Why does my plane dive at high altitude

[Cerberus738]

i have a feeling it just needs more control surfaces but maybe you peoples out there have some better insight. it controls fairly well at low altitude. as soon as i get above 13000 it starts to fall and pitch eventually locks all the way up and the only option i have is going back down. it keeps falling towards to prograde marker until at 23000 vertical velocity is going down. once it reaches 19000 it starts to climb back up again. pitch is locked the whole time. in a way its nice cause i can just point the plane in the direction i want to go and it just cruises up and down between 23000 and 1900.... but i would like to have control. also i would like it to have a cruising altitude of 30000 but i just can't get it that high.

Edited February 17, 2014 by Cerberus738

[MajorThomas]

I would say this is pretty normal for aircraft. Eventually as you rise in altitude the air becomes too thin to give you the lift your plane was designed for, and you stall out. As you fall the air thickens, lift returns and you can fly again.

Cruising altitude of 30,000 m may be very difficult to achieve, the air is very thin there!

[Cerberus738]

so i need more wing area? the wings are ridiculous already!!! it is 70t at take off so maybe that will fix the problem

[MajorThomas]

Maybe I should strengthen my statement that no aircraft could fly there. 30k m is about where I'd put the "Karman line" on Kerbin.

It's basically a no-go zone: too low for orbit, too high to fly. It's about here that the RAPIER engine used by spaceplanes switches over to "rocket engine" mode (from air breathing to using internally-tanked oxidizer).

[Red Iron Crown]

I think your cruising altitude goal of 30km is unrealistic. That's almost 100,000 feet, well beyond the service ceiling of even high performance fighter aircraft and into exotic rocket plane territory. The air is really too thin for sustainable aerodynamic flight at that altitude.

[Jimbimbibble]

In ksp the aerodynamics model is very unrealistic, which is causing this to happen. Wings should be producing more lift when you're supersonic but that doesn't happen in ksp. If you're into planes I'd strongly recommend using Ferram Aerospace Research. It will make your planes fly more realistically (and harder to design well).

[Claw]

It looks like you have plenty of wing area (probably more than you need).

I often find this is due to over reliance on flight control surfaces for main body lift, or that you have a lawn dart that your flight control surfaces can compensate for at low altitude.

It's a long explination I can get into if you want, but I would recommend trying two things to narrow down your problem.

From the picture, it seems like your CoL is too far behind yor CoM.

What is your pitch trim state right after takeoff when flying level? If it's already pitching up a lot that will tell you something about your CoL and CoM.

Starting out by trying to fly your plane with the CoL a bit closer to the CoM and see how that goes. That will obviously have other consequences (because it looks like your CoM moves backward a lot), but just try it to see how it flies.

Let me know how that goes before I recommend flight control changes. You probably don't actually need so many, but I can guess why you have them.

If you are able to provide a craft file, I can probably be more specific in my recommendations (if you want).

Good luck

[Claw]

In ksp the aerodynamics model is very unrealistic, which is causing this to happen. Wings should be producing more lift when you're supersonic but that doesn't happen in ksp. If you're into planes I'd strongly recommend using Ferram Aerospace Research. It will make your planes fly more realistically (and harder to design well).

While the KSP model is not ideal, it isn't always directly to blame. Lawndart behavior and loss of lift due to elevons are real phenomina. FAR is a more realistic model (from what I hear), but straight off blaming the KSP model is counterproductive.

Wings do not produce more lift simply because you are supersonic. In fact, all kinds of other bad things happen at supersonic speeds. Mach tuck (nosediving while tryung to get supersonic) has killed many pilots, and that was aerodynamics in the real world.

[capi3101]

Actually, for your payload, you may have too much wing surface going on. As a rule, a pair of swept wings can generate sufficient lift for about six tonnes worth of aircraft. With the proper engine and intake configuration, you can fly that into space...

Also, how many intakes do you have on that thing, and what type.

Also, if it this is meant to be a spaceplane, where are your rocket engines? You need mostly jets but you'll have to have a couple of rockets if your goal is orbit.

[vetrox]

More horizontal speed!

as long as the prograde marker is above the horizon you are fine. You may find you need to pitch up rather than staying locked the whole flight.

Edited February 14, 2014 by vetrox

[ComradeGoat]

Are the bottom engines reducing power, thus casting asymmetric thrust, causing you to pitch forward?

[Cerberus738]

Hey guys! I didn't abandon this thread! The girlfriend figured I could go at least one day without crashing something into another planet.... i disagreed with her but she won

That plane is not meant to go into space. I've just started building planes and have been doing a lot of messing around on Kerbin. This particular plane is meant to deliver that orange tank to various airbases around the world. After reading all the posts I think my goal of a high altitude cruise, high speed delivery vehicle is unrealistic.... and disappointing i would like to understand the wing dynamics better. i just assumed more lift would help. It had trouble getting off the ground with 1 set of wings so i added another layer. it still didn't lift very well so i added the third layer. it performs well at low altitude but is way to slow to get around the planet.

Claw.

Thanks for all the suggestions! I'm about to load the game and do some messing around. It does not lift off the runway on its own. i have to pull up on the stick to get the nose up. does angling the wings up and down help this? I'm not entirely sure what you mean by my pitch trim state at take off? You mean do i have to have hold the pitch up or down to make it fly level?

All engines are at full power.

[Claw]

Hey guys! I didn't abandon this thread! The girlfriend figured I could go at least one day without crashing something into another planet.... i disagreed with her but she won

Haha, yes. They must win time to time. Especially when it's V-day...

That plane is not meant to go into space. I've just started building planes and have been doing a lot of messing around on Kerbin. This particular plane is meant to deliver that orange tank to various airbases around the world. After reading all the posts I think my goal of a high altitude cruise, high speed delivery vehicle is unrealistic.... and disappointing

This isn't an unrealistic goal (in KSP anyway).

i would like to understand the wing dynamics better. i just assumed more lift would help. It had trouble getting off the ground with 1 set of wings so i added another layer. it still didn't lift very well so i added the third layer. it performs well at low altitude but is way to slow to get around the planet.

There are lots of reasons this could be happening beyond needing more lift. More wings help, to a point. After that they aren't really helping and just add to the part count. A reasonable rule of thumb is 0.5 to 1.0 points of lift rating per ton of aircraft. It looks like you have 80+ points of lift per wing layer.

Claw.

Thanks for all the suggestions! I'm about to load the game and do some messing around. It does not lift off the runway on its own. i have to pull up on the stick to get the nose up.

You are welcome. I'm sure you can meet most (if not all) of your goal. Not lifting off the runway on it's own is pretty typical of planes, especially big ones.

does angling the wings up and down help this?

Yes, but angling up isn't necessarily the best option. It's not a bad option either, but big planes will often require pitching up to take off. EDIT: I will caveat this by saying if you're aiming for a long range cruiser, angling up the wings can make it easier to fly. I didn't mean to say don't do it. Give it a try. It can help, but don't expect it to fix all your other problems too.

I'm not entirely sure what you mean by my pitch trim state at take off? You mean do i have to have hold the pitch up or down to make it fly level?

Yes that is what I mean, in a way. Try it this way. Launch your plane and takeoff. Level off around 500m (so your prograde marker is on the horizon) and take a look at your trim indicators. If the SAS is already putting in a lot of trim, that's going to be a problem at altitude. It's one of many things to look at.

All engines are at full power.

The question ComradeGoat asked about the bottom engines pertains to engine flameout. The last set of engines you place on your craft can experience thrust rollback before they flameout completely. If it turns out the bottom set is the last ones you placed and they are rolling back, it can cause your plane to pitch over.

Your engines also look like they might be clipping together, this can cause phantom forces.

Anyway, there are lots of reasons this could be happening. Based on the pictures, I don't think you need more wing or control surface. If anything you have too many control surfaces at the back and you're probably envoking the control surface AoA bug. Also your CoM is too far forward. These two things coupled together are probably causing your nosedown at altitude.

Edited February 15, 2014 by Claw

[capi3101]

Might suggest you take a look at keptin's illustrated thread on spaceplane design. Helped me out a lot.

It's not necessarily an unrealistic goal; in fact things are simplified somewhat because you're not going to space, and now that we know that we can give you advice that's better tailored to your specific situation.

General rules - one engine per 9 tonnes of payload (anything not a wing, intake, or jet), one Mk1 or Mk2 fuel tank per jet (preferably Mk2 if you can manage it), one pair of swept wings per six tonnes of craft, one Ram intake per tonne of craft (or at least no fewer than three intakes per engine; 6-8 is better).

EDIT: (Hope OP doesn't mind a momentarily hijacking of the thread) - BTW, Claw, Auk VIII needed the third set of forward canards to hold its pitch angle; without it, I'd have to sit there and hold the S key the whole way up. And it'd still slowly pitch downward until finally starting to go downwards.

Edited February 15, 2014 by capi3101

[Cerberus738]

Well this project is officially abandoned. I've messed around with it and now i'm at the point where i can't even get it off the ground. Rage quit. Just gonna drop the stupid thing from space.

i did just read that page. and i tried all of the suggestions on it. having lost my original design i'm too angry to continue. back to rockets

[Claw]

Well this project is officially abandoned. I've messed around with it and now i'm at the point where i can't even get it off the ground. Rage quit. Just gonna drop the stupid thing from space.

i did just read that page. and i tried all of the suggestions on it. having lost my original design i'm too angry to continue. back to rockets

*sadface*

Take a break. Come back and start over when you're ready.

[Cerberus738]

*sadface*

Take a break. Come back and start over when you're ready.

ah patience. I will go back to it. Its just sooooooooo frustrating!!!

[Claw]

ah patience. I will go back to it. Its just sooooooooo frustrating!!!

This I cannot deny.

[Hodo]

My only suggestion is FAR.

The stock KSP aerodynamic model is like flying through thick and chunky potato soup.

Unfortunatly the craft you designed also would not work in FAR. It creates to much drag on the front of the craft and would fly like pushing a barn door through the air.

[Cerberus738]

Well this project is officially abandoned.

It seems that a secret division of the government did not follow orders.....

I had to redesign from the ground up... BUT after reading the suggestions from everyone and that page on spaceplane design I had a successful test flight!!! It's stable at 27000. goes sufficiently fast and it can land with no power!! its very exciting.

I think a major problem with the other design is that the center of thrust was above the center of lift and center of mass. as the air got thin it just kept pushing it over. also it seems that having wings too far forward causes problems at high altitude.

It still has a large amount of wing area... i'm happy with it the way it is but if anyone has any suggestions on making it sleeker and possibly faster?? i'm open to suggestions!

Also... its not meant to go into space.

[Taki117]

It seems that a secret division of the government did not follow orders.....

SNIP

Also... its not meant to go into space.

Then why have the ridiculously large Jumbo64 on there at all? Here's the thing, that's a hell of a lot of dead weight, even just filled with liquid fuel. If you want larger liquid fuel tanks I suggest checking out the KSP Interstellar mod, they have liquid fuel tanks in 2.5 and 3.75m sizes. (And lots of other cool stuff, like a thermal powered turbo jet that doesn't require any fuel)

[Claw]

It seems that a secret division of the government did not follow orders.....

I had to redesign from the ground up... BUT after reading the suggestions from everyone and that page on spaceplane design I had a successful test flight!!! It's stable at 27000. goes sufficiently fast and it can land with no power!! its very exciting.

I think a major problem with the other design is that the center of thrust was above the center of lift and center of mass. as the air got thin it just kept pushing it over. also it seems that having wings too far forward causes problems at high altitude.

It still has a large amount of wing area... i'm happy with it the way it is but if anyone has any suggestions on making it sleeker and possibly faster?? i'm open to suggestions!

Also... its not meant to go into space.

Looks pretty good.

It still looks like your CoL is pretty far behind your CoM, but if it's working then leave it alone. 1600 m/s at 27km is pretty fast. If you can manage to tweak your design enough to get up to 32km, then your speed/efficiency will really bump up.

The CoT not inline with the CoL isn't really a problem. The CoT not inline with the CoM is, but can be overcome to some extent as shown below.

Javascript is disabled. View full album

[Armchair Rocket Scientist]

I think your cruising altitude goal of 30km is unrealistic. That's almost 100,000 feet, well beyond the service ceiling of even high performance fighter aircraft and into exotic rocket plane territory. The air is really too thin for sustainable aerodynamic flight at that altitude.

Sustainable purely aerodynamic flight, yes. However, 30 km is also the altitude where it starts becoming reasonably for an airbreathing craft to simply cruise at orbital velocity and use its engine to overcome drag. Seriously, SSTOs commonly fly level at 30+ km and hypersonic speeds. Take an SSTO and replace the rocket engines and tanks with more jet fuel, and you'll likely get a hypersonic aircraft that can deliver a payload anywhere on Kerbin and return.

[MaverickSawyer]

Indeed. I started with a HST, removed some fuel tanks and swapped them for the LFO tanks, added a pair of OMS engines (AIES MODOC-2s, to be precise), and flew to orbit and back. She's a beauty.

Cruised at altitudes in excess of 20 km for more than 10 minutes just building up speed. Spent 4 of those minutes above 30 km with a single TurboJet fed by twin Ram intakes. Flameout occured about 36.5km, so sustained flight is possible up there, but it's tricky as hell, and you have to let Mechjeb handle the controls and spend your time monitoring vertical speed and CoM shifts as fuel burns off.

[Claw]

Sustainable purely aerodynamic flight, yes. However, 30 km is also the altitude where it starts becoming reasonably for an airbreathing craft to simply cruise at orbital velocity and use its engine to overcome drag. Seriously, SSTOs commonly fly level at 30+ km and hypersonic speeds. Take an SSTO and replace the rocket engines and tanks with more jet fuel, and you'll likely get a hypersonic aircraft that can deliver a payload anywhere on Kerbin and return.

Yes, this. For me it seems much more noticeable at 32km. At that point, wings don't matter and it's all about having a bit of thrust fed by 3 or so intakes per engine. I just completed a challenge with a wingless SSTO VTOL that circumnavigated Kerbin (below 68km) on about 72 units of fuel including a VAB landing. Most of that fuel was used up getting to altitude.