Heated Pitot Wire

shiney · #1 02-29-2008, 12:31 PM

Should I run my heated pitot wire seperate to the rest of my wing wire bundle? I'm sure I have seen this recommended somewhere on a builders site but can't find it. Is there a reason for doing this and if so, what is it?

Shiney

rfinch · #2 02-29-2008, 03:43 PM

High current wires should be run at least a foot separate from low current wires; i.e. power wires should be separated from sensor wires. I would include shielded receive-only coax antenna wires with sensor wires, and transmit coax with power.

Run a ground wire too BTW. Near the negative terminal on your battery, make a short run to a grounding terminal block...then run grounding wires from the block to the remote areas (wing tips, tail). Don't depend solely on the airframe for grounding.

RV8RIVETER · #3 02-29-2008, 06:58 PM

In an RV I would not worry about running pitot heat wire in a bundle. It is a constant current wire and only time it is going to generate rf is the instant it is turned on and then when it is turned off, as the field rises and collapses. Also, you probably aren't going to be using it much anyway as well, unless things are very different in England.

The main reasons for seprating wiring is rf and in "big aircraft" they try and minimize the possibility of a high current wire shorting to a lower current one in case of a chafed wire bundle (miles of wire that don't get inspected very much).

But there is nothing wrong with seperating it if you feel like it. It's your airplane.

Bubblehead · #4 03-01-2008, 06:05 AM

Just don't route it close to the compass! The previous owner of my -8 did that and the compass would always show a NE heading if pitot heat was on! I'm rerouting the wire now.

RV8RIVETER · #5 03-01-2008, 07:45 AM

Very true, the larger the current the larger the magnetic field.

shiney · #6 03-01-2008, 08:33 AM

so, in conclussion it appears you think its ok to route it in the bundle to the wing.

gmcjetpilot · #7 03-01-2008, 05:08 PM

Quote:

Originally Posted by shiney

so, in conclussion it appears you think its ok to route it in the bundle to the wing.

Conclusion correct!

Wires that cause or "send" out RFI (radio freq interference) tend to have a current that varies at some frequency, not a constant DC "resistive" load, like a little heated pitot. Also we are talking DC not AC, so there's no AC hum from a pitot supply wire. My story and I'm sticking to it.

Side note: Electromagnets (like a motor) are "inductive" loads and have coils. Switching an inductive load on or off, can "blow back", ie a collapsing magnetic field. These circuits, like the master relay (an electromagnet) need those "fly back diodes". However a "heater" turns electrical energy into heat (not a magnetic field), so a heated pitot is classified as a resistive load. An electromagnet makes a lousy pitot tube and a heated pitot a lousy electromagnetic.

To be fair a heated pitot might be a little of both load types. At first it might act as a inductive load, but as the element heats, it becomes resistive. It's a grey area, but the rule is, if it causes a magnetic field its "inductive" (motor); if electrical power is turned into a different kind of energy (heat), it's resistive.

Non Sequitur:
Q: Why heated pitot? Do you really need it? It's just my pet peeve, VFR pilot and planes with heated pitot tubes; I'm not criticising only saying are you going to get any use out of it. IFR, yea sure. I have meet a few builder that felt they needed it because the C-172 they learned in had it. It's not a total waste just not something that's strictly needed by FAR's or for VFR operations. If you are a VFR pilot/plane and get into icing you have other problems.

L.Adamson · #8 03-01-2008, 05:37 PM

Quote:

Originally Posted by gmcjetpilot

Q: Why heated pitot? Do you really need it? It's just my pet peeve, VFR pilot and planes with heated pitot tubes; I'm not criticising only saying are you going to get any use out of it. IFR, yea sure. I have meet a few builder that felt they needed it because the C-172 they learned in had it. It's not a total waste just not something that's strictly needed by FAR's or for VFR operations. If you are a VFR pilot/plane and get into icing you have other problems.

When I purchased my heated pitot/static tube (thoughts of IFR), sometime in the later 90's, it was 6.7 times cheaper than the going prices these days, including the mount.

It looks kind of cool, and certainly looks a lot better than the stock bent aluminum tubes of the day; which I've never been fond of. That will be my VFR reasoning, in case you ever wonder...

L.Adamson --- RV6A