Alright, I know a tiny bit about maximum density altitude and such. But how do they come up with that? Is there a formula that they use, or did some unlucky bastards have to test it to find out?
Say I have a single engine Cessna and decide I want to go to the moon. What’s happening to the craft once I get above it’s ceiling rating. As I continue to try to climb, what’s going on? Does it just go into a stall and that’s the end of it?
What if I try the same thing in a 747?
I am not an airplane guy, but I doubt anything really dramatic happens.
My guess is that as you approach the ceiling for the specific aircraft you will notice that now it cannot climb at the same rate as when you were at a lower altitude. Eventually you reach a point where it is impossible to climb anymore.
Same like a car in an infinite runway. The acceleration decreases as speed increases and eventually acceleration becomes zero and you reach top speed.
In gravity we thrust.
Are you sure? Surely this could be checked experimentally. Well, not with an infinite runway per se, but what if we built some sort of apparatus that had, I don’t know, rollers and a flexible belt that wrapped around them and used that as a surface? Anyone think I’m on to something?
I was taught that it is the intersection of the Vx (best angle of climb) and Vy (best rate of climb) lines. See here (fig 10):
Since the lines are straight, you could determine their slope and y-intercept without ever getting close to the actual absolute ceiling.
Obviously it’s more complicated in practice, but I’d suppose this gives a useful first approximation,
Nothing constructive to share here, other than a Calvin and Hobbes comic on a similar topic:
Says who?
Oh, and to answer the second part: your rate of climb gets closer and closer to 0, so you never actually reach the ceiling. In some aircraft, approaching this point this is no big deal; in others you enter what’s called the “coffin corner”, where the stall speed (which increases with altitude) meets with the speed of the critical mach number (which decreases with altitude). Passing either of these boundaries entails a dramatic loss of lift and can therefore be very dangerous (though as far as I know, this effect really only applies to high-performance subsonic aircraft).
Most piston powered aircraft can get a little higher if you fiddle with things but in reality, pretty much what is already said.
Add in the gradual loss of power as the air thins out & you are on the ragged edge of control.
Normally small aircraft , even if you are clumsy on the controls won’t get too far out of hand if you stall them.
Doing a test, I got a 1970 Cessna 310 Turbo to 31,500 feet. It snowed in the cockpit for a bit, the heater flamed out, the right engine finally quit and the oil in the prop hub was so cold & thick that I could not feather the engine.
ATC center was having a good laugh at our so called flight plan and asked when we passed 18,000’ on the way down if we thought we could get the engine back. I said I was working on it. Got it to fire up about 15,000’ and warm enough to bring it up to decent power settings by 12,000’. Rest of the decent into TUL was normal.
Had to tell the boss to pass on that job as we could not hang the 32,000’ we would need and have enough control to actually fly the line.
Had a normally aspirated Cessna - 180 up over 18,000’ & that took a lot of tricks with flap and moving weight around & low fuel.
Most important thing to take with you is oxygen…
The most basic level:
Aircraft engines work by moving air. The higher you go, the less air there is, so the less your engine can do. Eventually your engine can’t keep you moving fast enough to keep going up.
If your engine can propel you without air, you’re no longer an aircraft, you’re a spacecraft.
I think the service ceiling is formally defined as the density altitude at full gross weight at which your climb rate diminishes to 100 fpm.
Sometimes it is not just due to performance. The pressurisation system and emergency oxygen equipment can have an effect.
The Dash 8 has a service ceiling of 25,000’. Even if it had the performance to fly higher (which it normally doesn’t), it is not legally allowed to go any higher because it does not have drop out oxygen masks for passengers, and they are required for operations above 25,000’
In addition to that, the pressure hull has a maximum pressure differential between the outside air and the inside of the cabin. You may, in a light aircraft, have the engine performance to climb above the certified ceiling but you can’t increase the cabin differential pressure and so the cabin altitude will start climbing above what is considered safe/comfortable for passengers.
On the flip side, at high weights you may not be able to get anywhere near the service ceiling. Some variants of the Avro RJ-100 are certified for 35,000’ but I’ve never had one above 31,000’ because we fly them at high weights and high temperatures so the performance is diminished. We generally wouldn’t fly the RJ any higher than the altitude where we still get a 500 foot per minute rate of climb because you need some performance margin for manoeuvering and in case you need to use the anti-ice systems or if you encounter any turbulence.
It will never take off
Si
That was a major factor for the Lockheed SR-71 plane. So that at its top 80,000 feet altitude, the engine inlet cone ‘spikes’ provided about 70% of the power, with the engine itself providing only 30%.
Something similar was happening with the concorde, I don’t understand the physics of it though.
Do you mean 70% of the compression for the jet engine?
Very interesting story, GusNSpot. I’d love to hear about more of your adventures someday. I certainly agree with your last point…
I recall a news story a few years ago about a couple of guys who took a small jet aircraft (Gulfstream or some such) up as high as they could go. They hit 41,000 feet before the engines flamed out; they could not get the engines restarted on the way down, and they crashed and died.
Oh yeah, here it is.
41,000’ was its certified maximum altitude. There was nothing particularly wrong with being up that high, it was the way the pilots went about it that got them in to trouble. It is also a demonstration of the maximum certified ceiling not necessarily being an appropriate altitude to actually fly at unless conditions are perfect.
Neil Armstrong flewthe X-15 rocket plane on a ballistic trajectory to 207500 feet (plan was 205000)
Which gives me the image that he would have been stuck in space or broken up as an uncontrolled meteor later.:dubious:
So it’s not just the performance of climb/engine/wings it’s the control-ability or the safety factors (oxygen for passengers, coffin corner) that make a difference
