Low and Slow (Aircraft)

[Confutus]

I had been away from KSP for a while, partly because I found aircraft to be too hard and finicky to design and build at the early levels.   Hopefully my observations will be useful to other newbies at aircraft design.  Although there is a tutorial that helps significantly, it wasn't quite enough.

At present I'm trying a custom career game without reverting flights or respawning pilots.  Fuel is cheap, but I can't afford to lose parts off vehicles. I can't afford to lose whole vehicles, and I really can't afford to lose pilots, so I'm taking baby steps in developing aircraft. My philosophy is to begin with simple configurations, a low fuel load, low thrust (increasing with successive tests) and only adding one part at a time, since every change produces a new complication. As instabilities develop in taking off before liftoff, at low altitudes and slow speeds, I hope to fix them, and work up to better performing craft.

The simplest configuration I have found that  flies uses a total of 18 of the first tier of aircraft parts (45 science)   Using a tricycle landing gear configuration,   a fuselage with a Mark 1 cockpit, empty fuel tank fuselage, and tail cone;  2 type C structural wings,  elevons as ailerons,  one engine; and two tail fins in a v configuration with the engine between them.   (A parachute is optional since taking off is one thing,  landing is something else.) [I have an image, but haven't quite figured how to upload it.]

My first designs way back when  frequently veered off the runway and crashed.  Since I started using the snap feature to lock the angle of mounting the nose wheel to vertical, it's not nearly such a problem.  The next problem crops up in tests of increasing power.  At about 30 kph, the nose wheel starts to lift off, and then drops back down. At about 40 kph, the whole vehicle will lift off, but drops back down. It takes about 50-60 kph (at about 40% power) to get sustained flight. Can any experienced kerbal aircraft designers tell me what this behavior is called and what causes it?  The thing will fly, since the ailerons and tail fins give it just enough attitude control, but It's such a beast that I can hardly recommend it as a trainer below about 500 m of altitude.

I do expect to be continuing flight tests with different configurations.

 

[OHara]

That sounds like a typical first airplane.  From your description, I built what  is in the image at right.  With just one Juno engine, it has just enough power to fly. 

Turning off roll authority on the tail-fins is helpful, because ailerons do the job alone with less wallowing, in case you don't know that already.

This forum doesn't hold images itself, but users put images at other places like imgur.com and link to them there.

I don't think there is a particular name for sinking back down on the runway after first liftoff.  Probably you pitch up a bit when the main wheels leave the ground, which will make you slow a bit; in that case coming back down is called stalling if SAS held your pitch fixed, or porpoising if you fly using alt-S trim and the nose dips back down.

It is difficult to land without hitting the tail on the ground, with that first straight-tapered tail-cone.  I crashed a bunch but succeeded with a 70m/s approach, touching down at 55m/s.

[Confutus]

That's close. Here it is.  I guess it's porpoising because the nose pitches up and down both before and after  first liftoff, and stalls a couple of times.  SAS isn't able to hold the pitch steady.

[OHara]

That is very little wing !

You did say "structural wing type C" but I thought you must have meant the larger type-C panels. 
You might like a version with (4-to-6 times) more wing area, because with bigger wings deflecting more volume of air, you can take off, and later land, at slower speed.

[UmbralRaptor]

2 hours ago, Confutus said:

That's close. Here it is.  I guess it's porpoising because the nose pitches up and down both before and after  first liftoff, and stalls a couple of times.  SAS isn't able to hold the pitch steady.

It's probably suffering from poor pitch authority, given the the butterfly tail has all the control surfaces well back/forward of the center of mass. It's somewhat hard to say with the center of lift and center of mass icons off, but there's the possibility of it being tail-heavy/unstable in some configurations. The off-axis thrust is potentially also an issue.

 

Playing around with some alternate configurations, I got this thing (which is probably rather overweight for flitting around, and presents some entry/exit issues.

[Confutus]

I'm guessing that the porpoising would indicate an unstable configuration for pitch authority.  The CM and CL are approximately beneath the parachute. I knew that the ailerons should be placed as far from the CM as possible to give greater roll authority, but I didn't stop to think that might apply to pitch and yaw as well, so I should probably move the tail fins back. The craft is so short, there isn't a whole lot of room without it doing dire things to the CM and CL.  I should also probably move the engine back to get the thrust axis closer to in line with the CM and CL.  The tail fins are barely adequate to give me pitch control in flight, so for my incremental flight test program, I meant to try adding a couple of elevators to them anyway.  This will  probably also mean tweaking the placement of various parts. Thanks for the suggestion, it's helpful.

[Confutus]

1 hour ago, OHara said:

That is very little wing !

You did say "structural wing type C" but I thought you must have meant the larger type-C panels. 
You might like a version with (4-to-6 times) more wing area, because with bigger wings deflecting more volume of air, you can take off, and later land, at slower speed.

Amused hmmm.  This is why I have a test program. Bigger wings *are* in it. I had tried a biplane configuration, but the lower wings were so close to the ground, the slightest roll tore them off.  That's a hard nope. I've seen pictures of biplanes with higher wings (now I know why) and struts connecting them, but in my current game, i don't have all those parts yet. 

[Confutus]

The testing program continues; I'm trying to change only one thing at a time. I have several versions with tweaks to the engine, tail, wing, and landing gear placement, but the low power trial runs haven't shown any major differences and there is still porpoising. Adding elevators to the tail fins hasn't shown much change yet, either. The most successful version so far has used type B wing connectors instead of the type C; with the engine and tail fin placement unchanged from the first version.  This one actually lifted off at 35 % thrust, at a speed of about 38 m/sec. I cut power and landed it, still on the runway, without breaking anything or killing anyone.

[Zeiss Ikon]

Incremental improvement.  You're on the right track.

If you haven't done so, you might look at the one or two stock airplanes supplied in the game.  They're tested and known to fly well...

[Confutus]

5 hours ago, Zeiss Ikon said:

Incremental improvement.  You're on the right track.

If you haven't done so, you might look at the one or two stock airplanes supplied in the game.  They're tested and known to fly well...

TY. I have looked at the stock airplanes and I'm working up to them. Most of them have parts I haven't unlocked yet. I take this as not just a game, but an exercise in learning (follow the paved road, or go bushwhacking through the wilderness? Each has advantages)  and in aerospace engineering philosophy and practice. Mine is design, build, fly, and test, heavy on the test. It's much too easy to overbuild and go on to the next glamorous design without fully understanding how my vehicles behave and what their limitations are.

[Zeiss Ikon]

Given the limitations of the user controls in the game (reasonably adequate for VTO rockets, even for Mun/Moon landings; much less so for aircraft that really need four-axis control all the time), I don't consider it "cheating" to design and test aircraft in an environment with reverts and respawns (like Sandbox, even).  I've built two airplanes recently in my current RO career; one a basic science gathering craft using only starter-node parts, the other a transonic X-1 equivalent that requires the X-1 cockpit and rocket engine, in the first-tier aviation branch.  The latter takes so much runway to get airborne with full tanks that the testing that approaches all-up flight has to be all-up flight, because there's not a chance of getting a rejected takeoff or bunny hop stopped again without going into the sea (or into the hills on the other end of the runway).

Neither design could have been developed to their current "flies good!" status in a no-reverts, no-respawn setting.  I'd have been out of pilots before either plane got off the runway for even a bunny hop, and hiring more Kerbals would have bankrupted the Program at that stage.

All that to say, you're doing this the hard way.  Learn to build and fly airplanes in KSP the KSP way (i.e. by blowing stuff up and respawning the killed crews) before you commit to no-reverts, no-respawns play.

[Confutus]

33 minutes ago, Zeiss Ikon said:

All that to say, you're doing this the hard way.  Learn to build and fly airplanes in KSP the KSP way (i.e. by blowing stuff up and respawning the killed crews) before you commit to no-reverts, no-respawns play.

But I like doing things the hard way.  Sure, I could build high performance craft with starter-node parts, but I really don't like to crash and burn umpteen dozen times before I get something that works well enough to use. That's more frustrating than fun. It tickles my fancy to learn how to control my creations at something like the same pace I am learning to build them.  If I'm doing something edgy I can always go to sandbox mode, blow stuff up,  and kill kerbals to my heart's content and call it simulations, but I prefer the challenge of developing things with  limited resources.

[HvP]

@Confutus Hi there.

If there's "one neat trick" to building a plane that can fly "low and slow" and remain stable it is this: add a few degrees of incidence to the wings. A positive angle of incidence means that when viewed from the side you rotate the wings so that the leading edge is ever so slightly higher than the trailing edge. This allows the wings to produce more lift at slow speeds without having to constantly pitch the nose upwards.

Adding incidence does add more drag to the wings during flight, but not as much as pitching the whole plane does, and that drag is usually compensated by extra lift. It also has the advantage of requiring no extra parts.

Edited January 14, 2020 by HvP

[Confutus]

6 minutes ago, HvP said:

@Confutus Hi there.

If there's "one neat trick" to building a plane that can fly "low and slow" and remain stable it is this: add a few degrees of incidence to the wings. A positive angle of incidence means that when viewed from the side you rotate the wings so that the leading edge is ever so slightly higher than the trailing edge. This allows the wings to produce more lift at slow speeds without having to constantly pitch the nose upwards.

 

I am indeed planning to include tweaking the angle of incidence, further along in the flight test program. First, I want to see what happens with straight wings of various sizes and configurations at various throttle settings.  There are a lot of variables to play with; it's more systematic to change only one thing at a time.

[Laie]

Your question made me mess around a little, and here's what came of it:

Wings angled at 5°, which already seems a bit steep. Note the rear wheel which serves as tailstrike preventative. Only one control surface, there's plenty of magic torque. Has a nose-down attitude on the ground for easier landing, but still manages to take off at 20m/s.

There is only one engine, the tanks have intakes at both ends (because no other nose cone available at that techlevel). Endurance should be on the order of 30-40 minutes at 80m/s.

[RizzoTheRat]

Re angle of incidence, one real world thing that a lot of KSP players aren't aware of, is that on a stable aircraft design the tailplane is generally at a negative incidence to give downforce not lift.  this means that if your aircraft pitches up (not that we have gusts in KSP) the tail's downforce decreases and will pitch it back down again.  Canard designs are generally harder to make stable as they need to provide lift, but be set up to have a smaller increase in lift with pitch than the wing does.  Note there are very few successful canard aircraft before fly by wire and computer controlled stability, but by generating lift not downforce a canard design gives you less drag.

Also the usual adage that the COM should be ahead of the COL isn't 100% true.  In order to fly in a straight line the COM and COL need to be exactly on top of each other, and you use elevators to adjust the force on the tailplane to achieve this.  Put the COL of the wing behind the COM and then place the tailplane with negative incidence to bring the COL forward to close to the COM should make for a nice stable design.

Of course all this is for docile handling stable aircraft, and all goes out the window when you start talking about agile supersonic high TWR fighter type aircraft 

 

Edited January 14, 2020 by RizzoTheRat

[Confutus]

10 hours ago, Laie said:

Your question made me mess around a little, and here's what came of it:

 

 @Laie Wow. Funky. Very Kerbal. I never would have dreamed of something like that.