Eighty mile an hour combat has been around for several years. During
that time, I have managed to finish in the top three a few times. I had
always looked at it as a way to fly another event with a minimum investment
in equipment. Take an engine that was no longer competitive in fast, and
do something to make it go 80 mph. As far as a plane, take a worn out fast
plane and fly it at 80 mph. We progressed from worn out fast engines to
using the Magnum 36 that we reworked for control line use.
A couple of years a go we started flying FAI models on 60 ft lines. These had the advantage, and still do, of turning very tight. Against the 35 engines, on worn out fast planes, it is a distinct advantage.
Having long ago abandoned the worn out fast engines for the Magnum, I was looking to replace the Magnums. This year in practicing for the Bladder Grabber, I had the crankshaft fail in one of them. On another one, the bearings began to fail, and when I opened it up the crankshaft was cracking in the identical pattern as the one that had failed. I was now down to two Magnums. When Jeff went to Texas this year, he brought back an OS Max 25 FX. I was impressed with this engine; I decided to order one. Since then, I have picked up three more from eBay.
These are RC engines with carburetors and a remote needle valve. The following details the steps you need to go through to modify the engine. It will take about one hour to make the modifications. The first step is to put a glow plug in. This is to protect the engine from metal chips during later steps.
The OS Max .25FX in side and rear views.
Notice the part line in the casting between the back plate and remote
needle valve assembly. Remove the back plate/needle valve assembly. Set
the bolts and case aside.
Using a band saw or Dremel (I use a band saw), remove the remote needle valve assembly from the back plate.
Back Plate and Needle Valve Assembly on Band Saw
Needle Valve Assembly removed from Back Plate
Next using the Dremel, clean up the cut. I use the drum sander for this process.
Back Plate Cleaned Up
Wash off all metal chips from the back plate using alcohol and replace the back plate.
Back Plate Replaced
Next, remove the carburetor. I have tried using a metric Philips head screwdriver, but these screws are really set tight. I use a pair of vise grips to set a tight grip and break loose the screws.
Remove the Carburetor
Once the screws are out, pull the carburetor off. Take and close the
engine so that the crankshaft port is closed. Take and use alcohol to wet
down some pieces of paper towel and put them in the venturi hole and the
exhaust. The reason for this is to keep metal chips from getting into the
Wet Paper Towel in Venturi
The venturi is made from a one inch long nylon spacer. The spacer is .5" OD and .328" ID.
Home Depot part number 30689 87298 5.
.5" OD and .328" ID.
Next, place the standoff into the venturi opening and push it down. Now mark on both sides the hole position for the needle valve. Remove the venturi and drill one side of the hole for the needle valve using a Number 21 drill bit.
Wash the venturi with alcohol and put it aside.
Using the Number 21 drill bit, drill out the needle valve holes in the engine to accept the new assembly. Next, put the venturi in and drill the back side hole. Put the venturi in and check to see if the needle valve assembly will go thru. If it will not, then carefully relieve the bind.
Taper Reamer to Relieve Hole
Turn the engine upside down and carefully remove the wet paper towel and clean out the venturi to remove any stray metal chips from drilling. I use a Q-tip moistened with alcohol.
Metal Chips Caught on Wet Paper Towel
The next step is to rework the needle valve assembly.
Super Tiger Needle valve assembly 22070153 ( Tower Hobbies PN LXFY25)
Valve Body before Rework
Now take the needle valve body and carefully grind the end where the compression nut goes. You want to remove all material back to where the threads start.
Needle Valve Body after Grinding
Next, put the assembly in a vise and tap the needle valve body for a
2-56 screw. Be careful not to break the tap in the needle valve body.
Take and insert the venturi. Push the needle valve body through, and put on the needle valve nut. Position the needle valve hole so that it faces down, and tighten the nut. The hole will not be in the center, but this does not matter since you will be running on pressure.
Using a 2-56 screw and washer, apply lock tight and insert it into the body. Tighten the nut and remove the paper towel from the exhaust port. The engine conversion is complete.
Completed Needle Valve
You will need to find a motor mount that will work. I have been using the ones I got from Mejzlik. Because the spacing of the 25 FX is different from the Nelson, you turn them upside down and the spacing is perfect. The last step is to mount a remote needle valve assembly.
Engine Mounted to Metal Mounts
Your prop wrench will not work on this engine. You will need to use a 10 mm wrench.
Your prop wrench will not work on this engine. You will need a 10 mm wrench. An APC 8x5 on a full size AMA fast combat plane (10%) fuel should clock at 80 MPH without a streamer.
Since the original writing of this article, we have noticed that the engines have a tendency to go lean. Actually more than a tendency, the damn things go lean. These are running on 10% nitro. Increasing the nitro to 23% helped but did not eliminate the problem. Don McKay was having more than his share of problems. I did have problems with the engine going lean if I did not let it heat up and then set it off with a slight gurgle. I removed the venturi and the engines ran better but still had a went lean.
At the Bladder Grabber this year, Mike Moreland, Jeff Dawson, Jeff Rein, Don McKay and myself discussed the solution. It comes down to engine setting. You have to let the engines get warm. Start the engine and set the needle so you have the rich gurgle. By the time you walk to the circle (approximately 14 seconds), the engine gurgle will have disappeared. Reset the needle to gurgle and then launch. If you are competing where the pilot starts their engine and then dashes to the handle, you need to wait for the gurgle to go away and let the pitman reset the needle before launching. You risk not being the first up, but you should have a good setting.
Do not use the venturi insert, up the nitro to 20%.
You will need to control the speed through your choice of props. Start
out with the 8X5 as a baseline and see what speed you are getting. If you
need to slow down move up to a 9x4. Experiment with diameter and pitch to
get the best performance for your plane.
This page was upated Oct. 21, 2009