up-dec7c.htm
Made some progress on flame holders today. Using propane, a basketball inflating needle, and some sheet metal (roof flashing) I tried several shapes and sizes. The shape that worked best was a flat plate broadside into the air stream (from a hair dryer). This is, of course, a high drag shape. Second best was a strip about 1" wide bent into a "V" with the edges curled around a little, almost into a tube. The flame held for a variety of positions behind the sheet metal. I tried spraying some kerosene through it but with no luck. The temperature was about 50 F and the kerosene didn't even want to light, but that was without a flame holder. Tomorrow I hope to try it with butane and kerosene again in different configurations. I also hope to try out the glow plugs I bought as ignition sources. These run on 1.5 volts and cost only $3.50 US. I did not find any tubes of the size I'll need so I'll just have to keep looking, possibly at K-mart, for something cheap and lightweight.
I've thought of a way to adjust the fuel flow for a jet engine. Use inlet air pressure for course adjustment and a bimetalic strip set to increase fuel flow when the exhaust temperature is too low for fine adjustment.
I stopped by the model airplane shop today, looked at parts, talked to the proprietors, and bought some metal tubing, a tube bender, and two glow plugs. The proprietor said that the transmitters have a line of sight range of about two miles, though I doubt it's that far. He said it was definitely out of sight for a large plane. A new 6 channel transmitter will cost between $180 and $250, depending on brand, etc.
I hefted a 0.61 engine (they seem to have the highest displacement per dollar of the model airplane engines) and it seemed quite heavy. It's probably the same horsepower/lb as the Ryobi. I like the Ryobi because it's much cheaper per horsepower (about half that of the model airplane engines), the weight of avionics will be a smaller percentage of total weight, and the Ryobi uses cheap gas/oil. I like the 0.61 because it's cheaper per unit ($80 vs $150) and the total vehicle size will be smaller.
Pasadena will be hosting the 21st Annual Model Sport & Hobby Show on 9-11 January. I hope to attend.
Flameholders are no longer a problem. I built three today and operated with three different fuels, propane, butane, and kerosene.
This morning I picked up a glass chimney for a kerosene lamp as a see-through combustion chamber and it worked great (for $2.45 US). It's 2" at one end, 3" at the other, 4" around the middle, and about 8" long. Using it I was able to see how the flame behind the flameholder reacted to being at various places within the combustion chamber and to different air flows.
My first flameholder was a cone, about 35 degrees angle and 2" across the bottom. It just fit within the small end of the combustion chamber. I made it out of an aluminum soda can and used a high temperature solder to hold the edges together. Next I took a brass tube and notched two holes in the side similar to a basketball inflating needle and soldered it into the bottom of the cone. This worked well with propane and air from first a hairdryer, next a vacuum cleaner exhaust, and third air blown from the air compressor. Air velocities were not measured. I then tried kerosene which was harder to light but once everything got warm (the brass tube fed the fuel straight up the tailpipe into the back of the cone and was, therefore, in the combustion area). At no time did the solder, aluminum, or brass begin to melt.
My second attempt was with a similar cone made of thin brass sheet and a slightly larger tube (about 20mm ID). I ran both propane and butane through this one (it fit the butane nipple better). The flame was easily maintained with both fuels. The propane was regulated to a fairly low pressure but the butane was not. With the Butane I ran from a small flame, not much more than a large candle, to flames shooting over a foot out the tailpipe. I then tried trimming the cone to see how size affected the flame. I was pleased to find that the smaller cone (1") held the flame better and disturbed the airflow through the combustion chamber less.
My third attempt was to take a 2" water pipe straight fitting, drill a small hole in the side for the basketball needle, and a small (3/8" by 3/8") wedge of sheet steel mounted pointy side into the wind and wrapping around the needle. This I used with propane only and the vacuum cleaner exhaust. The flame was very stable, small because of the limited pressure, and a nice blue. In all cases today I started the flame with a propane torch then either added the blown air or moved the flameholder into the blown air.
I tried starting the kerosene with one of the glow-plugs I picked up yesterday but could not get either of them to glow. When I was a kid they glowed. I also tried one of the kerosene heater electric lighters I have (similar to a glow-plug but much larger and more fragile) which ignited the propane torch but was unimpressive.
Continuous ignition is the next step to building working pulse and ramjet engines. A couple of weeks ago a friend showed me a barbeque lighter that is like the manual piezoelectric butane kind but it uses a small battery to operate a pulsed (1 Hz) spark. The whole thing was very lightweight and the electronics very small. The only downside is the $39.95 US price tag, though these will probably go down as more Yuppies buy them. I'll have to get one to see if it will suit my needs. They are small enough to run dual ignition systems in case one fails in flight.
After that there will the fuel control problem.
I scavanged my daughter's old Subaru car engine for two carburetor venturis (low and high speed) and the fan. I'm not sure how I'll use the venturis but my mind is working on it. The fan is nylon, had 7 blades extending from a ring about six inches across, and about 12 inches across total. It was an interesting idea to use it instead of a propellor, despite it's weight, until I dropped it and broke one of the blades. Oh well. The venturis and their needle valves are soaking in vinegar to clean off the corrosion and crud from sitting in the weather for a year (I use it to scavenge parts when her car breaks down).
I've picked up some parts and done some reading. I've found no one to help me in the short term on the model Pogo (nor even to answer my mail messages to companies). The place with the Ryobi engines was closed for a week so I couldn't order any of those. In the meantime I looked around for materials to make a duct for it (they recommend a 16" two-blade prop) and decided that that's garbage can sized. So I stepped back and am looking at starting with a smaller version, maybe an 0.41ci engine. The servos and receivers look light enough to work with that engine but haven't bought yet. I've found, from the usenet, that it's best to buy avionics from one of the big houses like Hobby Lobby rather than local because of the prices and selection. This means a trip or a delay waiting for delivery.
I've been looking at ways to eliminate the need for gyroscopes. The two primary means are lowering the CG and moving the Cd up. By pointing the engine up (rather than down) I move the engine weight lower. I can also put the fuel tank and avionics low (this will be closer to the engine and flight controls). There are a variety of shapes I can use for the upper part (rocket mount) that have a high cross section drag. This way, when the vehicle starts to tip and travel horizontally, the high drag (well above the CG) will tend to bring it back upright.
My schedule for starting has really slipped but I have a few ideas that should make it go better when I do start.
Other interests:
I reviewed the dimensions of the pulsejet plans I have and
noted that:
With this in mind I built a pulsejet using a large coffee can (6" x 6"), 3" blued home heating exhaust pipes, a motorcycle carburetor and reed valve assembly, and aluminum flashing interfaces. Instead of fueling it up with gasoline I used propane to test the combustion process and ability to draw air through the reed valve assembly. I used a barbeque grill piezo-electric igniter to start the combustion process.
The results were less than outstanding. While the combustion explosions never seemed enough to damage the construction (which was good) they didn't appear strong enough to move the assembly either.
I think the problem is that the fuel-air mixture is not correct. I also think that the reeds in the reed valve are much too stiff for this type of engine. The carburetor may also be too large, though the throat is dimensionally similar to the design I have and goes to a motorcycle with a much smaller piston cylinder.
Planned activity is think on this more, replace the reeds with something less stiff, and fuel the carburetor with gasoline. No schedule on this as yet. Too many domestic duties.