VTOL lift issue

[Majorjim!]

Hi all,

 

 Its's not often that I cannot figure something out in KSP but this one is a head-scratcher..

 

 I have made a flat rising VTOL (not my Thrust rig) That has a perfect COM/COT, no surfaces. It tips forward everytime when lifting off..

Anyone have an idea of what could cause this? I cannot share the craft file, sorry. I tried artificially offsetting the COM but it had no effect. This feels like some kind of phantom force but I cannot see where it comes from.

 

MJ

Edited December 12, 2015 by Majorjim

[sal_vager]

Not seeing the craft will make this difficult.

All I can think of is uneven thrust build-up which could happen if you mix jet engine types, for example my attempts to have a Goliath at one end and Wheesleys at the other of a vtol I was building ended in failure, due to the engines spooling up at different rates.

I changed the design to a single engine type and was able to make it work.

Craft file.

[Majorjim!]

37 minutes ago, sal_vager said:

Not seeing the craft will make this difficult.

All I can think of is uneven thrust build-up which could happen if you mix jet engine types, for example my attempts to have a Goliath at one end and Wheesleys at the other of a vtol I was building ended in failure, due to the engines spooling up at different rates.

I changed the design to a single engine type and was able to make it work.

Craft file.

No its not that.. It's a single, or rather two clipped junos. They fire directly between the COM. It's a replica of the LLTV.

 I just don't understand how with the COL and COM lined up it could do this. What could I be missing?

[Majorjim!]

OK, I accounted for the mass of the seated kerbal 0.93t. Even when perfectly balanced COM/COT it can pitch via RCS better forward than back.

 

 The Four RCS ports that provide control are equally spaced either side of the center of mass. I just dont get it.

[KerikBalm]

8 hours ago, Majorjim said:

No its not that.. It's a single, or rather two clipped junos. They fire directly between the COM. It's a replica of the LLTV.

 I just don't understand how with the COL and COM lined up it could do this. What could I be missing?

Ummm... maybe its a typo, but which are you looking at the CoL (the Center of Lift) or the CoT (the Center of Thrust)?

The CoL has nothing to do with the placement of your Junos

[sal_vager]

I have a thought, it may be air starvation causing one engine to lose thrust.

The LLTV/LLRV is not really designed for forward flight and was intended as a vtol test vehicle, so it won't have much airspeed, it turns out this is important in 1.0.5

The intake air resource isn't handled in the same way as 1.0.4 so the intake air gauge was removed as being redundant, the air is now based in intake area, intake angle, your speed and air density at your altitude on the current planet/moon.

But when stationary or hovering your airspeed is zero/near zero, so all intakes needed a base speed air value so you could start moving, effectively this is the intake sucking in air.

The Juno's maximum air need is 1.402 per second, so two of these need 2.804, it's not a lot so one small circular intake per juno should be enough even when stationary, but a single intake will cause one engine to run at reduced thrust and even flame out.

I'm guessing you have a juno in front and behind the CoM which would be why you pitch forwards, but are you using two intakes or maybe a single 1.25m circular intake for the look?  The 1.25m intake has the same base speed as the small intake so you'd still need two of them.

So to avoid this I think you need extra static intake air to feed the junos, you could clip in a few more small intakes and that should provide enough air even if stationary at full throttle.

Here's a pic of what I mean, with one intake I don't have enough static air so one of the junos loses power.

[Majorjim!]

4 hours ago, KerikBalm said:

Ummm... maybe its a typo, but which are you looking at the CoL (the Center of Lift) or the CoT (the Center of Thrust)?

The CoL has nothing to do with the placement of your Junos

No it's not that. I know the editor very well indeed. Thanks though.

 

3 hours ago, sal_vager said:

I have a thought, it may be air starvation causing one engine to lose thrust.

The LLTV/LLRV is not really designed for forward flight and was intended as a vtol test vehicle, so it won't have much airspeed, it turns out this is important in 1.0.5

The intake air resource isn't handled in the same way as 1.0.4 so the intake air gauge was removed as being redundant, the air is now based in intake area, intake angle, your speed and air density at your altitude on the current planet/moon.

But when stationary or hovering your airspeed is zero/near zero, so all intakes needed a base speed air value so you could start moving, effectively this is the intake sucking in air.

The Juno's maximum air need is 1.402 per second, so two of these need 2.804, it's not a lot so one small circular intake per juno should be enough even when stationary, but a single intake will cause one engine to run at reduced thrust and even flame out.

I'm guessing you have a juno in front and behind the CoM which would be why you pitch forwards, but are you using two intakes or maybe a single 1.25m circular intake for the look?  The 1.25m intake has the same base speed as the small intake so you'd still need two of them.

So to avoid this I think you need extra static intake air to feed the junos, you could clip in a few more small intakes and that should provide enough air even if stationary at full throttle.

Here's a pic of what I mean, with one intake I don't have enough static air so one of the junos loses power.

It's not that either as the two jets can only just lift the craft at full throttle and I had to add another intake before as one was flaming out.

 Thanks for the replys but i am still stuck as anything. It really makes no sense at all.. Is there a mod that shows the COM in game?

[katateochi]

If you are stumped then this has gotta be an odd one! My only thoughts are;

What happens if you hyper-edit it into space and just let it sit without SAS on? Does it spin on its own? 
I've just been having issues with a v small VTOL that wanted to roll left all the time. I'd done something I've done 100's of times before and clipped an SAS module inside something (using a cubic strut) but with this particular design that was causing phantom forces. Maybe a clipping problem? 
 

[Majorjim!]

Just now, katateochi said:

If you are stumped then this has gotta be an odd one! My only thoughts are;

What happens if you hyper-edit it into space and just let it sit without SAS on? Does it spin on its own? 
I've just been having issues with a v small VTOL that wanted to roll left all the time. I'd done something I've done 100's of times before and clipped an SAS module inside something (using a cubic strut) but with this particular design that was causing phantom forces. Maybe a clipping problem? 
 

I thought that too, I will test in orbit. Hack grav showed no effect though.

 I was thinking, even though the COM is lined up with the COT could it be that even though that is the case, that there is simply more mass upfront? Perhaps the motor is not in the exact center of the craft. To the VAB!

 

 I will report back Kat.

[katateochi]

17 minutes ago, Majorjim said:

even though the COM is lined up with the COT

I used to think that the 3 markers were 100% accurate, but my faith in them has been shaken a bit lately.  There was a bug (last version or one before?) where COT markers only referenced the last engine to be placed, or something like that. That resulted in really messed up COT markers, but I think that got fixed.  
Recently (in 1.0.5) I've had some really odd COL marker issues caused by wings being placed in unusual ways (actually not that unusual, just not perfectly straight).  This plane flies really nicely but it's COL marker would make you think otherwise

I've not had any reason to question the COM marker (we're still friends) but if the COT is lying to you then that could account for it. 

Oh and another thing (that I found with my little VTOL), when I launched it from the SPH the clipped parts would push each other out of alignment slightly and cause the craft to become asymmetrical. BUT if I reverted to launch then it would load up correctly and everything stayed aligned.  Try launching your craft, then revert to launch right away and see if that changes anything.  

[KerikBalm]

1 hour ago, Majorjim said:

No it's not that. I know the editor very well indeed. Thanks though.

I figured it was probably a typo, but it never hurts to ask.

While I can't imagine why you'd have problems if CoT aligns with CoM... and that is a strange problem... I assume some reaction wheels of moving the CoT forward a little bit should make it flyable, even if the problem is inexplicable...

[Majorjim!]

Just now, KerikBalm said:

I figured it was probably a typo, but it never hurts to ask.

While I can't imagine why you'd have problems if CoT aligns with CoM... and that is a strange problem... I assume some reaction wheels of moving the CoT forward a little bit should make it flyable, even if the problem is inexplicable...

No worries man, I appreciate the help.

I wont be adding any reaction wheels as it wold ruin the handling, ie just using RCS.

Just now, katateochi said:

I used to think that the 3 markers were 100% accurate, but my faith in them has been shaken a bit lately.  There was a bug (last version or one before?) where COT markers only referenced the last engine to be placed, or something like that. That resulted in really messed up COT markers, but I think that got fixed.  
Recently (in 1.0.5) I've had some really odd COL marker issues caused by wings being placed in unusual ways (actually not that unusual, just not perfectly straight).  This plane flies really nicely but it's COL marker would make you think otherwise

I've not had any reason to question the COM marker (we're still friends) but if the COT is lying to you then that could account for it. 

Oh and another thing (that I found with my little VTOL), when I launched it from the SPH the clipped parts would push each other out of alignment slightly and cause the craft to become asymmetrical. BUT if I reverted to launch then it would load up correctly and everything stayed aligned.  Try launching your craft, then revert to launch right away and see if that changes anything.  

 

 Yeah I suspected it could be bugged indicators but the two junos are clipped into each other and are thrusting directly down through the center of mass. I am really thinking it's just because the COM is not actually the center of the craft, but it's close.. Hmm. I have an evening free, so I will try a few things.

 Thanks again for the help chaps.

Edited December 13, 2015 by Majorjim

[Majorjim!]

Right i just tested an idea. I loaded the craft with a seated kerbal and zoomed in with the camera to see where the COM was. Guess what..

 It's a tad too far forward. This is odd as I over compensated for weight over the rear of the craft. So either the stated weight of a kerbal is incorrect or the COM indicator is giving a false reading. I will try to offset it further and see if it fixes the tipping.

[Majorjim!]

OK quick update,

 

 I am getting inaccurate COM indicators in the VAB. I can see it's off as the camera is fixed on a different COM ingame. This could possibly be due to the no offset limit mod but I am unsure. I just have to test the COM ingame now not in the VAB.. Ho hum.

[SlabGizor117]

Could there be uneven drag?  I realize it's the LLTV not a plane, but is their anything asymmetrical aerodynamically, but balanced, weight-wise?  

Edited December 13, 2015 by SlabGizor117

[Majorjim!]

Just now, SlabGizor117 said:

Could there be uneven drag?  I realize it's the LLTV not a plane, but is their anything asymmetrical aerodynamically, but balanced, weight-wise?  

No nothing. The tipping is immediate, before the front legs have left the ground. I can see it's a disparity between the editor COM indicator and the actual COM as experienced ingame.

[SlabGizor117]

Hmm..  Not sure, you may just have to start from scratch again if you can't figure it out..

[Raptor9]

There is a "bug?" that causes asymmetric thrust starvation when static. @BlueCanary and I have been discussing this issue off and on trying to recreate pre-1.0 VTOL's like the one below.  Both of us would throttle up and it would flip over to one side, even with all the reaction wheels fighting it.  It appears that you need a lot of intakes to generate thrust while stationary to prevent one jet engine from robbing thrust from another, but once you start moving, intake air increases and it's no longer an issue, so you can throttle up.

Edited December 14, 2015 by Raptor9

[Majorjim!]

Just now, SlabGizor117 said:

Hmm..  Not sure, you may just have to start from scratch again if you can't figure it out..

It is either a game bug or the no offset mod have caused it. I am building in a completely different way now and i think the game flips out sometimes.

 I can confirm I am getting different COM information from the editor to ingame.