keptin

Basic Aircraft Design - Explained Simply, With Pictures

251 posts in this topic

Hey KSP folks! The Space Plane Hangar remains a conundrum for quite a few, and since it's one of my favorite parts of KSP, I wanted to shed some light on concepts that might help others build their own successful aircraft.

In retrospect, this is something between a design and basic concepts of flight tut, but whatever, here goes!

Basic Aircraft Design .craft pack - Here is a basic .craft pack I've put together with some stupid-simple designs taking different approaches.

And the easiest plane to fly ever, the EZ Trainer Mk7 "Just hold the S-key" Edition (to download: right-click, save-as).

~The tut is an embedded image, you should see it below this text. It may take some time to download.

Alternate Mirror: https://dl.dropboxusercontent.com/u/66999462/AircraftDesign.jpg

GqLQktX.jpg

Translations:

French guide, by Varsass - http://kerbalspaceprogram.fr/forum/viewtopic.php?f=10&t=2758

Translation Permissions:

You have my permission to translate this guide into other languages if one does not already exist. Please be respectful to the original content by maintaining its indented meaning if possible. The font used for the guide is Humor Sans and can be downloaded here. You may use different fonts for your translation as needed.

Please include a link to this thread in your post for your translated version. In addition, please private message me a link to your translation so I may include it here. Thanks!


v28.1 - Added section for Angle of Incidence, asymmetric thrust compensation, landing gear positioning, minor fixes
v25.1 - Minor fixes and rephrasing, spelling corrections
v25 - Expanded drag force, landing gear, CoL relative to CoM, Added section on AoA, several other fixes
v20 - Clarified the relationship of CoL to CoM, neutral stability
v19 - With an additional 50% material; center of thrust, center of lift, edit fixes on existing articles
v11 - Turbo infused with 100% more knowledge!
v7 - Initial Release

For a more realistic drag model get

Ferram Aerospace Research.

If you're looking for more aircraft parts, check out my own mod, Kerbal Aircraft Expansion (KAX)

.

uFPqHRa.jpg

Edited by keptin
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Hmm, I just changed hosts, it should work now. I added some mirror links at the bottom of the OP if it doesn't.

*10/9 Edit* - Updated the tutorial with 100% more knowledge!

Edited by keptin
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Good job, glad to see you added a much needed section on CoT, hopefully more people will know what causes their jets to start spinning out at high altitudes (or weird orientations).

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Humorous, and informative. Even more veteran KSP pilots can pick up a thing or two here. This deserves to get stickied.

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Fantastic work keptin, you've been stickied :D

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Amazing work! I've been playing for a long time and I think I made one successful SSTO out of 20 or so designs by luck. This will REALLY help!

+rep

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Very good, but I want to make one little correction:

The section on "distance the CoL is behind the CoM" is correct up until "Too Far" which doesn't equal flip-happy; if the CoL is too far behind the CoM that should be "Lawn Dart / LOL how does pitch up?" "Flip happy" should correspond to "Anywhere in front of" which would help make the information a little more consistent for new airplane designers.

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This deserves to get stickied.
Fantastic work keptin, you've been stickied :D

That didn't take long now did it. Rep on both Keptin and Sal for that one.

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Thanks ferram, I'll clarify that in the next update by differentiating "CoL too far behind CoM" and "CoL too far in front of CoM" as having two separate behaviors. I like the lawn dart analogy.

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This is probably one of the most helpful things i have found on the forums, as have only gotten two space planes into orbit! Thanks :)

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Just one point on drag - more parts does not mean more drag. Drag is proportional to mass, which means the acceleration of each part is determined solely by its drag value, and the mass weighted average of all the drag values determines the overall drag value. When you add a part, you essentially pull the craft's drag closer to that part's drag.

For example, a SRB has 0.3 drag. add a rocket nosecone (with 0.3 drag) and the craft's drag will stay at 0.3. Add wings (about 0.02 drag) and the craft's drag will go down. Add a cupola (0.4 drag) and the craft's drag will go up again.

TL;DR: KSP drag is weird but not weird in the way you think it's weird. Unless I'm mistaken.

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Very nice tutorial, good read and informative.

You missed a point on dihedral angle though. Even with its primitive drag model positive dihedral will tend to roll a plane back level again which is key if you want to fly without SAS (which i find makes landings much easier).

Hope to see more.

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TL;DR: KSP drag is weird but not weird in the way you think it's weird. Unless I'm mistaken.

The way drag is calculated in KSP still confuses me and I feel I need an Explain Like I'm Ten on this so I can ELI5 in the tut.

Ok, for the sake of an example, say our craft consists of nothing but 2 fuel tanks, each with 0.2 drag. How will this compare to a craft made of 4 fuel tanks, drag wise? Now, what if we have a craft with 2 fuel tanks, one with 0.2 drag and the other with 0.1 drag?

What is the relationship between the drag value, mass, and number of parts, and how does that affect actual performance?

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As far as I can tell, drag is just The drag value (ex. 0.2) times mass (2). Then it is applied to every part in the craft as a negative acceleration aiming retrograde (not actually sure about the last parr)

What I would like to know is what the drag coefficient in thrme configs stands for.

Btw, AMAZING thread! Inspired me to make a U-2 remake.

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I think your confusion is that Supernovy is talking about the arbitrary "maximum_drag" factor that goes into the drag equation, and that you are thinking in terms of "drag force" which does increase with the number of parts. Here's the drag equation:

drag force = 0.5 * air_density * surface_velocity2 * maximum_drag * mass * FlightGlobals.DragMultiplier

This is worked out for each part, where maximum_drag, mass and surface_velocity are part-specific, air_density is based on altitude, and the DragMultiplier is a constant factor that can be ignored for our purposes.

Assuming all fuel tanks have the same mass for this:

For your example of 2 fuel tanks vs. 4 fuel tanks, both craft will have a maximum_drag of 0.2; the average maximum_drag of each vessel is 0.2. However, the craft with 4 tanks will have twice as much drag as the one with 2 tanks, because it has twice as much mass.

For a craft with two fuel tanks, one with maximum_drag = 0.2, the other 0.1, the average maximum_drag of your craft will be 0.15 (mass averaged), and the total drag force will be 75% of the original 2 fuel tank craft.

I think you understand it fine, and the explanation in the tutorial is correct; however, make sure to differentiate between total drag force and the maximum_drag value.

Edited by ferram4
typos

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Awesome, thanks, that was exactly the rundown I needed. I think I'll expand on drag a bit in the tut and differentiate between those values.

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As long as the center of lift is inside the center of mass, it should be a pretty stable aircraft.

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Let me put it this way: there is no difference in aerodynamic properties (e.g. terminal velocity) between the 2 0.2 drag tanks and the 4 0.2 drag tanks. The force may be different, but the mass dependence on drag exactly cancels out. For example, dropping a full fuel tank and empty fuel tank through the air, both will hit the ground at the same speed. Adding more parts will not make drag slow you down more.

Ferram4's entire post is correct, but I want to point out that the drag properties of a KSP craft can be determined solely through the maximum_drag values, just like you don't need the mass of an object to figure out what rate it will fall at under gravity.

AmpsterMan: I believe the maximum_drag value represents the drag coefficient, which depends on shape. The mass dependence then represents the surface area. The actual values, however, seem to be chosen without respect to shape.

If I have something wrong, please correct me.

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Awesome tutorial! I think it achieves its goals exactly, and will certainly help visual thinkers get in the air.

Not sure about the font though... But that's just personal taste.

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Great summary pic. Some nitpicks / comments:

  • KSP lift model is a simple curve - wings give tiny ift at zero AOA, lots of lift at 20 degrees, and less lift after that.
  • What this means is Canards don't stall before the main wing in KSP, unless you give them a notch of trim-up rotation in the SPH. Same goes for elevons (but less important).
  • Wing aspect ratios don't matter much without FAMod, the same wing pieces give the same lift. Only thing that changes is your plane is harder to roll.
  • Wing sweep does matter in KSP, but not like in real life. Stock KSP wings only produce lift from air flow perpendicular to the part (not the plane!). This has two effects.
  • 1) If you rotate wing panels 90 degrees, they won't do anything.
  • 2) If you make swept wings, with wing segments rotated, your plane will have less lift when going straight, and yaw-roll coupling (when it goes sideways, one wing side will generate more lift than the other, rolling the plane). Due to this, swept-forward wings are actually more stable in KSP!
  • Winglets and Canards apply force at their mounting point (not their apparent center), so canards-on-wings can act like almost perfect rudders (no yaw-roll coupling)
  • The most common complaint about tricycle landing gear is "why does my plane swerve off the runway". The answer:
  • 1) Your rear gear isn't straight or
  • 2) your plane's rear is lifting off first, putting all the weight on the front wheel (wheelbarrowing).

Great explanations though! I'd also love to help with an advanced guide.

Edited by antbin
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