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My spaceplanes fly like a lawn dart, no matter how I build them.


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Ahem, this is proof to the contrary :P

It's only an issue if CoL moves far in front of CoM.

Well... Be careful with this. If the CoL is even a little bit in front of CoM, the aircraft will want to nose up, which encourages a stall, instead of nosing down, which discourages a stall. It's less pronounced when they're close, but it's not what you want to aim for. CoM in front of CoL will always be more aerodynamically stable than CoL before CoM, though the latter may still be flyable.

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LD: That plane is the most stable design I have ever made... and that's with CoM behind CoL for almost the entire duration of the ascent to orbit; the only exception being after it switches to rocket mode for the last phase. Granted in most cases you do want CoM behind CoL; that plane's an exception. It actually becomes much harder to fly during the descent when CoM creeps in front of CoL. Doable, but much harder.

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He never posted a pic of his craft so no clue what you are talking about here.

Trust me, I've had many aircraft that have flown like lawn darts and in almost every situation the problem simply wasn't enough lift towards the front of the craft to help haul up the weight of the forward chasis and keep it pointed in that direction. I have over 525 hours invested in this game so far.. I've experienced things that just based on description can probably tell you what the cause is. Sure, he hadn't posted any pictures, but he didn't need to. I could tell just based on the description. It's either that, or I'm psychic, you pick.

I wish to note.. his pictures.. PROVED ME PERFECTLY CORRECT.. the center of lift was sooo far to the rear it's hilarious.

Truth is while having a center of mass near lift is important, even more important is having just enough lift in general for both halves of the aircraft to keep the craft on an evenly balanced trajectory when in level flight. You can't really overpower lift. It just results in more control It increases fuel costs so not good for career designs, but you can always stack on more engines unless you're in one of the realism sims.

I prefer to build practically though.

So in summary, yes.. yes... I knew exactly what I was talking about.

Edited by Maelstrom Vortex
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...CoM in front of CoL will always be more aerodynamically stable than CoL before CoM, though the latter may still be flyable.
Almost - the former is stable, the latter is always unstable. Which I guess is technically more and less stable, if you want to be specific. It's still flyable in KSP if it's only a little unstable, but only because of two things: One, the acceptable AoA range before stall seems to be pretty generous, if there is even stall in the first place. Second, the atmosphere in KSP is perfectly predictable, in that it's unmoving. It's small changes in airspeed, which translates into small changes in lift, which translates into small changes in pitching moment, that are the real danger when it comes to longitudinal static (in)stability.

That said, there are real-world aircraft that are intentionally unstable in this mode, first among them the F-16 Fighting Falcon, and many of the modern high-maneuverability fighters that followed. But these have flight computers and fly-by-wire control systems to deal with the constant small deviations from steady state that occur.

Back to the topic, though, KristopherKerlumbus, you also want to make sure you understand how the center of mass is going to move as the fuel is depleted. This can exacerbate any problems you already have.

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Seriously; what is it that you want to achieve?

If it's a good Mk3 spaceplane then do that - no-one else has yet.

I have build a few SSTOs of that size or larger before there were MK3 parts using the B9 parts which are the same size as the B9.

I mean this was from last year.....

Javascript is disabled. View full album

I am sure I can repeat the design with the new parts easy.

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I have build a few SSTOs of that size or larger before there were MK3 parts using the B9 parts which are the same size as the B9.

I mean this was from last year.....

http://imgur.com/a/4vqwI

I am sure I can repeat the design with the new parts easy.

Thank you and Wanderfound - Those are the first I've seen posted (not that I've looked very hard). No surprise to see it is you two presenting them :-)

Happy New Year.

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Thank you and Wanderfound - Those are the first I've seen posted (not that I've looked very hard). No surprise to see it is you two presenting them :-)

Happy New Year.

Likewise. :)

Have another one:

screenshot58_zpsa30efb21.jpg

http://forum.kerbalspaceprogram.com/threads/90747-Kerbodyne-SSTO-Division-Omnibus-Thread?p=1610271&viewfull=1#post1610271

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Firstly, thank you all for your help, I'm making great strides in building, and when I've felt confident enough, I'll definitely start posting pictures of my creations. ^_^

As a slight addendum, though, I think I already know the answer to this, but I just want to make sure--CoM near the rear of the craft will make it "tail-drag" as it goes through the air, and a CoM near the front of the craft will make it correspondingly nose-heavy, right? The ideal situation is slightly to the rear?

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Firstly, thank you all for your help, I'm making great strides in building, and when I've felt confident enough, I'll definitely start posting pictures of my creations. ^_^

As a slight addendum, though, I think I already know the answer to this, but I just want to make sure--CoM near the rear of the craft will make it "tail-drag" as it goes through the air, and a CoM near the front of the craft will make it correspondingly nose-heavy, right? The ideal situation is slightly to the rear?

Well... yes and no.

The center of mass is placed where it won't move. You build the plane so that the CoM is in the middle of the fuel tanks.

What you're adjusting is the center of *lift*. And yes, the center of lift should be either perfectly centered in the center of mass or just barely behind. If there's any imbalance at all, you want it to be slightly nose-heavy.

Best,

-Slashy

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Firstly, thank you all for your help, I'm making great strides in building, and when I've felt confident enough, I'll definitely start posting pictures of my creations. ^_^

As a slight addendum, though, I think I already know the answer to this, but I just want to make sure--CoM near the rear of the craft will make it "tail-drag" as it goes through the air, and a CoM near the front of the craft will make it correspondingly nose-heavy, right? The ideal situation is slightly to the rear?

Here is a simple way at looking at CoM and CoL.

Your Center of Mass is your center of gravity of the craft, if it were to sit on a head of pin centered on this it would sit perfectly balanced not tipping in any direction.

Now the Center of Lift is your lever point that will move that. If you have the CoL to far forward of the CoM, the craft will want to flip nose up when it flies. If it is to far back it will be to nose heavy and it would want to let the nose fall. It would be like holding a broom handle at the very back.

When an aircraft flies it is balanced on its CoL, this is what is is holding it in the air. It is a very important point on any aircraft, if this is shifted tofar one direction or the other it will affect the handling of the craft.

Some real aircraft have their CoL vs CoM in different locations in respect to each other, and depending on what they are designed to do. Like for example the old IJN Mitsubishi A6M Zero, that plane had its CoL almost on top of the CoM. It was designed to be as nimble as possible but still stable in easy to handle. But other aircraft like the Boeing 777, has its CoL a fair ways away from the CoM, but this craft has the handling charecteristics of a schoolbus, and is quite easy to maintain in level flight for long distances. But some newer 5th Generation fighters like the F-22A Raptor have their CoL slightly ahead of the CoM. But they have computers compinsate for the unstable flight characteristics of them.

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Original statement:

Well... Be careful with [CoL in front of CoM]. If the CoL is even a little bit in front of CoM, the aircraft will want to nose up, which encourages a stall, instead of nosing down, which discourages a stall. It's less pronounced when they're close, but it's not what you want to aim for. CoM in front of CoL will always be more aerodynamically stable than CoL before CoM, though the latter may still be flyable.
LD: That plane is the most stable design I have ever made... and that's with CoM behind CoL for almost the entire duration of the ascent to orbit; the only exception being after it switches to rocket mode for the last phase. Granted in most cases you do want CoM behind CoL; that plane's an exception. It actually becomes much harder to fly during the descent when CoM creeps in front of CoL. Doable, but much harder.

The emphasized lined was really my main point. I don't doubt your claim that it's a stable aircraft. As you state, though, that plane is an exception, and I was just trying to give good general advice.

Almost - the former is stable, the latter is always unstable. Which I guess is technically more and less stable, if you want to be specific. <snip>

Totally agree, I was just trying to be diplomatic. Calling the latter always unstable flies in the face of what armagheddonsgw is saying (see above).

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