Can not land heavy space plane - breaks on touchdown!

[barsandcat]

Help!

How do you land heavy space plane?

It is really hard to get it going parallel to the ground at 60~80m/s!

Craft file: https://drive.google.com/file/d/0B2dwd4kcTgMlY3dwY08zVHdBVUU/view?usp=sharing

[diomedea]

Change the forward landing gear to a steerable part (the larger steerable being the LY-35 medium gear). With all fixed gear (LY-99 as you're using), yaw is only controlled by the rudder, but it loses effectiveness at slow airspeed like 60 m/s Of course you'll need to adjust the vertical displacement of all gear to have the plane not pitching downward while taking off. Also, be gentle with that steerable gear when landing, touch down with the main gear (aft) first and rotate slowly to make ground contact with the forward gear.

[ForScience6686]

how far out do you begin your approach?  what speed does your craft stall at?   are you using flaps to increase low speed lift?   

The bigger the craft the harder it can be,  but most often the problem is too much vertical speed.   my advice is to setup a long approach and come in slow.  I see many people just shoot straight for the runway and try big flares.   You have much less control this way.   You should be approaching with a positive aoa  and use the throttle to control altitude,  pitch to control speed.  I also will decrease spring rate and increase damper to allow the plane to squat into the landing and reduce bounce back.   also helps prevent the nose gear from being forced down.

[barsandcat]

11 minutes ago, ForScience6686 said:

how far out do you begin your approach?  what speed does your craft stall at?   are you using flaps to increase low speed lift?   

The bigger the craft the harder it can be,  but most often the problem is too much vertical speed.   my advice is to setup a long approach and come in slow.  I see many people just shoot straight for the runway and try big flares.   You have much less control this way.   You should be approaching with a positive aoa  and use the throttle to control altitude,  pitch to control speed.  I also will decrease spring rate and increase damper to allow the plane to squat into the landing and reduce bounce back.   also helps prevent the nose gear from being forced down.

Flaps! Apparently no, I don't use them, and it looks like they are exactly what I need

[bewing]

And sometimes ditching the plane into the ocean just offshore and then driving it to the runway works well. You can land most planes in the water going quite a bit faster than on dry land, so you may have more control. You need to have some fuel remaining for the taxi run, and it takes some extra time.

[ForScience6686]

I'm guessing you lack lift.   it should be possible without flaps,  but they help with lift as well as increase drag as you should be coming in under power and drop throttle upon crossing over the runway.

[Kryxal]

Also consider landing heading west so there's no real fear of an overshoot ... the land around KSC is plenty flat on an overshoot for a good distance, till it rises up to regular grasslands.

[AeroGav]

I had a quick look at your ship, can't play long as i'm at work early tomorrow.

I think the structural weak point is the wing segment holding the main fuselage to the quad mount nacelles. Maybe a pair of gear legs attached to the main fuselage direct, and some smaller ones out on the wings nearer the tips with the body legs reaching down slightly further and carrying most of the weight.

Landing speeds do seem a little high,  if you come in at much less than 100 m/s you won't have enough lift to flare and will touchdown with high vertical speed and break the wings,  if you come in faster the horizontal speed could do you in.   And that's with it nearly empty (!)

Nearly everything I build stalls below 40 m/s loaded. That said, my heaviest designs are only 120ton gross,  think yours would pass that easily if you fill her up.

If I had more time I'd try to analyse why my ships seem to have better low speed handling.    Feel free to compare your ships to some of mine though, eg.

https://kerbalx.com/AeroGav/Wyvern-Offroad

120 ton with empty CRG-100 bay, taildragger design, capable of landing on Duna.

 

https://kerbalx.com/AeroGav/Kerboliner-Stretch-36-Super

110 ton passenger SSTA,  really nice low speed handling but I need to rework the gear, i think the mains are too far aft of CG which makes for a bouncing/tumbling forward tendency when landing offworld (corrective action applied to the Wyvern design)

 

https://kerbalx.com/AeroGav/Star-Empress

110 ton interplanetary liner, not designed for offworld landing but from memory easier/slower landings than yours?

[foamyesque]

Some general advice:

The general principle for landing -- if you're going in deadstick -- is to maintain speed with a pitch-down attitude (5-15 degrees depending on wing loading and angle) and then, as you approach the runway, flare continuously so that you stay in just-under-level flight and gently drop on the runway. This produces the least amount of strain through the wheels; vertical speed is the killer on flat terrain, not horizontal.

Another problem is wheel/wing positioning; when your gear touch down, they will impose both a large vertical force and a rearwards drag. This will tend to torque the front down. With large craft, this torque, if not minimized, is actually capable of snapping a craft in half. It can be minimized through a large (and, ideally, distributed wing, so that there isn't differential strain along the length of the craft), which will act to keep the weight off the wheels at touchdown speeds, and through keeping the CoL, CoM, and rear gear alignment all close to each other.

Using additional rear gear can also help mitigate things, both by increasing the spring resistance so that the strain is imposed more gradually instead of bottoming out the suspension (which tends to jam the landing gear up into the fuselage and causes all sorts of other issues) and by reducing differential lateral strain. More wings is a better option for the first and balancing the cantilever on a pair of the gear carefully a better option for the second, but sometimes they aren't possible. You need to be careful to not use gear that's too large, though, or you can get one that has no give at all, which has the same undesired effect of translating the shock of landing directly to the mounting.

Flaps can work to give you additional low-speed lift and slow you down faster. They can make a very useful difference in your landing approach, since they're much more effective than the same mass in main wing in providing lift (with consequent advantages in other flight phases). They can however bleed airspeed in a hurry, so it takes some practice to get the timing down.

Avoid overstrutting. You want to anyway, because they're awful aerodynamically, but some floppiness in the craft actually helps absorb differential strains. Over-rigidifying causes brittle failures and can progagate stresses in weird ways.

 

For your craft specifically, @barsandcat, the solution's actually really simple: Move your center rear landing gear forward, so that it's in-line with the two wing-pod mounted ones. That way all your rear gear will touch down at the same time (and give you a nice wide wheelbase for lateral stability). That gives you the ability to runway land. I did some further tweaking and swapped out the XL gear for the Ls and moved all three rear gear forward, which gives it some rough-field capability as well as making it slightly lighter, making it rotate for takeoff easier, and making it more stable in the air.

Craft file here: https://www.dropbox.com/s/pjo2065xtcus9l2/KSP Refueler 8W10N VF.craft?dl=0

 

Edited November 24, 2016 by foamyesque

[barsandcat]

Thanks for help! By far the, most dramatic help, was from flaps. Even without changing plane (I had one mission in progress), just by right click and deploy 2 of 4 elerons I have increased successrate from 1/10 to 2/4!

[ForScience6686]

Nice work,  don't forget about them in the future now.  I tend to set them with the gear action group or assign there own action group if I have multiple sets of flaps.   helps gradually reduce the speed during the approach.

[Fearless Son]

If you are using a really heavy plane, then I would recommend supplementing your flaps with a drogue chute or two.  Try to keep your approach to the runway low and level, and as you pass over the threshold you can deploy the drogues and let them slow you down.  If you fly the plane flared just enough to keep a level flight then the chutes will drop your altitude along with your airspeed without having to change the orientation of the plane, and if you are already positioned right they should allow you to touch down with a gentle horizontal landing impact before all your weight settles.  As a bonus, the drag from the chute should be well behind the center of mass (indeed well behind the tail) so it will keep the craft really stable in those last critical moments, should help avoid the problem of an asymmetric landing that might tumble the plane too much (like when one tire hits the pavement before the other and gets too much weight on one point to handle and snaps.)  

[foamyesque]

9 hours ago, Fearless Son said:

If you are using a really heavy plane, then I would recommend supplementing your flaps with a drogue chute or two.  Try to keep your approach to the runway low and level, and as you pass over the threshold you can deploy the drogues and let them slow you down.  If you fly the plane flared just enough to keep a level flight then the chutes will drop your altitude along with your airspeed without having to change the orientation of the plane, and if you are already positioned right they should allow you to touch down with a gentle horizontal landing impact before all your weight settles.  As a bonus, the drag from the chute should be well behind the center of mass (indeed well behind the tail) so it will keep the craft really stable in those last critical moments, should help avoid the problem of an asymmetric landing that might tumble the plane too much (like when one tire hits the pavement before the other and gets too much weight on one point to handle and snaps.)  

 

 

Define really heavy; I haven't found chutes necessary in years (and back then it was for countering infini-glide), and I not-infrequently land machines up to, and past, the 200t mark. This one, for example:

 

[Fearless Son]

14 hours ago, foamyesque said:

Define really heavy; I haven't found chutes necessary in years (and back then it was for countering infini-glide), and I not-infrequently land machines up to, and past, the 200t mark. This one, for example:

Nice skills, landing that tanker.  

I do not mean to imply that chutes are necessary, just that they can be helpful.  This is especially the case early in a career or science mode game when you might be more limited in your part options.  For example, one might lack the heavy-duty landing gear that you have on your craft above.  Take that away and replace it with earlier, lighter, less robust tech and while you might still be able to take off, landing that much mass without snapping a wheel on touchdown might get a lot more complicated.  By contrast, radial drogue chutes are fairly early game tech and can be used as a crude kind of single-use airbrake to make landings more gentle.  

[fourfa]

Bring an engineer along, and they're not even single-use!

[Fearless Son]

19 hours ago, fourfa said:

Bring an engineer along, and they're not even single-use!

True, but they are single-use insofar as you will only be using them once per lift-off.  Having an engineer disembark to repack a parachute in mid-flight is certainly a very Kerbal thing to do, but like most Kerbal things to do it usually ends in an amusing disaster.  

Once the craft is safely grounded and at a complete stop then repacking chutes becomes much more viable.  

[ForScience6686]

17 hours ago, Fearless Son said:

True, but they are single-use insofar as you will only be using them once per lift-off.  Having an engineer disembark to repack a parachute in mid-flight is certainly a very Kerbal thing to do, but like most Kerbal things to do it usually ends in an amusing disaster.  

Once the craft is safely grounded and at a complete stop then repacking chutes becomes much more viable.  

Needing to repack a chute mid flight is very kerbal  like.   you should only need to do it after landing.   besides I only use this method in non powered Landers,  as I generally come in too fast to make sure I make the runway.