What is wrong with my plane?

[Colombo]

Hello,

I am total noob in KSP. I am trying to build an early plane that could pump me higher than 10k meters with basic engines.

However, it is highly unstable. So my question is: What is wrong with my plane? How could improve it?

Thank you.

[InterCity]

Hello and welcome to the forums.

I can't tell you for sure unless I see a screenshot with center of mass and center of lift visible, but planes are usually unstable when their center of mass (the yellow ball in editor) is too close to center of lift (the blue ball). I'd suggest either moving your wings slightly backward or moving your fuel tanks forward. Also, straightening up the wings might help, as it seems you have a slight V there. If nothing helps, tell us.

[p1t1o]

It is much easier for short aircraft to be longitudinally unstable, the first thing that I would do is make it slightly longer so that I can add a seperate tailplane behind the wings.

Tailless designs are fine, but they generally dont give you great pitch authority - either the control surfaces are too close to the point of rotation (CoM) giving very little moment arm, or the wing is so far back as to make the aircrat TOO stable (plane wants to just fly in a straight[ish] line whatever you do). You can add canards to give more pitch control, but one thing at a time, yes

As always, when making these adjustments you want to keep an eye on the CoM/CoL as mentioned above by managing where mass goes - the most common way to do this is manage where you put your fuel - not every tank needs to be full, but you also want to keep an eye on how the CoM changes as fuel is used up, try and keep fuel close to the CoM, where possible - it seems like there are a lot of contradictory rules, but finding the sweet-spot is the challenge

Also, I'd get rid of that rocket engine, and its associated fuel - for your mission, you'd rather save weight than add thrust, its probably making your CoM/CoL relationship a bit difficult too.

[capi3101]

Gonna echo what everybody else has said so far; from the look of things, you've got too much mass aft, to the point where your center of mass is behind your center of lift. You need the opposite to be true. Lengthening the fuselage would help to an extent (try a piece of structural fuselage if you have it), as would sticking a full fuel tank ahead of the cockpit.

I would also say that p1t1o is correct about the rocket engine - unless the purpose of your flight is to test that engine, ditch it. He's also right about the pitch authority issues that typically come with a tail-less plane design (i.e. no aft horizontal stabilizers); you might try canards (a pair of AV-R8s would do just fine; you don't need the "dedicated" canard parts) if that issue crops up. Though, I would advise against the canards until you've got your initial stability issues under control...

And if you are just doing the rocket as a test, it doesn't need fuel - you can deny it fuel, set its thrust limiter to zero, activate it through staging and it'll still count. If you don't need a rocket, consider changing the setup to a single engine - you don't need a lot of engine for a plane that small.

One final note about the Wheesley engines - if you're wanting to go much past Mach 1 (300 m/s, give or take), forget it. Those things top out around that speed.

[ForScience6686]

Just a guess here, but the use of the rocket engine is probably shifting your com back causing the instability. I would also guess that you have excess wing, I would just go with the delta wing in that setup and remove the extra segments. For early planes I went with more of a conventional plane design and no rockets until you get the better jet engines.

Edited September 10, 2015 by ForScience6686

[Chaos_Klaus]

As the OP mentioned, the rocket is there to get above 10km with the tech at hand. So it is absolutely sensible to use a rocket engine for high altitude flight. NASA did the same.

Also, the V shape of the wings actually makes the craft more stable. It's called dihedral wings.

Your real problem is propably the relation between CoL and CoM. CoM always needs to be in front of CoL. Also note that CoM is shifting while you burn your fuel. You can toggle these markers by pressing the weight and wing symbol in the lower left of the VAB screen.

[InterCity]

Also, the V shape of the wings actually makes the craft more stable. It's called dihedral wings.

In KSP, the aerodynamic model is not so precise, and from my experience it seems that dihedral wings only decrease lift in it. It's indeed correct that it works IRL.

About the rockets, I forgot to mention that, they might be actually better than jets in terms of stability, because they are lighter and will help you move the CoM forward - but it's a nonsense to have rockets AND jets.

[p1t1o]

As the OP mentioned, the rocket is there to get above 10km with the tech at hand. So it is absolutely sensible to use a rocket engine for high altitude flight. NASA did the same.

Also, the V shape of the wings actually makes the craft more stable. It's called dihedral wings.

Your real problem is propably the relation between CoL and CoM. CoM always needs to be in front of CoL. Also note that CoM is shifting while you burn your fuel. You can toggle these markers by pressing the weight and wing symbol in the lower left of the VAB screen.

Sure, but he's only going for 10km which should be easily be reached without it and it complicates the design.

Proper usage and positioning of dihedral wings can give a certain roll stability, but there is a question as to whether that works/is worth it in KSP, IIRC. The nature of the stabilising effect is not simply "roll stability", it involves roll/yaw coupling and improperly positioned/angled dihedral can result in various effects which can lead to loss of control.

[Colombo]

Thank you for your responses.

p1t1o: I apologize, if I did not spelled it correctly. But I truly need to fly above 10k, somewhere around 20k, with basic engines, as missions require it. It is truly just to push it up, above 10k, for a while. I found that it can get easilly into spiral. Is there something that I can do to alleviate this?

How I can know how many lift I need for my plane? (so if my wings are sufficient), is there mod that can visualize it? (so I wouldn't have to count that by hand).

Thank you.

[tbarcello]

Anyone else is thinking "I've seen this before!" ?

http://forum.kerbalspaceprogram.com/threads/133395-What-s-wrong-with-my-plane

Search feature is broken again

[Martin0]

IMO your Center of lift is too near your Center of mass, burning fuel the col moves forward and your plane becomes unstable.

You also appear to have too much control surfaces.

Try to make those changes:

[ForScience6686]

Thank you for your responses.

p1t1o: I apologize, if I did not spelled it correctly. But I truly need to fly above 10k, somewhere around 20k, with basic engines, as missions require it. It is truly just to push it up, above 10k, for a while. I found that it can get easilly into spiral. Is there something that I can do to alleviate this?

How I can know how many lift I need for my plane? (so if my wings are sufficient), is there mod that can visualize it? (so I wouldn't have to count that by hand).

Thank you.

http://i59.tinypic.com/2vifok6.png

I tried the same thing in early career for survey contracts and it was difficult on the basic engines. I held off and waited for higher tech, just my opinion though. I would guess you lose control when using the rocket engine. If so, drain your rocket fuel tanks in the sph to see how the com is affected. As far as lift, I think last I read was 1lift rating per 10 tons, could be wrong on that. I usually use test flights to verify enough lift. I would drop the wing segments and just use the delta to decrease drag,and I think you need to remove the second set of tail fins, the delta wing is your horizontal stabilizer in this case so you only need vertical. Also make sure only the outer control surface is set to roll, the inner should just be pitch, and then tail to yaw only. It would help to know when you are experiencing problems during flight.

[capi3101]

1 lift per 10 tonnes is the recommended amount for stock aero these days. For FAR, I have my own set of formulas that I use, and they aren't straightforward ones (there's a square root involved and everything)...

Mod? I'd suggest RCS Build Aid; if you're going to get serious about planes, I'd almost consider it essential. While the primary function of that mod is to help you set up RCS blocks so that your craft doesn't torque all over the place when you're docking, one of its features is a "Dry Center of Mass" marker, which shows you where your plane's CoM will be once it's out of gas - which in turn allows you to infer which direction your CoM will shift as the plane's fuel drains. You can use it to set things up so that either A) you can place your wings to where your plane will never lose longitudinal stability or not shift its CoM in flight appreciably.

Of course, that mod doesn't estimate how much lift you need. But it is a suggestion that might help out in this case.

[p1t1o]

Thank you for your responses.

p1t1o: I apologize, if I did not spelled it correctly. But I truly need to fly above 10k, somewhere around 20k, with basic engines, as missions require it. It is truly just to push it up, above 10k, for a while. I found that it can get easilly into spiral. Is there something that I can do to alleviate this?

How I can know how many lift I need for my plane? (so if my wings are sufficient), is there mod that can visualize it? (so I wouldn't have to count that by hand).

Thank you.

http://i59.tinypic.com/2vifok6.png

I would highly recommend the mod FAR - "Ferrams Aerospace Research".

Be aware that the main purpose of this mod is to greatly modify the aerodynamics simulation to more accurately resemble real life. Craft designed for stock aero will not necessarily fly with FAR, and vice versa.

However, this means that all of the "real-world" concepts for designing aircraft are better represented and will work out better, it makes designing a craft a bit more predictable, instead of having to make allowances for stock behaviour.

What FAR also gives you are lots of tools to help you - you need these because you it will also now simulate more effects that you will need to consider. For example, if you are building an aircraft intended to fly supersonic, you might want to take area-ruling into account to reduce drag in the supersonic regime (again, there are graphical tools to help you do this).

Dont worry though, its not too intimidating, it is entirely possible to cobble together flying craft without endlessly examining graphs and numbers!

The tools will be able to tell you things like how much lift you are producing, how stable your craft will be, how well you have done your area-ruling and many more things besides. One very useful thing I just discovered is that it will tell you, for a given speed, what angle-of-attck you need to stay in the air. If it is low, like 1-3 degrees, you know you are generating plenty of lift at that speed, if it is high, like 15 degrees say, then you know that something is amiss, maybe you are too heavy, maybe you need a bigger wing.

In-flight there are options to graphically display the lift and drag being produced by all the parts on your craft, it is quite a powerful mod.

As for spiralling on trying to achieve 10k, there are any number of reasons for this:

You could be unable to go fast enough to generate enough lift - I agree that reaching 20k on basic engine might be a push, but you should be able to make 10k, reaching 20k could possibly be done with a larger, 4 engine design.

Your fuel usage could be shifting your CoL/CoM relationship, damaging your stability.

You could be trying to go too high too soon, before you have built up enough speed - another thing the FAR tools will tell you is the lowest speed that will generate enough lift to keep your currently aircraft in the air.

The rocket engine - it is totally valid to have a rocket onboard, but it does add a lot of weight for an air-breather. It might boost you to a higher altitude, but once it is out of fuel, you may not be able to stay there if your aircraft is now in a speed/height regime that it cannot fly in (especially with the now-dead-weight rocket). It will also be very easy to over-speed, at worst causing total breakup, at best causing unexpected aero effects, for example, if your craft is not designed to be supersonic and your rocket boosts you to Mach 3! It would be something I would leave out until you are more confident.

One last tip, especially if you move to FAR (Im not sure if it is simulated in stock) - long, narrow wings (a low "aspect ratio" - ratio of length to average "chord" [the distance from trailing edge to leading edge]) have a much better lift/drag ratio than stockier, wider wings, delta wings included. This means it can help you to achieve higher altitude with slower aircraft and less powerful engines.

If you are at all serious about flying in-atmosphere in KSP, FAR is a must. No I am not being paid by them

Edited September 11, 2015 by p1t1o

[Tarheel1999]

IMO your Center of lift is too near your Center of mass, burning fuel the col moves forward and your plane becomes unstable.

You also appear to have too much control surfaces.

Try to make those changes:http://i.imgur.com/wbQO0mB.png

Another way to say this is the plane will become more unstable if CoM moves backwards when dry. You need to shift your your CoL backwards a little if this is the case. Try sliding the winglets on the front backwards a little. Changing control surfaces will improve control but is not necessary. Your fundamental problem is CoL is sitting right on top of CoM and you haven't factored in fuel drain. You have plenty of lift for that design. I would avoid installing FAR until you feel comfortable with stock aero.

Things to keep in mind:

1) The CoL should always be behind CoM.

2) The closer CoL is to CoM the more unstable/maneuverable the plane is.

3) Conversely, the further the CoL is from the CoM the more stable/less maneuverable the plane is.

The trick is finding the right balance of CoL and CoM.

Edited September 11, 2015 by Tarheel1999

[Alshain]

I would also remove that inner wing piece all together. You shouldn't need so much wing for a Mk1 plane. It's a fine balance, too little and you won't have enough lift, too much and you have more mass you have to lift. If you remove the canards and the inner wing, it should balance out and your CoL will be close to the same place. Then tweak the position of the wings to bring it back behind the CoM.

Edited September 11, 2015 by Alshain

[Val]

IMO your Center of lift is too near your Center of mass, burning fuel the col moves forward and your plane becomes unstable.

You also appear to have too much control surfaces.

Try to make those changes:http://i.imgur.com/wbQO0mB.png

I agree with most of your advice. Except, in my opion, emptying tanks of fuel is a bad strategy for making a craft stable. It will not change the Dry CoM and it will still be unstable when the tanks are getting empty. Dry Mass need to be moved forward or wings backward.