High altitude planes

[Marakaos]

Hey guys,

So I've created those two planes, both with weasley turbofan engine:

https://imgur.com/a/yR5iJ

aiming for high altitude flights. Having in mind planes like U-2 or any real high altitude planes, I came to conclusion that I need long and narrow wings. But, the plane of this design (first image) was able to achieve about 15km, only when my other design with delta-like wings (second image) achieved almost 20km :| Also, I could see from areo info that the long-winged one produce much more drag than the delta one. 

So my question is: is my plane design wrong, or isn't it just working in ksp, or is the reality of high aspect ratio planes different than I think?

I use FAR btw.

 

Edited January 20, 2018 by Marakaos

[foamyesque]

KSP doesn't care (much) about aspect ratio; the total lift is the same regardless of configuration. You do get certain inertial and lever-arm effects, but those aren't really relevant to 'do I have enough lift to stay up'. The drag differential is because your long-wing layout is using a bunch of struts, and the reason your delta-wing has a higher ceiling is because it has more wing area to provide lift.

EDIT: Didn't notice you were using FAR. I don't recall if they model aspect ratio, but either way, the struts are murder and the delta wing has so much more wing that it'll still have more lift.

Edited January 20, 2018 by foamyesque

[ARS]

Keep in mind that wheesley engine's performance decreases significantly as the altitude increases. It was meant for low altitude flight. At sea level, it produces 120 kn thrust, but at 15 km altitude, it only produces 13.7 kn of thrust, barely enough to provide any thrust to climb higher

Long and narrow wing has less surface area to provide lift, and at higher altitude, the atmospher is so thin that the lift generated by long and narrow wings is insufficient for sustainable flight. Delta wings are able to work better since the lack of lift per square area is compensated by having larger area to provide lift

[bewing]

Welcome to the forums. As said above, the Wheesley is specifically designed as a low altitude engine. As you found out, it basically konks out at 15km. To get higher with the low tech jet engines, you need rocket assist. If you upgrade to the panther (or get one on a contract) that engine can get you much higher.

 

[Laie]

10 hours ago, Marakaos said:

I use FAR btw.

FAR is supposed to take aspect into account, but I'm not very experienced with it and cannot advice you as to how it works. However, it seems as if your delta wing has has more area, might that be it?

[Foxster]

Don't be too surprised if KSP's aero model breaks down in edge cases. Its an approximation missing some elements. 

It is usually best for initial designs to be based on logic and real-world physics and then be prepared to experiment with non-logical alternatives for optimisation. For example, the RAPIER engine doesn't seem as the first choice as the "best" high altitude air-breathing engine but in long-distance and high-altitude challenges it is. 

[Snark]

On 1/20/2018 at 1:42 PM, Marakaos said:

weasley turbofan engine

This is your problem-- as folks have pointed out, it's not a high-altitude engine.  Not only does it conk out at high altitudes, but also it conks out at high speeds, and you need to be able to go fast to fly up high.

The Rapier, for example, works great as a high-altitude engine, precisely because it can go so fast. Note that KSP planes get one substantial speed benefit that's much more pronounced than IRL aircraft, due to the freakishly small planet sizes:  they're actually flying at a large fraction of orbital velocity.  This lowers their effective "weight"-- for example, a craft flying at 1400 m/s on Kerbin is effectively flying as if gravity were only 2/3 of its actual value.  Since gravity is effectively lower, they don't need as much wing area to maintain altitude-- which, in turn, means less drag, which makes for more efficient flight.

The other thing to bear in mind is that tuning the AoA (angle of attack) of your wings makes a difference.  The easiest "default" way to build a plane in KSP-- i.e. if you just go into the SPH, slap together some parts like Legos, and go-- is to attach them with zero AoA.  This works, but it's not optimal.  That's because wings need to have some AoA to the airstream in order to generate much lift.  If you've attached them to your fuselage with zero AoA, what that means is that when you fly, you're going to have to have the entire plane pitched slightly above in order to generate lift.  That's sub-optimal, because having the fuselage pitched like that means it'll have more drag than it otherwise would have.

The ideal case for optimally efficient flight (which is what you want if you're trying to maximize cruise altitude) is when your wings are mounted to the body, pitched up just enough that when cruising in level flight, the body of the aircraft is pointing perfectly .  Unfortunately, there's no simple rule to tell you what the angle needs to be when you're building the plane-- it depends on lots of variables, such as your weight, drag, intended cruising altitude, intended cruising speed.  Basically you just need to fiddle with it and try some test flights.  How to know if you've nailed it perfectly:  when you're cruising in level flight, you want the marker to be precisely centered in your navball cross-hairs.  If it's below the cross-hairs, you need a little more AoA.  If it's above the cross-hairs, you need a little less.

(Disclaimer:  I've never used FAR myself; all of the above is based on my experience with stock aerodynamics.  But my guess is that these same general principles would probably apply to FAR, and that all that would change would be the numbers involved.  That's just a guess, though-- perhaps someone more familiar with FAR could weigh in.)

[harits]

hi

[Gargamel]

On 1/23/2018 at 11:48 AM, Snark said:

If it's below the cross-hairs, you need a little more AoA.  If it's above the cross-hairs, you need a little less.

Isn't that backwards snark?   If not, please explain why, which I know you have no problem doing! 

[Snark]

42 minutes ago, Gargamel said:

Isn't that backwards snark?   If not, please explain why, which I know you have no problem doing! 

No, I think it's correct, unless I've got a brain fart going, here.  Rationale:

• Make sure we're on the same page with terminology.  When I say "AoA", I refer to the AoA of the wings, relative to the plane body.  If the wings are mounted perfectly horizontally on the plane body, then AoA = zero.  Normally you want the wings tipped slightly up, thus a positive AoA.  What we're discussing here is "how to figure out if you have the right amount of AoA."
• You know you've nailed it correctly if, when you're in level flight, your marker is precisely centered on the crosshairs.  This means that your velocity is perfectly horizontal, and also your plane (as defined by the cockpit, fuselage, etc.) is also perfectly horizontal.
• Suppose that the marker is slightly below the crosshairs.  For example, suppose that the plane is oriented perfectly horizontally so that the navball crosshairs are centered on the "horizon", but the marker is slightly below that.  What that means is "the plane is horizontal, but it's losing altitude".  That means you don't have enough AoA on your wings, because increasing the wing AoA would give additional lift and thereby raise the marker.
• Conversely, if the marker is above the crosshairs, it means you're climbing relative to the direction the plane is pointing, which means you have too much AoA on your wings and should decrease it.

If that's all a case of Too Many Words™, think of it this way:  Suppose you just build a plane the "default" way and don't put any AoA at all on the wings.  You will find that when you're flying around, will always be slightly below the crosshairs on the navball.  So, to correct " is below crosshairs",  what you need to do is to add a little AoA to the wings.

[Loskene]

3 hours ago, Snark said:

No, I think it's correct, unless I've got a brain fart going, here.  Rationale:

• Make sure we're on the same page with terminology.  When I say "AoA", I refer to the AoA of the wings, relative to the plane body.  If the wings are mounted perfectly horizontally on the plane body, then AoA = zero.  Normally you want the wings tipped slightly up, thus a positive AoA.  What we're discussing here is "how to figure out if you have the right amount of AoA."
• You know you've nailed it correctly if, when you're in level flight, your marker is precisely centered on the crosshairs.  This means that your velocity is perfectly horizontal, and also your plane (as defined by the cockpit, fuselage, etc.) is also perfectly horizontal.
• Suppose that the marker is slightly below the crosshairs.  For example, suppose that the plane is oriented perfectly horizontally so that the navball crosshairs are centered on the "horizon", but the marker is slightly below that.  What that means is "the plane is horizontal, but it's losing altitude".  That means you don't have enough AoA on your wings, because increasing the wing AoA would give additional lift and thereby raise the marker.
• Conversely, if the marker is above the crosshairs, it means you're climbing relative to the direction the plane is pointing, which means you have too much AoA on your wings and should decrease it.

If that's all a case of Too Many Words™, think of it this way:  Suppose you just build a plane the "default" way and don't put any AoA at all on the wings.  You will find that when you're flying around, will always be slightly below the crosshairs on the navball.  So, to correct " is below crosshairs",  what you need to do is to add a little AoA to the wings.

AoA built into the wing relative to the plane body is called the angle of incidence, just to help clear up. AoA itself refers to the angle of the wing (or the whole plane if the AoI is 0) relative to the airflow.

If the wings are flat and your plane steadily loses altitude in level flight at a given speed (as most designs will by default), it means you either need to pitch up slightly, increasing the angle of attack and keeping your vertical velocity at 0 by flying up slightly to compensate for the pull of gravity that lift isn't compensating for. Or you need to put a little bit of angle of incidence in your wings, which will reduce drag since you don't have to pitch the entire plane up, but will be stuck at a fixed angle meaning you'll have to pitch up or down anyway during certain phases of the flight that the plane isn't optimised for. I generally don't fuss with AoI unless I'm making a long range cruiser.

So if you want to make a plane go as fast as possible, you want to minimise both angle of attack and angle of incidence necessary to sustain level flight at your target speed and altitude. This can be done with moar boosters or more calculated wing and body design, depending on the aero model you use.

Edited February 8, 2019 by Loskene
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