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  1. #1

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    Engine getting too hot

    This is a question for Bill Baxter. I am posting it here based on recommendation from Gray Beard from the Beginners forum

    I have a 75ax with a MAS K series 14x8 prop in a GP Reactor 46 for a couple of weeks now. The engine details are as follows

    1) This is an old engine which was used on UCD60 a couple of years back. On that airplane, the tank centre was nearly 2" above the carb and I was never able to run the 75 properly.
    2) We never actually did a full break-in for the 75. We ran it rich for a the first 3 tanks of flying. The engine wasnt loaded heavily in that period as that airplane was mainly 3D funfly and my first 3D airplane so I didnt do too much nose-up flying while I was learning the airplane. But we never could tune the engine properly, probably due to the tank height difference and I had to put the engine into storage. ( I have now lowered the tank and installed another engine with full break-in on that airplane and now it flies OK)
    3) Before storing, I thoroughly washed the engine by immersing in plain alcohol for a couple of nights (with the o-ring parts removed) and then thoroughly oiling with light machine oil. I have never opened up the engine, only removed backplate, carb and plug befor immersion. Everything is tight
    4) The fuel I had used then was 20% castor-methanol with no nitro.

    Now coming back to the present

    1) I had to install the 75 on the Reactor as my earlier 55ax did not have sufficient power for hovering this 2.8kg airplane
    2) Started with a MAS K series 14x4 but we quickly realised that the engine was over-revving so put in a 14x8. Prop and spinner are balanced
    3) Having opened both needles during the washing, I set the LSN at 1.7 turns out from fully closed carb and HSN at 3 turns. Then I tuned the HSN by ear at full throttle, it turned out to peak at about 2.5 turns, set it a quarter turn from peak for flying. (For the LSN I used the pinch test- when the fuel line is pinched it bains RPM for a few seconds before beginning to struggle)
    4) When flying basic pattern without a cowl couple of weeks back it behaved OK but a bit rich. I didnt do too much hovering then, just some installation flights
    5) This last Saturday, I fitted the cowl and flew (The cowl already has a lot of cooling openings, both inlet and exhaust. Any more and there wont be any cowl left). It kept losing power in the vertical and finally cut and I had to deadstick. When I tried to tune at full throttle and then came back to idle, it immediately cut. It was also extremely hot, you could feel the radiant heat just sitting in front of the engine and inspite of a breeze. I also got black oil in the exhaust and compression was very soft when hot, you could turn it over by hand. Compression returned as it cooled
    6) Fearing overheating, therefore, on Sunday, I again flew without cowl and tuned the HSN by holding the model vertical. It tuned at about 3.2 turns out. It still lost power in vertical but didnt cut
    7) Then I again ran the engine on the ground alternating between 4 turns out to 2.8 turns out for about 2 minutes. As before, it cut as soon as I idled, the compression was soft, got black oil in the exhaust and engine was very hot

    The fuel I am using is Klotz 100 (80-20 synthetic castor mix) with 10% nitro (3.5l methanol + 500ml nitro + 1 bottle i.e. 1l Klotz 100). New OS #8 plug. Tank is at same height as carb

    Note that when I put in the 75 it was because I was having exactly the same problems with the 55 on the same airplane with the same batch of fuel earlier. (On the 55 I had used all props- 12x4, 12x6, 12x8, 13x4, 13x6 with the same problems before I gave up on it)

    Another small piece of Info- when I put the 75 ax together after washing, I accidentally over tigntened one of the carb screws and it has stripped. It wont fall out but wont stop turning when you try to tighten. I have currently put it with some teflon tape. The O-ring on the carb neck is OK and there shouldnt be any leakage as the engine cuts off when the baffle closes fully

    Whew, that was quite a bit of information in one go!!!

    Bill, can you help me understand and resolve my problem?

    Ameyam
    Preflight check is optional , picking up the pieces is consequential

  2. #2
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    RE: Engine getting too hot

    When you said the engine was "overrevving", what RPM were you running.  If you're running below the mid-12,000's there is no way the engine was over-revved.  If you prop the engine for between 11,000 and 13,000 RPM when the engine is leaned to just rich of peak RPM at full throttle, you'll be OK, no matter what propeller you're using.  However, the engine will be quite noisy.  O.S. suggests rather large propellers to give a low noise profile because many RC fields in Japan have very strict noise regulations.

    Any model engine will become hot enough to cause a severe burn if you touch the cylinder and cylinder head while it's running or just after it's been stopped.  It will take several minutes for the engine to become cool enough to touch after stopping.

    If the engine will sag in RPM when the aircraft is pitched to the vertical, you're going lean.  You have to correct the fuel-flow difficulty.  If the engine has an actual problem, it's best diagnosed on a test fixture.  There, you can put everything in the open and see how things run.

    For cooling, you need to m ake sure that any air entering the cowling actually passed through the fins of the cylinder and cylinder head.  If you can see the air outlet when you look into the air inlet, you can be assured that the air is bypassing the cyilnder.  The air outlet should be at least 2 times large than the air inlet.  In many airplanes, some kind of baffle is needed to direct airflow through the cylinder fins.

    The oil mixture should be 18% by volume of the fuel, and the Klotz 80/20 synthetic/castor is a good oil.

    Finally, the exact number of turns on the needle valve when you set your engine is completely irrelevant.  The engine tunes wherever it tunes.  However, changes in the weather from day-to-day can cause significant changes in the needle setting.  Even flying from a cool morning to a  hot afternoon will require noticeable changes in the needle setting.  The needle needs to be double-checked before every flight, even if you only perform the "pinch" test or lift the nose of the model to the vertical.

    On the .75, you can try 11 x 8 or 9 props, 12 x 6, 7, or 8, even a 13 x 4 or 5 if you want to hover.  Keep peak RPM below 13K and you should be fine.





    Bill Baxter, Manager Hobby Services/Futaba Service/North America
    3002 N. Apollo Dr. Ste. 1 Champaign, IL 61822 USA
    Service Phone: 217 398-0007
    Email: hobbyservices@hobbico.com

  3. #3

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    RE: Engine getting too hot

    ORIGINAL: Bax

    When you said the engine was ''overrevving'', what RPM were you running. If you're running below the mid-12,000's there is no way the engine was over-revved. If you prop the engine for between 11,000 and 13,000 RPM when the engine is leaned to just rich of peak RPM at full throttle, you'll be OK, no matter what propeller you're using. However, the engine will be quite noisy. O.S. suggests rather large propellers to give a low noise profile because many RC fields in Japan have very strict noise regulations.


    Ok, I understand that we need to quantify this. I never actually tach my engines, dont even have a tach. I will borrow a tach and an infra red temperature gauge this weekend and revert

    ORIGINAL: Bax

    Any model engine will become hot enough to cause a severe burn if you touch the cylinder and cylinder head while it's running or just after it's been stopped. It will take several minutes for the engine to become cool enough to touch after stopping.

    If the engine will sag in RPM when the aircraft is pitched to the vertical, you're going lean. You have to correct the fuel-flow difficulty. If the engine has an actual problem, it's best diagnosed on a test fixture. There, you can put everything in the open and see how things run.



    Actually, touching the engine was just a analogy. Its the same story with the 75ax now and the 55ax earlier. It gets very hot, drops black oil in the exhaust and the compression is so soft you can turn over compression with just a finger on the prop. It wont even start in this condition. Once it cools down compression is back


    Since the common factors here are the fuel, airplane and tank, I will replace them. I am brewing 0% castor klotz fuel tomorrow and will put in a Sullivian 14oz flexitank and bubbleless clunk assembly that I had some success with the 55ax earlier. I have already flushed the carb on monday- basically held it under plain alcohol with tubing inserted in both the fuel nipple and needle holes and blew air in all directions. Also pushed a thin blunt throttle wire into the holes to ensure nothing is blocked. Lets see

    ORIGINAL: Bax

    For cooling, you need to m ake sure that any air entering the cowling actually passed through the fins of the cylinder and cylinder head. If you can see the air outlet when you look into the air inlet, you can be assured that the air is bypassing the cyilnder. The air outlet should be at least 2 times large than the air inlet. In many airplanes, some kind of baffle is needed to direct airflow through the cylinder fins.


    There is no cowl right now. Even with the cowl, the engine is mostly outside it


    ORIGINAL: Bax

    The oil mixture should be 18% by volume of the fuel, and the Klotz 80/20 synthetic/castor is a good oil.


    Its 20% oil content- 1l oil in 5l of fuel



    ORIGINAL: Bax

    Finally, the exact number of turns on the needle valve when you set your engine is completely irrelevant. The engine tunes wherever it tunes. However, changes in the weather from day-to-day can cause significant changes in the needle setting. Even flying from a cool morning to a hot afternoon will require noticeable changes in the needle setting. The needle needs to be double-checked before every flight, even if you only perform the ''pinch'' test or lift the nose of the model to the vertical.

    On the .75, you can try 11 x 8 or 9 props, 12 x 6, 7, or 8, even a 13 x 4 or 5 if you want to hover. Keep peak RPM below 13K and you should be fine.





    I realise that the number of turns is quite meaningless. I had just given it as a reference.

    I didnt know that lower size props could be used on this engine. I have 15x4, 14x8, 14x6, 14x4, 13x6, 13x4. I will report the RPM and temperature with the first first 5.

    Do you think the stripped carb screw will be creating any problems? Remember, I have put the screw in with some teflon tape. It doesnt tighten but doesnt come out either. The other screw and carb o-ring are fine

    Ameyam
    Preflight check is optional , picking up the pieces is consequential

  4. #4
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    RE: Engine getting too hot

    Bill, I might be out of line posting here and if so please accept my apologies.

    A 14X4 "may" be OK (My opinion is it's too much prop) but a 14X8 "is" too much prop for the 75AX. That alone can cause overheating. I would be looking at something in the 13X4 to 13X6 range.
    Spektrum DX8i, DA DLE SuperTigre OS FOX Saito Enya Jett TT: John 3:16


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