please help with airplane design

[hawk_za]

i cannot understand what i am doing wrong i designed a plane using only the most entry level parts

my center of lift is slightly behind my c.o.m and my c.o.t is inline and centered with the c.o.m

 

yet upon takeoff the craft will always veer to the right and oscillate out of control ..

it's almost like the tail lifts and the nose starts to wheelbarrow

yes thew landing gear is as straight as i can perceive it to be

http://s000.tinyupload.com/?file_id=48906261464886715871

link to my craft

this is a problem i have had ever since the introduced flight to ksp and i cant be this stupid so seeing as i am,

 

i need help

any advice i can get i would appreciate

 

one extremely frustrated hawk

[DrLicor]

hey, I'll be glad to help, but can't test your file right now.

Can you upload some screens with the CoM, CoL and CoT?

 

 

Also take a look a this, it will help you:

 

Edited September 20, 2016 by DrLicor

[Val]

31 minutes ago, hawk_za said:

it's almost like the tail lifts and the nose starts to wheelbarrow

If the main landing gear is not almost directly below CoM, then that's basically what's happening.

For a landing gear with tricycle setup, put the nose wheel as far forward as possible and main gear below CoL.

Edited September 20, 2016 by Val
Clarifications

[hawk_za]

Thanks drlicor have that as a jpeg on my desktop and have watched numerous you tube vids in an effort to id this problem having said that i am currently on build 1.2 which i know is experimental but this is a long standing issue ive been having regardless of version no which is what bring me to the conclusion im the idiot

http://feed1.tinypic.com/rss.php?ua=VTyuneK6W3l8d%2BG%2BJ7AX8w%3D%3D

album for pics of craft

hi val i just tried your advice and it did not help same problems persisted loss of control at 80 m/s due to unknown oscillations and veering

[DrLicor]

Alright, had a look at your screens.

Firstly I would make sure your center of lift is further to the back, your plane will be more stable.

Also recommend to adjust the place of your landing gear. Make sure there just a bit behind the center of mass. 

Are you in career, because I would wait a bit untill you researched better plane parts. 

The plane youre trying to build is quite small, wings a bit to big for the size of the body, etc.

And I never had any great landings with those.

 

I'll post some screenshots of crafts I made later on.

Edited September 20, 2016 by DrLicor

[hawk_za]

okay so i did attempt the advice you game me make a mk 2 now it appears to veer off only when i try lift the craft using the s key

 

here are the images

http://tinypic.com/a/b3tvs5/1

its almost as if the plane does on have enough lift

[DrLicor]

Mk2 map shows the same images..

Did you use another landing gear?

The lift is good on the first plane, maybe a bit more control surfaces.. like flaps on your tail

[hawk_za]

hey drlicor i just wanted to say thanks for taking time out of you day to help me with this

http://tinypic.com/a/b3tvs5/1

okay tested the link should be good.

craft does look similar but I'm determined to create a flybal craft with these parts as otherwise my career mode gets allot harder as its purpose is to collect science from the rest of kerbin as for landing thats what the parachute is for :>

[capi3101]

Are you adjusting the friction settings on your gear at all? If not, try 0.4 for the nose gear and disable the brakes. Use 1.4 for the rear gear and set the brakes to full (200%, I mean).

Myself, I'd use a few pieces of structural fuselage to lengthen your plane, and then add a dedicated tail with pitching surfaces aft and as far above the main wing as you can manage. As it is, you're not getting a lot of torque from those elevons you've got - they are WAY too close to your CoM - so your pitch authority is going to be absolute dreck. I'd wager your initial assessment of your plane is correct: the rear of the plane is lifting first, causing the front gear to wheelbarrow before you've reached takeoff speed. 

If you don't have structural fuselage, try a drained FL-T400 tank instead. You can even keep some of the of the oxidizer in it if necessary if it turns out you need additional ballast up towards the nose.

Edited September 20, 2016 by capi3101

[Rodhern]

Surely much of the KSP fun comes from building contraptions more exotic than the their classic real world counter parts. Nothing wrong with that. But, you may be raising the difficulty level a fair bit by not having a more traditional tail plane. All pitch control for your plane is done by the wing, the wing attached control surfaces and reaction wheels, right?

edit: I was too slow, capi3101 already made a better explanation of the same thing.

Edited September 20, 2016 by Rodhern
obsolete answer

[DrLicor]

Haha you're welcome  

Alright..

Usualy what I did when I wanted to collect science from kerbin was this:

MK2 as cockpit, 2 long MK2 fuselage, a short mk2 cargobay (to put the experiments in), mk2 bicoupler with 2x J-404 "Panther" Afterburning Turbofan.

2 tail fins on top of the bicoupler, for the yaw

For wings I used procedural wings, so I don't know the specs... place the wings from the back to 2/3 from the back. Make sure you add big elevations on the wings. The wings should like like this(only the shape!):

 

As landing gear the biggest of the stock game. (1x LY-60 Large Landing Gear and 2x LY-99 Extra Large Landing Gear)

1x  LY-60 Large Landing Gear in the front

2x LY-99 Extra Large Landing Gear on the wings just behind the CoM (main gear)(play a bit with the height, to level the gear)

If the plane doesn't takeoff well, you can always add 2 fins on the cockpit.

 

I'll post some screens of my plane later this day. 

 

 

 

[Val]

 

Judging by above picture, the issue is either

• Inverted controls because of swept wings. The hinge axis of the Control Surfaces goes right through CoM, so it can't decide whether it should act as if it's mounted in front or behind.
• Elevators are too close to CoM, so they have too little leverage. This leads to lack of control.
• Vertical stabilizer is too close to CoM, so it has too little leverage. This leads to large control movements that are causing the craft to roll instead.
• Combinations of above.

 

[capi3101]

2 minutes ago, Val said:

Judging by above picture, the issue is either

• Inverted controls because of swept wings. The hinge axis of the Control Surfaces goes right through CoM, so it can't decide whether it should act as if it's mounted in front or behind.
• Elevators are too close to CoM, so they have too little leverage. This leads to lack of control.
• Vertical stabilizer is too close to CoM, so it has too little leverage. This leads to large control movements that are causing the craft to roll instead.
• Combinations of above.

 

Hadn't thought about that first bullet point. Remedy's the same as I recommended earlier for the bottom three points.

[hawk_za]

36 minutes ago, capi3101 said:

Are you adjusting the friction settings on your gear at all? If not, try 0.4 for the nose gear and disable the brakes. Use 1.4 for the rear gear and set the brakes to full (200%, I mean).

Myself, I'd use a few pieces of structural fuselage to lengthen your plane, and then add a dedicated tail with pitching surfaces aft and as far above the main wing as you can manage. As it is, you're not getting a lot of torque from those elevons you've got - they are WAY too close to your CoM - so your pitch authority is going to be absolute dreck. I'd wager your initial assessment of your plane is correct: the rear of the plane is lifting first, causing the front gear to wheelbarrow before you've reached takeoff speed. 

If you don't have structural fuselage, try a drained FL-T400 tank instead. You can even keep some of the of the oxidizer in it if necessary if it turns out you need additional ballast up towards the nose.

okay used this advise set the wheels and extended the fuselage and added dedicated pitch rudders at the back ....that helped allot now this clucking plane actually get off the runway ..but i still don't understand where i was going wrong

i suspect it had to do with pitch authority and adding additional tail fins

http://tinypic.com/a/b3tvsg/1

pics of the mk 3

 

url for the mk 3 http://s000.tinyupload.com/?file_id=02786391389693917338 

 

ps big thanks to all that are helping me through this

@Val thanks for the expanded info i am starting to see the picture so if i understood correctly things are to close to each-other and to the c.o.m

Edited September 20, 2016 by hawk_za
futher reading

[Kryten 2X4B 523P]

Those outer control surfaces on the wings appear to be behind the centre of lift, which on those wings I think reverses the action (out at the moment, cant check in game - so in effect youre diving rather than climbing.) Im assuming theyre your pitch control surfaces.

Try rotating those wings using the rotate tool so theyre not "swept", but rather at 90 degrees to the fuselage - just enough so that those control surfaces are ahead of the centre of lift. - and add a tailplane

 

Edited September 20, 2016 by Kryten 2X4B 523P

[DrLicor]

4 minutes ago, Kryten 2X4B 523P said:

Those outer control surfaces on the wings appear to be behind the centre of lift

 

almost all surfaces are behind the centrer of lift. but indeed, with swept wings, the game messes up with the pitch. 

Most likely when the CoL is near the CoM in my experience

[Kryten 2X4B 523P]

indeed - those wings in particular seem to hit that "sweet spot" that seems to screw it up...

[luizopiloto]

You need some wing strakes or LERX to add lift in front of the main wings to add pitch authority and stability for the plane...
In KSP physics it works more like fixed canards. A pair of wings alone doesn't create the torque needed to pitch the craft, another lift surface in front, or tail is needed.

Edited September 20, 2016 by luizopiloto

[capi3101]

6 hours ago, hawk_za said:

okay used this advise set the wheels and extended the fuselage and added dedicated pitch rudders at the back ....that helped allot now this clucking plane actually get off the runway ..but i still don't understand where i was going wrong

i suspect it had to do with pitch authority and adding additional tail fins

http://tinypic.com/a/b3tvsg/1

Yeah, you see your plane is like a big lever, and your center of mass is the fulcrum point. In order to get it to change its attitude along any of its three axes, your control surfaces apply torque, which is a cross-product between the amount of force they produce and the distance from the fulcrum at which that force is applied. Increasing either of those two factors improves the available torque, which makes it easier for your plane to turn in a given direction. Your pitching surfaces provide the force. By moving them further away from the center of mass (the fulcrum), you increase the torque - you improve your angular acceleration with less linear force required.

Anyway, what was happening is that you had competing forces working on the tail end of your plane - your control surfaces were wanting to push the tail of your plane downward (thus lifting the nose), while the main wings were wanting to push it upwards (lowering the nose - your main wing also provides some torque). Because the control surfaces are so much smaller than the main wing, the wing provides a much greater linear force, and there wasn't a large enough lever arm present for the pitching surfaces to properly counteract the torque the wing provided. Long story short, the wing won the battle between the competing forces and lifted up the tail end of the plane first as soon as the plane reached sufficient speed, causing you to wheelbarrow on the front wheel. Slip, bang, boom. 

 

I think I've got that right; might want to get somebody in here who did a little better in angular kinematics in physics in college...

[Van Disaster]

In the air, control force is size of control * distance from CoM ( everything is proportional to speed, so ignore that a minute ). Static forces are *also* size of surface * distance from CoM - this is why when your CoL is all the way aft the plane wants to nosedive, because the back is being pushed up & it rotates around CoL. Your main landing gear has to be near CoM because when you're on the ground the pivot is the gear, rather than CoM - if CoM is in front of the gear the plane has a nose-down force all the time, and you have to counter that with your pitch controls which when you're going quite slowly are not very effective ( proportional to speed, remember? ).

Next thing is static stabilizing force, which is what's working against rotation and inertia so your plane doesn't wildly oscillate - just wave something big & flat around, it's that resistance - and just like that it's area & distance from CoM again.

The tailless craft will fly - it may not be very controllable because it's a bit short on natural pitch stability, and because of that lack of stability it has to be balanced very carefully. Tailless craft are usually deltas or close relatives of because the wing shape is naturally more stable than straight wings.

This sort of thing does work though ( although in stock it'd need a vertical tail too ) you just have to balance everything properly:

Spoiler

( also whacky stuff )

Spoiler

I should add something about the CoL-behind-CoM thing - if it's behind, the slower you go the less ability you have to control pitch, so the nose naturally pitches down because your big static wings are pushing up through CoL & your little pitch surfaces are not strong enough to counter, you speed up and you get control back again. On top of that as you pitch down your AoA decreases & reduces your lift, which reduces the pitch-down force.

If CoL is in front of CoM, as you slow down & lose pitch authority the nose will naturally pitch *up*, so you lose even more speed, even more ability to control pitch, and so your wings gain more lift by increased AoA and it pitches even more, until you stall. Of course if CoL is right on CoM then your craft is neutral ( until one of them moves a bit as you maneuver ) & you don't need big control surfaces - however it's also not naturally very stable.

There's an answer to that - move your wings upwards so CoL is above CoM. The more you pitch up, the more CoM will move ahead of CoL.

Edit: here's something I threw together in 1.2 just as a 5 minute demo, it's perfectly happy going where you point it, with reaction wheels disabled.

Note where the wheels are, and how far from CoM all the lifting surfaces reach.

 

Edited September 20, 2016 by Van Disaster

[DrLicor]

@Van Disaster

nice design, but if youre out of fuel, does the CoM move behind the CoL? Resulting in a flippy aircraft?

When I'm building planes, I use them for far flight with automaticly leads me to a supersonic plane.

Most of the time I stick with design like these:

 

I hope @hawk_za can build his planes flawness now, with all those tips and tricks from everyone  

If you stay to the basic rules, you'll be fine:

• centre of mass further away than 1/3 of the tail.
• centre of lift behind the centre of mass, further away means more stable, less manoeuvrable.
• be sure your centre of thrust is in line with center of mass, otherwise it will result in torgue.
• place the gear as stated above, just behind the center of mass, but make sure it stays just behind the center of mass al the time, even when you're empty.
• swept wings are for more manoeuvrability in high speed.
• the further away your flaps are from centre of lift, the bigger the effect will be.

Let us know when you build something succesful!

[Van Disaster]

8 hours ago, DrLicor said:

@Van Disaster

nice design, but if youre out of fuel, does the CoM move behind the CoL? Resulting in a flippy aircraft?

 

I hadn't even considered that because of the 5 min demo thing, but ( and I suspect your question was slightly rhetorical to make a point ) no, CoM would move forward if I spent time flying. In general my real aircraft don't have any CoM shift unless I pump fuel around - given I run FAR which has constant CoL shift that is often pretty necessary.

 

8 hours ago, DrLicor said:

• centre of lift behind the centre of mass, further away means more stable, less manoeuvrable.
• be sure your centre of thrust is in line with center of mass, otherwise it will result in torgue.
• place the gear as stated above, just behind the center of mass, but make sure it stays just behind the center of mass al the time, even when you're empty.
• swept wings are for more manoeuvrability in high speed.
• the further away your flaps are from centre of lift, the bigger the effect will be.

I explained CoM/CoL above - CoM too far back will mean it overwhelms pitch authority ( your ability to pitch up in this case ) at slower speeds which is debatably "stable". Heared a rule of thumb that your main gear wheels should be about 20 degrees back from CoM - ie, draw a line vertically through CoM, rotate it 20 degrees and that's where the actual wheels should be. STOL aircraft with huge pitch authority can be further away, but not too much. Not sure stock cares about wing sweep or even aspect ratio, although swept wings will be less draggy at speed even in stock I should think.

The last point there needs some elaboration: the further away your flaps are from CoL, the bigger the unwanted effect will be. They will act as pitch controls no matter where you put them,but if you put them on CoL there's no lever arm so there won't be any torque.

One last thing - disable the cockpit reaction wheels, they'll get in the way of aerodynamics. If you're building a spaceplane then just stick the toggle in the action group you're using to toggle your space engines. Generally I just turn them off all the time because I dislike how unrealistic they are, but they're there to be used, you just don't really want them in atmosphere.

Edited September 21, 2016 by Van Disaster

[hawk_za]

wow guys and gals thanks so much for the effort you have all put in i am now understanding were i went wrong and why. So man designing a plane is no joke :>

but thanks to all the members here i can now attempt it and understand the forces involved :>

 

very much less frustrated hawk  :>

 

*edit* okay just built a plane now and it looks remarkably like my mk3, but the difference is now I'm understanding why the parts go where they go and how the c.o.m c.o.l and landing gear all interact to crate a stable take off

 

again i cant say thank you enough ever since about .18 i have not understood this but now i do.

And it is in NO small part thanks tho the explanations in this thread

Edited September 22, 2016 by hawk_za
futher info

[capi3101]

Glad we could be of assistance to you. If you do have any further questions about how to plane, just ask.