Old Iron Sides Article & Plans
July 1970 American Aircraft Modeler

July 1970 American Aircraft Modeler Table of Contents These pages from vintage modeling magazines like Flying Aces, Air Trails, American Modeler, American Aircraft Modeler, Young Men, Flying Models, Model Airplane News, R/C Modeler, captured the era. All copyrights acknowledged.

Old Iron Sides

Jerry Thomas

The most intriguing, yet frustrating, type of model airplane is that flown in Jet Speed. Jet has the greatest unexplored potential; but the difficulty is that its powerplant cannot be put on a block, like a two-cycle engine, to see if a modification will increase its speed. It must be test-flown to study the effect of air flow on the tune of the engine.

Ed Fisher and I, when going team in Jet in 1967, started with great expectations. We succeeded in just burning valve after valve. This was the same basic engine I had used for years, so we felt that neither the plane nor the tank were the cause. To double check, the engine was installed on Rollie Hilesland's and Keith Loutocky's plane, which was running well. The engine continued to burn valves. The flojector, which had been used for 18 years, was replaced, and the ship turned a nice flight of 169 mph. A few weeks later the plane won Jet Speed at the Nationals in California at 174.86 mph.

Old Ironsides Dynajet-powered Jet Speed model.

Rear view.

Both wood and metal versions are shown on plans. Detail above is wing, fuselage, and control unit assembly. Wing spar is welded steel tubing arranged so that Mono-line control wire goes out through the tubing.

A word of caution: Model jet engines are pulse jets which run extremely hot and should be handled by an adult. They must be launched immediately after starting, or they will actually melt. They burn raw white gas and make a very loud noise. But, they are fast, relatively simple, and fascinating. Author demonstrates best starting position and suggests plenty of practice without starting. Have C02 fire extinguisher handy.

Improved performance possible by very careful alterations to valve stop. With file or lathe remove metal to permit valve to open slightly higher and therefore longer. This step, and a tuned front end are about the only really practical modifications for the otherwise stock Dyna-Jet. It works, holds record.

The Thomas/Fisher team. Dyna-Jet is made by Curtis Dyna-Products of Indianapolis, Indiana

A first attempt at modifying an engine was to increase the valve lift by turning back the valve stop. This removed .030 to .060 in. from the stock at the outer edge of the valve stop, as shown on the plans. I settled on .040, which I still use, as best for power and reliability. Beyond this, whole leaves are lost during a flight, and valves must be changed quite often. When running a rich metering jet, five or six flights can be had before the edge of the valve starts to chip. This improvement, with Mono-line, put me in the 155 mph bracket.

When the valve stop is turned to get more lift, or opening, from the valve, continue to check with a leaf from an old valve to get as much contact between the valve and valve stop as possible. Those who don't have a lathe can, with some patience, file ten "flats" to get the extra lift. Mark the .040 in. on the edge at one point. File and keep checking until the leaf makes contact as above. Then make a template of this curve and, using a valve to mark the valve stop for leaf position, have at it! Take time to do a good job.

Metering jets is a critical item. To check metering jets with wire drills, I set my standard at a No. 60 drill for a No.6 metering jet. This is done as a check to be sure sizes are really going up or down when testing. Metering jets have been known to vary in size for a given number. The Dyna-Jet instructions recommend a No.4, but this varies with the plane and tank. I used either a No.5 or No.6 for years, and then found that this airplane and tank are best with No. 11, which is the same size as a No. 55 drill. Testing is necessary to determine the right size, since it varies with the tank, CG, etc. Sound is no guide. It's done strictly by clocking the plane. Our plane is usually going faster when it sounds erratic. When it leans out, it slows down.

The flojector does not have the hose connector as on a stock flojector. With practice it is easier to start a jet, since the plane need not be shifted. Note, before cutting off the hose connector, it points between the holes and is in line with them. This is the starting point. By taking the position shown in the picture and looking down into the intake, it is easy to judge by the spray of fuel, when the right place is reached. In case of flooding, with flames coming out the tube, simply point the tube on the tire pump at the valves. This will clear it and many times the engine will start. To modify the tire pump, put 1/8-in. or 5/32-in. brass tubing in the hose, and bend it slightly to come in at the approximate angle of the original connector. A medium or small diameter tire pump gives enough Volume of air and doesn't tire the person pumping as quickly.

Squatting over a jet model when starting may seem dangerous, but the engine can't blow up because of the open combustion chamber. Flames sometimes come close to the lower extremity of the body, but in 20 years I have yet to be burned or to set any part of my clothing on fire.

When I decided to build a tank around the intake, the result was a tank longer than the intake in order to get the necessary Volume of fuel. After much work, this design seemed a lost cause. Then a fellow launching the model made a chance remark that one certain tank seemed to have more thrust on the ground. After some thought, and much searching, I ended up with the end of a salt shaker taped to the inside of the intake to duplicate the length of the tank and flew it on my regular plane. This most unscientific adaptation jumped the speed from 155 mph to 167 mph, A turned intake, similar to those in the picture, evolved and turned 176 mph using the old .024 in. Mono-line. For those without a lathe, that old salt shaker and some epoxy could make a satisfactory substitute.

On the tuned intake, good results are obtained by using the length (shown on the drawing) of three inches from the valve face to actual intake. The venturi opening and minimum diameter are the same as a stock Dyna-Jet. To try tuning the engine, the overall length could be shortened by .25 in. and a 1.5-in. inside diameter tube attached to the cowl. Many combinations can be tried on a jet. It does takes patience and one change at a time.

Getting the most out of a jet is not complete unless a good seal is created between the head and tube. If the head isn't a good "rattling fit" in the tube, carefully squeeze the tube until it is round and the head goes in easily with no bind. Then lap the lock ring and tube face on wet-or-dry paper until the surfaces are flat. The threads aren't perfectly true to the faces, so any misalignment will let the ring seat flat on the tube. It may sound like unnecessary work, but a year ago I was lazy when the head wouldn't lock solidly so it could not be turned, and I let it go as good enough. It meant 12 mph between that flight and the next when the head was seated and tight. The Scotch electrical tape that fairs the cowl to the tube is a good check on seal, because a leak will cut the tape like a knife.

The face that the valves seat on should be flat and have no nicks from burned or chipped valves. Find a flat plate glass window in a junk yard and on this, with No. 280, No. 320, and No. 400 wet-or-dry paper, depending on how deep any nicks might be, the head can be lapped in, even at the field. Using the paper wet simplifies the job.

To help eliminate burned valves, know the metering jet sizes and run the fastest rich metering jet possible. A No. 12 can be run but the No. 11 is faster. Beyond a No. 12, size of the flojector hole must be increased as it becomes the metering jet.

Two types of bodies are shown on the plans: the one used on the actual airplane; and the other a combination of a brass tank and a wood body, for those who don't have use of machinery or materials to make the all-metal body. Drawings for both bodies are clear enough to build from without detailed written instructions. Do remember to glue a piece of asbestos along the top of the wood body. Any good piece of straight-grained hardwood (no balsa!) will do - spruce is good.

Don't get excited about the fact that the fuel line does not go to the rear of the tank. Believe me, it works. The shorter fuel line gives much better running without a tendency to cut out. The fuel goes to the back of the tank on takeoff. Since the tank is almost full, there is no trouble getting fuel. In flight, the fuel is 30 times heavier so it presses to the outside of the circle if the model is tracking straight, and will go forward when the model starts to slow down.

The tank on the actual model was a piece of spun tubing obtained from an aircraft salvage firm, but the one shown is turned from 1 1/4-in. 24ST aluminum bar stock, as is the nose of the tank. The seal on the tank nose is a 1 1/8-in. neoprene "O" ring which can be obtained from a plumbing supplier. It must be just a light push fit since the fuel will tend to swell the "O" ring. Screws holding on the nose and the front of the skid go into the tank, but no leaking problems have developed.

The tubes are soldered to the two 1/4-in. brass screws after the screws are put into the tank nose. Drill the 1/8-in. holes in the screws first and countersink the nose just enough to get the screw to seat. Then file off the excess. Wind a strand of soft wire around the end of the fuel tube and solder. This helps obtain a seal to eliminate running lean and burning a valve. Change the neoprene tubing for every flight session.

On the brass tank, the exploded view shows how parts go together. Tin the edges and fit the top, bottom, and end inside the side piece and sweat-solder together. Note the nut plate on the inside of the bottom of the tank. Solder it in for the front skid bolt.

The material for the engine mounts and elevator horn is stainless steel from automotive hose clamps. It's a bit hard to drill but well worth it. This also could be used on the wing-body bracket with the wood body, or .030-in. brass would suffice. When putting bolts in to hold the wings, use bolts long enough to go through the nuts and take all the load.

On the 4-40 machine screws, use hex heads or Allen bolts, which are much higher strength than normal iron or brass screws. At least use them on the wing-body screws, if nowhere else.

Several different materials have been shown on the plans should it be impossible to come close to the original design. The spar on the original is 1/4-in. stainless steel tubing heli-arc welded together for the offset. It is easy enough to use 1/8-in. 24ST aluminum, or 1/4-in. maple for the spar. The original tail was .06-in. magnesium, but .05- or .06-in. 24ST aluminum could be used. Do not use a plywood tail on the all-metal model because of the low location and dihedral necessary. However, on the wood body this could be used.

The low tail keeps the pushrod inside the tail boom (boom is 5/8 in. thin-wall 24ST aluminum tubing) and puts the pushrod in tension for up control. At these speeds, if the pushrod is in compression, or pushing to get up, the pushrod bends and little or no control is possible. There is no worry about getting "down" control with the high thrust line. Note the 3/32-in. pushrod. The ends are carefully bent "U" shapes, which will stay on the horns very nicely.

The wings are made of .010- or .012-in. 24ST aluminum. Bending the LE and gluing or riveting the TE is something that comes with practice and doing. Practice with scrap pieces until satisfied with the LE radius. When gluing or riveting the TE, make sure it is clamped or held on a flat surface. When "potting" the epoxy tip, wax the spar so it will separate. It pays to redo any of these steps until satisfied with the results. The same is true with setting up the engine. With each repetition techniques are perfected and speed gained. We started this season turning in the low 170's, and then just kept going faster. We finally turned 179.93, 181.75, and then 182.67 mph for our three officials, and a new record with a .031-in. line on September 28, 1969.

The starting technique is very simple. Either standing or squatting with the model between the legs, grasp the left wing and rest the right hand on the cowl, pointing the brass tubing on the tire pump into the intake. With practice, knowing where the holes in the flojector are will be easy and a couple of pumps (slow and long) should result in a spray. Soon it shouldn't take more than a few to start the engine. Then just throw the hose to the side. The person with the starter box has by then released the button and walked (or run) away, pulling the clips off automatically. Grasp the right wing while setting the model on the ground, making sure that the model is pointed straight ahead. Don't get excited and release any old way. It's awfully hard on flyers to time their leaps for getting out of the way of poor releases.

Don't waste time on the ground. The tank on the all-metal plane allows 14 to 15 laps. We originally got 12 laps. Counting one lap to get on the pylon, it takes at least ten laps for a clocking. We made a new nose in case Ed ever did have a problem getting on the pylon.

The fuel we use has given those extra few mph's. After trying many things, we still use as our basic fuel benzene with propylene oxide. By varying the benzene propylene mixture we consistently go faster. Using straight benzene will do an excellent job at less cost.

Care in handling these, or any chemicals, cannot be over-emphasized. Chemicals are dangerous if not handled with common sense. Both of these fuels are highly volatile (especially the propylene oxide). They should be stored in a cool dry place where there is no open flame or chance of sparks from switches, etc. With any volatile chemical (one that evaporates rapidly), fumes develop and could go off with a bang. This is true of paint thinner (benzene), lacquer thinner, etc. Preferably they should be stored in their original containers, or in dark glass or plastic bottles.

Mix fuel outside where there is plenty of fresh air. Avoid fumes, spillage and skin contact. We mix ours at the flying site (plenty of fresh air) so we know exactly what percentage mix we have.

There is no fantastic reason for our ship's going as fast as it has. It is simply a clean, grooving airplane that lets all the thrust propel it forward. It is a little larger than many jet designs, but with care it comes out between 30 to 32 oz., with the all-metal design. Old Ironsides weighs 30 oz.

Old Ironsides Plans

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