Building STOL aircraft for Stock Aerodynamics

[SSgt Baloo]

I've got a lot of mods full of aircraft parts, and I love building airplanes, but my attempts to build planes that get up off the runway at low airspeeds* have been hit-or-miss, with the emphasis on miss. I can build lightly, after a fashion, and I've tried using more wing, with some success (see example, below), but most of the really promising birds I've built to fly slowly, end up also accelerating slowly and sinking into the sea after departing the end of the runway, when they don't swerve off the runway and crash.

This one flies well, but it's, well, huge!


Same plane, different angle:

* and continue to fly without crashing, an important caveat.

[Snark]

I've had some good luck with ultralights-- that is, no cabin, just a command chair perched on a Mk0 fuselage with a Juno.  It's so light that it gets off the ground really easily.  Runs on bicycle landing gear under the fuselage (gear is offset upwards so that the fuselage sits low to the ground).

[Val]

In my experience, the best solution for STOL in KSP is a long thin main wing, with moderate Angle of Incidence (~2°), plus large trailing edge flaps to be deployed for extra lift at take-off and landing speeds.

Delta wing and canard configurations are poorly suited for STOL in my opinion.

[Rocket In My Pocket]

If realism isn't a factor here just take some sepratrons attached to radial decouplers/hardpoints and stage them as you take off. Once they've done the job, get rid of them.

Some should be mounted facing backwards, some facing down. I use this concept a lot when designing Kerbal biome hoppers that will need to make a short take off after landing in whatever biome.

Heck if you put enough of them on there you can even get a one time use vertical take off.

(On a side note, it's not that unrealistic as it has been done, it's just not terribly common...or safe.) https://en.wikipedia.org/wiki/JATO

Edited September 7, 2016 by Rocket In My Pocket

[SSgt Baloo]

I think part of the problem is that I'm using a propeller engine that produces only about 5 KN of thrust. That means light construction is of the utmost importance. I did manage to build a small plane that flies relatively well, though without any claim to greatness except that I managed to keep it's weight down to just over 1.7 tons at launch. Its unstick speed is about 31.[fraction].  Here's an image of it as its wheels leave the runway.



The speed shown is about 70 MPH. Because of the leisurely rate of acceleration, this thing isn't really STOL, but at least it's stable enough that when I touch it down again, I can apply the brakes hard (intermittently, at first, to avoid nosing over), so stopping in a hurry isn't a problem. I suppose I should pick another engine with a bit more thrust, if I want this thing to leap off the ground.

Although someone suggested flaps, I suspect they are only a help when you have enough thrust to become airborne even while they are deployed. The current powerplant is not powerful enough to do that. The "Merlin" from the same parts pack may have enough thrust to make that a possibility. It has a whole *gasp!* twenty kilonewtons of thrust. 

More images of The Feeble Beast here: http://s28.photobucket.com/user/SSgtBaloo/slideshow/7SEP2016

[Val]

8 hours ago, SSgt Baloo said:

Here's an image of it as its wheels leave the runway.



The speed shown is about 70 MPH. Because of the leisurely rate of acceleration, this thing isn't really STOL, but at least it's stable enough that when I touch it down again, I can apply the brakes hard (intermittently, at first, to avoid nosing over), so stopping in a hurry isn't a problem. I suppose I should pick another engine with a bit more thrust, if I want this thing to leap off the ground.

Although someone suggested flaps, I suspect they are only a help when you have enough thrust to become airborne even while they are deployed. The current powerplant is not powerful enough to do that. The "Merlin" from the same parts pack may have enough thrust to make that a possibility. It has a whole *gasp!* twenty kilonewtons of thrust. 

If you give the main wing 5° Angle of Incidence (or maybe even 10°), then you can reduce the take-off speed.

[eddiew]

I agree that putting an angle on the main wing works wonders. I also have a lot of luck with biplanes in stock aero, because it stops them being incredibly wide while still delivering good lift. Don't really have a good small example, but this massively heavy thing lifts off at around 95 when loaded, and as little as 50 when nearly dry.

Spoiler

Upper wing is inclined ~3 degrees, lower wing is flat. Results in a much smaller visual profile  

More relevant is probably something like this which follows a classic bush-plane design. A thin(ish) main wing, with the tailfins doing the majority of control. IIRC,this takes off at about 45m/s, which made it a good runaround for doing local science and anomaly surveys. A variant with twin junos under the wings also works, but I never screenshotted that - pretty sure it would lift off even sooner though by virtue of being lighter.

Also remember you can increase the pitch authority to 150% in the tweakables menu. Makes a huge difference for a lot of my builds  

[Val]

This one weighs 4.3 t at take off and the engine produces ~18 kN. Takes off and climbs at ~29 m/s with full flaps. It has 5° Angle of Incidence on the main wing.

[foamyesque]

I have a super basic stock plane design. Takes off at ~40m/s. How long the runup to that takes depends on where I tweak the engine thrust limiters to, but with them at max thrust it's about half-way between the first hashmark and the solid one when it lifts off. Landing is also very short. Only parts in the Aviation node or earlier are used:

 

It's my standard early-game Kerbin survey contract thingie. Carries a material bay as part of the fuselage, and other science bits can be stuck on as needed.

.craft file, if interested:

https://www.dropbox.com/s/knwdp2hxwp22tuf/Basic Plane.craft?dl=0

[Northstar1989]

To get off the ground at a very low take-off speed, you want to produce lots of lift.  But this doesn't necessarily need to be on the ground- in fact the more lift you produce on the runway, the more drag you experience there as well.  If your wheels don't produce too much friction (mind you can tweak the friction way down in 1.13) then you are better off relying on the normal force of the ground on the wheels to support your plane rather than lift while it is still on the runway so you can get up to takeoff speed as quickly as possible.

Try, basically, to build in as much lift and as little weight as possible.  The more thrust and the less drag/friction you have on the runway the better as well, as it will help you get up to speed more quickly.  Only try to nose up once you have enough speed for take-off (this will take a little trial-and-error and reverting)- trying to nose up before this point will only create additional drag from your control surfaces- and in the case of rear elevators, will actually produce negative lift that will press you more firmly into the runway (increasing friction with it).

Flaps help because they produce additional lift when deployed.  Flaps should ideally go either forward of or at the center-of-mass however, as placing them too far back will push the nose down and make it harder to nose off the runway...

 

Regards,

Northstar

Edited September 9, 2016 by Northstar1989

[Val]

I made a craft similar to yours, @SSgt Baloo.

Similar 5-6 kN thrust, but with less wing and slightly heavier 2t.

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It's viable to make it take off at 25 m/s.

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It can't takeoff very well with flaps as you also mentioned.

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But flaps can be used for short landings.

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[fourfa]

This thread got me playing with STOL biplane designs

https://kerbalx.com/fourfa/STOL-Biplane

This particular one had me pleasantly diverted for a few hours, proximity flying in the mountains and pulling 47G maneuvers.  Had to scrape Jeb out of the cockpit with a chisel after.  Rotates and takes off at 25m/s, on it's way to Mach 2.87.

[Azimech]

Three STOL's for carrier cargo operations: light, medium and heavy.

https://kerbalx.com/Azimech/77I--CBLC1-Naval-Light-STOL

https://kerbalx.com/Azimech/77I--CBMC1-Naval-Medium-STOL

https://kerbalx.com/Azimech/77I--CBHC1-Naval-Heavy-STOL

Good luck!

[SSgt Baloo]

Several people have remarked that between 2 and 5 degrees of incidence in the main wing is helpful. Due to nerve damage, my hands are uncalibrated (and just a bit shaky). How do you get exactly 5 degrees of incidence, anyway?

Edited September 12, 2016 by SSgt Baloo

[Corona688]

The rotate tool in the SPH.  I think alt or shift or something while rotating a part with qweasdf also makes minute changes.

[fourfa]

Shift-QWEASD gives you 5 degree increments, that's the best way in vanilla.

There are mods to make these adjustments easier and more precise - interested in that?

[SSgt Baloo]

17 minutes ago, fourfa said:

Shift-QWEASD gives you 5 degree increments, that's the best way in vanilla.

There are mods to make these adjustments easier and more precise - interested in that?

Which ones are they?

 

[fourfa]

I use Editor Extensions Redux.  Extremely powerful, has settings for 1* and 5* angle snap.  Also includes NoOffsetLimits, which allows infinite offsetting of parts, and allows surface mounting any part.  The combination opens a lot of creative possibilities and actually helps reduces part count a lot.

If you just want more angle control, try Part Angle Display

[SSgt Baloo]

2 hours ago, fourfa said:

I use Editor Extensions Redux.  Extremely powerful, has settings for 1* and 5* angle snap.  Also includes NoOffsetLimits, which allows infinite offsetting of parts, and allows surface mounting any part.  The combination opens a lot of creative possibilities and actually helps reduces part count a lot.

If you just want more angle control, try Part Angle Display

I think I'll give Editor Extensions Redux a try. It sounds as if it'll do what I want, once I figure out how to use it.  

[SSgt Baloo]

fourfa: Editor extensions redux is turning out to be a great help. Thanks!

I have figured out that speed upon unstick is less important then distance traveled until then. The slowest flyers I've made so far take way too much distance before leaving the ground, and they don't climb with any enthusiasm. One of my best efforts thus far has unstuck less than 100 meters from the launch point (99, to be exact), but I don't know if that's good, bad or mediocre. When I notice the wheels leaving the surface, I hit <F3> and see how much distance the plane covered. I forgot to take pictures, but thanks for sharing, everybody. Lots of good ideas to stea..., er, I mean, benchmark. 

[Corona688]

So they just have a hard time reaching speed?  Seprotrons

[SSgt Baloo]

Well, after much experimentation, these are my two greatest successes. Both can take off in less than 60 meters, and I've gotten the biplane version off the ground in less than 50, but that was only once and I have not been able to duplicate that stunt since, so just over 50 seems to be the limit.


The monoplane version has its L/G and engines suspended at the ends of the FAT 445 elevons I'm using as flaps for these. Each wing half consists of one type C wing connector at the root, and two wing connector type Bs, with a rounded wingtip that came with one (or another) Aircraft parts mod pack. Ailerons are also FAT-445s. Each horizontal stabilizer half consists of two type E wing connectors with FAT-445s as elevators, and the end caps are also type E connectors, with Elevon 3s as rudders.


The Biplane version moves one Type B wing connector to replace the FAT-445 flaps, and places those flaps on the wing's trailing edge. Otherwise, these planes are almost identical.

While I put ladders at both the front and rear of each plane, they aren't much use for climbing down, as I can't figure out how to align them with the invisible ladders on the doors when they EVA. The rear ladders are actually useless, since they can't be used to climb down or up. The front ladders can be used to get close enough to the front doors to board, so if they need to get out and do something, they just have to jump down and climb back in later. If anyone has tips for how to place the ladders so they don't block, but facilitate the downward climb from the cockpit door, please share.

More images here: http://s28.photobucket.com/user/SSgtBaloo/slideshow/16SEP2016

Edited September 16, 2016 by SSgt Baloo

[Val]

50 minutes ago, SSgt Baloo said:

Well, after much experimentation, these are my two greatest successes.

Beautiful. Especially the monoplane version. I really like that one.

---

In the meantime I went in a completely different direction with almost VTOL. It's a STOL, but with VTOL engines that have <1 TWR.

It became VSTOL (Very Short TakeOff and Landing).

Craft

[SSgt Baloo]

I have been avoiding talking about using seperatrons in the builds I am making because once they are used, they can't easily be reset and used again (if at all). It's the same reason I haven't been equipping these planes with drag chutes. What if you want to take off and land more than once in the course of a mission? Also, adding rockets to a plane can make it that much more complicated. As my age advances, I find myself getting a more and more of a head start on befuddlement without even trying, so I want to keep the operation of these things pretty simple and straightforward - just like any other plane except shorter take-offs and landings.

Not that there isn't a place for rocket boosters and drag 'chutes. Until KSP gets steam catapults, there are few other ways to "throw" a high-performance plane into the air in a short distance, with enough force to make it stick.  Likewise, we don't have arresting gear either. Anything intended to land in a specific small space (such as a seagoing vessel smaller than your local airport) needs drag chutes.

I have also noticed that my favorite propeller engine for going fast and high (the Bumblebee, from the Airplane Plus mod) doesn't work as well, in this application, as the Kraken from the KAX mod. I guess it starts producing more thrust sooner, even if the other engine gives the plane a higher top speed. I couldn't get either of these guys airborne in as short a distance with the Bumblebee (about 250 meters with that one, I think).

Now to experiment with single-seaters. 

[Baybrawler]

The best thing I could say is add more power! Either super light aircraft or super powerful engines give the best STOL capabilities. Also, for landing use as much braking as possible, and thrust reversers along with drogue chutes if possible/reasonable.