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  1. #1

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    TT 40 Engine Rules of Thumb?

    To anyone,

    I am just wondering general rules of thumb for changing stuff on a Thunder Tiger 40 between three different variables:

    1. Larger diameter x Small pitch Prop vs. Small diameter x Large Pitch Prop
    2. Lower head with thinner shim than stock (0.014-0.017 vs 0.003 to 0.005)
    3. Colder or hotter plugs

    Hear is the fixed parts.

    1. Course is short - 2 poles at 400 ft.
    2. Plane is relatively draggy compared to a Q500 - Club 40 Sky Raider Mach II or LA Racer 40
    3. Plane is relatively heavy compared to a Q500 - 4-1/2 lbs.

    OK, let me add the last variables of weather:

    1. Hot, and humid vs. Cooler and dry (75% humidity vs. 25% humidity), 0 to 600 ft above sea level.
    2. Windy vs. Calm (15mph vs. 5mph)

    What should be the rules of thumb for the first three items based on the second three fixed items to account for changes in weather of the last two items?

    Any input is appreciated.

    Thanks,

    Mike W

  2. #2

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    RE: TT 40 Engine Rules of Thumb?

    Mike, here are some of my basic rules of thumb. They are based on my experience and opinion; I have no facts to support them.

    Very windy, use a large diameter low pitch prop; 10X4 or 10.5X4.5
    Moderate wind, less diameter and a little more pitch; 9.5X 4.5 or 10X5
    Calm wind, small diameter max pitch; 9X6 or 9X6.5

    Also, when you change props you should adjust your flying style. With lower pitch props you can pull tighter in the turns; with higher pitch props you should not pull as tight. FYI: if you have a pitch gauge, use it; not all props will have the pitch as marked.

    We cannot change the humidity or height above sea level but we can play with glow plugs and head shims. So far I have not experimented with head shims but I would think that as humidity goes up so should the head shim.

    As for glow plugs, use the one that gives the highest RPM in the weather conditions you have. Medium-Hot or Hot is best because of the idle requirement. Cold plugs do not idle well.


    Mike thanks for starting this thread. I would like to see what others have to say.

    Bob

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    RE: TT 40 Engine Rules of Thumb?

    Hey Mike, looks like no-one else is interested in sharing info on this subject. That’s sad; because sharing information is fun and what will keep our event competitive and levels the playing field.

    Bob

  4. #4
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    RE: TT 40 Engine Rules of Thumb?

    I only race the TT 40 PRO in 424 which we only get to use the APC 9x6 prop, but here is some general information. If the plane is heavier than 3.75 lbs and draggier than a quickie, I would be looking at 9.5~10 inch diameter props that I could spin at or slightly over 16,500. You either get more diameter (thrust) or pitch (speed). If you look at AMA racing classes, you will notice the cleaner the airframe, the smaller the diameter and higher pitch props are used. You have to decide if you can fly smoother and carry more pitch, or if you yank it around the poles hard and need more diameter. As a rough rule of thumb, if you add 1" of diameter, you must subtract 1" in pitch to maintain the same RPM. The formula works both ways if you are using the same manufacturers props.

    No matter which prop you end up using, there are a lot of basics to racing you should be conscious of. I have posted a document I put together a couple of years ago for a racing clinic I did here in Florida. There is quite a bit of information in it I think you may be able to adapt to your form of racing. I put a section on engine set-up, break-in and lots of other goodies to help out the new guys. Take a look at it and see if it helps any.

    Good luck and I hope you enjoy racing as much as I have!
    Attached Files Attached Files
    Scott Smith
    NMPRA 86t - District 7 VP

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    RE: TT 40 Engine Rules of Thumb?

    Mike, experiment, practice and take notes of everything.

    IF race day props are not to be issued then I would suggest to get a bag of 9x6 APC's and run them all to see which will yield the best RPM's,,, all 9x6's are not the same,,, then keep the best one for Race Day use only. Never take off at peak RPM or else you will go lean in a few laps, slow up, get lapped and possibly cook the engine. Take off RPM should be around 500 down from peak. Change out the muffler thru bolt to a 60 muffler bolt and have extra muffler gaskets on hand,,, if your muffler is leaky, your losing RPM's. Debur the baffle inside the muffler.

    Find out what the club uses for fuel on Race day and practice with that only,,,

    Learn to read your plug. By doing that you will know how your engine is running. A Golden honey color look to the plug with a shinny coil is what you want. IF there's no color to it,,, its running rich. Suggest to procure some jewlers eye magnifiers to get a better look at the coil in the plug,,, if it looks blistery, little spec's or bubbles or any de-formities of the coil, it ran to hot or lean. The condition of the coil will tell all after each run.

    Other than that, there's really not much one can do to a one stock engine class of Racing. Don t try to drill out the muffler exhaust port, port match the exhaust and intakes, enlarge the carb or change the angle of ports to include the crank port, shave out the exhaust rib(that will really make the tt pro 40 zing) or run without the baffle,,,

    it will get noticed

    BV
    If it ain t leaking oil, then somethings wrong,,, USMC 82-03, RETIRED!

  6. #6
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    RE: TT 40 Engine Rules of Thumb?


    ORIGINAL: Bill Vargas
    Other than that, there's really not much one can do to a one stock engine class of Racing. Don t try to drill out the muffler exhaust port, port match the exhaust and intakes, enlarge the carb or change the angle of ports to include the crank port, shave out the exhaust rib(that will really make the tt pro 40 zing) or run without the baffle,,,
    BV
    One would have to wonder how many of these "noticeable" things get done since you can't actually see the exhaust rib without taking off the muffler or the crank ports without taking off the carb, etc. etc. How often does the winning engine actually get torn down? Not very often I imagine. Racing is racing and there is always a temptation to push the rules or read between the lines. (Yeah, right, like NASCAR was originally supposed to be STOCK cars...) A good race CD needs to be savvy about engine modifications. This is one advantage of an "unlimited" class like 428. There CAN'T be any engine "cheating" because anything goes. The fast guys, who are also usually the best pilots and have the best-prepared airplanes, know all the tricks and how to get away with them if they wanted to. It's really up to the personal integrity of the individual. Fortunately the vast majority of racers have a high degree of personal integrity and would not cheat at racing "toy" airplanes. I've been accused of cheating before ( I didn't). But it's STILL nice to know what the "tricks" are anyway.
    Gary James
    AMA CD 68845, NMPRA 15I, RCCA #908

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    RE: TT 40 Engine Rules of Thumb?

    Gary, 428 is NOT or has never been "unlimited". It uses a stock motor and has very specific rules.

    The top 5 finishing motors having been getting tore down at the Nats for several years now and measured and examined for tampering. I'm not aware of anyone being found with an illegal motor, and when you have Dennis and Dub going over all the motors with Calipers, you know they are doing a good job.
    Dave Norman
    29w

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    RE: TT 40 Engine Rules of Thumb?

    Gary, Racers are just that, Racers! Always looking for more speed, as you say the tricks are out there,,, like spooning the crank or running presure off a tap coming from a backplate, just to mention a couple more. Its really not all about the engine if you got the thumbs to make up for the lack of engine Hp.

    And thats where Practicing to fly the poles comes in,,, forget about Racing each other,,, just Race the poles,,, practice, practice, practice


    BV
    If it ain t leaking oil, then somethings wrong,,, USMC 82-03, RETIRED!

  9. #9
    GSJames's Avatar
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    RE: TT 40 Engine Rules of Thumb?


    ORIGINAL: daven

    Gary, 428 is NOT or has never been ''unlimited''. It uses a stock motor and has very specific rules.

    The top 5 finishing motors having been getting tore down at the Nats for several years now and measured and examined for tampering. I'm not aware of anyone being found with an illegal motor, and when you have Dennis and Dub going over all the motors with Calipers, you know they are doing a good job.
    Dave: I think you might be misunderstanding me. As a newbie to RC Pylon, I am VERY impressed with the high degree of personal integrity, helpfulness and honesty of the competitive racers. I especially want to publicly thank you for taking one of your Sunday afternoons two years ago to help guide me in my initial year. THANKS! The point that I was actually trying to make (and apparently did so rather poorly) is that by KNOWING where the tricks are, the honest competitor can often detect when someone else is being less than honest. Knowledge is power. If a person wins by cheating, they are not a winner, just a cheater. I am not surprised that the NATS-winning engines are inspected. As a former NATS CD (CL Combat) I would have expected nothing less. Certainly the guys doing the inspecting (Dennis and Dub) are the best that there is!

    Since 428 and 422 are SO FAR BEYOND my present skill level, I was unaware that they used stock engines. I incorrectly thought that it was pretty much "anything goes" engine-wise as long as displacement, pressure bladders and muffler rules were met. Thank you for correcting me. Perhaps someday I will be able to fly at that level, perhaps not...

    I am enjoying my new event, but the best part is the ability to associate with some very fine people.
    Gary James
    AMA CD 68845, NMPRA 15I, RCCA #908

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    RE: TT 40 Engine Rules of Thumb?

    No worries Gary, just didn't want that misconception out there. I enjoyed that afternoon hanging out in the shop drinking some good beer. I'm now South of the cities, a little over an hour away, stop by next time you are in town.
    Dave Norman
    29w

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    RE: TT 40 Engine Rules of Thumb?

    OK, I may need to give out more information.

    For one, the only modifications allowed to the TT40 is head shim thickness, prop size, and/or glow plug types. You can't take the baffles out of the muffler. You can change out bearings if you wish, for worn or new engines.

    Fuel used is Ritch's Brew Pylon 15%, all synthetic oil.

    By the way, APC 9 x 6 sport prop does not do all that great on this short of a course with a heavier, draggier airplanes of the Sky Raider or LA Racer.

  12. #12
    Ken Erickson's Avatar
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    RE: TT 40 Engine Rules of Thumb?

    Small addition:

    The National rules allow also use of a remote needle valve, other than what came on the plane or add one to an engine which did not come with one. Well, not any, you can't change the mixture in flight, but who would want to do that?

    I have three (3) Thunder Tiger carburetors which have had the needle valve assembly break out of the carb body. Rather than buy new carb body with the same weakness, I JB Weld a piece of the 1/8th in. aluminum tube, which came with the tanks, into the broken hole/socket and use an OS remote mount and needle assembly. Works fine.

    Ken Erickson

  13. #13

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    RE: TT 40 Engine Rules of Thumb?

    Hi!
    1. The prop of choice is depended on many variables. Such as airframe drag, weight and what type of performance you want (speed or climbing performance).
    Let's take an example:

    In Sweden we race Q-500 pylon with side exhaust .40 engines using 80/20 fuel with 10x6 plastic props.
    The engine of choice for the last two decades has been the Webra .40 GT.
    That engine turns a RAM or Graupner Cam prop (svept tips) at around 14000 rpm on the ground.
    Max speed is around 160-180km/h.
    We are not allowed to use Tettra "Bubbleles tanks.

    That plane would go faster if we changed the prop to a 9x6 or allowed 15% nitro! But it would probably climb bettetr with a 10x6 prop.

    If you are using no nitro fuel and want better performance without doing radical things like raising exhaust timing you can: 1. Raise compression by removing a head gasket and at the same time use a colder plug.
    Those to things goes together...low compression means hotter plug a high comprssion means colder plug.

    Then you ask : What is high compression? And what is low colmpression? And what is cold and warm plug?
    This is something you learn by racing!!

    Take the first example again!
    In my Webra GT .40, With which I have won many races, I use a Nova Rossi 4 glow or Rossi 3 plug and standard 0,2mm head shim. Using an OS 8 or Enya 3 (which are good sport plugs ) will not perform as good.The difference is not that much noticable , only 100-300rpm. But easily recognized both by listening to how the engine sounds and how the plane performs in the air. It is possible that there is better plugs available but in my 30 years of racing I have not found any better ones.

    If I would run that engine on 15% nitro fuel I would probably had to use a much colder plug like a Rossi 5 or 6 plug to obtain better performance and at the same time probably lower compression adding a 0,1mm head shim.



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