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[1.x.x] On the particulars of Center of Lift and Center of Mass on winged craft


Val

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On 12/1/2016 at 0:12 AM, AeroGav said:

From the debug menu, I enabled drag data in right click menus, and made a note of lift and drag on the wing sections at different angles of incidence.

Mach 1.7                
                 
Incidence Angle 1 2 3 4 5 6 7 8
Lift 19.5 39.7 59.9 7.63 9.15 10.68 7.32 8.14
Drag 7.8 8.29 9.1 0.88 1 1.13 0.77 0.86
L/D ratio 2.5 4.788902 6.582418 8.670455 9.15 9.451327 9.506494 9.465116
                 
                 
200 M/S                
                 
Incidence Angle 1 2 3 4 5 6 7 8
Lift 8 16.1 24.21 35.98 42.73 49.48 59.93 66.72
Drag 0.55 0.58 0.64 0.73 0.83 0.94 1.24 1.39
L/D ratio 14.54545 27.75862 37.82813 49.28767 51.48193 52.6383 48.33065 48

Man, THIS.  I've been redesigning some planes based on this info, and it's changed everything.  I find I can do with less wing surface, now that they are making their max effort at 6-8 degrees.  Lower takeoff and landing speeds, it seems easier to make the plane stable even with the CoL very close to the CoM, and with fewer drag surfaces I'm getting higher speeds, lower mass, and making orbit with less effort.  Thank you so much!

Is this the right place to talk about flaps?  I'd like to really optimize some high-speed low-drag designs with flaps to add lift for takeoff and landing, and help get landing speeds down on Duna without building vast gliders.  Problem is - the usual placement of control surfaces at the rear of the main wing (or god forbid at the back of a delta wing) induces a colossal pitch moment once deployed.  OK, fine - I'll move them to the underside of the main wing, so their lift vector lines up with the overall lift vector when deployed (which, incidentally, is hard to visualize in the SPH, so it's a lot of back and forth to launch and F12).  So now there's no pitch moment, but the amount of extra lift is small, but the amount of extra drag is huge.  That's because - DUH, I now realize - additional pitch beyond the ideal 6-9 just puts it into stall.  So used that way it's just an airbrake, and effectively increases my vertical speed on landing, making the problem WORSE.

Is there a way to use flaps effectively in KSP?  Perhaps set the deploy authority so when deployed they match the 6-8 on the main wing, undeployed they are flat to prograde for minimal drag, and scale the amount of control surface until the control authority is right instead of just monkeying with the slider?  Use front and rear pairs to cancel the pitch moment - or does that just add part count and drag for no benefit beyond simply adding more wing?

Thanks

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Further food for thought on canards vs. tail planes:

Tail plane control surfaces decrease in AoA as you pitch; max deflection produces weaker lift the higher your craft AoA

Canards are the opposite: AoA increases the further your craft is from prograde. Stalls of canards still create a retrograde force as drag increases and now you are pulling back on the nose!

I currently believe the secret to stable canard designs are enough passive stability that the canard cannot overcome it even in the worst case (radial ship orientation with control plane perpendicular to airflow). A sufficiently large, tailless, delta wing helps here.

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21 minutes ago, fourfa said:

Is there a way to use flaps effectively in KSP?  Perhaps set the deploy authority so when deployed they match the 6-8 on the main wing, undeployed they are flat to prograde for minimal drag, and scale the amount of control surface until the control authority is right instead of just monkeying with the slider?  Use front and rear pairs to cancel the pitch moment - or does that just add part count and drag for no benefit beyond simply adding more wing?

You can "limit authority" on the item used as a flap so that when "deployed" the total of it's angle and the wing it attaches to + airplane body aoa does not exceed 30 degrees.

You can put flaps ahead of cg to balance the ones behind and get around trim problems , eg. put some on the canards as well as main wing.

Note that flaps put on the leading edge of the wing can't hinge down like real life slats  - they have to deploy upwards if you want them making lift!   

Which brings us onto the main problem - they don't work like real flaps do, which are only a small percent of total wing area, but increase lift across the whole wing by increasing the apparent curvature.  In KSP the act of deflecting only affects the lift produced by the flap itself, so the overall effect on landing speed is not worth the trouble.

What might be better - on a Duna airplane, with v  large wings, you have the problem of making too much lift during the speedrun at 22km.  Lowering the nose to suppress this, if the wings have incidence, creates negative body aoa and high drag.  Some kind of spoiler might help here?  

28 minutes ago, ajburges said:

Further food for thought on canards vs. tail planes:

Tail plane control surfaces decrease in AoA as you pitch; max deflection produces weaker lift the higher your craft AoA

Canards are the opposite: AoA increases the further your craft is from prograde. Stalls of canards still create a retrograde force as drag increases and now you are pulling back on the nose!

I currently believe the secret to stable canard designs are enough passive stability that the canard cannot overcome it even in the worst case (radial ship orientation with control plane perpendicular to airflow). A sufficiently large, tailless, delta wing helps here.

But, what happens when the main wing stalls and creates retro torque on a tailed design? The main wing is ahead of cg, not by as much as a canard, but it is larger.

The test i did, with two otherwise identical planes on re-entry,, with same relative cg + col,  the canard and tailplane were able to hold the nose up radial out to the exact same altitude before aero forces forced them unstalled.   These were both very stable aircaft, not able to pitch much more than 10 aoa however.

You can try out my little "ghoul" jet ,  it is so small, the cockpit reaction wheels can force the nose to 45 degrees aoa at low speed.  The main wing is at 5 degree incidence so will be stalled by 25.

https://kerbalx.com/AeroGav/Ghoul

20170108040657_1_zpsaeom1obm.jpg

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  • 1 month later...
On 18/1/2017 at 9:08 AM, Boris-Barboris said:

AC is CoL by definition in my vocabulary. There is another way to call things though - let AC be AC and CoL be blue dot in KSP. Then CoL is approximation of AC under long list of assumptions.

This. I think it is the source of most of the arguing in this thread. The 'blue dot in KSP' may be called CoL, but it has little to nothing to do with 'the resultant of adding all the aerodynamic force vectors acting on the ship at any AoA' (and there are some simplifications in that definition, I know, but let's not confuse the readers unfamiliar with vector calculus further). That thing would actually be called the Center of Pressure (CoP), at least by me, and it is the thing that is important for stability, and can by the way be influenced by control surfaces and/or other things that change the lift/drag characteristics. @Val calls it Aerodynamic Center (another name I've heard for it), and it is the thing KSP shows... when you fix it by installing your awesome Correct CoL mod. Thanks for that mod, BTW, very useful. :)

The problem is that the 'blue dot in KSP', on a stock install, only takes into account the lift force created by parts with lift rating, and disregards body lift and drag like they didn't exist. We all know that in this thread at this point, I think, but we don't know that it is the thing the other guy is talking about! :rolleyes:

BTW, just to confuse things further and condense everything into a sentence: the thing to do for stability to get a maneuverable yet still stable plane is to put the CoL (the real one, not the 'blue dot of KSP') on top or slightly in front of the CoM, but put the CoD (Center of Drag) behind it. That would mean the CoP (which is short of an average of the two) is behind the CoM, so stable, but still as maneuverable as it can be. All of this for a given static AoA, of course, then you would have to evaluate stability in the rest of the AoA envelope, and then repeat everything but looking at it from a dynamic point of view...

 

Rune. We need a CoD indicator, that's what we need! :P

Edited by Rune
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5 minutes ago, Boris-Barboris said:

You've got everything backwards :confused:

Or we are misunderstanding each other further. :wink: I think I get that what you think I should have written instead is 'the thing for a maneuverable yet still stable plane...'. I sort of implied this was a recipe for stability, when it's the edge case that binds the region of stability.

 

Rune. Don't worry, I get the thing. I may not be able to express it correctly in a foreign language, but I actually studied it in uni. :wink:

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  • 2 weeks later...
  • 1 month later...

Love this guide. I hit some of the issues explained recently, with a spaceplane that was stable subsonic but flipped on re-entry. Discussing it with someone I managed to resolve the issues but the guide here really illuminates what I was previously stumbling towards.

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  • 1 year later...

I re-opening this. 

I was playing for last week with Space shuttle stock configs.  

I managed to get a much better re-entry so it is no longer flipping or doing let's try spinning that is a good trick. 

I did it via adding a bit more weight on the front ( cabine and airlock)  as before the front was too light so during that 40 degree pitch on re-entry it starts flipping and going to spin and out of control. 

Now it is a bit of opposite.  The re-entry si good no flipping or spinning but due to added weight to front (  I removed the monoprop from OMS and engine mount  and put it only to front cabine)  the front is a bit heavy and when atmosphere is starting to hit the shuttle I am having a hard time to keep that 40 degree pitch. It is simply going down and when I manage to keep at least 20 degree pitch I am happy. I have to use RCS and keep pressing E to keep that 20 degree at least.

E7vNTLI.png

So as you can see via config I was able to put CoL to the center I think :) And CoM is in the center too when the monoprop is empty in front.  But due to added weight as I wrote the shuttle tends to put the nose down and not holding the pitch as I want.     So I was thinking what if in this case of shuttle I need to have CoL a bit either in front of CoM or a bit behind of CoM ? That somehow it can not be in the perfect center with CoM in this case.  Could it be better so it will keep that 40 degree pitch during re-entry ? 

Or simply I need to  still play with weight in the front parts to makethe weight more even between front weight and back weight ?  

 

Edited by Jovzin
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  • 1 month later...

What I like to do in those cases where you have an aircraft where 2/3 of its flight profile is great - but then say on re-entry, excessive stability causes nose-prograde lock and overheating-

It can be a huge advantage to have fuel tanks staggered both in the extreme fore and aft ends of your aircraft.  It's harder to design around than near-CoM fuel setups, but it gives you a powerful tool to dynamically alter CoM on re-entry, which you can fine-tune to get your desired re-entry AoA.

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  • 1 year later...

Hey, I read this while I was developing my some heavy SSTOs and its been pretty impactful, but something I've been wondering is how you get the perfect angle, since in the stock game the lowest you can rotate object with the rotate gizmo is 5 degrees on snap, and on the free rotate you cant really figure out exact angles. How did you get your perfect angles to exact degrees? 

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On 4/19/2020 at 7:41 PM, Its_Marc said:

Hey, I read this while I was developing my some heavy SSTOs and its been pretty impactful, but something I've been wondering is how you get the perfect angle, since in the stock game the lowest you can rotate object with the rotate gizmo is 5 degrees on snap, and on the free rotate you cant really figure out exact angles. How did you get your perfect angles to exact degrees? 

I'm glad you had use of it.

I used a mod to allow me to rotate it in 1 degree increments. I think, it was called Editor Extensions.

I'd start out at 5 degrees. Time it to 450 m/s in level flight at sea level (if it'd even do that). Then go back to hangar and and make small adjustments up or down, test again to see if would shorten the time.

In my opinion, exact angles aren't really required, though, except for reproducibility, like if you have to pull the wings off for a redesign, and want to put them back on at the same angle.

I haven't actually played KSP for a while, so I can't really recommend a current mod to help you. Sorry.

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