[Bubblehead]

The RV-8 I bought has a heated pitot. When flying on Saturday I decided to turn it on to see if it drew current. When I did the compass swung to 060! The compass is a panel mount unit, so maybe the electrical circuit runs close to it.

I've emailed the previous owner to find out who manufactured the pitot, but has anyone else out there had this problem? If so, how did you solve it?

I'll post mfr info when I receive it.

 

[vlittle]

Pretty common: large loads (lights, pitot) can cause compass deflection. Pretty tough to eliminate without rewiring.

If I ever find a need to use the compass, I turn off the heavy loads first.

 

[jmjula]

Have you traced the pitot wire from the switch to the floor? Does it have a service loop in it near the compass? The loop could also be a combination of wire routing and the return path through the fuselage, which won't be obvious. Current flowing through a wire produces a magnetic field. If there is an extra service loop in the wire, and add in a large current, then you will have a larger magnetic field.

If there is no obvious loop, then I would try re-routing the pitot wire in a different way that keeps it further from the compass, and is the most direct shot to the wing. This will minimize the overall loop size, thus shrinking the magnetic field produced. It might even take routing a ground wire in parallel to the power wire instead of using the fuselage as a return path.

 

[Bubblehead]

Good suggestions. I'm going to trace the wire this weekend when I'm finishing up the condition inspection. I'll bet the wire runs close to the compass. That's not the short way to the pitot, but it might be the easy way!

 

[Bubblehead]

wire size and shielding questions

With the bad weather I'm doing a lot of work on the -8. One item to tackle is tracing the wire to the heated pitot. It looks like it goes under the panel and around to the left wing. If so I'll reroute it aft and under the pilot's seat along the spar.

I reread the instructions for the heated pitot and it does not say what kind of wire should be used. It looks like the wire is just plain wire same as other accessories but shouldn't it be shielded to help reduce the magnetic field generated by the high currrent?

I need to check the current to make sure the wire size is correct. What have others used for the power to the pitot?

 

[L.Adamson]

I need to check the current to make sure the wire size is correct. What have others used for the power to the pitot?

My pitot tube is heated for pitot & static lines. #12 wire (non-shielded) & a 20 amp fuse.

I don't like panel mounted compasses for the obvious reasons of electrical interference, such as yours.

L.Adamson

 

[Sparky]

Unshielded

Wire for pitot heat power leads does not need to be shielded. Shielded wire is usually used to protect the wire from external emf (electro motive force or electro magnetic fields) for very low signal level wires (microphone, low level audio, sensor signals, etc), and to keep high level noisy (ie pulsed voltages such as strobe light drive voltages, mgneto p-leads, etc.) contained within the wire (in this case it doesn't so much contain the magnetic field as it does provide a capacitive "short" to transigent (quickly changing) voltages).

High current wires such as pitot heat, landing lights, or any main power wires are fairly constant current DC voltages and therefore don't generate a lot of noise, so they are not normally shielded. However, the high current generates a magnetic field around the wire. The higher the current, the more magnetic field (this is how hall effect current sensors work and, for that matter the basis of how electric motors and generators work -- so it's a good thing).

Usually this does not cause problems unless the magnetic field encroaches upon something (like a compass or magnetometer) that out of necessity is sensitive to magnetic fields. If the wires are kept an appropriate distance (a foot or more, if possible, at currents we're talking about) from the compass (magnetometer, etc) we should be OK. Unless, like mentioned above, there are loops in the wire, which could amplify the magnetism (a little difficult to explain).

Also, if there is a ground return wire from the pitot heat to a central ground block, it would carry the same current as the power side and would generate the same magnetic characteristics as the power wire. Usually, high power grounds are grounded to the aircraft frame (in aluminum a/c such as RV's) near the load (pitot tube, landing light, etc.) rather than returned to a central ground, but since this is a homebuilt a/c there is an almost equal chance it is returned. So you need to check the ground side of the pitot heat as well as the power side. If it's grounded at or near the pitot tube - no problem.

Your problem is almost certainly due to wire routing issues. A lot of panel mounted compasses have this or similar problems. If you can't get to the root of the problem, you may need to relocate the compass.

I hope this helps and doesn't confuse you more.

 

[vlittle]

Paul is right.

Theoretically, it's possible to minimize the magnetic interference by running a ground lead for the pitot and lights and twist it into a pair with the power lead.

Practically, however, you won't be able to eliminate the interference. Many certified aircraft, including transport jets have this problem.

Now, aside from the compelling need to seek perfection, is this really a problem? If you have a calibrated magnetometer elsewhere in the aircraft, you'll never need to use the compass except in a scenario where the magnetometer fails or you lose all power.

So a simple placard to remind you that the compass is only accurate with lights and pitot heat off should suffice.

I only look at my compass when it needs the annual calibration. Otherwise, it is the GPS or EFIS that gives me heading info.

If you plan to fly IFR you may want to have several calibration charts for each configuration of lights and pitot that affects the compass. Otherwise declare victory and move on.

Vern

 

[Bubblehead]

Reroute is the route!

It's not a matter of perfection or calibration, or even placarding. I want to have all my instruments and gauges working properly at all times. Calibration won't work because when the pitot heat is on the compass stays at 060! I don't have a magnetometer anywhere in the plane so I depend on the compass to set the DG and have the GPS496 for track information. Yes, it's a backup for the compass too. The DG drifts more than I would like so I need the compass until I either "go glass" or replace the DG.

I appreciate the suggestions but placards are a sore spot with me. They are usually not helpful, only a CYA. When things are going to s*** in the cockpit the last thing I want to do is read a placard! I'd rather have the systems and components working right.

I think the solution to the problem will prove to be rerouting the wire. It looks like it goes from the right sub panel (this is a -8) to the panel, then along the bottom of the panel to the left (Port) side then back to the wing. It should be no problem to reroute that aft then to the wing. If that doesn't take care of it I'll move the compass to the left side of the panel,.

The wingtip lights don't seem to effect the compass but I'll reroute those wires too.

I'm off to the hanger! Thanks for the suggestions.