Jump to content

Odd question for any aerodynamics experts


Echo38
 Share

Recommended Posts

So, don't ask me how I got to thinking about this, but: if a Second World War fighter were orbiting Earth in a decaying orbit, then, upon entering the atmosphere, it would burn up. Right? But what would happen if heat weren't a concern? Imagine for a moment that the airplane & everything inside it were not subject to problems of temperature.

 

At this point, then, my main concern would be exceeding structural limits. Let's imagine that the airplane in question is a P-38, which had a rather low critical Mach, even for a WWII fighter. So, in the absence of heat from atmospheric friction, then, as soon as the controls began to have the slightest amount of responsiveness, as air molecules started hitting them, I would attempt to keep the airplane's dive as shallow as possible, with throttles at idle* and propeller levers at full (to maximize drag).

 

Would the airplane break up due to exceeding structural limitations? At such a thin air density, and coming from any sort of orbital velocity, its Mach number would be far, far higher than I'd expect a WWII to withstand. Normally, the drag of the propellers (even when pitched for maximum drag) would keep a prop fighter from getting anywhere near the speed of sound. I've also heard it theorized often that the wing shape of Second World War fighters is such that the airframe would be destroyed if it were able to reach such a speed. But, assuming this is true at normal altitudes for such an aircraft, does it still hold true at extremely low air densities, such as in low-Earth orbit in upper atmosphere? Coming from an in-space orbit, it would already automatically be greatly exceeding the speed of sound as soon as it entered the most outer edge of the atmosphere.

 

And, also, if the airplane were somehow able to avoid structural failure during re-entry, would not the "Mach tuck" effect cause the airplane to inevitably (and soon) enter a straight-down nose dive? At normal altitudes, the pilot can "ride it out" until the wing regains lift and the control surfaces become responsive again in the denser air below. But what would happen in a prolonged dive at extremely low air density?

 

My grasp of the relationships in low air density is shaky. I know that IAS decreases farther and farther below TAS as a WWII fighter climbs, due to the air density. But it gets complicated beyond that. Max-alpha turns at high altitudes have a much higher TAS, and a much wider turning circle, than they do down low. I don't know what happens to turn times. This much can be experienced in a high-fidelity simulator like DCS. Beyond this, beyond the realm of what a pilot of a propeller aircraft can experience, my understanding of low-density aerodynamics is even more dim. Care to discuss the presented problem?

 

* Normally, this would cause the engines to freeze from overcooling, but recall that this thought experiment has the condition of temperature not being a factor.


Edited by Echo38
Link to comment
Share on other sites

I'm not a scientist, but I like physics. Physics is good, physics is everything.

 

Your question does not make sense, in term of physics, but it's interesting enough to try to give a reasonable answer. Let's cut it to the chase..

 

The sound is a 'matter' wave, and for it to propagate, it needs a medium to propagate within, unlike the electromagnetic waves, for example. We very know that the air density depends on altitude and humidity (of the air). We also know that above ~160km altitude the air density is so low that the sound waves won't propagate.

 

You assumed that our prop plane was in orbit already, that means, it somehow got there, so, for Low Earth Orbit (LOE) objects it should have a speed of ~7.8km/s.

 

Depending on which orbit the plane decays from, you'll have different behaviours of the plane when re-entering. Taking out the "temperature" from the equation means that you'll have no 'air' friction. Let's say you were @ 1000km above the Earth when you decide to 'put your nose down' and dive towards it. You're now in Exosphere, travelling @28000km/h, and unless there's no force acting upon your plane you don't feel you're moving. To reach a lower orbit you have to, yes, accelerate the plane using some rocket boosters that should fire in the opposite direction of travel, as the plane control surfaces are useless for now. After several orbit changes to lower ones, you'll reach the Thermosphere @ ~600km altitude, and the first thing you'll notice are the sparks from the heavily ionized gases getting in contact with your plane, but nothing too destructive for the airframe, yet. The 'air' is still too thin to have any impact on the crate.

Getting lower into the Thermosphere, things will change dramatically. In the lower part of the Thermosphere, @ approximately 120km altitude there's a 10km layer of sodium atoms which gives our atmosphere the 'airglow' in the night starlight. This is where the Mesopause begins.. together with your problems. This is the coldest place 'on Earth', with temps averaging -100C, depending on various factors. You still can't 'fly' the plane but you'll notice it starting to lose speed. The air density starts to increase exponentially and so does the parasitic drag. By the time you'll reach the Stratosphere you'd disintegrate at current speed and that is due to the air pressure being too high for the airframe to withstand such forces. So, temperature is not the worse enemy.. air is.. :)

Specs:

Asus Z97 PRO Gamer, i7 4790K@4.6GHz, 4x8GB Kingston @2400MHz 11-13-14-32, Titan X, Creative X-Fi, 128+2x250GB SSDs, VPC T50 Throttle + G940, MFG Crosswinds, TrackIR 5 w/ pro clip, JetSeat, Win10 Pro 64-bit, Oculus Rift, 27"@1920x1080

 

Settings:

2.1.x - Textures:High Terrain:High Civ.Traffic:Off Water:High VisRan:Low Heatblur:High Shadows:High Res:1920x1080 RoC:1024 MSAA:4x AF:16x HDR:OFF DefS: ON GCI: ON DoF:Off Lens: OFF C/G:390m Trees:1500m R:max Gamma: 1.5

 

Link to comment
Share on other sites

You can actually test this using X-Plane. Get a P-38 or similar plane, disable damage model if necessary and put it in orbit and fly it to the gound while recording accelerations of the plane. Contollability won't be realistic as I doubt X-Plane can simulate shock waves properly in this scenario but drag and lift should be in the ballpark.

DCS Finland: Suomalainen DCS yhteisö -- Finnish DCS community

--------------------------------------------------

SF Squadron

Link to comment
Share on other sites

After several orbit changes to lower ones, you'll reach the Thermosphere @ ~600km altitude, and the first thing you'll notice are the sparks from the heavily ionized gases getting in contact with your plane, but nothing too destructive for the airframe, yet. The 'air' is still too thin to have any impact on the crate.

Getting lower into the Thermosphere, things will change dramatically. In the lower part of the Thermosphere, @ approximately 120km altitude there's a 10km layer of sodium atoms which gives our atmosphere the 'airglow' in the night starlight. This is where the Mesopause begins.. together with your problems. [...] You still can't 'fly' the plane but you'll notice it starting to lose speed. The air density starts to increase exponentially and so does the parasitic drag. By the time you'll reach the Stratosphere you'd disintegrate at current speed and that is due to the air pressure being too high for the airframe to withstand such forces.

 

Right, that's what I was getting at with the whole "won't it disintegrate upon reaching thicker air" part. So, the question is, is there any way (with the given condition of "magical" immunity to temperature problems) to go from the, uh, thermosphere to the mesosphere gradually enough that the force of the relative wind won't break up the bird? At some point, the control surfaces will be responsive enough to have at least some effect, at which point couldn't the pilot try "skipping"?

Link to comment
Share on other sites

Right, that's what I was getting at with the whole "won't it disintegrate upon reaching thicker air" part. So, the question is, is there any way (with the given condition of "magical" immunity to temperature problems) to go from the, uh, thermosphere to the mesosphere gradually enough that the force of the relative wind won't break up the bird? At some point, the control surfaces will be responsive enough to have at least some effect, at which point couldn't the pilot try "skipping"?
I doubt that a pilot can 'steer' the plane at those re-entry speeds, taking into consideration the small deflection surfaces of the aircraft. The space Shuttle actually uses the 'air' to decelerate the ship during the entry. If we ruled out the 'heat' then we also ruled out the friction. In these conditions, it's highly unlikely that any aircraft or spacecraft would survive such a re-entry without further tweaking of the laws of physics.

 

Sent from my HTC One using Tapatalk

Specs:

Asus Z97 PRO Gamer, i7 4790K@4.6GHz, 4x8GB Kingston @2400MHz 11-13-14-32, Titan X, Creative X-Fi, 128+2x250GB SSDs, VPC T50 Throttle + G940, MFG Crosswinds, TrackIR 5 w/ pro clip, JetSeat, Win10 Pro 64-bit, Oculus Rift, 27"@1920x1080

 

Settings:

2.1.x - Textures:High Terrain:High Civ.Traffic:Off Water:High VisRan:Low Heatblur:High Shadows:High Res:1920x1080 RoC:1024 MSAA:4x AF:16x HDR:OFF DefS: ON GCI: ON DoF:Off Lens: OFF C/G:390m Trees:1500m R:max Gamma: 1.5

 

Link to comment
Share on other sites

I know you said, "Don't ask", but how exactly did this question come to your mind?

Hardware: T-16000M Pack, Saitek 3 Throttle Quadrant, Homemade 32-function Leo Bodnar Button Box, MFG Crosswind Pedals Oculus Rift S

System Specs: MSI MPG X570 GAMING PLUS, GTX 1070 SC2, AMD RX3700, 32GB DDR4-3200, Samsung 860 EVO, Samsung 970 EVO 250GB

Modules: Ka-50, Mi-8MTV2, FC3, F/A-18C, F-14B, F-5E, P-51D, Spitfire Mk LF Mk. IXc, Bf-109K-4, Fw-190A-8

Maps: Normandy, Nevada

[sIGPIC][/sIGPIC]

Link to comment
Share on other sites

If we ruled out the 'heat' then we also ruled out the friction.

 

No, specifically heat effects and not friction. IRL, atmospheric friction unavoidably causes heat buildup, of course, but I specifically omitted only temperature as a concern in my hypothetical scenario. The idea is (like) turning off "damage from extreme temperatures" in a simulator's difficulty options.

 

So, I'm well aware that it's physically impossible IRL, but I mean specifically (and have meant since the beginning) a hypothetical scenario where all physics are true-to-life, except that the aircraft and everything within it are immune to overheating and overcooling. Thus, friction is still just as much a thing, except that it magically isn't causing a heat buildup in the aircraft (or, more accurately, the aircraft is somehow undamaged by the heat buildup).

 

At which point, that changes a few things, and I'm wondering if any other "difficulty options" would need to be "turned off" in order to make such an unusual re-entry possible, and how it would work under the given conditions.

 

Can I now "skip" off of the upper atmosphere to gradually reduce my speed without exposing myself to atmo dense enough to cause structural damage? If not, then could I if I had a more appropriate form of control (e.g. a reaction control system)?

 

In short, and rephrasing the original question: what problems other than temperature damage are preventing a P-38, launched from low-Earth orbit, from successfully landing without destroying itself, and what would be the minimum changes necessary for that safe landing to occur? (And I mean a normal landing, not a parachute splashdown.)

 

I know you said, "Don't ask", but how exactly did this question come to your mind?

 

I suppose I have a peculiar imagination.


Edited by Echo38
Link to comment
Share on other sites

The main problem as I see it would be you have no way of controlling the attitude of the object as it re enters (without a RCS). It could well be travelling backwards any combination of revolution around any axis. Ignoring the thermal effect the aero dynamic pressures would rapidly increase past the structural strength of the airframe.

Link to comment
Share on other sites

Yes, but at those speeds the pilot has a (very) small window when he can actually fly the aircraft. And there's also the gravity he has to fight with.. As I said before, there are too many IFs involved in the scenario :)

Specs:

Asus Z97 PRO Gamer, i7 4790K@4.6GHz, 4x8GB Kingston @2400MHz 11-13-14-32, Titan X, Creative X-Fi, 128+2x250GB SSDs, VPC T50 Throttle + G940, MFG Crosswinds, TrackIR 5 w/ pro clip, JetSeat, Win10 Pro 64-bit, Oculus Rift, 27"@1920x1080

 

Settings:

2.1.x - Textures:High Terrain:High Civ.Traffic:Off Water:High VisRan:Low Heatblur:High Shadows:High Res:1920x1080 RoC:1024 MSAA:4x AF:16x HDR:OFF DefS: ON GCI: ON DoF:Off Lens: OFF C/G:390m Trees:1500m R:max Gamma: 1.5

 

Link to comment
Share on other sites

Gravity wont be an issue if the airplane is in orbit..aka orbitting..which means zero gravity, push & pull equal out to 0. You can orbit anywhere, even 1m above ground..if you can fly 7 km/sec.

 

The plane gets ripped in pieces is my guess

Gigabyte Aorus X570S Master - Ryzen 5900X - Gskill 64GB 3200/CL14@3600/CL14 - Asus 1080ti EK-waterblock - 3x Samsung 980Pro 1TB - 1x Samsung 870 Evo 1TB - 1x SanDisc 120GB SSD - Heatkiller IV - MoRa3-360LT@9x120mm Noctua F12 - Corsair AXi-1200 - TiR5-Pro - Warthog Hotas - Saitek Combat Pedals - Asus PG278Q 27" QHD Gsync 144Hz - Win11 Pro/Linux - Phanteks Evolv-X 

Link to comment
Share on other sites

First line of the OP said "decaying orbit," which means that the trajectory is such that the craft will eventually enter the atmosphere and either burn up or impact the ground, if no sufficient corrective action is taken.

 

ooops, I seem to have stumbled over language barrier there :doh:

 

Still, I guess a WWII airframe will collapse for many reasons, not because of the friction induced heat alone but because of hot/cold material stress; the freezing cold up there where shadow is and the intense heat where sunlight strikes the surface. For sure his canopy glass will shatter pretty early after getting beamed there 1st place, so will any fabric covered surface if any, metal sheeting might hold 30 sec. longer :cry:

 

Anyway, some Hollywood writer will eventually show us what happens if, with TomCruise at the controls :megalol:

Gigabyte Aorus X570S Master - Ryzen 5900X - Gskill 64GB 3200/CL14@3600/CL14 - Asus 1080ti EK-waterblock - 3x Samsung 980Pro 1TB - 1x Samsung 870 Evo 1TB - 1x SanDisc 120GB SSD - Heatkiller IV - MoRa3-360LT@9x120mm Noctua F12 - Corsair AXi-1200 - TiR5-Pro - Warthog Hotas - Saitek Combat Pedals - Asus PG278Q 27" QHD Gsync 144Hz - Win11 Pro/Linux - Phanteks Evolv-X 

Link to comment
Share on other sites

Why disintegration though? The drag is equal to drag coefficient*air density*velocity squared*surface area. If air density is sufficiently low, air drag should be proportionally low, depending on the other factors.

Intel i7-950 @stock, Asus P6X58D-E, 3x4GB Corsair Vengeance, Asus GTX 580, Corsair 120GB SSD, Corsair HX 750W PSU

[sIGPIC][/sIGPIC]

Link to comment
Share on other sites

That was the other thing I was getting at. Presumably, the aircraft doesn't immediately start taking measurable damage the moment the first air molecules start to hit it, no matter how high the relative velocity. So, if the air begins hitting the airplane hard enough to approach being damaging, then it must be hitting the wings & control surfaces hard enough for them to be at least somewhat effective. Right? In which case, one ought to be able to do something akin to the "atmospheric skipping" technique in which the craft repeatedly leaves and re-enters the very edge of the atmosphere, normally used to allow a spacecraft to cool down.

 

In this case of "heat doesn't matter," the idea would be to just keep it at that spot where there's just enough air density—just enough force—to give lift & elevator authority, but not enough force to break the airplane. Just skirting the very edge of the atmosphere. Or is there some additional problem that I don't understand, such as some fundamental difference between a small amount of air molecules at an extremely high relative velocity and a large amount of air molecules at a comparatively low relative velocity?

 

The only clear problem I can think of, other than temperature, would be "Mach tuck" forcing the airplane into a nosedive, despite all efforts to raise the nose to regulate altitude.


Edited by Echo38
Link to comment
Share on other sites

You're obviously chasing your tail.. It is "possible" to control the aircraft in your 'heat stripped theory' but definitely not in a prop plane. The pilot would need a FBW system to control the aircraft, because at those speeds, when reaching the upper levels of the stratosphere, any small movement of any control surface would disintegrate the plane.

Specs:

Asus Z97 PRO Gamer, i7 4790K@4.6GHz, 4x8GB Kingston @2400MHz 11-13-14-32, Titan X, Creative X-Fi, 128+2x250GB SSDs, VPC T50 Throttle + G940, MFG Crosswinds, TrackIR 5 w/ pro clip, JetSeat, Win10 Pro 64-bit, Oculus Rift, 27"@1920x1080

 

Settings:

2.1.x - Textures:High Terrain:High Civ.Traffic:Off Water:High VisRan:Low Heatblur:High Shadows:High Res:1920x1080 RoC:1024 MSAA:4x AF:16x HDR:OFF DefS: ON GCI: ON DoF:Off Lens: OFF C/G:390m Trees:1500m R:max Gamma: 1.5

 

Link to comment
Share on other sites

Why disintegration though? The drag is equal to drag coefficient*air density*velocity squared*surface area. If air density is sufficiently low, air drag should be proportionally low, depending on the other factors.

 

Yes but you're also going proportionally faster and wont be able to slow down enough to prevent an extreme level of drag, esp. not if you actually want to reenter as that demands a certain angle or you're just going to skip back out into orbit.

Link to comment
Share on other sites

Instead of talking about hypothetical scenarios, I'd like someone to actually calculate or better yet simulate the scenario as then we will get a much more accurate result. Space Shuttle experiences only about 4 G deceleration upon re-entry so I don't think it's unthinkable to consider a possibility that P-38 shaped space vehicle can't do the same. Once again, we need calculations to know.

DCS Finland: Suomalainen DCS yhteisö -- Finnish DCS community

--------------------------------------------------

SF Squadron

Link to comment
Share on other sites

Yes but you're also going proportionally faster and wont be able to slow down enough to prevent an extreme level of drag, esp. not if you actually want to reenter as that demands a certain angle or you're just going to skip back out into orbit.

 

So what is the velocity-to-air density ratio for the slowest reentry possible?

Intel i7-950 @stock, Asus P6X58D-E, 3x4GB Corsair Vengeance, Asus GTX 580, Corsair 120GB SSD, Corsair HX 750W PSU

[sIGPIC][/sIGPIC]

Link to comment
Share on other sites

So, don't ask me how I got to thinking about this, but: if a Second World War fighter were orbiting Earth in a decaying orbit

One question is exactly what is the orbit? If it starts from high enough, it will probably never slow down to the point where it's controllable.

 

At this point, then, my main concern would be exceeding structural limits. Let's imagine that the airplane in question is a P-38, which had a rather low critical Mach, even for a WWII fighter. So, in the absence of heat from atmospheric friction, then, as soon as the controls began to have the slightest amount of responsiveness, as air molecules started hitting them, I would attempt to keep the airplane's dive as shallow as possible, with throttles at idle* and propeller levers at full (to maximize drag).
You probably wouldn't have control because your Mach number would be enormous. As soon as the air was dense enough to matter, you would find yourself past critical Mach and unable to pitch or slow down.

 

Would the airplane break up due to exceeding structural limitations?
That would be down to dynamic pressure, so while the P-38 would be going insanely fast up high, it could be structurally fine without any heat concerns. It would probably feel less stress than it does in normal flight at extreme altitudes because there would be so little air. What happens at medium altitudes is more difficulty to say. You'd need to factor in the drag to see how much it slows down the plane while dynamic pressure builds up.

 

I guess something you could use as a rough guide would be to calculate the dynamic pressure for structural failure and then compare that with the terminal velocity for a given altitude. Plug the velocity into the dynamic pressure to see if the terminal velocity exceeds the structural limit.

 

The Spach Shuttle avoided the issue by maintaining really high AoA flight to provide drag and some lift. The P-38 wouldn't be able to since it wouldn't be capable of maintaining very high AoA's and would not be able to generate lift at high Mach.

 

 

And, also, if the airplane were somehow able to avoid structural failure during re-entry, would not the "Mach tuck" effect cause the airplane to inevitably (and soon) enter a straight-down nose dive? At normal altitudes, the pilot can "ride it out" until the wing regains lift and the control surfaces become responsive again in the denser air below. But what would happen in a prolonged dive at extremely low air density?
This would be a big problem as it would take effect long before you slowed down. If the aircraft became stable, it would probably nose dive to destruction.

 

That's my take on it anyway. To get a real answer you'd need to simulate it.

Awaiting: DCS F-15C

Win 10 i5-9600KF 4.6 GHz 64 GB RAM RTX2080Ti 11GB -- Win 7 64 i5-6600K 3.6 GHz 32 GB RAM GTX970 4GB -- A-10C, F-5E, Su-27, F-15C, F-14B, F-16C missions in User Files

 

Link to comment
Share on other sites

So what is the velocity-to-air density ratio for the slowest reentry possible?

 

No clue, but you'd probably at least be going Mach 25 or above once you reach the karman line. In other words way past the critical mach number of the P-38, which is a problem considering that means zero controllability.

 

Also as exorcet points out, the P-38 is not capable of exceeding ~15 deg of AoA, thus it has no way of effectively slowing down. Couple that with the loss of control and it's a death trip irrespective of heat build up.

Link to comment
Share on other sites

You probably wouldn't have control because your Mach number would be enormous. As soon as the air was dense enough to matter, you would find yourself past critical Mach and unable to pitch or slow down.

 

Yeah, I was afraid of that. So I guess we all agree that this craft would be unable to skip and would inevitably enter a nosedive (or at least be uncontrollable) as-is. I wonder if this part of the problem could be solved with an RCS and perhaps an unusual CoG, well aft of normal.

 

That would be down to dynamic pressure, so while the P-38 would be going insanely fast up high, it could be structurally fine without any heat concerns. It would probably feel less stress than it does in normal flight at extreme altitudes because there would be so little air. What happens at medium altitudes is more difficulty to say. You'd need to factor in the drag to see how much it slows down the plane while dynamic pressure builds up.

 

I guess something you could use as a rough guide would be to calculate the dynamic pressure for structural failure and then compare that with the terminal velocity for a given altitude. Plug the velocity into the dynamic pressure to see if the terminal velocity exceeds the structural limit.

 

Yeah, this sort of thing would be the most helpful in answering my main question. Does anyone know how to do this (assuming it's interesting enough to do for a lark)?

Link to comment
Share on other sites

I don't know about the P-38's structural limits. If you can find a never exceed speed, you can back out a dynamic pressure from that. For the terminal velocity, you can use a calculator:

 

http://www.calctool.org/CALC/eng/aerospace/terminal

 

Though you'd need to know some values relating to the P-38, or whatever aircraft. Weight and areas should be straight forward. Density can be looked up on charts on the internet or books. The CD can be tricky because it will change with Mach and the aircraft's orientation, and I'd be surprised to see any CD value for a WWII aircraft over Mach 2. You'd probably have to guess.

Awaiting: DCS F-15C

Win 10 i5-9600KF 4.6 GHz 64 GB RAM RTX2080Ti 11GB -- Win 7 64 i5-6600K 3.6 GHz 32 GB RAM GTX970 4GB -- A-10C, F-5E, Su-27, F-15C, F-14B, F-16C missions in User Files

 

Link to comment
Share on other sites

 Share

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...