Basic question on biig Space Plane Wings

[DundraL]

While its true you can just keep adding wings there is a point you want to add struts from piece to piece, and beyond that, if you want to make a truly huge aircraft, you need to add structural supports inside the wing, then strut the wing pieces to that. For example:

Note the big diagonal lines on the wings: Those are I-Beams that the wing pieces are strutted to in order to keep the wings from just bending.

Also dont forget to keep an eye on where your center of mass/lift are when your fuel tanks are empty!

[el_coyoto]

Concerning the "enough wings?" bit, I took note of results I found in some thread (that I can't find any more) on my old trusty notebook dealing with the lift rating/mass ratio and its relation with rotate speed at take-off.

The main objective of the thread was to help people with building planes that take off before the end of the runway, and I think it's a nice trick to use to guess wether your plane flies badly because it has not enough wings or if it's another problem.

The thread defined "lift rating" as the sum of lift number of aero parts divided by the mass of the plane.

The author provided numerical results which I reproduced fairly accurately on a few tests, so it seems like a nice approximation of how much wing you'd need on a plane :

[table=width: 500, class: outer_border]

[tr]

[td]Lift rating[/td]

[td]Test result[/td]

[/tr]

[tr]

[td]0.36[/td]

[td]failed to take off, end of runway, 110 m/s[/td]

[/tr]

[tr]

[td]0.68[/td]

[td]Rotate @65 m/s[/td]

[/tr]

[tr]

[td]1.06[/td]

[td]Rotate @45 m/s[/td]

[/tr]

[/table]

So, if at some point in the SPH I want to check if my plane has enough wings without leaving the SPH, I quickly sum the lift of the biggest aero parts (laziness) my design uses and check if it has a lift rating that I like.

For a SSTO, I usually go with ~0.5 : it will need quite a bit of runway to take-off, will require more speed while landing to avoid stalling, but more lift would mean more drag, which is a real problem with spaceplanes...

[Alshain]

Concerning the "enough wings?" bit, I took note of results I found in some thread (that I can't find any more) on my old trusty notebook dealing with the lift rating/mass ratio and its relation with rotate speed at take-off.

The main objective of the thread was to help people with building planes that take off before the end of the runway, and I think it's a nice trick to use to guess wether your plane flies badly because it has not enough wings or if it's another problem.

That's sort of what I do also, I use engineer to mass the plane though. Anyway, back to topic. I don't believe his issue with lifting is because of a lack of wings, it's more a matter weight distribution and possible landing gear position. The craft is just a little over-designed and needs to trim the fat as I mentioned earlier. That's something you learn in time, I still have a few of my early planes and I load them up and think "how in the world did this thing ever get off the ground?".

I think it's a tendency to pack a lot of fuel because your afraid you might run out, but that becomes diminishing returns on planes really quickly. It helps to know before hand what your building the plane to do. If you building a plane to get to a station in LKO, don't build it with enough fuel to get to the Mun. Likewise, it takes time to learn exactly how much fuel a plane will consume, DeltaV guidelines don't work well like they do with rockets because of the Jet engine efficiency.

Edited October 3, 2014 by Alshain

[Wanderfound]

Takeoffs for FAR users:

* Estimate or experiment to discover what speed you can reach before the end of the runway. Enter this speed in Mach on the Data & Stability Derivatives analysis screen. Leave pressure at 1.225 and temperature at 20. Select whatever flap setting you want to use, if any.

* Look at the "level flight" figures on the right of the screen. If the AoA listed there is below 10° or so, you should be able to rotate and lift off easily so long as you haven't messed up your gear placement. If the AoA is less than the resting ground pitch of the plane, then it will self-launch as soon as it gets to the right speed.

To demonstrate:

That's showing that this plane, when the flaps are set to 2, requires a 5.7° angle of attack to maintain level flight at 110m/s.

Edited October 3, 2014 by Wanderfound

[Alshain]

Wanderfound, I think the OP is stock aero. He never mentioned FAR anyway.

Edited October 3, 2014 by Alshain

[Wanderfound]

Wanderfound, I think the OP is stock aero. He never mentioned FAR anyway.

Yeah, I know. Writing for the lurkers.

[davidpsummers]

Thanks for all the comments.

I hadn't worried about COM/COL because I was just playing around with attaching wings.

What are the restrictions on air intakes? Can you mount them radial and mount the engine inline? Or do the airtakes have to be inline with the engines.

On you get speed and altitude and turn on the rockets, what direction do you point? 45 degrees like with a rocket or closer to the horizon?

Is it just my imagination, or does the crew outside of the SPH drive crazier than the crew outside of the VAB?

[Alshain]

Intakes can be mounted anywhere you like.

When you turn on the rockets, you want the raising of your Periapsis to take priority over Apoapsis which will raise naturally on it's own, so aim 10 degrees or so and throttle way back to save fuel, which is very important at this stage. Keep the apoapsis just ahead of your plane till it hits 70km. I like to keep my time to Ap at 1 minute the whole time if possible, but once your Pe gets close to positive you can ignore that. When you hit 70km Ap, ideally you want your Pe to be positive, that will reduce your final circularization burn to almost nothing and save a ton of fuel.

[davidpsummers]

Intakes can be mounted anywhere you like.

When you turn on the rockets, you want the raising of your Periapsis to take priority over Apoapsis which will raise naturally on it's own, so aim 10 degrees or so and throttle way back to save fuel, which is very important at this stage. Keep the apoapsis just ahead of your plane till it hits 70km. I like to keep my time to Ap at 1 minute the whole time if possible, but once your Pe gets close to positive you can ignore that. When you hit 70km Ap, ideally you want your Pe to be positive, that will reduce your final circularization burn to almost nothing and save a ton of fuel.

How does throttling back save fuel? Because you aren't quite clear of the atmosphere yet?

Some other random questions....

If you see air friction flames, is the normal or are you too low for your speed?

Why do you have your COL just _behind_ the COM. One tutorial said it was to avoid "excessive maneuverability", whatever that is. I know that as long as I have them close, it doesn't seem to matter?

[Wanderfound]

How does throttling back save fuel? Because you aren't quite clear of the atmosphere yet?

Some other random questions....

If you see air friction flames, is the normal or are you too low for your speed?

Why do you have your COL just _behind_ the COM. One tutorial said it was to avoid "excessive maneuverability", whatever that is. I know that as long as I have them close, it doesn't seem to matter?

A jet engine at full throttle will flame out (obliging you to light the fuel-hungry rockets) at a much lower altitude than the same engine at partial throttle. Ideally, you want to shut down to one engine, then gradually wind the throttle back just in front of flameout until there's no more throttle to wind.

Flames imply high drag, but it doesn't matter much on jet engines. Flames during rocketry is wasteful, though.

CoL: to a certain degree, lift is synonymous with drag. An object moving through the air will tend to rotate so that its centre of drag is behind its centre of mass. Think about which bit of the dart has the metal and which bit has the feathers.

With KSP's overpowered torque wheels, it is possible to get away with CoL slightly in front of CoM, particularly in stock aerodynamics. Without the torque, any deviation from straight flight would eventually be magnified into a complete flip if not rapidly contained by control surfaces.

Set your design so that CoL is overlapping but behind CoM, whether the tanks are full or empty. CoM closer to CoL = more manoeuvrable; CoM further from CoL = more stable.

EDIT: just realised you were asking about throttled-down rocketry. I don't tend to do that myself, but it does give you more time in which your wings are generating lift, although the value of that will depend on your ability to avoid stalls.

Edited October 4, 2014 by Wanderfound

[Ruedii]

I didn't mention before, but your best stock rocket engine for space planes is the aerospike. You can use your rocket to improve takeoff thrust as well, and then turn it off once you are off the ground.

[Alshain]

I didn't mention before, but your best stock rocket engine for space planes is the aerospike. You can use your rocket to improve takeoff thrust as well, and then turn it off once you are off the ground.

I hate terms like "best". It's so subjective. I like the aerospike a lot, but I wouldn't say it is "best" (nor would I say it isn't "best"). I have a great Kerbal rescue plane in my exchange thread that uses 48-7S, and several that use RAPIERS. They work every bit as good as the aerospike. Don't limit yourself to just one option, or even one definition of "best", because that aerospike has no gimbal and is crap for steering once you leave the atmosphere.

[Ruedii]

Another thing I forget to mention one thing before, it seems that while the wheels can land at very high forward velocities (well above 150m/s), the do have a limited vertical speed (less than 10 m/s to be absolutely safe.) So the faster your plane lands, the lower it's landing angle.

This usually means you have to take a faster approach after you pass the mountains to get down to your standard glide angle. If you are using FAR, you will likely have to use both types of flaps to improve your wing lift and increase your craft's drag.