Kerbal Aerodynamics 101 - By Professor Lynch [1.0.0]

[Lynch]

​Hello dear Kerbonauts,

All my calculative posts include;

Kerbal Aerodynamics 101 - By Professor Lynch [1.0.0]

Kerbal Rocketeering 101 - By Professor Lynch [1.0.2]

Kerbal Orbital Mechanics 101 - By Professor Lynch [1.0.4]

It is Professor Lynch speaking...

Ever wondered about how much Lift and Thrust you need on your planes? If you are thinking small as a few tonnes, that might be trial and error, but lifting 200 - 300 tonnes heavy planes is a serious case. What this Thread will offer are;

A formula, in which you will enter your Mass,

As a result;

Take off Velocity

Wings required ( in Type B standards )

Thrust Limiter adjustment in order to maintain a cruising speed

Once I see enough interest I will be posting my calculations. Perhaps I can even script an application so that you can just download and use it as a Mod maybe in the future.

Edited October 14, 2015 by Lynch

[Frank327]

My impression was that you should aim for 1:1 lift to mass ratio and one turbojet engine for every 15 tonnes (?).

[UmbralRaptor]

Are there supposed to be pictures with the formulas? There doesn't seem to be anything, even in the page source.

[Lynch]

ATTENTION: The below aren't for FAR/NEAR or any modded versions. These work under Stock and Stockalike part modded KSP. Basically, anything that doesn't mess with games math calculations can use it.

Okay, it might work for you perhaps; let's calculate your standards so to see if it fits.

Assume we use 1 Turbojet and total Mass is 15 tonnes:

Mass = 15

Thrust = 225

Horizontal Forces

Then:

Weight = (15*9.81) = 147.15

Drag = 0.5 * 1.223* V^2 * 0.008 * 15* 0.2

The ideal take off speed is 90-100 and I take it 90 m/s personally. This is also the speed which your cruising will occur, can change +/- 10 depending on altitude.

Drag = 0.5 * 1.223 * 8100 * 0.008 * 15 * 0.2 = 118.87

Thrust at 100% = 225

We would want Thrust = Drag, so 119 / 225 =%52 Thrust Limiter

Vertical Forces

We would want Lift = Weight, so Lift = 147.15

147.15 = 0.5 * 1.223 * 8100 * 0.008 * 15 * Clift

Clift = 0.2475 , this is from 0.2 to 0.35 most cases, heavier you are higher it is; but it is the Clift required, not the Clift you have since you have no wings.

Wings will be designed according to that number.

The calculation of Clift is much complicated, an I will only be doing those under enough interest.

So after approximations, using 2 Lift Rated, Type B Rectangular Wings; you will generate 14kN Lift Force per wing.

Lift = Weight , 14 * n = 147 ; n = 10 that is 10 Type B rectangular wings, 5 left 5 right.

I personally use %10 elevon (0.66 lift) per wing, that is 5 wing on left, and 5*%10*0.66 = 0.33 elevon per side and %25 of a side wing on tail, and %50 the elevon for rudder.

All these calculations are made by me, Lynch.

I also use the same nickname on Steam.

The calculations in order to find the most accurate Clift is probably done by me; yet if you believe you are calculating Clift far more accurate. Feel free to share with me.

- - - Updated - - -

Well, I wanted the tension to go up and make a giant release.

However, I just read that yesterday, Squad had posted they will be making the calculation process different.

So there you go , mega -CHEATSHEET- for KSP.

Force of Drag = 0.5 * 1.223125 * Atmospheric Pressure * Velocity^2 * Coefficient of Drag * 0.008 * Mass

Force of Lift = Air density * Velocity * Coefficient of Lift * f(AoA)

BEAR IN MIND IDEAL TAKE OFF VELOCITY = TERMINAL VELOCITY = 100m/s

Whereas;

Atmospheric Pressure = 1.000 * e^(altitude/-5000)

Coefficient of Drag = (Mass of n(1) * Drag of n(1) + Mass of n(2) * Drag of n(2) ... Mass of n(final) * Drag of n(final))/ Mass of n(1) + Mass of n(2) + ... + Mass of n(final)

Air Density = 1.223125 * e^(altitude/-5000)

Coefficient of Lift = Lift Rating - Mass , for example Wing Connector Type B , Mass = 0.1 , Lift = 2 ; thus Coefficient of Lift = 1.9

Using all these formulas you can calculate 4 forces of aerodynamics, Thrust, Drag, Lift, Weight.

You are welcome!

- - - Updated - - -

KerbAlculator Application Download link!

http://www.megafileupload.com/en/file/602622/KerbAlculator-rar.html

README

CHANGE MASS AND THRUST ONLY OTHERS FOR ADVANCED PLAYERS

x Type B Wing Connector means , the Lift rate of it, you can place a Type C but the lift rate halves, so you must double the quantity.

x Elevon 5 means the same aswell for the Elevon category.

Edited January 25, 2015 by Lynch

[Pecan]

Lots of numbers and teasers - please show me your 15t 1-turbojet spaceplane and tell me:

a) Where the rocket-power comes from

What orbit you're circularising at

c) How much fuel (LFO and oxidiser) you use

d) If you're not talking about spaceplanes so the above don't apply; what are you talking about?

Edited January 26, 2015 by Pecan

[GoSlash27]

I'm also not following the base assumptions here.

Is this just showing an equillibrium point at takeoff where t=d and l=w? If so, what's the utility of it? Nobody launches planes just to have them limp along at 100 m/sec at sea level...

Scratchin' mah head,

-Slashy

Edited January 26, 2015 by GoSlash27

[Lynch]

This is a result of my trial.

I wanted to go to Duna with a SSTO MK3 or MK4 spaceplane. Circularise at 70.000 LKO and 315615 Duna Orbit, then drop my ship down to base and collect my 5 people team. Come back , dock, and undock and dispose. Then come back to Kerbin orbit.

Thus, i wanted a fancy, no clipping ship.

Seperating 20 tonnes for my dunar lander, 80 tons would be my shuttle. But lifting it and landing it was a big case so I did this.

It is basically a calculator for your ship to fly in kerbin atmosphere.

You did not have to be so offensive, I worked days on these calculations.

[Pecan]

I'm not intending to be offensive, I'm letting you know I don't understand from your description what this application is meant to be telling me, or why.

For a start I don't know how you get from:

"Mass = 15"

To:

"Weight = (15*9.81) = 147.15"

Definitions of 'weight' that I am used to would say that under 1g acceleration (on the surface of Earth or Kerbin, for instance) the weight would be 15t. Why is your value so much higher?

Then:

"The ideal take off speed is 90-100"

Why? What do I lose if I use more runway and take-off faster? How about if I point up at 40m/s?

"This is also the speed which your cruising will occur"

What cruising? Why am I cruising?

"52% Thrust Limiter"

Why not just throttle-back? Why not just go faster?

"I personally use 10% elevon (0.66 lift) per wing ... 50% the elevon for rudder"

Which is about control, not lift unless you're doing some very strange things. How does this relate to the rest of the package?

(Apart from all that I'm probably missing something because I don't even know what a Mk4 spaceplane is.)

[Alshain]

1. Wrong forum.

2. There are rules and you haven't followed a few of them.

[Lynch]

I'm not intending to be offensive, I'm letting you know I don't understand from your description what this application is meant to be telling me, or why.

For a start I don't know how you get from:

"Mass = 15"

To:

"Weight = (15*9.81) = 147.15"

Definitions of 'weight' that I am used to would say that under 1g acceleration (on the surface of Earth or Kerbin, for instance) the weight would be 15t. Why is your value so much higher?

Then:

"The ideal take off speed is 90-100"

Why? What do I lose if I use more runway and take-off faster? How about if I point up at 40m/s?

"This is also the speed which your cruising will occur"

What cruising? Why am I cruising?

"52% Thrust Limiter"

Why not just throttle-back? Why not just go faster?

"I personally use 10% elevon (0.66 lift) per wing ... 50% the elevon for rudder"

Which is about control, not lift unless you're doing some very strange things. How does this relate to the rest of the package?

(Apart from all that I'm probably missing something because I don't even know what a Mk4 spaceplane is.)

Weight is not calculated in tonnes or kg s its kN and it is F = m * a , which a = 9.81 m/s2

The ideal take off speed is 100m/s because under 1 atm pressure and 1.223 air density which is at 0m altitude ; the Gravity is equal to Air Drag at only 100m /s which prevents you from burning against gravity or drag at 100m/s and making it an optimal TERMINAL VELOCITY.

Cruising is for landers; once you come back from an interstellar journey you may want to burn at 52% to prevent more than enough wasting, and let the good ol terminal velocity do its job. You are not cruising, you my friend; are probably not playing this game the hard way either. This guide is for people who do not clip or air hog.

Control part is for a reason, scaling those mass into 200 tonnes or 400 tonnes, try to fly those masses without much of a thrust; how much lift and control would you need? I just calcd it.

MK IV is a Stockalike part, which i rarely use but still like it.

- - - Updated - - -

1. Wrong forum.

2. There are rules and you haven't followed a few of them.

1.Thank you alshain but you aint no moderator, I have spoken this with one of em.

2.Your comments had no relation with my thread, and you haven't followed me.

[KerikBalm]

It is Professor Lynch speaking...

Being smug is not a good way to make your first post on the forums.

Weight is not calculated in tonnes or kg s its kN

The standard SI unit is Newtons, not KiloNewtons - but since engine thrust is liste in kN as well, it is convenient to use kN, as you do.

Thrust at 100% = 225

We would want Thrust = Drag, so 119 / 225 =%52 Thrust Limiter

The Turbojet only produces about 115 kN at 100 m/s -> if the rest was correct (it isn't) you'd want over 100% thrust, not 52.

It produces 50% thrust at a standstill, and 100% thrust at 1,000 m/s

Drag = 0.5 * 1.223 * 8100 * 0.008 * 15 * 0.2 = 118.87

This ignores the drag generated by the wings.

You also don't show anywhere the AoA you are assuming.

BEAR IN MIND IDEAL TAKE OFF VELOCITY = TERMINAL VELOCITY = 100m/s

NO IT IS NOT

That only applies for VERTICAL ASCENT. If you have a high L/D, your gravity drag losses are minimal.

It is much much more complicated for an aircraft deriving lift from wings, climbing at an angle.

Of course, if you are climbing vertically, wings do nothing for you.

The speed you want to go, is the speed at which your climb rate is maximized. An aircraft can climb with a TWR well under 1:1

You basically want to take your best L/D speed/AoA - and then calculate the steepest climb you can do while maintaining that... well and you need to adjust for the cos of your climb angle, as at a 90 degree climb, you are generating no useful lift.

You did not have to be so offensive, I worked days on these calculations.

#1) he was not offensive, and this is the internet - even on these generally nice and (overly?) moderated forums, a "thicker skin" is required.

#2) That you worked a long time on this only goes to show the importance of making sure a project is well planned before embarking on it. It would have been better to get input before doing a lot of work, instead of after.

[DocMoriarty]

The real rules which make the plane also space worthy if it has the engines is the following since 0.24.x:

1 RAPIER per 12t total take off weight (TOW)

3 Ram Air Intakes per engine or 2 Shock Cone Intakes (with only 1 center engine on a light craft 1 shock cone is sufficient too).

1 lift rating per TOW

And the ideal take off speed is around 50-70m/s, not 90m/s (which indicates that either your rear gears are too far behind CoM or you have less lift than needed). That ensures you have enough lift for high altitudes. Of course alignments and stuff must be right.

Alignments:

Example craft: http://www.docmoriarty.com/ksp/mods/SPN Raven 550 0_00.craft

- CoL (blue arrow) must be a little behind and above CoM (yellow/black sphere)

- CoM must not move when tanks are emptied, ideally it stays the same all the time. It is most important that CoM doesn't move behind CoL. Try this by setting fuel levels of tanks in SPH empty and full.

- CoT (pink arrow) must be exactly in the center behind CoM

- Set fuel as needed (see image). A SSTO usually needs more liquid fuel than oxigen for the atmospheric part of it's flight. Saves weight.

- rear gears must not be too far behind CoM. Ideally CoM is in the middle between start of gear and the 2 little vertical lines painted on the gear bay or it will be hard to rotate for takeoff.

- to help take off gears should be attached in a way that the plane already has an angle of attack on the runway (rear gears higher, front gear lower)

- bow wheel: disable brakes, enable steering

- set control surfaces to control only what they are meant for, example: ailerons should only be active on roll, not on pitch or jaw

That was a crash course in space craft design. You don't really need to know more.

Edited January 27, 2015 by DocMoriarty

[Lynch]

The real rules which make the plane also space worthy if it has the engines is the following since 0.24.x:

1 RAPIER per 12t total take off weight (TOW)

3 Ram Air Intakes per engine or 2 Shock Cone Intakes (with only 1 center engine on a light craft 1 shock cone is sufficient too).

1 lift rating per TOW

And the ideal take off speed is around 50-60m/s, not 90m/s (which indicates that either your rear gears are too far behind CoM or you have less lift than needed). That ensures you have enough lift for high altitudes. Of course alignments and stuff must be right.

Alignments:

http://www.docmoriarty.com/ksp/gfx/alignments_01.jpg

- CoL (blue arrow) must be a little behind and above CoM (yellow/black sphere)

- CoM must not move when tanks are emptied, ideally it stays the same all the time. It is most important that CoM doesn't move behind CoL. Try this by setting fuel levels of tanks in SPH empty and full.

- CoT (pink arrow) must be exactly in the center behind CoM

- rear gears must not be too far behind CoM. Ideally CoM is in the middle between start of gear and the 2 little vertical lines painted on the gear bay or it will be hard to rotate for takeoff.

- to help take off gears should be attached in a way that the plane already has an angle of attack on the runway (rear gears higher, front gear lower)

- bow wheel: disable brakes, enable steering

- set control surfaces to control only what they are meant for, example: ailerons should only be active on roll, not on pitch or jaw

That was a crash course in space craft design. You don't really need to know more.

Thanks mate, I found your reply useful.

- - - Updated - - -

Being smug is not a good way to make your first post on the forums.

The standard SI unit is Newtons, not KiloNewtons - but since engine thrust is liste in kN as well, it is convenient to use kN, as you do.

The Turbojet only produces about 115 kN at 100 m/s -> if the rest was correct (it isn't) you'd want over 100% thrust, not 52.

It produces 50% thrust at a standstill, and 100% thrust at 1,000 m/s

This ignores the drag generated by the wings.

You also don't show anywhere the AoA you are assuming.

NO IT IS NOT

That only applies for VERTICAL ASCENT. If you have a high L/D, your gravity drag losses are minimal.

It is much much more complicated for an aircraft deriving lift from wings, climbing at an angle.

Of course, if you are climbing vertically, wings do nothing for you.

The speed you want to go, is the speed at which your climb rate is maximized. An aircraft can climb with a TWR well under 1:1

You basically want to take your best L/D speed/AoA - and then calculate the steepest climb you can do while maintaining that... well and you need to adjust for the cos of your climb angle, as at a 90 degree climb, you are generating no useful lift.

#1) he was not offensive, and this is the internet - even on these generally nice and (overly?) moderated forums, a "thicker skin" is required.

#2) That you worked a long time on this only goes to show the importance of making sure a project is well planned before embarking on it. It would have been better to get input before doing a lot of work, instead of after.

I know the SI unit, the SI for the game is kN though, so telling me what we already know?

Thrust is 225 at stock game as far as i remember. I couldn't run the game while writing that bit of the part, yet its irrelevant cause you can input any thrust into the calc.

AoA is said to be 0.1 between 10 - 20

[GoSlash27]

Lynch,

I appreciate the effort, but you have a lot to learn about how KSP aerodynamics works.

Your calculator is based on a lot of false assumptions (too many to list here), and designs to a useless criteria. Nobody has a use for an airplane that generates just enough thrust and lift to maintain level flight at sea level and 100 m/sec.

Check out this chunk of script that Numerobis kept from KSP .18: https://github.com/numerobis/KSP-scripts/blob/master/jets.py#L122

After the aerodynamics overhaul and part rebalancing we'll all be relearning this subject together. I look forward to your contributions in that effort.

Best,

-Slashy

[KerikBalm]

I know the SI unit, the SI for the game is kN though, so telling me what we already know?

I'm explaining to the other guy as much as to you. FWIW There is no SI for the game. SI is very specific.

http://en.wikipedia.org/wiki/International_System_of_Units

Thrust is 225 at stock game as far as i remember. I couldn't run the game while writing that bit of the part, yet its irrelevant cause you can input any thrust into the calc.

Its not irrelevant, and it shows you don't understand how the jet engines in the game work. The thrust value listed in the game files, and in the game window is 225 - but it does not produce 225 kN of thrust on the runway. In the .cfg files, you'll see its flagged to use a velocity curve, and that velocity curve scales its max thrust.

It only produces that thrust at 1,000 m/s. At 0 m/s (and 100 m/s won't be very different), it produces only 50% thrust.

The thrust curve is different for the basic jet (which produces 100% thrust at 0 m/s, and 0% thrust at 1,000 m/s)

If you are telling people to set thrust limiters, then you need to understand how the engine thrust works.

Also, you should note that the effective ISP of turbojets can reach a ridiculous 40,000s value. Fuel optimal ascents don't really matter even factor in the jet fuel, its so small.

What you really want for spaceplanes, is to calculate how to get your plane flying as high and fast as possible before switching to rockets (and then optimize for weight... doubling the dry weight of your craft to get it going 10 m/s faster is going to cost you more than 10 m/s of dV from your rockets).

Its really not useful to know how much thrust one needs to have to putter around at sea level at 100 m/s

[Lynch]

Best regards to everyone on their answer to my post, this was a limited calculation, you can always over-calculate to take off from sealevel and turn it into a spaceplane, i prepare design calculations; you can advance them later on.

I am preparing a new post up on calculations; for whom interested I await you on the links of the 1st post of this thread. =)