February Centerfold

Grumman Wildcat Landing Gear, Dave Engel, Chicago IL

The centerfold for February is really only part of a plane – specifically the landing gear of Dave Engel’s Wildcat. Dave built the landing gear from plans by Bob Carlson offered through Model Airplane News plans service. The gear was welded up from tiny tubing made of 304 stainless steel, aluminum and brass. All of the joints are articulated and pivot to allow the landing gear to fold up, sucking the wheels into the sides of the fuselage. The strut oleos really work giving the plane a soft bouncy ride on take offs and landings. A scale cable drive system retracts the gear into the fuselage via a servo pulling on the cable. In the full scale Wildcat, the pilot had to winch the gear up using the same kind of cable system, but powered by a hand crank located on the right side of the cockpit – 29 turns of the crank were required, all while flying left handed! Work on the landing gear assembly from inception to completion required the better part of two years. Dave then went on to build a Wildcat (from another set of Model Airplane News plans) to sit on the exquisite landing gear.

But if you own an older engine that’s acquired that golden brown patina from years of castor oil, and you sometimes decide to just put the plane away “wet” rather than properly cleaning it (we all have time pressures and /or aching knees), then you are also correct in your belief that W­D 40 can ruin your engine. You guys should just keep the W­D 40 away and use “Snake Oil” or Air tool oil, or Marvel, or motor oil, or whatever else you have laying around. Dan Socha once showed me a rather knarly antiqueish looking engine that he acquired at a swap shop. The engine ran fine right up until it saw its first dose of W­D 40. Other club members have had similar results!

And now to really digress …

This whole story reminds me of a car I used to own, much maligned and denigrated by the mainstream public and automotive press; but much loved by the enlightened few who knew her secrets. The car was a 1976 Mazda RX­4 with a 13­B Rotary engine. That little 13­B Rotary had only 79 cu inches of displacement (1300cc), but it produced 140 hp in a car that barely weighed 2000 lbs. She was a rocket. Five speed transmission. After burner exhaust, Recaro racing seats. 60 series tires. Pirelli P­5 treads. Lots of performance packed in a small nondescript white sedan, which never attracted any unwanted attention from the local constabulary.

The problem was in the Rotary engine. The rotor (piston) was triangular in shape and wobbled around in an ellipsoidal casing on an eccentric shaft. The rotor formed more than 40% of the surface area of the combustion chamber, which presented a problem for cooling. The combustion chamber casing was cooled with a standard water jacket and radiator system. In order to cool the rotor, the engineers made the rotor hollow, and pumped oil through the rotor via over-sized oil journals in the eccentric shaft. This hot oil was brought outside the engine and run through an oil cooler on the bottom of the radiator. Two thirds of the radiator was dedicated to cooling water, one third for cooling the oil.

The problem came in how to lubricate the tip seals (piston ring equivalents) out on the apexes of the triangular shaped rotors. The solution was to bleed oil from inside the rotor directly to the base of the leading edge of the tip seals. Innocent enough. But, we now have an engine which quietly consumes part of its cooling supply. The engineers got around this problem by super­ sizing the oil capacity of the 13­B. It took six quarts of oil to fill the engine ­ two quarts for the radiator, three quarts for the crankcase and rotors, and one extra quart to allow for burn ­off between oil changes. On average, the 13­B burned off between ½ and one whole quart of oil every 4,000 miles trying to keep its tip seals intact.

Since the oil was supposed to be changed every 3,000 miles, this was deemed to not be a problem. The problem was, the average American in the 1970s was too lazy, or too cheap, or too stupid to change his oil every 3,000 miles. Sometimes they liked to stretch it to 5,000 miles, and the manufacturers of synthetic oils claimed as much as 15,000 miles was possible. But, if you tried doing this in a rotary, the engine invariably ran low on oil, increased its operating temperature due to reduced cooling efficiency, burned up its tip seals, lost compression, and refused to run. Few 13­Bs made it to 60,000 miles. Mazda warranted the rotary for 50,000 miles. My RX­4 had 130,000 miles on it when I sold it, and she still ran strong.

But I was a maintenance freak back then. I even changed my own spark plugs.

One of these days I am going to have to try the O.S. Wankel Rotary in an airplane …

Is the Rotary a good engine? Depends on who you ask. W­D 40 is a lot like that. Use it regularly; it will reward you with a clean and strong engine. Slack off, or start late with the maintenance, and it will bite you in the wallet.

Ok. That horse is dead …

Dave also pointed out one of the operational advantages of some of the 4­stroke designs. Many use a pressurized carburetor / fuel system to deliver fuel to the cylinder. The pressure regulator in the fuel system supplies a steady flow of fuel, which allows the carburetor to meter fuel as a fixed fuel­to­air ratio, which is independent off the vagaries of the local weather system, the current temperature, or the altitude of the field you might be visiting. This means no futzing with the high­ speed idle adjustment every time the thermometer moves 10 degrees or a low­ pressure system moves in.

Speaking of needle valves, Dave showed us a modification that he performs to all of his engines. He no longer uses needle valve extensions to bring the needle valve outside the engine cowling. The extension leads to premature carburetor failure due to the long lever­arm it introduces into the engine configuration. As the engine vibrates, the extension whips the valve around causing it to enlarge the hole in the valve seat enough to require adjustment. The needle valve is screwed in a little to maintain proper adjustment, and the whole process starts over again. Eventually, the needle can no longer be screwed in far enough to properly adjust the idle. At this point the needle and the valve set need to be replaced.

Instead he modifies the end of the needle valve (possibly involving some grinding, hole drilling, and tapping) to accept a socket head screw, which is secured with lock­tite or cyanoacrylate­glue. After the modification, a standard ball­head hex driver can be inserted through a small hole in the cowling, and into the end of the needle valve. The larger size of the hex driver handle allows for finer and more­ positive adjustments than is normally possible with a standard needle valve extension.

Finally, Dave talked about cleaning engines. He used to use a product called Demon Clean, which worked well, but he believes is no longer produced, which is probably a good thing as it was corrosive enough to eat its way through the metal can it was shipped in. So now instead, Dave uses oven spray­ on oven cleaners to remove the baked on crud from the outside of his engines. He mentioned that you should only buy the “aluminum safe” (read the label) oven cleaners unless you do not mind that the engine will be developing a black or gray tint. Dave showed us the engine he learned this fact on. It was originally unevenly discolored as the cylinder head had darkened where the oven cleaner had been applied. So he later went back to “even out the tan” by applying the cleaner to the entire engine. The effect was not dissimilar to the expensive red or blue anodizing treatments found on some of the fancier looking engines. All in all, it was a great speech, and Dave was able to easily handle the unruly audience. Good job and many thanks.