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  1. #1

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    THRUST VS. PLANE WEIGHT?

    I have read that with a 17-6 (or 17-8?, I forgot) prop a DLE 20 has about 14 pounds of thrust. If this is true and if this engine is installed in an airplane that weighs less than 14 pounds, say 13 pounds, would the airplane have unlimited vertical, or is this oversimplifying things??

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    RE: THRUST VS. PLANE WEIGHT?

    Greatly over simplified. Unlimited vertical generally arrives when you have greater than 1.5-1 P/W ratios At less than 1-1 or only slightly greater you will be able to climb, but not unlimited. You still need to overcome drag.
    Never lie, and never minimize or gloss. Tell it like it is and let the pieces fall where they may. The truth always wins.

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    RE: THRUST VS. PLANE WEIGHT?

    Yes. If the thrust is more than the planes weight, even by a small margin, the plane will be able to climb with the nose pointing straight up, and it will keep going until the air thins a little (say 5-10,000 feet) causing the engine to lose power, then it stops climbing. In practice, a plane that can barely climb will be more likely to want to fall off to one side or the other, so a degree of overpowering is desirable for true vertical performance from rest (eg: to pull smartly away from a prop hang into fast straight up vertical flight). Generally for this kind of performance - as seen typically in 3D type aerobatic aircraft you want about 50% more thrust than the weight of the plane.

    However, even if the thrust is a little less than the planes weight - say 14lb thrust on a 15lb plane, then pulling up vertical from level flight will see the  aircraft going a long way vertically before its speed falls off.

    Thrust can be calculated with maths for every prop at a known rpm, or by entering the known rpm in any of the on-line thrust calculator programs, or more directly by simply holding the plane back with a spring balance scale during full power runnning.

  4. #4
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    RE: THRUST VS. PLANE WEIGHT?

    Very large can of worms
    (But far better than an oil thread - at least this can be interesting )

    I will use simple terms here to explain it

    To move the plane vertically requires the engine to overcome a few things.
    1 - the weight of the plane
    2 - the Acceleration force to actually make the plane climb (even a steady rate climb is an acceleration)
    3 - the Drag of the plane -
    That Drag includes
    A - The induced drag caused by the prop wash
    B - The drag from the control surfaces being used
    C - The "effective drag" caused by things such as engine thrust angles

    To make it really simple to work out?
    Use a Thrust to weight Ratio of at least 1.5:1 and you will have "unlimited vertical" for practical purposes.

    3W, BME and DA all the way. Proven power, Proven reliability and Proven support equals much cheaper in the long term.

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    RE: THRUST VS. PLANE WEIGHT?

    1:1 thrust is fine for a non-performance application. Energy management is still very critical because there is nothing left to counter drag once the nose is pointed up. All my old planes with 1:1 would generally climb vertically about 100' from take-off or 200" from high speed and then just stop until it fell over. All that was a long time ago when planes tended to be very heavy. Nobody has to learn energy management these days and it shows by how many crash when their engine quits.

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    RE: THRUST VS. PLANE WEIGHT?

    So my 71/2 pound Reactor Bipe with a DLE-20 with a Zoar 17x6 prop with 14+ pounds of thrust should be ok?

    Steiny

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    RE: THRUST VS. PLANE WEIGHT?



    That 6" prop will loose thrust very quickly as soon as any plane speed enters the picture.



    Those thrust figures can only be calculated well at static plane condition (no forward speed). Between zero speed and pitch speed*1.1 the thrust falss off to zero. So with a pitch speed of 50 mph, at 26 mph you are at 7lbs thrust. Add some extra drag because the plane has sped up, and you are left with only 6lbs of thrust. 



    The plane will go vertical unlimited, but probably not like on rails because your prop pitch is too low.

    Pe, (www.mvvs.nl), MVVS, MOKImotor, RCexl, MTW, Xoar, Mejzlik.
    Blessing in ignorance? There is sanctuary in analysis.

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    RE: THRUST VS. PLANE WEIGHT?

    I guess my question is this: I'm building a 4*120 which should weigh around 12.22-12.75 pounds when done. If a DLE 20 has 14lbs of thrust with a 17-6 prop, then will the plane have plenty of power? I've had some say the DLE 20 would have more than enough power, some say it would be light on power, some say a DLE 30 would be better, some say the plane would be ridiculously overpowered with a DLE 30. The answers I've gotten are all over the board...

  9. #9

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    RE: THRUST VS. PLANE WEIGHT?


    ORIGINAL: Tired Old Man

    Greatly over simplified. Unlimited vertical generally arrives when you have greater than 1.5-1 P/W ratios At less than 1-1 or only slightly greater you will be able to climb, but not unlimited. You still need to overcome drag.
    What a drag drag is. Mean O'l Mr. Gravity....
    KEEP UM FLYIN

  10. #10

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    RE: THRUST VS. PLANE WEIGHT?

    ORIGINAL: psgugrad

    I guess my question is this: I'm building a 4*120 which should weigh around 12.22-12.75 pounds when done. If a DLE 20 has 14lbs of thrust with a 17-6 prop, then will the plane have plenty of power? I've had some say the DLE 20 would have more than enough power, some say it would be light on power, some say a DLE 30 would be better, some say the plane would be ridiculously overpowered with a DLE 30. The answers I've gotten are all over the board...

    You need to define "plenty of power". For a high lift wing, as found on a 4*, then yes, there would be an abundance of power. Don't forget that most full scale aircraft fly with power to weight ratios of 0.2 to 0.4-1. So it will fly with significantly more power than a full scale would, and require very little, if any, energy management. The plane will fly with much greater authority than it would have with "scale" P/W ratios.

    However, if you wanted to do a lot of 3d stuff, which a 4* is most assuredly not designed for, there would be a need for greater power levels. Lest we forget, at 1-1 a plane can hold it's own, hover but little in reserve for a pull out. So energy management and maneuver planning would be required, or more appropriately, the user would have to "know how to fly".

    We have a modeling "generation" that has learned how to fly with propeller thrust and very large flight surface areas as the primary means of establishing maneuverably. If the planes did not leap off the ground without any appreciable T/O roll they are considered underpowered. Nothing could be further from the truth because even the average trainer .40 is over powered, but they serve their purpose and fly extremely well. This generational definition of required power is grossely incorrect because few full scale aircraft need the same P/W for flight. It's how we fly the planes the power requirements. If flying a warbird or something like a Cub or T-Craft a 1.5 or 2-1 P/W ratio is absolutely ridiculous and unneccessary.

    Only 3d requires high P/W ratios, and if those that fly purely 3d ever try to fly scale they'll find themselves in a quick world of hurt. better to really learn how to fly and not just aim a propeller. For those that find a little rub in that you should know I'm primarily a 3d flyer, but also have no problem flying large, heavy aircraft with a minimum of available power (or NO power) when the need or desire arises.
    Never lie, and never minimize or gloss. Tell it like it is and let the pieces fall where they may. The truth always wins.

  11. #11

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    RE: THRUST VS. PLANE WEIGHT?

    You will need a DLE30, or a MVVS26 for that weight, and it will be marginal for 3D, especialy if you insist on American style in cowl mufflers.
    Just to putter around, the DLE20 will be fine.

    ORIGINAL: psgugrad

    I guess my question is this: I'm building a 4*120 which should weigh around 12.22-12.75 pounds when done. If a DLE 20 has 14lbs of thrust with a 17-6 prop, then will the plane have plenty of power? I've had some say the DLE 20 would have more than enough power, some say it would be light on power, some say a DLE 30 would be better, some say the plane would be ridiculously overpowered with a DLE 30. The answers I've gotten are all over the board...
    Pe, (www.mvvs.nl), MVVS, MOKImotor, RCexl, MTW, Xoar, Mejzlik.
    Blessing in ignorance? There is sanctuary in analysis.

  12. #12

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    RE: THRUST VS. PLANE WEIGHT?

    Pe,

    A 4* has a modified Clark Y airfoil. It's got more lift capability than it does weight by a large margin. It won't have unlimited aerobatic capability with a 20, which a 30 would provide, but the weight of a 30 is a lot on a 4*. Then again, my opinion has a 20 as too much weight also but what can one do? When they first came out we were flying them (kit builds) with Saito 1.50's and they had pretty much unlimited aerobatic capability. Even an OS 1.20 pumped 4 stroke was more than enough for them at heavier kit build weights.
    Never lie, and never minimize or gloss. Tell it like it is and let the pieces fall where they may. The truth always wins.

  13. #13
    Moderator w8ye's Avatar
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    RE: THRUST VS. PLANE WEIGHT?

    The "Four Star" has a simi symetrical airfoil and a Clark Y is a flat bottom
    Attended the CutFinger Institute of DirtNap University for years but never did graduate....
    Recipient, Mangledhand award August 2008
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  14. #14

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    RE: THRUST VS. PLANE WEIGHT?

    I kinow the 4*. The airfoil is like an Eppler 197! It flies exceptionally well on the wing with a MVVS .61! We had one in our club thus powered, and it flew a modest Imac program.
    Having said that, there is the "BUT". The guy said 12 - 13 lbs flight weight which is heavy for the 4-star. If you want to 3D that kind of weight, you will need a bit more power than that so it will fly on the prop.
    I will fly any plane if it has a static thrust of half the plane weight, or even less power if the plane is designed for it.

    BTW, did the oils arrive?


    ORIGINAL: Tired Old Man

    Pe,

    A 4* has a modified Clark Y airfoil. It's got more lift capability than it does weight by a large margin. It won't have unlimited aerobatic capability with a 20, which a 30 would provide, but the weight of a 30 is a lot on a 4*. Then again, my opinion has a 20 as too much weight also but what can one do? When they first came out we were flying them (kit builds) with Saito 1.50's and they had pretty much unlimited aerobatic capability. Even an OS 1.20 pumped 4 stroke was more than enough for them at heavier kit build weights.
    Pe, (www.mvvs.nl), MVVS, MOKImotor, RCexl, MTW, Xoar, Mejzlik.
    Blessing in ignorance? There is sanctuary in analysis.

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    RE: THRUST VS. PLANE WEIGHT?

    Not yet. You did include a billing with those, I hope? The timing would be just about perfect if they arrived in the next week or so.

    I have a difficult time picturing someone performing 3d with a 4*. Almost seems a travesty but it's been done with Cubs and T-Crafts. I personally believe there's some things that are and were meant to be flown in a anner that's non 3d.
    Never lie, and never minimize or gloss. Tell it like it is and let the pieces fall where they may. The truth always wins.

  16. #16

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    RE: THRUST VS. PLANE WEIGHT?

    I'm guessing 12.25 - 12.5 pounds ...yes that's a bit heavy for a 4*120 but remember that's with a gas engine and an extra battery pack. Wish I could hear from a guy who's put either a dle 20 or dle 30 in a 4*120.

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    RE: THRUST VS. PLANE WEIGHT?

    I forgot to add: I'm a mediocre pilot so I won't fly 3-d anyway....I just poke holes in the sky. I'd like to use the dle 20 as I prefer the beam mounts....just concerned about the plane being underpowered. My first plane was a Kadet LT-40 with a .61fx and the 2 seemed a great match. Never once did I say to my self "this thing is way overpowered" It seemed just right and couldn't imagine what a dog it would have been with an fp40!

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    RE: THRUST VS. PLANE WEIGHT?


    ORIGINAL: psgugrad

    I forgot to add: I'm a mediocre pilot so I won't fly 3-d anyway....I just poke holes in the sky. I'd like to use the dle 20 as I prefer the beam mounts....just concerned about the plane being underpowered. My first plane was a Kadet LT-40 with a .61fx and the 2 seemed a great match. Never once did I say to my self ''this thing is way overpowered'' It seemed just right and couldn't imagine what a dog it would have been with an fp40!
    If you think the LT-40 with a.61 in it is a "great match" then you'll need the DLE30 in that 4*! Dang.... my wife's LT-40 with a .46 in it is way overpowered at just under a 1:1 p/w ratio. Seriously, if it were mine, I'd put the 30 in the 4* .... if not for the power .... the larger prop disc will give noticable prop braking on steep landing approaches and downlines. You'll need a taller landing gear though so consider that when you make the final engine choice.

  19. #19

    RE: THRUST VS. PLANE WEIGHT?

    The DLE20 will fly the 4 Star the way it was designed to fly, the DLE30 makes it ridiculous as it doesn't 3D for crap, it's a waste of power and prop clearance could be an issue if you fly from a grass runway.
    Now go outside and play

  20. #20
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    RE: THRUST VS. PLANE WEIGHT?

    Yep, I had one with a Zenoah G26 on it and for my tastes as a sport flyer that was more than enough. It would even hover OK. I just feel from your description of your LT-40 you would be happier with the 30. IE Grossly overpowered.
    Spektrum DX8i, DA DLE SuperTigre OS FOX Saito Enya Jett TT: John 3:16

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    RE: THRUST VS. PLANE WEIGHT?


    ORIGINAL: jedijody

    The DLE20 will fly the 4 Star the way it was designed to fly, the DLE30 makes it ridiculous as it doesn't 3D for crap, it's a waste of power and prop clearance could be an issue if you fly from a grass runway.
    Jody, I tend to agree with you but I felt the OP really had his mind made up in advance that the 30 was the best choice! My original post was perhaps a poor attempt at poking fun at that choice.

  22. #22
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    RE: THRUST VS. PLANE WEIGHT?


    ORIGINAL: aussiesteve

    Very large can of worms
    (But far better than an oil thread - at least this can be interesting )

    I will use simple terms here to explain it

    To move the plane vertically requires the engine to overcome a few things.
    1 - the weight of the plane
    2 - the Acceleration force to actually make the plane climb (even a steady rate climb is an acceleration)
    3 - the Drag of the plane -
    That Drag includes
    A - The induced drag caused by the prop wash
    B - The drag from the control surfaces being used
    C - The "effective drag" caused by things such as engine thrust angles

    To make it really simple to work out?
    Use a Thrust to weight Ratio of at least 1.5:1 and you will have "unlimited vertical" for practical purposes.

    Wow thanks,, I learned (realized) something today.
    There is always someone that has thought about something
    more than I have, cool, Thanks Tim

    You know your landing gear was up when it takes full power to taxi to the ramp.

  23. #23

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    RE: THRUST VS. PLANE WEIGHT?

    I think we get too wrapped around the power thing. Enough power is all that's really necessary, but we have to define what we want to do with an amount of power. With gas engine specifically there's all too often the perception it's all about doing 3d stuff, when there are so many other types of flying that require much less power. Somebody has a light whiz bang 390 that he wants to hang on a prop and torque roll while the guy at the next flight station has a 48 pound Bf 109, and the guy in the next one has a 160" span Taylor Craft. Then you look back at the beginning of the flight line and you see the guy with the Trainer 40 and right next to him is a guy with a pylon mounted .048 on a 72" span glider. Not a one of them has the same power requirements.

    So many different needs but so often completely misunderstood. Sometimes it's fun to just sit back and consider all the opportunities.
    Never lie, and never minimize or gloss. Tell it like it is and let the pieces fall where they may. The truth always wins.

  24. #24
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    RE: THRUST VS. PLANE WEIGHT?

    I love 3D and all things over-powered, but I think for sport flying on that plane, the DLE20 would be great! It is called the 4 Star 120 after all...

    I maidened 2 planes for a buddy this past summer: a Phoenix Decathlon with a Magnum .52, and a TF Cessna (81" wing) with a Magnum .91. Both were at the low end of the recommended power by the manufacturers. The Decathlon was a turd; it would barely pull a loop, even with a slight dive entry, but the Cessna suprised me. It had plenty of power to pull a loop and flew very "scale", even somewhat sport-like. Both, however, flew "on the wing" and weren't bad to fly at all. By all reports, the DLE 20 is a match for 4 stroke 1.20's and then some, so it should fly that plane quite well...
    Crashing my way to being a better pilot!
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  25. #25
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    RE: THRUST VS. PLANE WEIGHT?

    I saw a Four Star 120 with a Super Tiger 90 and it flew fine for a fly around sport plane.
    Attended the CutFinger Institute of DirtNap University for years but never did graduate....
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