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How to keep Aircraft stable on landing?


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Hello, just wondering, how do I keep aircraft from tipping over and clipping their wings off before crashing completely when landing & taking off? It's been an issue that's been bugging me a lot lately, and has had Jeb stuck in orbit in a Space  Shuttle for two or three irl days now as every attempt at landing has had this result so I've been forced to hit F9 every time, any tips? I've heard the suggestion to have your wheels far apart but alot of planes aren't wide enough.

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A small checklist:

- Make sure the wheels actually point straight forward. In the SPH, use the rotate widget in "absolute" mode to achieve that. Otherwise you plane won't go straight.

- Put the wheels farther out. Design your plane so that it's possible to do it. The U2 on the runway has wheels out on the wing tips!

- Make sure you aren't building a wheelbarrow. I avoid this by tilting the wings so they have positive angle of attack when the plane is level, and they are applying their force just barely behind the center of mass. That makes it so that when I'm going fast enough to lift off, force comes off all the wheels at the same rate. A common problem is that the back wheels come off the ground before the nose wheel, which is unstable: a slight bump on the nose and you flip.

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Besides having the wheels on straight. The most important thing is to have the main landing gear positioned right by CoM. Just as having CoL right by CoM is best for flying characteristics, having the gear there gives the best driving charactetistics.

But since you're already in orbit and can't change your design, your best option is to land as slowly as possible in as flat area as posible.

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1 hour ago, Bluegillbronco2 said:

I would try and make the craft so the landing gears are farther apart so the weight is more distributed horizontally. Making the craft shorter also helps by keeping the center of mass lower to the ground.

It's Already pretty short, and yeah, I think I'll try putting structural pylons on the wings and putting the landing gear on those (the structural pylons are so the landing gear can still reach the ground). Thanks.

1 hour ago, numerobis said:

A small checklist:

- Make sure the wheels actually point straight forward. In the SPH, use the rotate widget in "absolute" mode to achieve that. Otherwise you plane won't go straight.

- Put the wheels farther out. Design your plane so that it's possible to do it. The U2 on the runway has wheels out on the wing tips!

- Make sure you aren't building a wheelbarrow. I avoid this by tilting the wings so they have positive angle of attack when the plane is level, and they are applying their force just barely behind the center of mass. That makes it so that when I'm going fast enough to lift off, force comes off all the wheels at the same rate. A common problem is that the back wheels come off the ground before the nose wheel, which is unstable: a slight bump on the nose and you flip.

I'm pretty sure they point forward, but I'll double check just to be sure.

As said before, I'll attempt to move the wheels out to the wings with the help of structural pylons.

What I assume you are saying is to basically have the wings pointed slightly upwards when on the runway, I was already aware of this and have the rear wheels placed slightly higher up then the front one so that the entire plane is slightly tilted up when on the runway, if it's something else that's a bit more complicated, then some example pics might help.

Anyway, thanks for the checklist.

51 minutes ago, Val said:

Besides having the wheels on straight. The most important thing is to have the main landing gear positioned right by CoM. Just as having CoL right by CoM is best for flying characteristics, having the gear there gives the best driving charactetistics.

But since you're already in orbit and can't change your design, your best option is to land as slowly as possible in as flat area as posible.

Alright, I'll start placing the front landing gear right in front of the COM (Moving the rear ones forward would mean I'd bash my engine on takeoff).

 

Actually, I can change my design due to having KIS/KAS Installed, which incase you don't know what they are ( although you likely already do due to their popularity) are mods that allow your Kerbals to hold and use equipment and build and move things mid-flight, so I may just send up a mission with a Engineer to fix the landing gear.

And coming in too fast might be my main problem, as this craft has successfully landed before, but often at a somewhat slower speed, as I've been coming in rather fast, which makes no surprise considering it's Jeb at the controls, and I have been landing in a relatively flat area, my designated landing area for most of my Spacecraft is the general area of the southern coast of the two large inland lakes/seas. with one of them being landlocked, it's mostly flat grassland, although there are some rolling hills and a few craters.

Thanks for the advice.

39 minutes ago, MircoMars said:

the above and disable breaking on the front wheel. 

the workaround would be a parachute landing... 

I'll start attempting landing with the front wheel brake off, thanks, that could wind up being really hepful, thanks mate.

And the shuttle does have a parachute to use for braking, just like the real one, although on successful landings I often stop before I can trigger it, but if a regular landing fails I might resort to a Parachute landing.

Thanks for the help.

 

Sorry for the late reply, was busy on other forum threads while I waited for people to reply to this one, I'll attempt to post a picture of the Spacecraft in question shortly.

Edited by DolphinDude3
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Jeb is back!

5SklS3c.jpg

Thanks guys! From both me and Sgt. Jeb .N. Kerman, the flight plan basically went like this.

1: Set Shuttle on Suborbital Trajectory hitting the landing zone.

2: Re-Enter, keeping my nose up and doing hard turns to lose speed.

3: Perform glide down to about 1 Kilometer up at about 300 m/s, pulling up hard at the end.

4; Moar maneuvering to lose speed, going only fast enough to generate enough lift to stay air-borne, If I remember correctly I also gave the engines a quick burst at this stage too, to help stay airborne.

5: Gently touchdown, using torque to stay stable and only using my parachute to brake as the actual airbrakes and landing gear like to crash the thing.

6: Engaged Brakes once I was going relatively slow, bringing the bird to a halt, Jeb crawls out and jumps in joy for a successful mission.

 

Oh, and for anybody curious, the Shuttle's mission was to rendezvous with a Mk-2 inline cockpit that was up there as part of a rescue and retrieve mission, after a previous mission recovered the Kerbal onboard, the plan was to have  Bill use KIS/KAS and attach parts to make it into a working spacecraft before then flying it down, a mission which was successful. We also refueled my first ever working SSTO Spaceplane, Atlas I, during our flight, just to ensure it had enough fuel to land (although there's a decent chance it could've gotten down without a refueling), all missions complete! Woho! Once again, thanks for the help guys and gals, it was useful in returning the shuttle and will likely be very useful in my future Aeroplane/Spaceplane related adventures.

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1 hour ago, DolphinDude3 said:

As said before, I'll attempt to move the wheels out to the wings with the help of structural pylons.

It's better to have the landing gear attached to the fuselage. Much more stable. Though for craft less than 30 t takeoff weight, it probably doesn't matter.

1 hour ago, DolphinDude3 said:

Alright, I'll start placing the front landing gear right in front of the COM (Moving the rear ones forward would mean I'd bash my engine on takeoff).

You won't bash your engine. Sounds like you're probably just used to craft with the gear too far back, where the nose lifts very suddenly and only when you give full control throw.

When you have the rear landing gear in CoM, you can lift the nose carefully and controlled.

It also helps to have Angle of Incidence. It reduces the angle you need to lift the nose to takeoff and land slowly. Plus it reduces drag in flight.

I've built very long craft (300+ t takeoff weight) some with just the small landing gear and was able to takeoff and land just fine without tail strikes.

Here's an example of a long craft.

Spoiler

 

J5zVaCF.png
Small landing gear in the middle of the craft.

V6qhyjZ.png
Lands before runway...

jdKNOQs.png
...and manages to roll up onto the runway without tail strike on the incline.

 

 

39 minutes ago, DolphinDude3 said:

Here it is, the Spaaaace Shuttle MK1R.

Mj6F2Jb.jpg

Did you manage to land it?

EDIT: Ninja'ed. Gratz on landing it. :cool:

Edited by Val
Ninjaed
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Another construction trick is to build a tail-dragger. Two wheels up front ahead of CoM, one in the tail. Then (a) you don't wheelbarrow, and (b) you don't tail strike. The trick is ensuring there's enough angle of attack on your wings so that you can lift off almost without needing to pull up -- that or build a canard plane, but it's hard to build a stable canard plane.

It's hard to build a tail-dragger with an odd number of engines, so I tend only to do it with my tiny early planes.

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On 12/2/2015, 12:02:18, DolphinDude3 said:

Jeb is back!

5SklS3c.jpg

 

One difference I can see between your ships and mine (which have proven to be very stable fliers) is a tail stabilizer/elevator. I'm not sure of the precise physical way to describe the effect, but I think it helps a lot to have a lift producing/pitch controlling surface way far back to balance the forward elements, even if it moves the COL upwards.

Some examples:

01qnKDP.png

Pontoons were ultimately too small, but it was nice and stable landing on the water. Deploying the tail elevators for final approach really helped to lower landing speed and keep the nose level.

sUcPRE2.png

Landing sequence of Kerbfleet's workhorse SSTO/intermunar shuttle since 1.0.0, the Gliido (this was from an Eve: Order Zero interlude)

iX9BpQD.png

...and going into the WAY back machine, Hummlebee (launched in v0.90, landed in v1.0.2). I found even this minimal tail helped balance quite a bit in both Kerbin and Duna atmosphere.

Not all successful plane designs have tails, but I tend to use them and they work out pretty well for me :) Good luck!

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On 12/1/2015, 9:09:46, Val said:

It's better to have the landing gear attached to the fuselage. Much more stable. Though for craft less than 30 t takeoff weight, it probably doesn't matter.

Just in case you haven't thought of it yet, a nice trick is to attach the gear to the fuselage, then use the offset tool to make them appear to be under the wings. That way, you get the best of both worlds:  stability of the wider stance, and durability of the tougher attachment.

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51 minutes ago, Zephram Kerman said:

Just in case you haven't thought of it yet, a nice trick is to attach the gear to the fuselage, then use the offset tool to make them appear to be under the wings. That way, you get the best of both worlds:  stability of the wider stance, and durability of the tougher attachment.

Thanks! I'll try to remember that for next time I'm building a Aircraft.

5 hours ago, Kuzzter said:

*Big post about Aircraft Tailfins*

Kuzzter replied to a thread I made!!! Woho! *Happily dances around the Astronaut Complex*

But thanks, I'll try putting on a tail stabilizer on my Space Shuttle and other Aircraft and see how it works! *Quickly fires up KSP to do so* Will report back with the results!

EDIT: I have returned with the results from test 1, the test was performed with the Space Shuttle Mk4, a Jet Engine equipped version of the regular Shuttle, although both quickly exploded due to being ripped apart by aerodynamics, quickly followed by a hydro-braking (note to self, they're Space Shuttles, not stunt planes!) during the short time they were intact no major difference was noticed between the normal one and the version equipped with a tail stabilizer, which may have something to do with the fact that the wings of the shuttle are at the rear, meaning we already have pitch control in that area, however tail stabilizers may still prove useful with Aircraft with wings farther forward, time to conduct test 2!

Btw, I forgot to mention in the original post (even although it is semi-important) I have FAR installed.

The craft that test 1 was done with.

GDrzzcS.jpg

QJgrsss.jpg

Edited by DolphinDude3
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On Tuesday December 01, 23:09:46 GMT-0600, Val said:

You won't bash your engine. Sounds like you're probably just used to craft with the gear too far back, where the nose lifts very suddenly and only when you give full control throw.

With proper landing gear placement and tail-mounted engines it is indeed possible to bash your engine, and tailstrikes are a very real thing IRL. But the solution to this is not to put your landing gear at the rear of the craft, but rather to figure out a safe rotation speed and rotate carefully and only after reaching that speed.
 

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4 hours ago, jwbrase said:

With proper landing gear placement and tail-mounted engines it is indeed possible to bash your engine, and tailstrikes are a very real thing IRL. But the solution to this is not to put your landing gear at the rear of the craft, but rather to figure out a safe rotation speed and rotate carefully and only after reaching that speed.

Yeah, that's kinda of what I was trying to say. Not that it's impossible to do tail strikes, but that you get much more control over how fast the nose lifts, when the gear is positioned correctly.

You said it much better.

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Regarding rear stabilizers - if you're going to have canards and rear stabilizers, i prefer to have tailplanes with the controls disabled or just put a small rear mount wing there, and do the pitching up and down with canard only, that way you get the passive stability of the stabilizer pushing the nose back into line but when pitching up to get more lift at low speed or thin air, the canard is contributing to total lift rather than subtracting from it.    

 

I'm not sure this is a thing really.., stability is probably determined by centre of lift vs centre of mass only. 

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On 2-12-2015 03:55:05, numerobis said:

A small checklist:

- Make sure the wheels actually point straight forward. In the SPH, use the rotate widget in "absolute" mode to achieve that. Otherwise you plane won't go straight.

- Put the wheels farther out. Design your plane so that it's possible to do it. The U2 on the runway has wheels out on the wing tips!

- Make sure you aren't building a wheelbarrow. I avoid this by tilting the wings so they have positive angle of attack when the plane is level, and they are applying their force just barely behind the center of mass. That makes it so that when I'm going fast enough to lift off, force comes off all the wheels at the same rate. A common problem is that the back wheels come off the ground before the nose wheel, which is unstable: a slight bump on the nose and you flip.

There's another easy way around this which makes your plane very stable on the runway: Tail draggers.

Basically you invert the gear so your single steering wheel is on the rear instead of on the front. That way you will never end up on the runway nose-first when taking off or landing, because the tail wheel gets lifted off the runway first, pitching the aircraft slightly forward until there is an equilibrium and your nose is fairly level with the horizon. From there, you can accelerate and gently pitch up to take off.

Tail draggers can fly very slowly if you build them properly and they need a little care to land since they land fairly horizontal (as opposed to nose up when landing a tricycle on the main wheels first), but once you get the hang of that, they're easy to fly and land.

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On 13. Dezember 2015 um 13:11:28, Stoney3K said:

There's another easy way around this which makes your plane very stable on the runway: Tail draggers.

Basically you invert the gear so your single steering wheel is on the rear instead of on the front. That way you will never end up on the runway nose-first when taking off or landing, because the tail wheel gets lifted off the runway first, pitching the aircraft slightly forward until there is an equilibrium and your nose is fairly level with the horizon. From there, you can accelerate and gently pitch up to take off.

Tail draggers can fly very slowly if you build them properly and they need a little care to land since they land fairly horizontal (as opposed to nose up when landing a tricycle on the main wheels first), but once you get the hang of that, they're easy to fly and land.

LOL, never flew a real taildragger, did you? They are a nuisance to taxi on the ground, they have a terrible tendency to bounce if you land "horizontal" like you suggested and are the most unforgiving beasts when it comes to landing. The reason they are used so much on bad runways (like bush pilots, wartime, remote islands etc.) is they reduce the risk of getting stuck with the single front wheel  - resulting in flips and heavy damage - and have a better overall weight distribution on the two main wheels which helps the airframe integrity on bouncy runways. 

However, you can flip a taildragger onto it's nose very easily if you apply the brakes too much. They are very hard to taxi because of the reduced sight and have a extreme tendency to perform unintended ground loops, because you steer from behind the centre of mass (like driving a car backwards).

Maybe it just works in KSP because taxiing is not a factor here.

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On 11 December 2015 at 2:28:20 AM, AeroGav said:

Regarding rear stabilizers - if you're going to have canards and rear stabilizers, i prefer to have tailplanes with the controls disabled or just put a small rear mount wing there, and do the pitching up and down with canard only, that way you get the passive stability of the stabilizer pushing the nose back into line but when pitching up to get more lift at low speed or thin air, the canard is contributing to total lift rather than subtracting from it.    

 

I'm not sure this is a thing really.., stability is probably determined by centre of lift vs centre of mass only. 

That approach could cause problems in FAR, because forward control surfaces stall much more easily than rear mounted elevators.

I've normally got moderate to high maximum deflection on the rear surfaces, with the canards set to about half of the rear deflection (and often some negative AoA setting as well).

 

For the landing stuff, most of it's already been mentioned:

1) Make your wheelbase as long and as wide as possible. Rear gear just behind CoM. Make heavy use of the translation tool.

2) Make sure the gear are straight (use the rotate tool set to absolute mode), and make sure that the part they're mounted to doesn't flex. This usually means fuselage instead of wing, although small light craft with narrow fuselages may be best off with wing mounts.

3) Disable brakes on the front wheel, raise the brake torque to maximum on the other gear. Default brake torque is much too weak (except perhaps for front wheels on taildraggers).

4) For heavy ships, use more gear. Support the CoM. Add tailstrike guard wheels if you're in the habit of bashing the engines on the ground.

5) Drag chutes work. Place them vertically close to CoM, as close to the tail as possible. Trigger them after touchdown.

6) Retrothrusters work too. Twitch and monoprop engines are good for this.

7) Avoid steering while landing as much as possible, but stay on the runway. If you do have to steer, be ready on the roll controls to cancel any tipover.

8) Land as slow, shallow and straight as you can. Use the whole runway.

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1 hour ago, Wanderfound said:

That approach could cause problems in FAR, because forward control surfaces stall much more easily than rear mounted elevators.

I've normally got moderate to high maximum deflection on the rear surfaces, with the canards set to about half of the rear deflection (and often some negative AoA setting as well).

 

One thing I discovered recently  is that rear control surfaces are better if your fuselage is prone to flexing.  Made a mk2 with lots of short components (inline cockpit, clamp-o-tron, fuel tanks and cargo bays) and it was somewhat bendy.   Pulling up hard makes the nose flex up, increasing the angle of attack and thus lift of the foreplanes even more in a positive feedback loop.  With tail controls fuselage flex will tend to oppose control surface deflection, limiting excursions.

Another problem you can have is body lift.   The Centre of Lift indicator in the Spaceplane Hangar doesn't take it into account especially if your wings have a positive angle of incidence.     My aircraft looked stable according to the indicators and flew well enough on the initial climb, but if you deviate more than a few degrees from prograde at high speeds, the body lift becomes significant and renders the A/C unstable.   

Canards may be more prone to this due to having more fuselage ahead of CG,  and of course having a lot of fuselage ahead of the wings is especially bad if that fuselage is flexing.

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One more thing I just noticed on this Canards vs Tailplane debate -

 

People who are having trouble with canard designs may be adding incidence to their main wing.   I just encountered this problem myself.   Aerodynamically stable canards are designed with an airfoil on the canard that stalls at a lower angle than the main wing , so the nose automatically pitches down at stall.   In KSP however, your Canard stalls at the same angle as the main wing, so manual recovery is necessary.   If you angle your main wings upward however, they are going to stall before the canards, and this is trouble.    

 

As low as 20 degrees angle of attack,  "diminishing returns" is starting to set in on the angle of attack/lift graph.    This means that as you continue to pitch up, your canards will gain lift faster than the main wing, moving the centre of lift forward and making the craft less stable.    This continues as the stall angle is reached, with the main wings loosing lift faster than the canard.   TL;DR -  the nose will pitch up at/near the stall.  Canards and angled wings don't mix.

There's three ways around this problem

1.  Stop angling your wings and messing with the stability of the airplane for a few % more performance.

2. Angle your canards to an equal or even greater degree than the wings.   Bear in mind type 2 fuselage pieces also lift however, and if you have more of those in front of the centre of gravity than behind it (you probably do, in order to balance heavy engines at the back), you may still have trouble.  

3.  Put a non angled lifting surface at the very back of the fuselage.   Yes, a tailplane . However, it does not need a control surface, it can just be a plain wing section, and you can do all your pitch control from the canards if you wish.  The main thing is however, that it will maintain lift after the angled wing has stalled and help to push the nose down.   Downside - this surface represents dead  weight when flying exactly on the prograde.  

 

Another problem with solution 2)  is that pitch control surfaces are a major source of drag already, this will make the situation worse.   My designs already have minimal fuselage drag, open nodes, radially attached junk, low intake drag (pre-coolers), and very low lift induced drag, due to having plenty of wing area , mostly flying under 5 degrees angle of attack.

The pitch control surfaces however, have to work much harder , often pitched at 10 degrees or so just holding the nose up in level flight.    Space planes typically have a very large fuel fraction, and it's often dissimilar types of fuel being burned at different phases of the flight (jet fuel, then rocket fuel), furthermore there is a payload fraction, that is going to be offloaded in orbit.    As a result centre of gravity shifts around a lot, and i often find myself playing it safe and keeping CoL well aft.   This means a lof of canard lift is required to hold the nose up.     Fitting oversized canards will reduce the induced drag from these surfaces running at high angle of attack , but makes it easy to overcontrol.  A space plane is not supposed to be aerobatic, it is supposed to be stable and cruise to orbit.    The best stock solution seems to be to use the Advanced canard, which only moves 10 degrees compared with 30 on most surfaces.  However it is tiny, so you need to mount lots of them, probably clipped inside one another, which isn't the most aesthetic.

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Maybe I'll be the only one who goes against the grain, but I actually recommend putting your landing gear as far back to your aircraft as possible and as far forwards.  This makes striking your tail and destroying your engine impossible while taking off and will make landing your aircraft far easier, since your'll have a far easier time having the landing gear touch the ground and begin braking.  Also, it makes the aircraft very stable both on the runway and on rough terrain, ensuring the aircraft's parts wont strike the terrain.  There are cons to putting the landing gear in the back though, the speed necessary to take-off will be dramatically increased, and if your aircraft doesn't have a lot if pitch authority, it might not even be able to take off, put that is the price of safety.  It's also recommended that you use two landing gear in the front, as opposed to just one as you see in most real aircraft, for some reason, just having one in the front causing instability and allows wobbling.

c9QJGNk6.jpg


 

Edited by Edax
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On 16-12-2015 at 9:32 AM, Old Foxboy said:

LOL, never flew a real taildragger, did you? They are a nuisance to taxi on the ground, they have a terrible tendency to bounce if you land "horizontal" like you suggested and are the most unforgiving beasts when it comes to landing.

Maybe it just works in KSP because taxiing is not a factor here.

That's less significant in KSP than it is in real life because of the third-person camera. If you fly IVA, they're a lot more difficult to handle because of the nose-up attitude on the ground.

@Edax: Your arrangement hardly takes off on lift, but more on sheer thrust alone. There is no way to rotate off the runway if the gear is too far away from the CoM and CoL, since your control surfaces would never be able to provide enough nose-up torque.

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5 hours ago, Stoney3K said:

 

@Edax: Your arrangement hardly takes off on lift, but more on sheer thrust alone. There is no way to rotate off the runway if the gear is too far away from the CoM and CoL, since your control surfaces would never be able to provide enough nose-up torque.

The Kerbal Way no doubt!

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