Re: Pietenpol-List: compass interference and swinging in the air.
Posted: Sat Dec 15, 2001 1:22 am
Original Posted By: "walter evans"
Group,It's a good idea to check all ferrous parts (not many of them ina wooden Pietenpol) with a small hand compass to determinewhether they have acquired a magnetic field. This would applychiefly to those parts in proximity with the magnetic compass.If you find them to be magnetized, any facility that does magneticparticle inspection (i.e. engine overhaul shop) will be able to"degauss" them for you before you install them on your airplane.All instruments should be installed with non-ferrous screws andnuts (usually of brass) if the compass is located among them, ornear them.Here in western Canada (and in the western USA) our land isdivided into a grid of section lines running in true directions N-Sand E-W. Roads and fencelines follow these making them handyfor "swinging" the compass in the air on a calm day. This methodcan be quite accurate to establish the deviation for the four car-dinal points: N, E,S, and W. (Deviation is the installation error inyour compass due to magnetic influences in the aircraft itself. Itcannot be eliminated on all headings, only minimized.)Step by step, this is the procedure:1. Figure out the magnetic heading for N, E,S and W using the local declination (also called "magnetic variation" by some people), which is provided by the isogonal lines on your aeronautical chart or sectional map. (Remember variation can be easterly or westerly depending on your location; if easterly, subtract it from the true hea- ding to obtain the magnetic heading; if westerly, add it to the true heading. For example, with a variation of 20 degrees E., the mag- netic heading would be 360 - 20 = 340 degrees. Ideally, your com- pass should read close to 340 degrees and the difference is the deviation, which will be shown on the compass correction card.) Place the magnetic heading figures you have calculated on a piece of paper to take along on your flight (Don't forget the pencil!).2. Make a cute little screwdriver by flattening and filing one end of a piece of 1/8" brazing rod. On your compass you will notice two ad- justing screws, one for N-S and the other for E-W. On the heads of these screws you will see a small dimple. With your new screwdriver turn each screw until its dimple aligns with another dimple beside the screw head. The compensating magnets in the compass itself are now in the neutral position.3. Now it is time to go flying (Don't forget that little screwdriver!). Make sure any electrical or radio equipment is "on" At about 1000' agl: a) Fly due north along the section line or road and adjust the N-S screw to eliminate all error, making the compass heading agree exactly with the magnetic heading you calculated. b) Fly due east and adjust the E-W screw to eliminate all error, as above. c) Fly due south and adjust the N-S screw to remove one half of any error. d) Fly due west and adjust the E-W screw to remove one half of any error. e) In turn, fly due N, E, S and W, recording your compass readings on that piece of paper.If your compass readings agree with the corresponding magnetic headingswithin a couple or three degrees (It's unlikely that anyone can fly acompasscourse more accurately than that in a Pietenpol), you are in business. Butyou have numbers for only the four cardinal points of the compass, and thedeviation for the intermediate points will have to be estimated or obtainedby plotting a "deviation curve", which I won't get into here. Now you canpre-pare your compass correction card.If you live where there are no section lines, you will have to use a"compassrose" or a "master compass" to "swing" your compass. Taildraggers musthave the tail up in flying position with the engine running. The procedureissimilar to that given above and you will be able to determine the deviationfor the intermediate compass points quite easily, and since you are on theground you won't have to wait for a windless day for good results. But youwon't have an excuse to go flying, either.Cheers,Graham Hansen (Pietenpol CF-AUN)________________________________________________________________________________
Group,It's a good idea to check all ferrous parts (not many of them ina wooden Pietenpol) with a small hand compass to determinewhether they have acquired a magnetic field. This would applychiefly to those parts in proximity with the magnetic compass.If you find them to be magnetized, any facility that does magneticparticle inspection (i.e. engine overhaul shop) will be able to"degauss" them for you before you install them on your airplane.All instruments should be installed with non-ferrous screws andnuts (usually of brass) if the compass is located among them, ornear them.Here in western Canada (and in the western USA) our land isdivided into a grid of section lines running in true directions N-Sand E-W. Roads and fencelines follow these making them handyfor "swinging" the compass in the air on a calm day. This methodcan be quite accurate to establish the deviation for the four car-dinal points: N, E,S, and W. (Deviation is the installation error inyour compass due to magnetic influences in the aircraft itself. Itcannot be eliminated on all headings, only minimized.)Step by step, this is the procedure:1. Figure out the magnetic heading for N, E,S and W using the local declination (also called "magnetic variation" by some people), which is provided by the isogonal lines on your aeronautical chart or sectional map. (Remember variation can be easterly or westerly depending on your location; if easterly, subtract it from the true hea- ding to obtain the magnetic heading; if westerly, add it to the true heading. For example, with a variation of 20 degrees E., the mag- netic heading would be 360 - 20 = 340 degrees. Ideally, your com- pass should read close to 340 degrees and the difference is the deviation, which will be shown on the compass correction card.) Place the magnetic heading figures you have calculated on a piece of paper to take along on your flight (Don't forget the pencil!).2. Make a cute little screwdriver by flattening and filing one end of a piece of 1/8" brazing rod. On your compass you will notice two ad- justing screws, one for N-S and the other for E-W. On the heads of these screws you will see a small dimple. With your new screwdriver turn each screw until its dimple aligns with another dimple beside the screw head. The compensating magnets in the compass itself are now in the neutral position.3. Now it is time to go flying (Don't forget that little screwdriver!). Make sure any electrical or radio equipment is "on" At about 1000' agl: a) Fly due north along the section line or road and adjust the N-S screw to eliminate all error, making the compass heading agree exactly with the magnetic heading you calculated. b) Fly due east and adjust the E-W screw to eliminate all error, as above. c) Fly due south and adjust the N-S screw to remove one half of any error. d) Fly due west and adjust the E-W screw to remove one half of any error. e) In turn, fly due N, E, S and W, recording your compass readings on that piece of paper.If your compass readings agree with the corresponding magnetic headingswithin a couple or three degrees (It's unlikely that anyone can fly acompasscourse more accurately than that in a Pietenpol), you are in business. Butyou have numbers for only the four cardinal points of the compass, and thedeviation for the intermediate points will have to be estimated or obtainedby plotting a "deviation curve", which I won't get into here. Now you canpre-pare your compass correction card.If you live where there are no section lines, you will have to use a"compassrose" or a "master compass" to "swing" your compass. Taildraggers musthave the tail up in flying position with the engine running. The procedureissimilar to that given above and you will be able to determine the deviationfor the intermediate compass points quite easily, and since you are on theground you won't have to wait for a windless day for good results. But youwon't have an excuse to go flying, either.Cheers,Graham Hansen (Pietenpol CF-AUN)________________________________________________________________________________