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Discussion in 'The light stuff area' started by ryanjames170, Oct 7, 2018.

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  1. Oct 8, 2018 #21

    blane.c

    blane.c

    blane.c

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    Be real, it is weight to power output AKA thrust. So what is the weight/hp/thrust all up weight which means carburetors or fuel injection, manifolds (intake and exhaust) if an engine doesn't have ignition timing to support hand propping then starter motor and generator/alternator too, in short engine with everything it takes to fly. Without the BS.
     
  2. Oct 8, 2018 #22

    lr27

    lr27

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  3. Oct 8, 2018 #23

    blane.c

    blane.c

    blane.c

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    A little light 20 degrees is normal in the traffic pattern or approach.
     
  4. Oct 8, 2018 #24

    BJC

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    Would you agree that the probability of an engine failure in a fleet of 200 airplanes is greater than the the probablity of one airplane having an engine failure?


    BJC
     
  5. Oct 8, 2018 #25

    ryanjames170

    ryanjames170

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    i will see if i can find it but someone did do a posting of a video for go carts but it might apply here for a idea of what the 212 predator can do HP wise
     
  6. Oct 8, 2018 #26

    ryanjames170

    ryanjames170

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  7. Oct 8, 2018 #27

    lr27

    lr27

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    Power and thrust are not synonymous. At low airspeeds, low rpm allows you to swing a big prop, which can produce more thrust for the same hp. Same general idea if you use two props at half power. There's a reason human powered aircraft props are so big. (11.3 feet for the Daedelus airplane) Two 10 hp engines, at 3,600 rpm, using appropriate props, will give you more thrust at low speeds than one 20 hp engine at 3,600 with the right prop. If you're willing to use a redrive, you can eliminate that problem.

    If you're flying a Davis DA-11, the above may not apply to you. ;-)
     
  8. Oct 8, 2018 #28

    lr27

    lr27

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    Considering that HF lists the shipping weight for the complete engine at around 38 lbs, I find the 30 lb figure believable for a stripped engine. Real question is how reliable are Predator engines?

    If you're flying a coordinated turn, then the engine won't really know you're banking. Can you really slip the airplane so hard that it tilts 20 degrees?
     
  9. Oct 8, 2018 #29

    Victor Bravo

    Victor Bravo

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    Ryan you have a very very good opportunity here to take advantage of a whole lot of brains and experience from the peop le on this forum. All you have to do is 'fess up and say what you really want to accomplish, and you will get several informed opinions from well educated and experienced people.

    So if yo WANT a twin engine airplane just to have a twin eng ine airplane, then this group can offer suggestions and ideas that will get you closer to that goal. If you just want to fly on a budget, we can give you educated suggestions on that too. If you NEED to meet Part 103, we know what direction to steer you for that too.

    So what is the most important thing you are trying to accomplish?
     
  10. Oct 8, 2018 #30

    Wanttaja

    Wanttaja

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    Easy. Imagine someone will give you $10 if you roll a die and get a six. That's a 1-in-6 chance of winning. But if you roll two dice, there's a 2-in-6 chance that at least one of the two die will show a six. Same thing on engines. If there's a 1:1000 chance that a type of engine will fail in a given flight, if you have two engines the odds are 2:1000.

    The other factor is the engine mounting location. If it's offset from the lateral axis of the airplane, loss of an engine will require immediate response to compensate for the off-axis thrust. There are ways to design around this (e.g., centerline thrust like a Skymaster), but the second engine means you have twice the chance of a *degraded* flight mode. Those can lead to more hazards than a complete failure in a single-engine plane. Maybe a given ultralight can fly on 15 HP if it's designed for 30, but the additional drag required to compensate for the off-center thrust may mean a harder time trying to fly it with a dead engine.

    Ron Wanttaja
     
  11. Oct 8, 2018 #31

    pictsidhe

    pictsidhe

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    All the industrial 200cc engines will put out around 9hp if you remove the governer and let them rev to around 5000. That's not the best rpm for driving a prop, but it's cheap hp. More highly tuned is probably going to mean even more rpm. I have a 212 predator stashed for some winter experiments. It does have some easy lb to shed.
     
  12. Oct 8, 2018 #32

    lr27

    lr27

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    Ron:

    That's not quite right. With two dice, there are 36 combinations you can roll. Of those combinations, 11 have at least one six in them. Not 12. You have 6 combinations that start with 6, and 6 combinations that end with six, but rolling 6 and 6 is common to both lists. If your engine's chances of failing are 1:1000, then if you assume that all engine failures are completely independent events that don't involve factors that affect the other engine as well, then your approximation is close to correct. But that's a questionable assumption. For instance, if the fuel systems are interconnected, you could just plain run out of gas, affecting both engines. If the systems are not interconnected, but you filled them both with the same bad gas, that will affect both systems too. If both have EFI or something and are connected to the airplane's electrical system, then they could both be brought down by the same electrical problem. It also seems like, if you forgot carburetor heat on one engine, you'd probably forget it on the other*. And so on. I'm not contesting that two engines makes the failure of at least one more likely, but it's not twice as likely.

    I don't disagree that, for asymmetrical thrust and a pilot who's not sufficiently and recently trained for it, the risk is increased rather than decreased. This seems especially problematic for an ultralight. Especially if Vmc is above stall. OTOH, if the aircraft can climb reasonably well and it's EASY to handle with one engine out, maybe the risk is reduced. Might be a tall order unless you have a push me pull you setup, i.e. both engines on the centerline. I seem to recall that the on the Cri Cri, if you lose an engine, the flow of the other one is supposed to stay attached to the canopy and gets directed at the rudder a bit crossways. I don't know how well this works. I've seen pictures of airplanes where the engines are canted outwards. I don't know how well that works either, and I'm sure it doesn't help efficiency. My guess is that centerline thrust might be the easiest to implement, but there might be an efficiency hit and cooling for the rear engine might be an issue.

    I'll admit that the second paragraph above may be off a bit, but it's based on a lot of stuff I've read. The first is too, but I've read about a fair number of accidents and their causes.
     
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  13. Oct 8, 2018 #33

    lr27

    lr27

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    5000 rpm with a 10 hp engine is like 3,500 with a 20 hp engine. The appropriate prop for the 10 hp engine is significantly smaller, so the tip speed is lower for any given rpm. Let's say you have a 60 inch prop on the big engine and two 42 inch props on the smaller ones. Tip speeds are about the same, and the total prop disc area is about the same. The kinetic energy imparted to the air is about the same. The two props will probably weigh less than the one big one, though the big one may have a slightly higher Reynolds number in its favor. I'd venture to guess that the two engine mounts, properly designed, would weigh less than the one big engine mount. If we could just deal with that engine out climb and control issue... I guess you could hop up the little engines a bit more, govern them to 5,200 rpm or something, and turn off or disconnect the governor if one of them failed. It might be loud but survivable as long as you didn't push the remaining engine hard enough to make it blow up.
     
  14. Oct 8, 2018 #34

    pictsidhe

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    I just asked javaprop about 10hp and 5000rpm
    It thought that a 1m prop could manage 79% propulsive efficiency at 63mph. At stall, thrust is nearly double. not too shabby. A pair of those wouldn't be awful. I suspect that it would need very well made airfoils, though. I had some 6 series foils already loaded. Re was a bit over half of the value of the foil polars used, so there's a few % drop already. Some care and 65-70% seems quite doable, which isn't too bad. 3500rpm didn't give much improvement at 63mph, though there was a 12% gain at low speed. Start trying to use more than 10hp, and efficiency will likely start to be hurt more at 5000rpm.
     
  15. Oct 8, 2018 #35

    Aerowerx

    Aerowerx

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    Your 1:1000 chance is based on studying 1000 engines (for arguments sake), of which there was a single failure. That does not mean that 2 engines magically doubles the odds. There is a 0.1% engine failure of either engine alone on any flight.

    If your logic was valid, then you would not be reading this post. Consider the gazillions of components involved in getting this from my computer to yours, and how the failure rate would build up.

    I have some small limited experience in failure prediction. Failure rates do not add linearly. They combine by Root-Mean-Square (IIRC).
     
  16. Oct 8, 2018 #36

    blane.c

    blane.c

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    Some of the reasons I advocate 3 or more engines, the asymmetrical thrust from any single engine loss is much less pronounced, 3 or more engine setups require less total hp than a two engine setups partially because you don't need to account for as much drag from asymmetrical thrust in the event of an engine loss.
     
  17. Oct 8, 2018 #37

    blane.c

    blane.c

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    I used to joke with my boss that he had the only 3 1/2 engine airplanes in the world, we'd go out with 4 and come back with 3 so a 3 1/2 average. He didn't think it was that funny. However the point is do you think that it was engine related or maintenance related?
     
  18. Oct 8, 2018 #38

    pictsidhe

    pictsidhe

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    Stop pussy footing around. The best option is clearly 200 cox 049s
     
  19. Oct 8, 2018 #39

    Dana

    Dana

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    I've never flown one, but I'm told the Kolb Flyer couldn't quite maintain altitude on one engine, but it could glide a L-O-O-O-O-NG way on one engine, giving you lots of landing options.

    With relatively low power and close-to-the-centerline engines, ultralight twins like the Lazair and Flyer don't have the asymmetric thrust issues that larger faster twins have.
     
  20. Oct 8, 2018 #40

    ryanjames170

    ryanjames170

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    i would like to find something that is Part 103 that can be flown on a budget..
    i was watching videos on youtube the otherday and say some kinda 4 engine ultralight with floats flying and it got me thinking.. smaller engines are cheaper, smaller, and lighter thus could maybe get me where i need to be with out pushing almost all of the cost into a darn engine.
     

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