxiied back so he didn't get near it (it would burn him through his glove). I compare this because the -37 was not that must faster than RVs so the cooling effect would be similar and you would likely need about the same amount of heat for adequate ice protection.

Here's an interesting paper, too: http://www.aeroelectric.com/articles/Pitot_Heater/Gauging_Pitot_Heater_Performance.pdf

You want it HOT...NOW

I figure, the only time I'm ever really gonna want pitot heat, I'm gonna want it HOT...NOW!!! Imho, there's no such thing as too hot/too fast when it comes to melting ice off critical components in flight.

)_( Dan
RV-7 N714D
http://www.rvproject.com

Thanks for the input

John, Brian and Dan,

Thank you for your input. Thanks for the aero-electric article. It indicates that when the tube is really putting out, the temp is 270c!!! That is HOT! It is clear that my tube with 12 volts isn't going to make it. Even with 24 volts I would have some concern.

I guess I will have to find a way to get more volts to my tube, or find another pitot tube.

Tony

Not hot enough

A quick note on Piper style (blade type) pitot tubes. There are two seperate heating elements in these blades. We generally turned it on for preflight, held the tube till it started getting warm, then let go and turned it off for the remainder of the preflight, as we were warned that full heat was only intended for use in flight.

Well, here is the upshot of that. A blade type running only one of the heaters doesn't get near hot enough, but you didn't know that only one heater was working during preflight, you just knew it was getting warm. I personally have had two cases where the blade started to heat in flight, but didn't get hot enough to prevent icing up in flight. (Guess what kind I DON'T have on my RV?) Just a word to the wise. I'm not saying that a Cessna/Beechcraft style tube with only heater is better than a Piper with two, neither works with no heat! I'm just saying at least you know what you have when it's cold at preflight.

Jeff

How hot does a pitot tube need to get?

I don't know how hot an electric heated pitot gets but you can't put your hand on it when it is on.

I have just come back from flying my RV and it has a Cessna type pitot which is the one advertised in most of the aviation papers such as Trade a Plane.

When I switched the pitot on it takes 15 amps at 12 volts so this should give you an idea of what to expect. Actually the 12 volts is 14.6 volts as that is what the alternator is putting into the battery, hence the heating power is 219 watts.

Barry

How hot?

Ice melts around zero degrees celsius.