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Xyphos

Stock VTOL Planes still have uncontrollable torque

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Posted (edited)

I've been experimenting a bit with Stock VTOL planes.
I'm using RCS Build Aid to display the WCoM, DCoM and Torque.
I've managed to align both CoM's almost perfectly with only 0.326 difference in torque.
I've added reaction wheels to counter the torque difference.
darn thing still pitches up uncontrollably and flips upside-down no matter what I do.

any advice?

 

UPDATE:
I think I fixed it. Craft file found here: https://kerbalx.com/Xyphos/Mk3-VTOL-SSTA

Edited by Xyphos
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Does your VTOL has wings? Are you sure that your wings does not have negative angle of attack before flipping starts? Remember that Mk2 fuselages generate noticeable amount of lift and torgue that flips your VTOL can come from offset between CoL and CoM.

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1 minute ago, 1greywind said:

Does your VTOL has wings?

considering it's a plane, I should hope so.... but it's a Mk3 plane. is it really a CoL issue?

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Posted (edited)

A picture would definitely help if you could manage it.

Is your vertical center of thrust in line with your center of mass? I usually check this by lowering the thrust on the horizontal engines to zero.

Also from personal experience, I've found that some of my vtol designs are more stable with less throttle when doing vertical take off. Rotating the lift engines so they are pointing out away from the COM by a few degrees can help stability as well. (However you will lose some power.)

Edited by Rocket In My Pocket

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It sounds like CoL. Sometimes you ahve to offset the CoM and CoT to counteract it.

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1 hour ago, Rocket In My Pocket said:

Rotating the lift engines so they are pointing out away from the COM by a few degrees can help stability as well. (However you will lose some power.)

Physically speaking this should not be the case, unless it's some consequence of ground effect (not in KSP) . I wonder if and how it really happens.

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I'd be interested to see it, if you can post a craft link... 

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Posted (edited)

28 minutes ago, SchweinAero said:

Physically speaking this should not be the case, unless it's some consequence of ground effect (not in KSP) . I wonder if and how it really happens.

In my experience it does not.

Arguably you gain a few degrees of gimbal if you are using gimbaling engines, but since you can not individually throttle engines what you gain on one side is your loss on the other. So I'd say point all engines straight down, with their combined thrust vector going exactly through the CoM obviously.

Edited by Dafni
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Posted (edited)

29 minutes ago, SchweinAero said:

Physically speaking this should not be the case, unless it's some consequence of ground effect (not in KSP) . I wonder if and how it really happens.

Let's say you have 4 lift engines placed around the COM, if they are straight up and down your plane is being lifted on a very narrow column of thrust making it tippy and unstable.

Angling them out widens the base of this "pillar" and provides more stability. It's basically the same reason that having multiple lift engines is more stable than just having one.

Edited by Rocket In My Pocket
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Just now, Rocket In My Pocket said:

Let's say you have 4 lift engines placed around the COM, if they are straight up and down your plane is being lifted on a very narrow column of thrust making it tippy and unstable.

Angling them out widens the base of this pillar and provides more stability. It's basically the same reason that having multiple lift engines is more stable than just having one.

Unfortunately this is not true in KSP. You dont get a "pillar" of thrust. You are always balancing on the same "needle" of thrust, as KSP simulates a combined vector of thrust just as such, a vector, no matter how far away from each other you tilt the engines.

Someone correct me if I am wrong and such a kind of phyisics has been introduced. Thats just how far I understood it last time I looked into how the game does these things.

Consequently the statement of having multiple engines resulting in more stabilty is also questionable. The only difference is how far away your CoT is from your CoM, as far as on-power stability goes for VTOLs (when you ignore aerodynamics obviously)

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Posted (edited)

1 minute ago, Dafni said:

Unfortunately this is not true in KSP. You dont get a "pillar" of thrust. You are always balancing on the same "needle" of thrust, as KSP simulates a combined vector of thrust just as such, a vector, no matter how far away from each other you tilt the engines.

Someone correct me if I am wrong and such a kind of phyisics has been introduced. Thats just how far I understood it last time I looked into how the game does these things.

Consequently the statement of having multiple engines resulting in more stabilty is also questionable. The only difference is how far away your CoT is from your CoM, as far as on-power stability goes for VTOLs (when you ignore aerodynamics obviously)

*Cough cough.*

Spoiler

If you're not doing it already, angle your VTOL engines outward

OIuC6L3.jpg

The above image is greatly exaggerated. 5-10° should be enough.

When the craft leans forward, the front VTOL engines start pointing more vertical, while the rear ones are less vertical. This means the front VTOL creates more downward force. It becomes a form of self-correcting balance.

Likewise if it leans in any other direction.

It does require at least 3 VTOL engines, to work in all 4 directions, but easier to do with 4 or more.

 

Edited by Rocket In My Pocket
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Posted (edited)

Just saw your pic/craft on KerbalX...  that's just gonna be tough!  You've got a lot of dispersed mass riding up on a pinpoint of thrust.  For bigger VTOLs I really think you have to have 2 or 3 points of thrust, and as you burn off fuel you've got the shifting COM issue.  This one might need to just go back to the drawing board a bit.

Edited by XLjedi

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1 minute ago, Rocket In My Pocket said:

*Cough cough.*

  Reveal hidden contents

If you're not doing it already, angle your VTOL engines outward

OIuC6L3.jpg

The above image is greatly exaggerated. 5-10° should be enough.

When the craft leans forward, the front VTOL engines start pointing more vertical, while the rear ones are less vertical. This means the front VTOL creates more downward force. It becomes a form of self-correcting balance.

Likewise if it leans in any other direction.

It does require at least 3 VTOL engines, to work in all 4 directions, but easier to do with 4 or more.

 

I stand corrected.

thanks for sharing.

(hangs head and blushes)

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Posted (edited)

38 minutes ago, XLjedi said:

I'd be interested to see it, if you can post a craft link... 

I think I fixed it, here's the download: https://kerbalx.com/Xyphos/Mk3-VTOL-SSTA

BUT - even if it's stock parts, it uses two mods for VTOL operation; "SASAG" and "Control From Here" - you can download those mods from my "useful mods" link in my signature below. SASAG is used to quickly set SAS to Radial-Out during VTOL flight and normal SAS during airplane mode. CFH changes the control point so the navball updates it's orientation, depending on the mode.

IMPORTANT: before flight, set the brakes and stage the RotoKlaw's decoupler, then switch vessels to the RotoKlaw and hold Q until it stows away and locks in place. The RotoKlaw is used for refueling from asteroids, and docking with other vessels that won't fit the shielded docking port.

it has enough delta-v and thrust to VTOL it's way to/from 10km Tylo orbit but if you're going to test it on Kerbin, either drain some fuel out of it or use the gravity hack, as it wasn't meant to VTOL on Kerbin. RCS is recommended for VTOL flight.

Edited by Xyphos
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though:

17 minutes ago, Dafni said:

..but since you can not individually throttle engines what you gain on one side is your loss on the other...

what gives? is this argument false then?

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Just now, Dafni said:

thanks for sharing.

No worries, that's what forums are for, discussing ideas that are often conflicting.

I'm never against the idea that I might be wrong, which is why I had to go digging for some kind of evidence that it wasn't all in my head lol.

(And who's really to say why it works, I might still be wrong about the wide pillar of thrust thing as Val says it's more about the engines producing more vertical thrust when tilted.)

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and back to the drawing board... VTOL works now that I moved the wings, but I need more RAPIERS and yet another rebalance before I can take it to orbit

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1 minute ago, Xyphos said:

and back to the drawing board... VTOL works now that I moved the wings, but I need more RAPIERS and yet another rebalance before I can take it to orbit

One thing I'd mention...  the takeoff TWR is like 1.07, which will get you off the ground.  You may find that having close to neutral thrust coming in to land vertically is going to result in a lot of destructive "pancake" landings.  Lower gravity locations, not so much an issue, but for vertical landing on Kerbin I like to have more in the 1.5 to 1.7 range.  But looks like a neat craft, thanks for sharing it!

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Posted (edited)

1 minute ago, XLjedi said:

the takeoff TWR is like 1.07

it wasn't meant to VTOL on Kerbin tho... I was just testing it with hacked gravity 0.62

Edited by Xyphos
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Posted (edited)

@Xyphos I didn't check your weight with near empty tanks either...  extra V-thrust might be overkill for a craft like this.

Edited by XLjedi

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check the full tank Tylo take-off then...

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Just now, Xyphos said:

check the full tank Tylo take-off then...

I think you've got it well under control!  Have fun!

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@Xyphos Yes Mk3 fuselages also generate lift (actually after "new" aerodynamics was introduced any part, including structural plates, girders, can generate lift if they have angle of attack, but Mk3 and Mk2 make lift better).

Sometimes it is a pain to balance CoL on VTOL so it can both take off vertically and land horizontally - remember that you can use flaps or air breaks in action group to do this in flight.

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30 minutes ago, Rocket In My Pocket said:

*Cough cough.*

 

Ermm.... but "downward force" doesn't mean anything? Say I have a vtol with 2 engines mounted 3m to the sides canted 10 degrees out. When the VTOL is level, they produce 0 net torque. 

Now let's say the VTOL tilts over to 10 degrees. The engines are still in the exact same positions on the ship, just on happens to be tilted 0 degrees relative to the surface of Kerbin (or whatever planet), and the other 20 degrees. They are still producing the exact same thrust and in the exact same positions on the ship, therefore they also produce 0 net torque. Your overall thrust vector is now tilted 10 degrees. The only way you would get any stabilization would be with special aerodynamics and/or ground effects, which we do not have.

You might want to take a look at this, as it is a fallacy that would have similar stabilization effects: https://en.wikipedia.org/wiki/Pendulum_rocket_fallacy

Try turning off SAS in a vtol with the engines canted out. If such a stabilization effect occurs, it should stay in a hover on its own. But it won't, it will slowly (or quickly) tip over until it crashes into something, in the exact same way as one without canted engines.

Given how close the engines are to the horizontal plane of the uh, plane, I'm guessing what might be experienced as a "stabilizing effect" is the engine gimbals having more power over the aircraft's attitude. (It's hard to explain why this works in words, I'll try to make a picture explaining it in a bit, perhaps when I get back from work)

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1 hour ago, EpicSpaceTroll139 said:

-snip-

I never said anything about downward force, I'm just quoting what a moderator said about it since it supports my conclusion that this is an actual tangible effect others see in the game and it's not just me being crazy. Lol.

As I said though; who's to say why or how it works, it's just a video game with a very arbitrary definition of physics.

The important thing for this conversation is that it does actually produce a noticeable improvement in stability which players can utilize.

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Posted (edited)

31 minutes ago, Rocket In My Pocket said:

The important thing for this conversation is that it does actually produce a noticeable improvement in stability which players can utilize.

I'm still inclined to think the effect is psychological, but I will now make sure to test it for myself. It might have something to do with the aerodynamic forces on vertical and angled nacelles with lateral velocity. 

Edit: the results are in and it seems @Dafni was right. Having engines far from the CoM and tilted toward the CoM are two ways to give their gimbals a longer lever arm - better control authority. If you lock gimbals, all-vertical and tilted setups work identically except for thrust loss. 

Edited by SchweinAero
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