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gonzxor5

Plane wont fly, help please?

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I've tried reading around, cant find anything to fix this. In the picture below the plane will not take off at all, until about 200m/s and then it flops backwards. If I put the center of lift more forward then it refuses to take off at all, if I put it more backwards it flops backwards at lower speeds. How do I fix this? Thank you :)

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You must make sure that center of mass is never behind center of lift at any stage of the flight. To me by looking at the picture I suspect that there is good possibility that goes back. You can test that by moving around fuel sliders and see. 

Also if you are just beginning with planes and SSTOs I suggest that you try first with smaller parts and see what it takes to get to orbit. Like Mk1 and 2 parts. And experiment it in sandbox mode.

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Avion on the wings is too small for your plane so it has a hard time lifting off. Also you dont need rocket engine. Try other jet engines. (im assuming u want to make airplanes and not SSTOs)

http://wiki.kerbalspaceprogram.com/wiki/Jet_engine

Also this is a very good read. It will make your plane fly wonder.

http://forum.kerbalspaceprogram.com/index.php?/topic/47818-basic-aircraft-design-explained-simply-with-pictures/

Edited by Kerbalmann

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2 hours ago, gonzxor5 said:

In the picture below the plane will not take off at all, until about 200m/s

I suspect that this is because you don't have enough control authority in the back for controlling your pitch. Also, make sure your back landing gear is almost right in line with the CoM, it should take very little effort to pivot your plane around the fulcrum point that is the back gear. 

2 hours ago, gonzxor5 said:

and then it flops backwards.

My guess for this part is that the CoL marker is deceptive in that it only shows the lift force, but doesn't take into account the drag forces from that big long fuselage being at a high angle of attack. 

So basically, your small control ability can't lift the nose until you're going fast, by which point any AoA the fuselage has will contribute huge drag forces, and the small control surfaces can't counter it. 

So I would suggest two things. First, put more control surfaces at the back for better pitch authority. Second, give your main wings some angle of incidence. This is where you angle your wings so that the front is slightly higher than the back, giving them some angle of attack while the rest of the plane is at 0 AoA. 

Hope this helps! 

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It appears you made the ship with the Space Shuttle in mind. Nothing wrong with that, but it's flight performance was related to a 'flying brick' for a reason! :P

I'm assuming you want to just make a regular plane to fly around the Space Centre and Kerbin. For this purpose, your ship is rather large and heavy, not to mention fuel-inefficient (I can't imagine you'd get much burn time out of that rocket engine). To fix this, you're going to want to lighten the load significantly. Use only one or two liquid fuel tanks, and replace the rocket engine with a jet engine. This lets you fly for longer and not have to carry as much fuel, reducing the amount of mass you have to lift (rockets, while very powerful, require that heavy Oxidiser, which is superfluous in Kerbin's atmosphere). I'd also recommend going down to a MK 2 or even MK 1 fuselage, as the MK 3 ones tend to be very large and heavy, useful more for very large ships. Lastly, if I've misjudged the mission of your ship and you do want a rocket plane, just let me know and I can revise this advice.

Moving on. For takeoff, I can see three problems from the picture. Firstly, it appears your landing gear are set up such that the nose points down on the runway (or in more aircrafty terms, a negative angle of attack). This is a good thing for ships which need to land safely (think the Space Shuttle), but will make lifting off extremely difficult at anything less than super high speeds, as you've seen. I suggest using smaller rear landing gear and a larger nose one, which should let the ship sit level on the runway, or even pitch the nose forward. Next, the rocket engine is attached further upwards of your centre of mass; this will produce a negative moment on the runway, and essentially fight any attempts at pitching the nose upwards. Move that down to be more in-line with the centre of mass, or follow my above advice to swap it out for a few smaller jet engines. Lastly, the wings of the ship are quite small as well. The Space Shuttle had small wings as well, but its primary goal was to land, and even with that it behaved like a flying brick, as above. You want larger wings for sure, try experimenting with the structural wings, I find the BigS wings are difficult to use in anything but Shuttle replicas. If you've followed all this advice yet you're still having takeoff problems, add a pair of canards to the nose of the ship. These will have the side-effect of making the ship more manoeuvrable, but they help immensely in getting ships off the runway sooner. They aren't magic though, and if you just slap canards onto that design it will likely still flip out.

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Just now, CalculusWarrior said:

It appears you made the ship with the Space Shuttle in mind. Nothing wrong with that, but it's flight performance was related to a 'flying brick' for a reason! :P

Ha yes, I took some looks from the space shuttle but not a 1 on 1 replica. Thank you for the advice. I will try to rebuild my shuttle to be more light and less bulky. I should have put a jet engine on as well, but I'm lazy and wanted lots of acceleration.

 

Just now, FullMetalMachinist said:

So I would suggest two things. First, put more control surfaces at the back for better pitch authority. Second, give your main wings some angle of incidence. This is where you angle your wings so that the front is slightly higher than the back, giving them some angle of attack while the rest of the plane is at 0 AoA. 

Thanks! I will try it out :)

Edited by gonzxor5

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Mk3 planes are hard to make. And having a big engine doesn't help very much. Some ideas:

  1. Just fly faster. Dangerous, but it works. 
  2. Add bigger flaps. Like waaaaay bigger. 
  3. Do it humblebee style. Ride a twin-boar LF booster to orbit, and manually throttle it using thrust limiter. @Kuzzter did it, but it's hard.

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For a quick fix, you could try emptying most of the rear fuel tank, keeping the front tank full. This will move the centre of mass forward and reduce your take-off weight and increase your runway acceleration. That might be enough for to get a short flight from your design.

For more detailed design advice, there's a nice simple pictures tutorial.

Good luck!

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Before finalizing your design, click on the rotate tool and turn the root part (i.e. turn the whole vessel) with the CoM and CoL markers active, to see what happens at different angles of attack.

This will tell you how stable it is: if CoL moves forward a lot when you do this, it means that when you start to climb, the change in centre of lift will force the craft to keep on climbing and, in the absence of rapid reactions and plenty of control surfaces, you will flip.

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