Wcmille

Optimal Aircraft design

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(I don't use FAR)

Are you putting your engines slightly below center of mass to counteract pitch down from lift?

How do you engineer ideal wing AoA?

How can I engineer level lift force at 100m/s?

How are you calculating sufficient air intake?

How are you calculating which landing gear size to use?

How can I calculate ideal cruising altitude and speed?

Edited by Wcmille

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7 hours ago, Wcmille said:

Are you putting your engines slightly below center of mass to counteract pitch down from lift?

No, this screws up the craft when I go to closed cycle mode on Rapiers/ when it gets high and aerodynamics start to become irrelevant

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How do you engineer ideal wing AoA?

Trial and error, it also depends on the size or Kerbin (I now play with a 3x size kerbin). Too much AoA can result in needing to fly "nose down" in the lower atmosphere, but generally for spaceplanes one should most concern themselves with the high and fast flight regime, in which case you don't want to have to pitch up so much. Here kerbin's size matters because in stock kerbin when youre going nearly 1,600 m/s, you don't need nearly as much aerodynamic lift as a 3x size kerbin (because you're much closer to orbital velocity.

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How can I engineer level lift force at 100m/s?

Well, you could go into the alt-f12 menu, have it display aero information, and look at how much lift a wing segment generates when your craft is going 100 m/s.

Each ton of craft will need 9.8 kN of lift at 100 m/s

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How are you calculating sufficient air intake?

Depends on the engine and intake, but I generally do 1 shock cone per 4 rapiers. Trial and error also works, if an engine starts to sputter on takeoff add more intakes

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How can I calculate ideal cruising altitude and speed?

as fast and high as possible. Rapiers wont do much beyond 28k though

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9 hours ago, Wcmille said:

engineer (...)

engineer (...)

 calculating (...)

calculating (...)

 calculate 

What now? I do the following:

1. Build

2. Test

3. Crash

4. Return and fix

5. Repeat from step 2

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@KerikBalm once I hit Alt-F12, what do I do next, to see the aero stats?

By cruising, I mean most efficient height and speed in terms of distance/fuel.

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I've found that if you really want to go a long distance, like to the other side of kerbin, ballistic trajectories are most efficient. Use rapiers or whiplashes to get up to ~1550 m/s and 28-30 km, fire rockets for a few hundred m/s, and glide the rest of the way.

Alt-f12, go to aero settings, theres one about displaying part's aero data in the right click menu.

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On 25.12.2017 at 2:28 AM, Wcmille said:

(I don't use FAR)

Are you putting your engines slightly below center of mass to counteract pitch down from lift?

How do you engineer ideal wing AoA?

How can I engineer level lift force at 100m/s?

How are you calculating sufficient air intake?

How are you calculating which landing gear size to use?

How can I calculate ideal cruising altitude and speed?

What I keep the same on all my aircraft is that I put the center of lift slightly behind the center of mass (how much that is depends on the speeds I expect that craft to be capable of), that I have at the VERY LEAST least one intake for 2 engines, most of the time 1 per engine, that I put the main landing gear slightly aft of the center of mass and that I try to keep their drag as low as possible. What I also do on the majority of aircraft is doubling the control surfaces and bind their opposite-of-each-other deploys to the brake so they form very effective split airbrakes, making any braking chutes for landing unnecessary.

 

Everything else is trial and error, but with the stuff described above whatever I build mostly ends up working on the first try. 

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On 12/24/2017 at 7:28 PM, Wcmille said:

Are you putting your engines slightly below center of mass to counteract pitch down from lift?

No.  The only time I have engines above or below the centerline is for (some) seaplanes or pure aesthetics as this causes problems in normal flight.

In general, I put everything on the centerline:  wings, engines, and control surfaces. 

On 12/24/2017 at 7:28 PM, Wcmille said:

How do you engineer ideal wing AoA?

99%, it gets zero built-in incidence.  The exception is when I build something weird with off-center engines.  But for that, it's usually the tailplane that has the incidence, not the wing.

NOTE:  "Angle of Attack" is the angle of the wing to the airstream.  "Incidence" is the angle of the wing to the fuselage.  The purpose of incidence is to let the wing have a slight angle of attack while the fuselage remains parallel with the airstream, at least for certain combinations of speed and altitude.

 

On 12/24/2017 at 7:28 PM, Wcmille said:

How can I engineer level lift force at 100m/s?

I'm not sure what you mean here.  Do you A) want the plane to maintain a constant altitude while moving at 100m/s, regardless of AoA, or do you B) want both constant altitude AND the fuselage being level at 100m/s?  Either way, I don't understand why you consider this a desirable quality to built into a plane,. because it will only work at a single specific altitude.

Because air gets thinner with altitude, the same wing moving at the same speed with the same angle of attack will produce less lift at a higher altitude than it does at a lower altitude.  So let's suppose you're doing Option B above.  You've built the plane with wing incidence such that at an altitude of 500m and a speed of 100m/s, it maintains constant altitude and the nose is level.  If the plane is above 500m, still level, and still at 100m/s, it will not be able to maintain altitude because it has less lift.  The only way to maintain altitude at a higher level would be to increase angle of attack, and because this will increase drag, you'll also have to throttle up to maintain 100m/s.  OTOH, if the plane is lower than 500m and still level at 100m/s, it will climb because the wing is producing more lift in the thicker air.  To maintain constant altitude, it will have to slow down.

 

On 12/24/2017 at 7:28 PM, Wcmille said:

How are you calculating sufficient air intake?

These days, you hardly need intakes at all.  As @KerikBalm says, 1 intake suffices for multiple engines.  If the plane is designed for speed (and trying to reach space), then paring intakes to the bare minimum is desirable to minimize drag and weight.  But if you're not trying to reach space, this doesn't matter and it's more about looks.

 

On 12/24/2017 at 7:28 PM, Wcmille said:

How are you calculating which landing gear size to use?

It depends on the mass of the plane.  I really don't know the numbers.  I typically start with the small ones and if they fail, I increase to the next bigger size.  One thing to keep in mind, however, is in actual service, the plane will almost certainly make ground contact with only 1 wheel initially.  So take the expected landing area into account.  On the runway, this doesn't matter so much, but if your plan to land out in the boonies, you typically need beefier gear.

On 12/24/2017 at 7:28 PM, Wcmille said:

How can I calculate ideal cruising altitude and speed?

It depends on the engine as each type works best at a particular altitude.

What you're looking for is the best combination of speed and engine Isp.  The faster the plane moves, the fewer seconds it needs to get from Point A to Point B.  The higher the Isp, the less fuel the engine burns per second.

In general, there's no single "optimum" way to build a plane, because planes are built to perform different tasks.  Instead, you just need to understand the basic principles and how they interact with each other, and apply those to the specific job you want your plane to do.

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I put engines below CoM when I'm building an atmosphere only plane for many reasons, one of them being counteracting pitch down. But, I use MechJeb's differential throttle setting for most of my planes when I am flying real high or going to space. This setting makes where your engines are less important, though you will see a major drop in power output if they are way off.

I never use any angle of incident on my planes. It makes the plane fly very different depending on speed and when you already have altitude as a variable, it makes it hard to make a plane fly the way you expect it too, IMO.

I trial and error landing gear. The bouncing wheel bug destroys things. Best to just play with settings until you dial as much out as you can.

You can look at the liquid fuel usage in the resources when flying and find a minimum usage when balancing the lowest possible speed you can fly at different altitudes. Since we don't know things like engine RPM and BSFC (brake specific fuel consumption) and many other factors that can play into fuel usage, we have to trial and error it. Basically slow(less fuel and power) and high(less drag) is good economy but you may not like the way it flies.

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I engineer my planes with the minimum amount of wings required for lift and have any devices stored internally in storage/cargo bays. My biggest enemy is drag and I optimize my SSTOs to have the least amount of atmospheric drag as possible which grants me at least 200-300m/s of d/v extra in orbit.

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