Airplane doesnt fly well

[Jetsim]

Isnt good turning essential in dogfightning?

I still wonder why making CoL closer to CoM improved pitch when it should get worse.

[Hodo]

Isnt good turning essential in dogfightning?

I still wonder why making CoL closer to CoM improved pitch when it should get worse.

Why?

The CoL is what is effectively turning you in a banking turn. It is thrust in a vertical direction as opposed to horizontal.

[Alshain]

I get the feeling your expecting it to turn like a Viper in Battlestar Galactica. If that's the case, your playing the wrong game.

[Hodo]

I get the feeling your expecting it to turn like a Viper in Battlestar Galactica. If that's the case, your playing the wrong game.

LOL true.

Real aircraft, F-15, F-16, and the Mig-29, all turn extremely well at a certain speed. The F-16 is one of the most nimble non-thrust vectoring aircraft in the air and it still cant turn 180 in less than 4 seconds at some speeds.

[Jetsim]

Why?

The CoL is what is effectively turning you in a banking turn. It is thrust in a vertical direction as opposed to horizontal.

People here said that pitch is leverage, and leverage is most effective from distance.

[RoboRay]

I still wonder why making CoL closer to CoM improved pitch when it should get worse.

Have you read those references we gave you earlier yet? Because that should have told you that you're making a lot of incorrect assumptions and helped you understand why.

[Alshain]

People here said that pitch is leverage, and leverage is most effective from distance.

Center of lift is not leverage. Control surfaces are leverage. Your center of lift is close to CoM for stability, you control surfaces are far from CoM for pitching.

[Jetsim]

Center of lift is not leverage. Control surfaces are leverage. Your center of lift is close to CoM for stability, you control surfaces are far from CoM for turning.

But naturally controllers moved closer to CoM, and that improved pitch = turning.

[Hodo]

As Alshain said.

Or let me put it this way.

Lift is just that the force that picks the craft up off the ground or moves it on the vertical access and not the horizontal access.

Pitch, is the leverage used to change the angle of attack, or point the nose up and down in relation to the crafts horizontal access.

Yaw, is the leverage used to change the slip angle, or point the nose left and right in relation to the the crafts horizontal access.

Most aircraft turn better when they roll and apply pitch up to bring the nose to the new direction. This unfortunately is at the cost of lift and speed.

Fighters work on this simple equation, Speed = Life. Or Energy = speed Speed = Altitude Altitude = Energy. You can either go up at the cost of speed, or go down and gain speed. You can turn, essentially going up sideways and lose speed, or you can go straight and gain speed.

In KSP most of that doesn't apply because of how over powered the jet engines are in relation to the real world.

A great example is, the F-100 used on the F-16 Fighting Falcon only generates 129kn of thrust in afterburner. But the stock basic jet engine creates 140kn of thrust. And the turbojet creates almost double that of the F-100.

- - - Updated - - -

But naturally controllers moved closer to CoM, and that improved pitch = turning.

No, you don't want your pitch authority close to the CoM. You want them near the end of the craft.

You want the CoL near the CoM.

[Jetsim]

Yeah I know that, but I had this delta wing plane with elevons, and first it couldnt pitch at all, then moved wings closer to CoM which naturally moved controller = elevon closer to CoM and plane pitched well. I dont understand.

[LethalDose]

Isnt good turning essential in dogfightning?

The ability to change velocity (both direction and speed) is what is needed for dogfighting. Dogfighting is also done almost exclusively at subsonic speeds.

I'm guessing you're just going full throttle on these jets, and has been pointed out, going supersonic almost immediately. That will tear the crap out of your aircraft in a heartbeat, in KSP/FAR andReal-life. What makes a good fighter pilots isn't making your aircraft go as fast as possible, it's knowing how to fly that fighter without tearing it apart.

And just as an aside since we're on the topic of fighters, turning, and speed, it's a fun time to mention EM Theory and USAF Col. John Boyd. Tangetial learning. You're welcome.

Also, why are we even talking about dogfighting, this is a space exploration game...

I still wonder why making CoL closer to CoM improved pitch when it should get worse.

People here said that pitch is leverage, and leverage is most effective from distance.

Yes, pitch is leverage. CoL has [almost] nothing to do leverage when maneuvering in any of the three axes (See below). For good pitch authority, you want powerful control surfaces far away from the axis (which goes thru the CoM) around which the aircraft is turning.

CoL/CoM location only has to do with leverage in that the further the CoL is from the CoM, the more pitch will be caused while cruising in the atmosphere (i.e. while not maneuvering). When the CoL is far ahead of or far behind the CoM, the vessel will tend to pitch up or down, respectively. This actually screws with your pitch authority! It does not help it!

You want the CoL just a little bit behind the CoM so if there's going to be any unwanted pitch at all, it's going to cause a dive, which conserves airspeed and therfore allows the pilot to regain control. If the CoM/CoL balance causes the craft to climb instead, you lose airspeed and can stall.

And if you go in to a stall, you're gonna have a bad time.

[James_Eh]

12 pages of good advice. Here's some bad advice. Start big. Keep adding things until it flies well.

[Pecan]

Jetsim - I've only just seen the small 'plane you posted 2 or 3 pages back. Sorry if I was slow, I thought we were still talking about the monster you'd posted in the first picture. You've got to admit that there's a LOT of difference :-)

Well done for persevering.

[MarvinKitFox]

I think you are confusing what is easy to know with what YOU already know.

Nah.

I'm looking at a person that has 4 pages of advice where *every single* post tells him to build smaller, and he insists on building yet bigger.

He gets good solid advice, then argues that he doesn't understand it, but they must be wrong because... he doesn't understand it.

You either have someone with a learning disability, in which case he is not really helpable,

or with an attitude problem of being unable to take advice, in which case he is not helpable,

or ... a troll

[Wanderfound]

In KSP most of that doesn't apply because of how over powered the jet engines are in relation to the real world.

A great example is, the F-100 used on the F-16 Fighting Falcon only generates 129kn of thrust in afterburner. But the stock basic jet engine creates 140kn of thrust. And the turbojet creates almost double that of the F-100.

OTOH, when you're building spaceplanes, it's not unreasonable to assume that you're using top-line components. How does a Kerbal turbojet compare to something like an SR-71 engine?

[NoClass]

OTOH, when you're building spaceplanes, it's not unreasonable to assume that you're using top-line components. How does a Kerbal turbojet compare to something like an SR-71 engine?

According to the J58 page on Wikipedia (blessed be the Font of All Knowledge) it's similar, but the KSP turbojet is still about 150% more powerful for its weight. The J58 is a 2700 kg engine thrusting 150kN wet and 110 dry. Of course KSP crams all the mass into a small exhaust section and calls that the "engine."

I didn't read too much into the page, as it's late and I have a short attention span, but I didn't see anything about fuel flow, which I suspect is much higher at full 'burner than KSP. I also suspect the thrusts at various airspeeds are radically different (especially for low a/s).

As an aside, props to Alshain for patient and respectful explanation of the basics.

Edited July 25, 2014 by NoClass

[Wanderfound]

According to the J58 page on Wikipedia (blessed be the Font of All Knowledge) it's similar, but the KSP turbojet is still about 150% more powerful for its weight.

I can live with that; after all, the J58 is a fifty year old design.

[Jetsim]

I appreciate good explanations here, but still 1 issue is beyond understanding.

If leverage (elevon) is best at distance from CoM, how can I have opposite effect when I moved wings (CoL) closer to CoM? Naturally closer wing has controller closer to CoM and having positive effect on pitch is contradictory with leverage principle.

[RoboRay]

I appreciate good explanations here, but still 1 issue is beyond understanding.

If leverage (elevon) is best at distance from CoM, how can I have opposite effect when I moved wings (CoL) closer to CoM? Naturally closer wing has controller closer to CoM and having positive effect on pitch is contradictory with leverage principle.

There are two unequal and opposing factors competing here that affect that.

One, positive stability, is determined by how far the CoL is behind the CoM. Your plane is like the arrow shot from a bow. If you try to throw an arrow sideways, what happens? The drag from the fins naturally turns the arrow so that the fins are behind the CoM. Lift is a form of drag, so the CoL also naturally "seeks" a position behind the CoM. The further back the CoL, the more aggressively the airflow tries to hold it behind the CoM. If a plane has the CoL way back, the plane can become very difficult to turn, meaning you must apply a lot of force with the control surfaces. If you move the CoL closer to the CoM, you don't need to apply as much force to make it turn.

However, that doesn't mean it's good to put the CoL right on top of the CoM. If you do that, the plane has neutral stability and has no preferred orientation in respect to the airflow. This makes it very hard to control because the nose will easily flip in any direction, even around backwards. Putting the CoL in front of the CoM would give negative stability, meaning the plane is always trying to turn around backwards and you will find it impossible to control.

The other factor is that the further your surfaces are from the CoM, the more effective they are for that axis of control. Moving your elevators forward gave them less leverage, but the plane also became less positively stable because you moved the CoL forward closer to the CoM, making it require much less force to control pitch.

So, the airflow applies leverage based on how far apart the CoL and CoM are, and the elevators apply leverage based on far apart they are from the CoM. Which one has greater effect depends on the design of the plane. In your particular design, the effect of moving the CoL forward helped you more than the effect of moving the elevators forward hurt you.

Edited July 25, 2014 by RoboRay

[Jetsim]

Thanks for explanation, but another detail raised question.

"The further back the CoL, the more aggressively the airflow tries to hold it behind the CoM"

Why doesnt CoL behind flip the plane from back to forward if it can flip plane backwards if CoL is in front of CoM?

[RoboRay]

Thanks for explanation, but another detail raised question.

"The further back the CoL, the more aggressively the airflow tries to hold it behind the CoM"

Why doesnt CoL behind flip the plane from back to forward if it can flip plane backwards if CoL is in front of CoM?

From above...

"Your plane is like the arrow shot from a bow. If you try to throw an arrow sideways, what happens? The drag from the fins naturally turns the arrow so that the fins are behind the CoM. Lift is a form of drag, so the CoL also naturally "seeks" a position behind the CoM."

[Jetsim]

From above...

"Your plane is like the arrow shot from a bow. If you try to throw an arrow sideways, what happens? The drag from the fins naturally turns the arrow so that the fins are behind the CoM. Lift is a form of drag, so the CoL also naturally "seeks" a position behind the CoM."

Cant understand.

[RoboRay]

I'm afraid I'm just going to have to go with MarvinKitFox's assessment.

Good luck everyone.

[NoClass]

Why doesnt CoL behind flip the plane from back to forward if it can flip plane backwards if CoL is in front of CoM?

You must ask yourself, "Jetsim, where does the air flow?" In flight, the CoL will always pull the tail of the plane away from the flow of air, and move behind the CoM.

If you were flying backwards, the air would be moving back to front, then the CoL would flip the plane backwards. Don't fly backwards!

[Hodo]

I can live with that; after all, the J58 is a fifty year old design.

Even the newest most powerful jet engines, are shy of the power of the Turbojet in KSP.

Here is a few to compare.

-D30-F6 from the Mig-31 Foxbat (top speed over mach 2.8), 93kn dry, 152kn afterburner.

-P&W F-119-PW-100, from the F-22 Raptor, dry thrust 104kn dry (roughly), 156+kn afterburner.

-NPO Saturn Izdeliye 117, from the Sukhoi PAK FA, 107kn dry (roughly), 176+kn afterburner.

And all of them have a ISP of almost 10 times that of the jet engines in KSP.