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Thread: Two cycles, Boy some folks sure hate them

  1. #31

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    Quote Originally Posted by Dana View Post
    A related issue I've been wondering about is aerobatics. I'm designing an aerobatic ultralight which will probably use a 447 or similar. I'd hate to have the engine seize coming out of a hammerhead or loop. The aerobatic Quicksilver Super used a 503 (with diaphragm carburetor) and I haven't heard of problems with it, though many 503's had oil injection.
    More anecdotal evidence perhaps but seems like one could examine the engine failures on Rans S-9's and S-10's as they are purposely designed for aerobatics and have reasonable service history available and compare to the straight and level fleet.

    I considered the same thing once, installing a two-stroke on something like a Baby Lakes. Outside the relatively low "TBO" I think it would perform satisfactorily but you can almost bet the bank on a power off landing in a relatively high wing load airplane at some point.

  2. #32

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    In regards to the lean condition while on a descent or during prop unloading manuevers, having flown with a rotax this info is not a secret, everyone who is diligent has indeed moved forward already. The problem arises on a long powered descent, NOT a landing approach on short final at idle power, the idle mixture is rich (or should be) dragging the airplane in on a low final has a possibility of causing a lean condition to occur, and it's a bad practice anyways. No reason to "count" on an engine failure, of course it can happen to any mechanical device, it's too bad that aviation continually has to reinvent the wheel so to speak.

  3. #33
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    Thanks!
    Quote Originally Posted by martymayes View Post
    . . . I'd be most concerned about putting a load on a two-stroke and not being able to keep the RPM in the proper range. . . .
    I agree which is why I'm looking at the 3502 which has a relatively flat power-rpm range compared to the nearly identical 3503 that has higher peak power but in a narrower band. A wider power range should allow a larger velocity change with an efficient prop.

    What I'll be doing next is to understand the prop-velocity-torque curves for a fixed pitch prop. Then I'll integrate the airframe, engine and prop curves to calculate the expected power:
    • 0-60 mph - acceleration to minimum flying speed at 1,150 lbs
    • 60-90 mph - acceleration to maximum rate of climb speed, at 1,150 lbs
    • 90-140 mph - expected maximum, level flight speed, 1,150 lbs
    Quote Originally Posted by Racegunz View Post
    In regards to the lean condition while on a descent or during prop unloading manuevers, having flown with a rotax this info is not a secret, everyone who is diligent has indeed moved forward already. The problem arises on a long powered descent, NOT a landing approach on short final at idle power, the idle mixture is rich (or should be) dragging the airplane in on a low final has a possibility of causing a lean condition to occur, and it's a bad practice anyways. . . .
    Were these failures using carburetor engines?

    My plan is to use fuel injection for the 3502 which has an intake, air pressure port to manage the fuel mixture. I'm not sure about an air temperature probe but will check. Regardless, the fuel injection should keep the mixture neither too rich nor too lean and all but eliminate carb ice risk.

    I also plan to use engine driven, oil injection. This ties the lubrication to engine RPM regardless of the throttle or fuel consumption. Our four-strokes have an oil pump so it makes sense that a reliable two-stroke would have one too.

    Bob Wilson

  4. #34

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    As far as I know the condition was/is with carbs,not sure how the fuel injection will help since the condition is high rpms with low ammounts of fuel which in a 2 stroke is a coolant just like running rich in a 4 stroke aids cooling. As long as awareness is there and an eye on the EGT's shouldn't be a problem and if this thread is any indication of your deligence I think you'll do fine.

  5. #35

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    Quote Originally Posted by bwilson4web View Post
    I also plan to use engine driven, oil injection. This ties the lubrication to engine RPM regardless of the throttle or fuel consumption. Our four-strokes have an oil pump so it makes sense that a reliable two-stroke would have one too.
    why does that make sense? aircraft engines run at pretty constant power most of the time. taxi is the only operation below 65% for most aircraft folks. why include a part which must be purchased, installed, carried around, calibrated, and presents a single-point failure mode when you don't require it? premixing oil&gas eliminates that failure point. granted, it requires thought when refueling. but mechanically it fits with Ford, Stout, Hooten, and Douglas - "simplicate and add lightness"

    besides, the hirth 3502 you like doesn't inject oil based on rpm. according to the hirth manual, the oil ratio on that and numerous other models depends on 2 variables:
    1. Number of rotations
    2. Throttle position (loaded condition)

    google.com/url?sa=t&rct=j&q=hirth%203502%20%22oil%20injection %22&source=web&cd=8&ved=0CF4QFjAH&url=http%3A%2F%2 Fhirth-motoren.de%2Fde%2Fdownloads-kategorie%2Fcategory%2F25-anleitungen.html%3Fdownload%3D219%253Agetrenntschm ierung&ei=_BJWT7LNHMGrsQLq9tHaCQ&usg=AFQjCNFddxsZ8 XUavXr0OfdNOFIQy10X7A&cad=rja

    so the oil going into the engine is reduced because the fuel going into the engine is reduced. well, duh, it does that with premix when closing the throttle, too. without added complexity, cost, and weight. plus, if oil injection is better for that engine, why didn't the people who build and warranty the engine require it instead of making it an option? i'd guess they put their money where their engineers are.

    your mileage may vary.
    Last edited by cdrmuetzel@juno.com; 03-06-2012 at 07:50 AM.

  6. #36
    bwilson4web's Avatar
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    Hi,
    Quote Originally Posted by cdrmuetzel@juno.com View Post
    . . . why include a part which must be purchased, installed, carried around, calibrated, and presents a single-point failure mode when you don't require it? . . .
    The oil is there to lubricate the moving parts and needs to be in proportion to the relative velocity of the moving parts to the fixed parts and proportional to the mechanical stress from producing power. The oil pump provides lubrication in proportion to:
    • rpm - the relative velocity
    • throttle - the mechanical stress

    In contrast, the throttle is not tied directly to the engine rpm and speed of the moving parts. In a descent, the engine and prop will turn rapidly but there won't be a significant flow of pre-mix gas to provide the lubrication for the moving parts. I want my moving engine parts to stay well lubricated regardless of throttle setting and in proportion to the moving part, speed differences.

    As for the other concerns, I can afford the oil pump; it will be factory and/or dealer installed; the part and supporting hardware filter, tube, and oil tank appears to be less than 2 lbs; calibration of the oil pump is another one of several calibrations needed for this engine and; the oil pump failure mitigates pre-mix fuel risks and maintenance.

    My understanding is pre-mix has about three week expiration date and needs to be drained and replaced. But I'm loath to dispose of the pre-mix by adding to my car tank due the risk to the catalytic converter and O{2} sensors. Furthermore, there are non-trivial safety risks when moving pre-mix around. In contrast, straight gas has a much longer tank storage life. The same is true of the oil in the tank.

    I appreciate that fixed operations with a lot of flying and similarly fueled craft can easily handle pre-mix. In contrast, my plane may sit for weeks or longer if health or other demands cut into my flying time. The oil pump avoid the expiration date risk of pre-mix.

    Thanks, Bob Wilson

  7. #37
    bwilson4web's Avatar
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    Propeller pitch analysis revealed:
    • need for faster prop rpm - to meet the dynamic range of 60-140 mph of the airframe
    • largest dynamic rpm range 60 hp and above - 60 hp is the rated power of the original engine
    It turns out that without doing some machining, I'm limited to the G50 gearbox and a 2.16:1 ratio. Since I'm limited to a 52" prop, this requires a greater pitch which either limits the takeoff-climb or cruise-top speed. . . . There is no free lunch but thankfully, no show stoppers.

    Bob Wilson
    Last edited by bwilson4web; 03-07-2012 at 07:52 PM.

  8. #38

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    Backing up a bit...

    "One radical thought is the BSR might fit in front of the firewall. "

    I'm not sure putting a chute in the high temp area of the engine compartment is a grand idea.
    The opinions and statements of this poster are largely based on facts and portray a possible version of the actual events.

  9. #39
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    Quote Originally Posted by Frank Giger View Post
    Backing up a bit...

    "One radical thought is the BSR might fit in front of the firewall. "

    I'm not sure putting a chute in the high temp area of the engine compartment is a grand idea.
    Good point as the VW installation had a cold-air pipe to a box around the gascolator. Fortunately, the two primary heat sources are well contained, the latest published sketch:


    After several interations, this appears to be the most practical approach using the existing engine mount anchor points. The Hirth radiator is to scale.

    This version keeps the engine upright and partially covers the stock exhaust and expansion chamber. There are options for a support bracket for the exhaust. There should be no problem with a cold-air feed into a light weight box located high in the steel tube, engine mount frame. This is especially true for the pyrotechnic rocket which needs a similar environment as the gascolator, fuel lines, and fuel injection controller. This would be a more challenging problem if I were using an air cooled engine.

    Understand this is a sketch and the final configuration won't be known until after I have all of the firewall-forward equipment: prop, engine, radiator and BSR. Like many things in life, there seems to be some 'margin of error' between the published and actual dimensions and weights.

    Having the parts will give me the exact weights and dimensions and allow form-and-fit tacking of the firewall forward assembly. Once the engine mount frame is welded up, we can double check the firewall forward weight and balance. Then we'll use foam to form the cowl plug, fiberglass the cowling, final assembly and check out. That is my plan (and God is laughing.)

    Both parachute recovery manufacturers recommend attachments that bring the aircraft 'flat' into the relative motion so the aircraft forms part of the drag needed to slow everything down. The firewall forward installation means these important straps will be shorter and engage early.

    Thanks,
    Bob Wilson
    Last edited by bwilson4web; 03-19-2012 at 03:31 AM.

  10. #40
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    Hi,

    Quote Originally Posted by martymayes View Post
    I'd definitely be interested in seeing such a review Bob. In particular, I'd like to see what kind of risk is incurred by flying at night in a homebuilt airplane with a converted auto powerplant or two-stoke engine.
    I finally got a little time today to put fingers to keyboard so I've posted the mission requirements review over in "Homebuilders Corner." Whether or not it does any good, well that is how the process works.

    Enjoy!

    Bob Wilson

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