Useful info  and restoration tips

                       what's on this page ----  Laser timer use     brake problems explained        corrosion control   weight and balance worksheet

1. Another use for the McAviation laser timer -- if you've got a degree wheel (if not, why not?),  After you've established  your timing mark and then set the engine on #1 cyl.  top dead center, clamp the laser timer on a tripod, jackstand or whatever, in front of the degree wheel on the engine and aim it at the 0 degree mark.
   Now, you can use the accuracy of the laser pointer to rotate the engine an exact nymber of degrees to arrive at the top dead center position of each cylinder in order to set the vavle timng, check compression, etc of each cylinder.    Obviously, just take the number of cylinders and divide that into 720 degrees, and that will tell you how many degrees to rotate the engine to locate each TDC in the firing order.  Example:  On  a 9 cylinder, each cylinder fires every 80 degrees, on a four cylinder, every 180 degrees.   This saves a lot of annoyance and wondering if the propellor is going to turn when you put pressure on the cyl. to check compression.  
   This is particularly good on taildraggers when the weighted pointer on some degree wheels doesn't give an accurate reading because of not hanging vertical.
2. Having Brake problems?  If they're grabbing , click here     if locking,  click here
   if spongy.   click here             Non- or poorly functioning brakes, click here      General issues,  click here   bleeding brakes   click here

•  Grabbing brakes.  If drum or disk brakes, the hydraulic cylinder could be leaking, causing soft spots or brake mud buildup on the shoes or pads.  Solution is to fix the leaking cylinders,  which is where we come in. Then, replace the shoes or padsI
• f you've got drum brakes, the possiblities are these, in order of probability and severity --
• drum might be out of round or rusted. Just a little rust can cause serious grabbing.
•  Drum may be out of round, a fairly common problem on old planes or cars; soloution is to turn the drum true on a lathe, taking off just enough to make it round.   If you don't have somone local to do that, we may be able to help. This however, will cause problem number one, rust. So, you'll like have to have the drum hard-chrome plated (not decorative chrome)  to ensure the problem doesn't return.
•  Another cause can be the shoes are grabbing at their edges during their servo action -- file the leading edges of the shoes to a taper  about an inch back of the  edge.  Remember, they might be asbestos -- don't bother suing us, we haven't got enough money to make it worthwhile, so wear a mask when filing.  
• Of course, the spring return mechanisms or shoe seats could be rusted, corroded, just plain stuck, keeping the shoes in contact with the drum.
• If disk brakes, the rotor could be rusty or warped.  Solution is same as above for drums, clean the rust, or turn the rotor true on a lathe, and have the rotor hard chrome plated, if it's not stainless steel. If you don't have somone local to do that, we may be able to help.
• On disk brakes, the cylinder or piston could be badly worn or corroded, causing the pads not to return properly because the piston is cocked in the cylinder and binding.  In steel cylinders, rust could cause the same thing.   The seal in a disk brake caliper piston could be pulled over the groove seat,  preventing the piston from returning. These problems will usually be accompanied by leaking.
• If brakes are locking, it's usually the same cause as above, but serious persistent locking can also be caused by equalizing valves in the master cylinder not funcioning properly, or the piston sticking, causing hydrualic pressure to remain on the wheel cylinder.  This requires a master cyl. overhaul.
• The wrong hydrualic fluid might be in use, especially if the brakes lock when hot, and stay locked for while, then unlock by themselves.  Some unapproved fluids, especially silicone based, can foam and expand when hot.
• If the brakes don't function as well as they used to, could be the following:
• Of course, ineffective brakes are most often due to  badly worn shoes or pads.
• Again, leaking seals, either at the wheel cylinders or the master cylinder.
• If leaking at the wheel cylinders, it's obvious, you'll see evidence around the wheel.
• If at the master cylinder, there might be evidence of leaking at the piston rod, but the leak could in internal blow-by leak, where one of  the  internal or check valves seals is letting fluid bypass. This problem often can be detected by excessive bubbling in the mater cyl.  reservoir when the pedal is depressed.   A little, short duration bubble or turbulence wave in the reservoir is often normal,  look for turbulence throughout the pedal travel.
• If the leaking is caused by excessive wear or corrosion of the cylinder or piston, you need us, if not, new seals and maybe honing  will do the job.
• MIght be a leaking hose or tube connection somewhere in the system,  no choice here but to search for evidence.
• A hose could have a weak and broken down wall, causing a brake line aneurism.  Look for swelling of a brake hose when brakes applied.
• If the pedal is spongy, usually due to air in the lines, which got there due to a leak somewhere, possibly in a line or the cylinders as described above.
• Spongy pedal can also be caused by bad valves or seals in the master cylinder.
• A weak brake hose can also cause spongy pedal.
• If the brakes are spongy when hot, it's probably because the fluid got hot from hard braking and boiled, now there's gas  bubbles in the system.  Aircraft hydrualic fluid when fresh doesn't cause this much, but if there's water contamination, the water boils.  By the way, if you're tempted to use some super whippy-dip auto racing fluid in your belchfire V8 homebuilt because landing speed is 120 knots, don't; although the boiling point is much higher,  the fluids are actually more hygroscopic. than usual.  Race teams change the fluid before every event.
• Silicone base fluids in your homebuilt might sound like a good idea, because the boiling point is high and non-hygroscopic, but it expands when hot, and can cause spongy pedal or brake locking in extreme cases. Brake locking is hell in a tail dragger.
• General -- if water gets in the system, ice can form causing, grabbing, locking, or non-function. If suspected, thaw it out, drain the system and refresh the fluid.  This isn't usually o problem with aircraft hydrualic fluid.  Auto fluid,  which is glycol based, is hygroscopic, which means it absorbs moisture -- it literally sucks it out of the air through the master cylinder vent.  Ever bleed the brakes on a car and wonder why the fluid looked rusty? ...that's why.
• If you're tempted to use some super whippy-dip auto racing fluid in your belchfire V8 homebuilt because landing speed is 120 knots, don't; although the boiling point is much higher,  the fluids are actually more hygroscopic. than usual.  Race teams change the fluid before every event.
• The wrong hydraulic fluid can cause all sorts of problems as discussed above -- spongy pedal, locked or grabbing brakes, non-functionality.  Reason is it might boil, freeze, expand. or just plain crap up your system.
• Bleeding the brakes.  Good idea to do it from time to time, especially if pedal is just a little spongy.  Aircraft brakes should be bled from the bottom up -- pushing the fluid in from the bleeder vavle up to the master cylinder.   This is due to just plain old gravity.  The master cylinder is higher than the wheel cylinder in aircraft, so the bubbles rise.  Takes a lot longer and uses a lot more fluid to bleed from the top down, if it works at all.   If you're working on a new system, and you know the fluid coming ouit during a bleed is perfectly clean, you can re-use it, but, not right away.  Put it in a clean tightly sealedjar and let it sit for a few days to let the microscopic emulsified air rise to the surface and dissipate.  Recently bled-through fluid will cause spongy pedal in a day or so.
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3. Use Stainless steel cotter pins, nuts, and washers where avaialble in aircraft grade.  they cost a little more, and the cotter pins are harder to bend and cut, but cotters, washers and nuts are the first things to rust due to the bending  on the pin, and the thrust on the washer and wrench on the nut.   Rusty hardware sure ruins the look of a freshly done airplane . 
4. Use anti-seize compound on bolt shanks going through deep holes to retard corrosion and make them easier to remove later.
5. Don't depend on stainless steel washers to prevent corrosion when used against aluminum.  Paint or anti-seize should  be used under them.to prevent moisture and dirt getting  trapped underneath.
6. If you've got an old airplane, check if there's an STC for use of Auto gas in it.  The modern 100LL still has too much lead in it for your engine's health, and the higher octane burns too slowly to fully burn during the combustion cycle and adds to carboning up the engine.  The EAA has done a lot of work on this.
7. Corrosion control in general -- keep it clean.    Although aluminum is wonderful, and it corrodes very little when open and clean, it has a bad habit of corroding when surfaces touch and are dirty, or when it's against a different metal.   Galvanic action occurs at seams when they're loose and dirt gets under.  Look for "smoky" rivets on aluminum airplanes -- when the rivet heads have a black sooty film around them and spreading back in the slipstream.  This means they're loose, have them fixed.  If you've got cowlings  or the prop off the plane, don't store them on a concrete floor for more than a couple days, concrete is alkaline and it attacks aluminum smartly.  Lift them off with a few planks.
8.                                   A sample weight and balance table for those that have forgotten
   
                          to figure your aircraft's CG, enter its empty weight and the empty CG as the arm for the                        aircraft.
                          enter the weight of oil, fuel, passengers, baggage and modifications if this is a alteration                        job, and their respective arms.  the arm is the distancein ches from the datum plane of the                        aircraft, given in the handbook.     multiply each weight and arm to get the moment for the                         item     Then add the weights to a total, add the moments to a total, then divide the sum of                         the moments by the sum of the weights to arrive at the CG ----don't add the arms to get CG                                       -- they're only multipliers to get the moments.
   
             Of course, it's even easier with our Aviation Mechanic's software product,  AVmechaniCalc

 If you're trying to find the CG of an empty airplane, drain the fuel and oil, add three rows to the spreadsheet for nose wheel (or tailwheel) and the left main and right main, weigh it in level attitude, record the weights and their arm from the datum, remember that the datum is often behind the nose wheel, so it's negative.  Then, total the wieghts, multipluy the arms and wights for the moments and proceed as above to find the CG.   Datum and leveling location can be found for older airplanes where the manual is lost by going to the FAA site and looking up the type certificate data sheet.  If you;ve got a homebuilt and don't know where the datum is, use the nose exactly (then all the arms are positive), or the wing leading edge, and measure everything from that point consistently, when the airplane is level.  Use plumb bobs to measure between points not having a line-of-sight between. Remember, anything forward of the datum is negative, anything aft is positive.

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