Help Building Large Aircraft

[Mister_Spaceman]

So, I've been playing KSP for a while. Close to two-thousand hours, in fact. Yet I still can't seem to make a functional cargo plane. All large craft that I build seem to be practically unable to fly, no matter how much thrust it has. I always take care to make sure to generally follow this guide when making aircraft, and I don't believe that aircraft size would change any of these basic principles. Large crafts are always sluggish, most likely do to weight, slow or unable to take off, due possibly to weight, TWR, or lack of lift. However, I normally have plenty of control surfaces, wing area, and engines. What could I be missing? How do I get my big planes to take off? Are there any tips or tricks you have?

[Serenity]

Can you post the aircraft you having problem with?Its usually easier to learn from the errors of your own craft, the community here is very ''gentle'' dont worry  .

Edited April 28, 2019 by Boyster

[Aeroboi]

Can you post a picture of a craft your working on right now? In any case I'm very experienced in building large SSTO's.
To be blunt, in fact it are heavier SSTO's that are more efficient, assuming you can make one efficient. The reason being is the involvement of weight and drag. Drag reduction is done through proper coning and placing every non fuselage, wing or landing gear part inside a cargo bay. Every structure from 1.25m to 5m has a cross section. Of course for 1.25m the cross section is 1.25m squared, unless your fuselage is on a pitch angle causing the wind to hit the underside of your fuselage tanks. Using a small incline of rotation of the wing chord line (wing indcidence:use internet search) makes sure the fuselage and nose stay at prograde causing a 1.25m cross section to stay close to 1.25m squared.

The thing with the drag model is that it scales up from 1.25m to 5m but the cool thing with weight is that it has momentum associated with it and the heavier the vessel the more momentum. That means the vessel wants to retain speed as it goes because something very heavy is less easier to deccelerate through drag then something lighter.
Therefore it is more efficient the heavier your space plane is. In any case it is far easier to break 400m/s due to the high momentum of a heavier space plane.
Try to use as few stacks as possible (stacks of fuselages)
If you want to design a spaceplane and you only need 1 stack in the middle then stay there. If you need more then 1 engine on a one stack space plane you can use the adapters. It is better to use 2 way adapters then to attach another fuel stack.

I"ve made this https://kerbalx.com/Aeroboi/Hearts-Chevron-96-Cargo-Lifter-2375MC
Maybe that's a bit to excessive but it's a example. How large a craft were you thinking off exactly? Other then for large space stations, mothership modules the space plane above is overpowered for normal applications. For most uses normal 2.75m or 3.75m one stack space planes using mk3 cargo bays can tug anything into orbit you need for a mission.
IIRC you can pack a tightly packed eve lander inside a mk3 cargo bay.
What kind of cargo do you intend to haul?

[Mister_Spaceman]

9 hours ago, Aeroboi said:

Can you post a picture of a craft your working on right now?
Try to use as few stacks as possible.
How large a craft were you thinking off exactly?
What kind of cargo do you intend to haul?

I don't have any pictures readily available, because I don't typically keep unsuccessful files. When you say use as few stacks, do you mean horizontal stacks, say, a central fuselage, with two fuselages radially attached, or "vertical", like a 1.25m, with 2.5 behind it, and so on? I typically use a typical cargo plane setup, see a C-5 or C-130. By large craft, I mean the general size of a Mk3 Cargo plane. I mean to haul typically cars/rovers around Kerbin for contracts. Opening KSP now, and I'll throw together a typical example of one to get some pictures.

[Aeroboi]

well, your fuselage is always horizontal so never vertical. So yes that is what I mean. When you attach 2 other tanks in 2 way symmetry to the center tank then it's called parallel attached tanks or boosters if you put it on decouplers. Depending on type of plane, mk3 in your intended case the best solution for aerodynamics is to put engines at the back. If you build the type of aircraft your speaking off I assume your using the Goliath or wheesley's under the wing.

Best is to scale amount of engines based on engine per mass. So if you got a lightweight airplane using 1 engine to lift 10 tons then a 100Ton airplane would need 10 of these engines. Do you?

 

Edited April 28, 2019 by Aeroboi

[Mister_Spaceman]

Iäve never reall thought about engine to mass ratio like that, I normally look at TWR, but I'm aware that a plane doesn't necessarily need 1+ TWR to fly. I think that looking at the Engine to Mass ratio like that may be helpful. I'm almost done cobbling together a arge (by my standards) cargo plane. I'll get back to you when I'm done.

[Mister_Spaceman]

Since imgur is being uncooperative, I'll post links to the images. I don't post images often, so I'm not sure if you'll have access to the images straightaway. If I do need to make the images publicly viewable, just let me know and I'll be glad to.
In SPH: https://imgur.com/U4ZRfl2
In flight, with engine data: https://imgur.com/m25Y5LI

This example took off just fine, for its size. In flight, however, it's extraordinarily sluggish, almost stubborn. I simple turn 90 degrees might take well over a minute, if you're trying to keep it banked at a reasonable angle.

[OHara]

I think you are doing things pretty much correctly.  Large aircraft are sluggish.

As you scale up, mass tends to go as the volume, with three factors of length-scale, and the wing area only by two factors of the scale, so the wing-loading gets higher.  

The time to make a full 360° turn is 2 π v / (g sinθ),
so about 6 minutes = 2 π 100m/s / (9.8m/s² sin10°)  if you fly 100m/s and bank 10°

[Snark]

On 4/28/2019 at 1:01 PM, Mister_Spaceman said:

I don't have any pictures readily available, because I don't typically keep unsuccessful files.

Yes, but seriously, we really need to see what you're doing in order to give constructive advice.  "Here's the issue with that design".

You don't even need to share .craft files, particularly-- just share a screenshot of the thing.  A picture's worth a thousand words, and aircraft design depends on so many interrelated factors that it's hard to talk about them "in general" without being so vague as to be fairly useless advice. 

Seeing an example of an attempt-- even a supposedly "failed" one-- will be really helpful.

[bewing]

1 hour ago, Snark said:

Seeing an example of an attempt-- even a supposedly "failed" one-- will be really helpful.

He did. Look up two posts.

 

[Snark]

58 minutes ago, bewing said:

He did. Look up two posts.

Ah, quite so.  Missed that.

On 4/28/2019 at 2:59 PM, Mister_Spaceman said:

In flight, however, it's extraordinarily sluggish, almost stubborn. I simple turn 90 degrees might take well over a minute, if you're trying to keep it banked at a reasonable angle

Lengthy advice below, but the TL;DR boils down to:  1. Make the tail bigger, with a lot more control surfaces; 2. Move the wing ailerons farther out towards the wingtips, 3. Consider maybe adding canards on the front, and 4. Make sure you have roll authority turned off for your vertical stabilizers.

And now for the Wall of Text™: 

Well, for one thing, it's got very little pitch authority.

Bear in mind that the effectiveness of a control surface (in terms of generating torque to rotate the craft about some axis) is directly proportional to its lever arm, i.e. how far it is from the center of mass along the relevant axis.

You don't have the CoM marked, but given the relative amount of lifting surface on wings and tail, I assume it's a reasonably safe assumption that your craft CoM is centered pretty close to the wing, yes?

In that case, any ailerons you have on the wings are going to be pretty much useless for pitch-control purposes, because they're right next to the CoM and therefore have very little lever arm to work with.  Which means you're entirely going to be dependent on tail elevators for pitch control... and it's hard for me to see just how much oomph you've got there, but it looks like a pretty small tail compared with the size of aircraft that you're trying to steer.

So, my suggestion would be to put more control surfaces on the tail, and/or move the tail farther back from the CoM if there's a reasonable way to do so.  You may want to consider giving more control authority (i.e. above the default 100%) to those elevators back there.

I'd also suggest adding some canards way up in the front of the craft, as far forward as possible, since that also has a pretty good lever arm.  More pitch authority there.

As for roll authority... is the craft rolling reasonably responsively, or is it sluggish?  If it's sluggish to roll, then you need to move those ailerons on the wings farther out towards the wingtips, to give them more lever arm to work with.  You've got them snuggled in close to the fuselage, which makes them less effective for roll authority.

Looks like the plane has vanishingly tiny yaw authority-- just a couple of itty bitty vertical stabilizers on the tail.  That may very well be plenty enough (you can fly a plane using just pitch and roll, without a lot of yaw).  Maybe you're fine there, maybe not-- I'd suggest addressing the pitch and roll issues first, and then see how you like how it flies.  Then consider whether you need additional yaw authority.

(One thing to make sure, though, is that you've got roll authority turned off for your tail's vertical stabilizers.  You want those to be doing nothing but yaw.  If you've got roll authority turned on, then they're going to generate a lot of unwanted yaw torque when they try to "help" with rolling, especially since you've chosen to put them sticking way up high above the craft body like that, which gives them a lot of lever arm for rolling.  Failing to turn off the roll authority can make the craft harder to steer and can cause the wing ailerons to end up fighting with the vertical stabilizers, making it harder to steer the craft.)

[adsii1970]

And to add to what everyone else and @Snark has said, an image would help greatly. Here are some trial and error lessons I've learned from making large cargo aircraft:

• The wings always need to be longer than what you think they do. Longer wings provide extra lift.
• COM is essential. How much cargo are you planning to haul? How much does your COM move with the load in/out?
• Always overpower the aircraft. Once you become airborne in a stable altitude, you should be able to throttle back.
• Never underestimate the angle of the wings. If you are looking at the aircraft from the nose perspective, the wings should have their tips raised; they should not be flat horizontal.
• Don't underestimate putting a "down angle" in the horizontal stabillizers. This will force the nose up.

But without any image to go on, I'm kinda shooting in the dark.

[Mister_Spaceman]

3 hours ago, Snark said:

put more control surfaces on the tail

adding some canards 

move those ailerons on the wings farther out towards the wingtips

the plane has vanishingly tiny yaw authority-- just a couple of itty bitty vertical stabilizers on the tail.

Most of this does appear to be the biggest problem, and a lot of it is due to my experience with smaller craft. Most of my planes have been smaller craft for completing contracts on the other side of Space-Africa. With the smaller craft, I've always thought less is more. I often don't even include rudders on them, since I very rarely use yaw. Next time I'm in need of a cargo plane, I'll be sure to consider all of this. The large-ish "ailerons" that are so close to the fuselage are actually flaps, courtesy of @blackheart612's Airplane Plus mod. A few parts are from his mod, actually. If I recall correctly, I have some admittedly tiny Elevon 2s in charge of roll authority. I'll upgrade those to the FAT control surfaces in the future. I haven't considered adding canards to such large craft before, but should other attempts fail to energize my planes' turning speed, you bet there will be some up there.

Thank you very much for your help, Snark and others, I'll hopefully be able to apply these tips to future aircraft!

[Kryxal]

The biggest things needed to turn faster are more bank angle, pitching up some to maintain altitude, and not too much speed.

[Curveball Anders]

1 hour ago, Kryxal said:

The biggest things needed to turn faster are more bank angle, pitching up some to maintain altitude, and not too much speed.

Note that if you get to a high bank angle and turn rate (if you have the structural integrity) you might actually need opposite rudder to keep your nose at the horizon (or actually over it since you 're most likely loosing height).