Can't achieve airplane lift/flight

[Lord_Archaic]

So out of all of my planes, I can't seem to be able to take off with them....Am I missing something or what? I've even tried putting a rocket engine on the back for extra thrust......and it just hits the end of the runway and into the ocean it goes....

 

Also, towards the start of the runway it starts to veer to tje left and I have to re-adjust and end up rolling it into the dirt.....

 

Any ideas as to how to fix this.....I'll try and get a photo up asap.

[Snark]

Hello, and welcome to the forums! 

So, "why won't my plane fly" is basically impossible to answer without seeing a picture-- post some pics and I'm sure plenty of people can offer advice.  It's quick and easy to do-- just put your photo on imgur.com or somewhere similar, copy the URL of the image itself (not the page, the image), then paste that image URL here in your forum post.  It will get automagically converted to an in-line image in your post.

As for your plane veering on the runway:  one possibility is that the landing gear's not perfectly aligned.  That's easy to happen.  To fix it, do the following:

1. Go into the SPH and open your plane.
2. Click the "3" key for "rotate", then click on your landing gear.
3. Press "F" to toggle between "absolute" and "relative" mode.  You want to set it to "absolute".  (It defaults to "relative", but once you set it to absolute, it'll stay that way for as long as you're in the SPH.)
4. Grab the rotate widget, then rotate your landing gear one "click" away from straight (on any axis, doesn't matter) and then back again.
5. Do this for all the landing gear on your craft.

That will guarantee that they're all perfectly aligned.  It may solve your veering-on-the-runway problem.  If it doesn't, post us a pic and we'll look into other possible causes, but that's the first thing to try.

Good luck! 

[bewing]

There is also a FAQ at the top of the forum listing -- and "why does my plane veer to the left on the runway and crash" does actually happen to be frequenly asked.

That veering is basically caused by a lack of directional stability on the ground. My typical answer is to reduce the friction on the front wheels -- I can explain that further if it becomes necessary. But there are many possible issues that can cause it, and each one needs its own fix, as Snark pointed out.

 

 

[Lord_Archaic]

The photo's of the plane I'm trying to launch.

[Nightside]

@Lord_Archaic, you are on the right track with your design but it needs a few adjustments.  The main problem is that the rear wheels are too far back. This will prevent you from being able to pull the nose up and lift off the ground. Take a look at some real planes or load up one of the stock example craft and you will see that the rear wheels are usually attached below and just behind the center of mass. This allows the center of mass to easily pivot.

also, the fixed wheels are difficult to tune, you may have better luck with the retractable gear. 

[Spricigo]

What make your craft take of is  a net upward force.  With wings and engines parallel to the ground you don't have enough upward force to overcome weight. (in part because how KSP's aerodynamic is,  it don't simulate wing shape so all lift comes from Angle of Attack ) 

There is two common ways to solve it:

(1)Have wings with some built in AoA (aka wing incidence). If you have enough lift to overcome weight you take off. 

(2)turn your craft up.  Yes, I  know is obvious. Yet pretty often inexperient players design planes unable to do it,  and that actually is your problem also. 

 Notice in your craft pitch is done (mostly)  by the control surfaces at the tail. It pitch up by forcing the tail down. But since  your craft touch the ground exactly below those control surfaces it only means the aerodynamic force is transferred to the ground resulting in bigger reaction force from the ground on your rear wheels. 

Personally I suggest to put some incidence in your wings, because it may also allow for a wider wheel base and avoids tail strikes. For this size and tech level I also prefer taildrager designs.

BTW those wings have better aesthetics but worse performance.  " looks good"  is as good as any other reason for pick a particular part, just notice what the  price is. 

 

[Kurtayn]

I'm no expert on building planes by any means, but you may want to try moving your rear landing gear closer to your center of mass. Having them back that far keeps the plane from nosing up.

[Snark]

10 hours ago, Kurtayn said:

I'm no expert on building planes by any means, but you may want to try moving your rear landing gear closer to your center of mass. Having them back that far keeps the plane from nosing up.

^ This is your problem right here, @Lord_Archaic.

Your airplane is starting on a perfectly horizontal runway.  Its wings are also perfectly horizontal, parallel to the ground.  So the only way to actually take off is to rotate the nose upwards from the horizontal so you can start climbing, right?

Well, there are only two ways you can rotate the nose upwards:  Either you need something in the front of the plane pushing upwards (like canards, for example), or else you need something in the back of the plane pushing the tail down (like the horizontal stabilizers on the tail).

However... "thing in the back that pushes the tail down" only works if the tail actually CAN go down.  Look at where you've got your rear landing gear:  It's right underneath the tail!  So it's not physically possible for the tail to go down, because it's resting directly on top of the landing gear, which are preventing it from lowering.

(Take a look at a real-life aircraft-- notice how their tails stick way out behind, without anything holding them up?  They put their rear wheels practically in the middle of the plane, just slightly aft of the center of mass.  This makes it so they can easily tip upwards on takeoff.  Since their tails stick out way behind the rear wheels, that gives them a nice big lever arm for pushing the tail down and rotating the plane up off the tarmac.)

And you don't have any control surfaces at all that are in front of the CoM to lift the nose up.  You do have some ailerons on the wings, but that's right next to the CoM which means they can't exert any meaningful torque.  So it's no wonder you can't take off.

So, some options:

1. You can add a pair of canards (the AV-R8 winglets work great for this) up at the front of the plane, as far forward as you can manage.  These can lift the nose up.
2. You can move your rear landing gear much farther forward-- put them just slightly behind the CoM.  (They can't be in front of the CoM or your plane will tip over when it's standing still, and drag its tail on the ground).
3. You can rotate your wings a bit so they have a little bit of AoA ("angle of attack")-- i.e. they're not perfectly parallel to the ground, but are tipped upward slightly.  This way, they can generate lift even when you're parallel to the ground.

You can do any or all of the above-- that ought to take care of the problem right there.  (If you were to do only #3, and not #1 or #2, that might be enough, but you'll likely have a pretty high takeoff speed before you get airborne.)

[Aegolius13]

21 hours ago, Spricigo said:

There is two common ways to solve it:

(1)Have wings with some built in AoA (aka wing incidence). If you have enough lift to overcome weight you take off. 

(2)turn your craft up.  Yes, I  know is obvious. Yet pretty often inexperient players design planes unable to do it,  and that actually is your problem also. 

One other way is to adjust your landing gear (e.g., using the move gizmo) so that the front of your plane rides a bit higher off  the ground than the back end.  That gives you more "free" angle of attack while on the runway.  But don't go too extreme or you can have a tail strike (especially if you move your rear landing gear forward, which as mentioned above is really important).

I can't tell for sure from the pics, but your engines look like they may be mounted a little higher than the center of mass.  This can cause a torque that makes the plane want to nose down.  You can either mount the engines a little lower, or try angling up a tiny bit (you want the purpose thrust arrow so that, if it were extended, it would go through your center of mass). But I would be leery of going too far down the angling road; off-center engines can do funny things, especially if your CoM shifts as you burn fuel. (There are about a thousand "why does my space shuttle replica not fly right?" threads on this issue).  

You can also get a little more leverage from your tail fins by placing them further back on the tail connector.  

[Spricigo]

@Snark for a craft of this size,  just a couple degree of wing incidence seems to be enough and not much a problem for takeoff speed. Specially since most of lift is on front of the CoM(if wing incidence is added) and a considerable rudder causing a pitch up tendency. 

@Aegolius13 now that you pointed out, I have the same impression of thrust vector passing above CoM. 

[foamyesque]

On 2017-06-22 at 8:47 PM, bewing said:

That veering is basically caused by a lack of directional stability on the ground. My typical answer is to reduce the friction on the front wheels -- I can explain that further if it becomes necessary.

 

That's still a kludgey solution. Better fix is understanding the aerodynamic loading being transferred through the wheels.

[bewing]

Perhaps. Or to remove the wings so that there is no aerodynamic loading, and see if the problem remains unchanged.

 

[foamyesque]

3 minutes ago, bewing said:

Perhaps. Or to remove the wings so that there is no aerodynamic loading, and see if the problem remains unchanged.

 

If it's unchanged, then you have a poor wheel layout to start with. Fiddling with the friction sliders is just such an immersion-breaking thing for me; I'll go way out of my way to avoid it, and I think I get better designs as a result. It's like someone solving a rocket's stability problems by strapping ten million reaction wheels to it instead of working to understand why it's behaving badly. =/

Edited June 24, 2017 by foamyesque

[Spricigo]

7 hours ago, foamyesque said:

... understand why it's behaving badly. =/

That basicaly is the first step in the way to solve any problem.

@Lord_Archaic https://kerbalx.com/Spricigo/KarlJatho   similar to your design but with thrust aligned to CoM,  wing incidence and inclined wheel base. Also 2 crew spaces for pilot & scientist. Very easy to takeoff and fly.

[Kryten 2X4B 523P]

looking at that, wheel layout is probably your biggest issue, as the rear wheels are way behind the COM.

However, Id make the following changes:

If youre intent on having the landing gear in tricycle formation (small wheel at front, two behind, move the rear wheels so theyre roughly in line between where the COL and COM are in your screenshot, and move them up the fuselage a bit so that the nose sits pointing upwards slightly on the runway. Personally, Id turn it into a taildragger with a small, steerable wheel on the underside end of the tail connector, and two LY-01 just in front of the COM (basically right where the cockpit meets the fuselage. This will give most of that "nose up" stance you want from the start. Remember to rotate both sets of wheels straight using absolute mode (f)

Move the tail fins right to the end of the tail connector. Move the horizontal ones down a little so theyre off the midline.

Move the engines down until theyre on the midline - where they are at the moment will be pushing the nose down.

Rotate the main wings forward a little, which should bring the COL back forwards. Rotate the wings upwards a notch to form a shallow V shape viewed from the front of the plane.

Set the control surfaces as follows, if you havent already - Elevons on the wings to roll only, vertical tail fin to yaw only, and horizontal tail fins to pitch only. drop the authority limiter on each down to about 20 to start with.

That should come pretty close to sorting your issues I think.

 

Edit: After having a bit of a play / recreate it should look like the following screenshots with the above changes - note it will still need control input to get off the runway.

 

However, if you change the landing gear as well to a taildragger like so, itll take off with no control input at about 70 m/s

Edited June 24, 2017 by Kryten 2X4B 523P
added screenies, clarified a couple of things

[Spricigo]

4 hours ago, Kryten 2X4B 523P said:

drop the authority limiter on each down to about 20 to start with.

 Nah! increase to 150% and go do some aerobatics.

Seriouly, for a plane that small probably using elevon 1 shuld be enough maybe just a basic fin for vertical stabilazer (either bank to turn or just rely on reaction wheels for yaw).

Quote

However, if you change the landing gear as well to a taildragger like so, it'll take off with no control input at about 70 m/s

Interestings. Mine is a bit heavier, with have more wing area, high tail and 3 degrees of incidence. Takes off with no control at 50m/s. A bigger difference than I was expecting.

[Kryten 2X4B 523P]

5 hours ago, Spricigo said:

 Nah! increase to 150% and go do some aerobatics.

Seriouly, for a plane that small probably using elevon 1 shuld be enough maybe just a basic fin for vertical stabilazer (either bank to turn or just rely on reaction wheels for yaw)

Heh :-) As you probably know, control authority is all about how far the surfaces are from the COM vs how heavy the plane is - basic leverage principles. As its pitching from the tail, thats a lot of leverage given those winglets used for horizontal stabilisers.  Likewise for the roll control on the elevons. But yeah, a different tail set up using just the elevons for control wouldnt need as much adjustment.

 

5 hours ago, Spricigo said:

Interestings. Mine is a bit heavier, with have more wing area, high tail and 3 degrees of incidence. Takes off with no control at 50m/s. A bigger difference than I was expecting.

70 ms was the point I noticed it was in the air, took me by surprise actually! The actual take off speed is probably a little less, and yeah, the difference is most likely due to the extra lift you have with the bigger wings. If memory serves, you need about 1 lift per ton on the main wings for light aircraft (might be a little more, dont have my spreadsheet handy). That plane is about 4 tons, so ideally 4 lift total. Those swept wings have about 1.4 lift each (again memory), so thats 2.8 lift, a little on the low side - so it has to go faster to generate max lift to get in the air. Be interesting to see some pics of your plane.

[foamyesque]

On 2017-06-25 at 0:22 AM, Kryten 2X4B 523P said:

Heh :-) As you probably know, control authority is all about how far the surfaces are from the COM vs how heavy the plane is - basic leverage principles. As its pitching from the tail, thats a lot of leverage given those winglets used for horizontal stabilisers.  Likewise for the roll control on the elevons. But yeah, a different tail set up using just the elevons for control wouldnt need as much adjustment.

That's mostly true, but incomplete: There's also mass distribution and air resistance distribution to consider. A glider-style wing will allow you to pitch much more easily than a delta will, all else equal, for example.

[Kryten 2X4B 523P]

13 hours ago, foamyesque said:

That's mostly true, but incomplete: There's also mass distribution and air resistance distribution to consider. A glider-style wing will allow you to pitch much more easily than a delta will, all else equal, for example.

Oh agreed, but in the interests of keeping it simple for someone starting out with aircraft and as a starting point, it bears out. No point overloading someone who is already struggling.

[doggonemess]

On 6/22/2017 at 5:37 PM, Snark said:

As for your plane veering on the runway:  one possibility is that the landing gear's not perfectly aligned.  That's easy to happen.  To fix it, do the following:

1. Go into the SPH and open your plane.
2. Click the "3" key for "rotate", then click on your landing gear.
3. Press "F" to toggle between "absolute" and "relative" mode.  You want to set it to "absolute".  (It defaults to "relative", but once you set it to absolute, it'll stay that way for as long as you're in the SPH.)
4. Grab the rotate widget, then rotate your landing gear one "click" away from straight (on any axis, doesn't matter) and then back again.
5. Do this for all the landing gear on your craft.

That will guarantee that they're all perfectly aligned.  It may solve your veering-on-the-runway problem.  If it doesn't, post us a pic and we'll look into other possible causes, but that's the first thing to try.

Good luck! 

I've been playing this game for years now and I NEVER knew that. Thanks so much for this!

L.

[Daniel Prates]

You most certainly have to place your main landing gear closer to your CoM. Actually, right below it, then offset it just a little so that it falls toward your smaller steering wheel, which is not there to support weight but rather just to steer. In your design, it is pretty clear that pitching upwards must be really hard with that design. Besides, look how close to the wheels your elevator is! A tough angle to pivot on. 

In any case, remember that lift is also a consideration. Is your craft too heavy for those tiny wings? Conversely, does it fly fast enough to generate the necessary lift? Lift pushes upwards while weight pushes downwards; more speed generates more lift; wheels on the ground generate drag;  so you must come to a situation where your maximum land speed provides enough lift to surpass the weight. Not all designs allow for that, hence, they fail.

I play with F.A.R., a mod that makes aircraft design more realistic (and difficult), and I promise  that plane there would never fly with FAR enabled!