high altitude flight

[miklkit]

I'm trying to build a high altitude plane and have found out that the rules are different. Things that work at low altitude don't work so well up high and the plane ends up going up and down while slowly losing speed until it can't maintain altitude anymore.
Basically I can hit a target altitude of 20,000m but can not maintain it.

This is the first version. At altitude the nose goes up and down until it goes out of control.
https://steamcommunity.com/sharedfiles/filedetails/?id=1933948095

This is the current version. It is stable but can not maintain altitude.
https://steamcommunity.com/sharedfiles/filedetails/?id=1933948668

What am I doing wrong?

[Azunai]

jet engine efficiency in this game is based on built in thrust curves. as a rule of thumb they all lose thrust at higher altitudes and at high velocities.

i don't remember the characteristics of the panther engine but i'm pretty sure that it's not suitable for sustained flight at 20km altitude.

the higher tech whiplash and rapier engines have a high enough ceiling to support stable flight at such altitudes IIRC

[controller gaming]

Whiplash is the engine you would want to use on a plane meant to go 20km. From my experience using a panther it starts to lose speed after 15-16 kms, a more suitable cruising height is 9-13km.

[MechBFP]

You aren’t doing anything wrong. You are simply running out of atmosphere (i.e. air) that high up.

[miklkit]

Ok, the panther engine is still not good enough. My question is really about controllability. Imagine that first plane but with the bubble cockpit and the old style round intakes. That would oscillate slowly between 16000m and 17000m but was otherwise fully controllable. Simply changing the intakes made it fly completely out of control and impossible to land. I was hoping that the new intakes would give it the added altitude.

My goal now is to build a stable plane with those intakes. The oscillations are so violent that the plane will flip upside down and try to fly backwards along with flat spins and such. It looks like the small wing version is the one i should concentrate on.

[MechBFP]

Originally posted by miklkit:

Ok, the panther engine is still not good enough. My question is really about controllability. Imagine that first plane but with the bubble cockpit and the old style round intakes. That would oscillate slowly between 16000m and 17000m but was otherwise fully controllable. Simply changing the intakes made it fly completely out of control and impossible to land. I was hoping that the new intakes would give it the added altitude.

My goal now is to build a stable plane with those intakes. The oscillations are so violent that the plane will flip upside down and try to fly backwards along with flat spins and such. It looks like the small wing version is the one i should concentrate on.

Is your center of lift behind the center of mass, both when the plane is full of fuel as well as empty?

[miklkit]

Yes, although they have only been flown with nearly full tanks. The longest flight got them down to a little more than half full.

[miklkit]

Remembering the FW TA152 and the U2 I put long wings on it and while its turn rate makes an airliner look like a fighter plane it is stable enough at 19000m. I didn't try for 20000m as it probably wouldn't do well.
Now to try it on one of those high altitude missions.
https://steamcommunity.com/sharedfiles/filedetails/?id=1934517923

[FistBones]

i dont know what kind of range youre looking for but i would make a significantly smaller plane. i recently used a single whiplash plane and was able to maintain a stable 21-22k meter altitude

[miklkit]

You may be correct and that 3 engine plane is a lemon.

I took a single engine panther plane, changed the air intake and it flew around the planet at 19,000 m and landed back at KSP with lots of fuel left over. It should be fine for those high altitude missions.

[Paladin Two-Seven]

Just sounds like the engines can't get enough air, in KSP high speed also generates air intake so I'm guessing what is happening is that you have the initial speed to get to altitude but the lack of air at altitude is starving the engine resulting in lower speed which further reduces air intake and it cascades until either the engine shuts down or you return to denser atmosphere at a lower altitude.

Also the crafts control surfaces are less effective up high making it feel sluggish.

[F'lar]

I also had problems with a high altitude plane until I decided to follow the K.I.S.S. principle. Now I have a plane that will fly around the world at an altitude of the low-20s. She has a horrible turn rate and oscillates a couple of thousand meters at cruise, but it's flyable. Best way to get down into thicker air and land is to perform what dogfighters called a 'Split-S'.

I use a Whiplash engine that has two air intakes for air supply.

https://steamcommunity.com/sharedfiles/filedetails/?id=1335577943

[miklkit]

I don't have that other stuff yet. The Panther engine can hold 19,000 steady and oscillates around 20,000. It flames out at 25,000.

Two low speed intakes VS one high speed intake is better?

[F'lar]

More air is better. Can't remember where I saw/read about it - Scott Manley maybe? - but they were talking about having two intakes/engine. My 'plane science' is more advanced than my 'rocket science' (career mode) because I got tired of seeing all the high-altitude missions I had on the books and wanted to finish them.

I do have designs that have 1 intake/engine - my SSTO, for example.
https://steamcommunity.com/sharedfiles/filedetails/?id=1640213502
https://steamcommunity.com/sharedfiles/filedetails/?id=1640214586

[miklkit]

This is my current best hi alt plane. All other versions are slower and lower and unstable in turns.
https://steamcommunity.com/sharedfiles/filedetails/?id=1937826505

I've built 2 rocket planes and they run out of gas way too fast to be good for anything.

[miklkit]

I decided to look into that two intakes thing, and it turns out to make 11-12 m/s difference at 19,000m with my plane. Or about the same speed but 1000 m higher.
https://steamcommunity.com/sharedfiles/filedetails/?id=1937931692

This is the same plane as the one in the above post but with another intake added.