Need some help on my uniquely designed airplane

[drtricky]

Some of you may know I made a previous thread regarding my aircraft "stalling". I'm making another thread here because I did not specify what I meant by "stalling".

So below is what my aircraft looks like (if you want the craft file to test it yourself, PM me):

Spoiler

 

Has ~1700 units of fuel. All panther engines are set to afterburning mode (excluding the one whiplash in the middle that has no such setting)

 

Now the current issue it has is that if I increase its maneuverability (turn radius and turn rate) by moving the CoL forward (but not ahead of the CoM), the plane becomes very prone to oversteering and consequentially stalling in a turn, I can increase my plane's AoA, but then it stalls at the increased AoA. It especially does this at altitudes above 1000 m at velocities over 200 m/s. So do you guys know (if) there's anything I can do to make my abomination plane able to maintain a tighter turn?

Oh, and this airplane was designed to function with stock aerodynamics (If that wasn't obvious already)

Edited March 15, 2016 by drtricky

[Nich]

Fighters are generally unstable and require computer control to fly.  At this point you could switch to a faster acting canard and fly with SAS on.  Big S parts have more deflection and faster actuator speed. Change the front nose cone to a precooler with an aerospike and put your control surfaces on that to give a larger lever arm.

[AeroGav]

Turn rate is determined how much lift your aircraft can generate relative to its weight, nothing else (unless the limiting factor is structural failure..).

Lift is determined by three things -

1) speed

2) angle of attack 

3) wing area

The trouble with increasing speed is that it increases the G force required to achieve the same turn rate in degrees per second.    

That leaves you with 2) and 3). 

 When you slide your wings backward a bit, to create a bigger gap between centre of lift and centre of mass,   your ship becomes more stable, but  you reach a point where that big tail fin you're using for a canard, becomes unable to pull the nose up to 30 degrees angle of attack, where lift reaches it's maximum.   In many ways that's a good thing because at 31 degrees the wing stalls.

That screenshot shows you're pushing things right to the ragged edge with centre of lift already, i'd never build a plane so close to being unstable - what happens when fuel burns off?

 

[drtricky]

2 minutes ago, AeroGav said:

 When you slide your wings backward a bit, to create a bigger gap between centre of lift and centre of mass,   your ship becomes more stable, but  you reach a point where that big tail fin you're using for a canard, becomes unable to pull the nose up to 30 degrees angle of attack, where lift reaches it's maximum.

I don't understand the part about the canard becoming unable to pull the nose up to the angle of attack. Can you elaborate more on that for me? 

 

2 minutes ago, AeroGav said:

That screenshot shows you're pushing things right to the ragged edge with centre of lift already, i'd never build a plane so close to being unstable - what happens when fuel burns off?

The CoM shifts slightly backwards (assuming all missiles have been fired). I balanced the fuel tanks such that the CoM with said 1700 units of fuel remains roughly equal to the CoM with fuel tanks empty.

 

[AeroGav]

11 minutes ago, drtricky said:

I don't understand the part about the canard becoming unable to pull the nose up to the angle of attack. Can you elaborate more on that for me? 

 

 

Canard = that tail plane you've got at the front, which controls pitch

Angle of attack =  the angle at which the airflow meets the wings - the bigger the angle, the greater the lift (and drag).  

On your Navball,  Angle of attack is the difference between the yellow prograde circle and the orange -w- symbol.

For example,  the prograde is showing 5 degrees positive,  so your direction of travel one of a 5 degree climb.

But the -w-  is right on the 20 degree mark,   this means your angle of attack would be 20-5 = 15 degrees

 

The further your Centre of lift is behind the centre of mass, the more stable is your airplane.   But, this also means the plane is more nose heavy, the more it wants to pitch down on its own.        Therefore,  the more force your front control surface needs to generate to hold the nose up at say, 25 degrees angle of attack.         At some point this control surface is going to be maxed out and unable to pitch up any more.

 

 

Edited March 15, 2016 by AeroGav

[drtricky]

58 minutes ago, AeroGav said:

At some point this control surface is going to be maxed out and unable to pitch up any more.

Now that I understand what Angle of Attack is, I can say the issue for me isn't that my aircraft can't achieve a high enough AoA, the issue is that if the aircraft's AoA is too high, then the airplane stalls. Does anyone know how to deal with this? I've seen planes that were able to make very tight turns with AoA's well beyond 25 degrees (with stock aerodynamics of course). And these weren't on-the-spot turns either, they were made at speed.

Edited March 16, 2016 by drtricky

[AeroGav]

In the real world, leading edge high lift devices delay onset of the stall to 25-30 degrees (Kerbal seems to already factor this in).   Delta wings have a very high stalling angle,  and leading edge root extensions (strakes) like no the F-18 hornet have this effect.

Back to Kerbal,  no.   Stock aerodynamics use the same curve for Angle of Attack vs Lift and Drag for all types of wing, all stall at 30 degrees.

Ferram Aerospace has much more elaborate drag modelling but all my designs seem to stall at similar angles, and leading edge slats/droops as used by airliners, have no effect on the stalling angle.  In fact they generate negative lift if you angle them down like in real life,  to get extra lift you need to angle them upwards 

 

The thing is,  drag increases faster than lift above 5 degrees AoA  and at 20+ it's going to be horrific.   Would it really help being able to swivel the nose another 10 degrees towards your target if you aren't actually turning faster towards him and are creating so much drag your speed washes away?

 

[drtricky]

17 minutes ago, AeroGav said:

leading edge slats/droops as used by airliners, have no effect on the stalling angle.  In fact they generate negative lift if you angle them down like in real life,  to get extra lift you need to angle them upwards 

17 minutes ago, AeroGav said:

The thing is,  drag increases faster than lift above 5 degrees AoA  and at 20+ it's going to be horrific.   Would it really help being able to swivel the nose another 10 degrees towards your target if you aren't actually turning faster towards him and are creating so much drag your speed washes away?

In dogfighting, having ways to slow down quickly is extremely helpful, both in reality, and with BDArmory fights. The issue with my plane isn't a loss of speed.

But are you saying that all planes have a limit for how hard they can turn in KSP? I've seen a plane that can make a turn at speed with its AoA well above 30 degrees.

Edited March 16, 2016 by drtricky

[Nich]

the old debate of turn fighter vs energy fighter

[AeroGav]

1 minute ago, drtricky said:

In dogfighting, having ways to slow down quickly is extremely helpful, both in reality, and with BDArmory fights. The issue with my plane isn't a loss of speed.

But are you saying that all planes have a limit for how hard they can turn? I've seriously seen a plane that can turn on a dime at speed

In Kerbal,  unmodded?  Or airshow footage?

There's several things that can be confused though when referring to lateral maneuvers -

1) Roll (and pitch) rate - how long to get into the turn, and how long between yanking full back stick and hitting max G load

2) Instantaneous turn rate, degrees per second of heading change

3) Sustained turn rate

4) Off Axis manuevres - where the turn rate doesnt increase but the angle of attack increases enough to point a little more at the target

 

Turn rate in terms of degrees per second,  is determined by two things -  Speed, and G - loading.

Going faster slows the turn rate, pulling more Gs increases it.  In real life, G load is limited to about 9 by human tolerance.

So,  a good fighter plane is able to sustain a 9g turn over as wide a range of speeds and altitudes as possible.  Maximum turn rate occurs at the lowest speed which you can generate enough lift to make 9g at.   More wing might let you do this down to slower speeds, but could harm straight line speed.     More wing also allows you to pull max g at higher altitudes in thinner air.

I'm not sure how things are with the latest generation of fighters, but i know that previous ones had a separate "instantaneous turn rate" to their "sustained turn rate".     When pulling up to maximum angle of attack they generate so much drag the plane loses speed even on full afterburner,  and within a few seconds it is going to slowly to maintain 9g any more.           So, being able to turn at maximum rate and not loose airspeed (unless you really want to, but usually it's better to do an S turn or vertical manuever if overshooting)  is another thing good fighters should be able to do - i think it's called "energy manuverability"     

[drtricky]

27 minutes ago, AeroGav said:

In Kerbal,  unmodded?

Of course.