Naked Science Forum
General Science => General Science => Topic started by: Just thinking on 13/08/2021 13:56:42
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Have you ever wondered how some planes can fly higher than others? I will list the contributing factors before explaining how 1. speed 2. lift 3. weight. For a plane to climb to a high altitude it must overcome the problem of the thin atmosphere. The plane must either travel very fast to generate the required lift or it must have very large wings to generate the lift these two factors also rely on the power of the aircraft and the weight. The Lockheed sr-71 is the highest flying air breathing aircraft 90.000 feet that has a speed of three times the speed of sound and has a swept delta wings and two powerful jet engines to accomplish this task it is a rather heavy aircraft but its engines compensate for this. Another high flyer is the Lockheed u-2 it can fly to 70.000 feet and travels at a speed just under the speed of sound it is much slower than the sr-71 as it flys on wings that are much larger and more delicate than the sr-71 it gains its high altitude due to increase lift and a lightweight ratio combined with a single and less powerful engine. So why can't a Boing 747 fly to these high altitudes well the 747 is not powerful enuff to heavy and lacks the lift. If the 747 was faster lighter or had larger wings it could. Are there any other reasons that could get a plane to fly so high? Thanks for reading.
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Please format your op with a question mark and ask a science question.
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Please format your op with a question mark and ask a science question.
There you go I have given you an opportunity to give an answer.
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The problem is manifold - indeed one of the problems is to do with the manifold!
A normally-aspirated piston engine can't get enough oxygen above about 20,000 ft. As you climb, you "lean" the mixture to keep the optimum fuel-air ratio but this obviously reduces the available power. "Service Ceiling" is the altitude at which the maximum rate of climb falls to 100 ft/min, below which it is difficult to maintain level flight in mild turbulence. Adding a supercharger can take you to 50,000 ft, but other problems arise....
Frank Whittle's undergraduate thesis explained why there is a limit to the speed of any propellor-driven aircraft, so being a sensible Peterhouse man, he invented the jet engine. And of course nobody took any notice until it was almost too late.
The jet effectively has a built-in supercharger in the shape of the compressor turbine and this can theoretically work wherever there is an oxygen molecule but...
Whilst you can compress the air to feed the flames, you also need ambient air to lift the wing. Lift is proportional to air density ρ and the square of speed L = ½ClρAv2 where A is the wing area and Cl the lift coefficient, is roughly constant. So to go high you need to go fast to stay up. Problem is that the speed of sound also depends on ρ, so the flight envelope has a nasty corner where the stall speed matches the speed of sound, and supersonic flight is a whole bunch of other problems.
Consequently jet airliners tend to fly at Mach 0.9 to 0.95 to avoid breaking the wings, and this generally gives you optimum range at around 40 - 45,000 ft. The U2 is essentially a powered glider, grabbing every ounce of lift at Mach 0.97 (or thereabouts - it's still a secret) with minimum fuel consumption so it can stay airborne for n hours (also a secret, but well beyond the bladder capacity of most pilots). The unpowered glider altitude record is 76,100 ft with a potential max of around 90,000 ft - light weight and long wings means you can fly slowly and well away from the sonic corner.
If you aren't worried about the airframe going supersonic, you build a Blackbird or a Concorde. Problem now is that the air is going past you faster than the flame front, so you have to slow down the air intake to the engines as the plane accelerates to stop the fire going out (probably the cleverest part of the Concorde design, and the bit that baffled Ilyushin and Boeing).
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If you aren't worried about the airframe going supersonic, you build a Blackbird or a Concorde. Problem now is that the air is going past you faster than the flame front, so you have to slow down the air intake to the engines as the plane accelerates to stop the fire going out (probably the cleverest part of the Concorde design, and the bit that baffled Ilyushin and Boeing).
That's all good information, Alan thanks for that. The Concorde had a unique solution for the air intake having cat flaps to reduce the flow and prevent blowout. What is the maximum altitude the Cessna 172 can achieve and at what speed?
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What is the maximum altitude the Cessna 172 can achieve and at what speed?
I'd have to check, but I think you could have put one in the cargo hold of the space shuttle...
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I'd have to check, but I think you could have put one in the cargo hold of the space shuttle...
That would get it up and going.
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Weight versus lift.
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What is the maximum altitude the Cessna 172 can achieve and at what speed?
Around 13,000 ft and 130 kt, depending on the model. The later standard versions with a 180 HP fuel injected engine accelerate better on the ground but don't go much faster or higher than the earlier 150 HP carburettor machines because they are heavier!
For sustained flight above 10,000 ft, supplementary oxygen is mandatory (for the pilot, not the engine) and this adds weight.
The Cessna 182 looks very similar until you get up close (it's a big brother with a 250 HP engine and variable prop) and tops out around 18,500 ft - worth fitting an oxygen system.
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Please format your op with a question mark and ask a science question.
There you go I have given you an opportunity to give an answer.
Don’t do it again, follow forum rules.
See how I have changed your subject. Easy , ok?
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The Cessna 182 looks very similar until you get up close (it's a big brother with a 250 HP engine and variable prop) and tops out around 18,500 ft - worth fitting an oxygen system.
The 182 sounds impressive the Cessna has come a long way. I had a 1/6 scale piper J3 cub about 20 years ago I think I fell asleep at the controls as I had a big mess to pick up in the field.
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Don’t do it again, follow forum rules.
See how I have changed your subject. Easy , ok?
You have made it too easy for me thanks Colin, I will try not to do that again. Have you any interest in aviation your input may be valuable????.
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The 182 sounds impressive the Cessna has come a long way.
The 182 actually a different bird with a different wing section and a longer fuselage. The 172 is still in production and is the most-produced airplane (around 50,000) of all time. The 206 is bigger still (6 seats) , and actually has more in common with the 172. The single wing strut and (idiotproof - don't ask me how I know) bow spring undercarriage is such a distinctive feature that most Cessnas up to the 850 HP turboprop Caravan look pretty much the same from a distance.
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The 182 actually a different bird with a different wing section and a longer fuselage. The 172 is still in production and is the most-produced airplane (around 50,000) of all time. The 206 is bigger still (6 seats) , and actually has more in common with the 172. The single wing strut and (idiotproof - don't ask me how I know) bow spring undercarriage is such a distinctive feature that most Cessnas up to the 850 HP turboprop Caravan look pretty much the same from a distance.
I'm more familiar with the 172 but they just keep getting bigger. The turboprop is a whole different ball game. I'm no pilot but they say if you can fly a Cessna you can fly anything.
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I think it's the other way around! The Cessna 152 is probably the most common basic trainer, the Aerobat version can give you a good grounding in recovery from "unusual attitudes", and it's easy to progress to the 172 as a popular rental aircraft, so you will find that most older pilots have flown a Cessna en route to whatever takes their fancy, but they are all very docile and forgiving compared with modern light aircraft and even a venerable hot ship like a Mooney (actually an older design than the 172) needs a bit more care and attention to detail getting on and off the ground.
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I think it's the other way around! The Cessna 152 is probably the most common basic trainer,
I get confused so many of the Cessna models look the same. Can I tell you a true story at some time in the early 70's I was about 12 and me and my mum went on a trip from Sydney to Tasmania in Australia we left Sydney airport in a Boeing 727 and on route flying south my mum asked the flight attendant if I could go up to the cockpit and have a look she came back and said yes the captain said your son can. So up I go to the cockpit flight attendant opens the door and introduced me to the flight crew the captain stands up and invites me to get in his seat. with the captain standing behind me he said turn the control to the left so I slowly started to turn left and the horizon started to roll to the right and I went for it I gave it a real good turn and I swear the plane must have rolled to about 30 degrees and the captain said ok ok that's it up you get the co pilot had to correct and said to the captain he is trying to take us back to Sydney all three flight crew started laughing and I was asked to return to my seat with a big thank you. When I got back to my seat I said to my mum did you feel the plane turning and she said yes it turned a lot and I was bosting that I was responsible for that. Anyway no big deal but a good memory.
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One should also note the benefits of high altitude flying. In particular, the lower air density would give less drag, and allow higher velocities and less fuel consumption.
Jet Streams?
For sustained flight above 10,000 ft, supplementary oxygen is mandatory (for the pilot, not the engine) and this adds weight.
I presume that is either supplementary oxygen, or a pressurized fuselage.
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One should also note the benefits of high altitude flying. In particular, the lower air density would give less drag, and allow higher velocities and less fuel consumption.
That's very true It's funny most people would think for a plane to climb higher the pilot would just pull the nose up we know that's not the case wee can stay level at max altitude but to go higher we need more speed.
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Can anyone shine some light on something that has fascinated me for a long time I have tried to research this but with no luck. OK, If you look at the horizontal fin/elevator on a 747 and on almost all large jet planes you will notice that the profile of the fin is like a wing only inverted. The only explanation I can arrive at for this is that it allows the plane to climb with the elevator in its neutral position macking for less drag. Another reason maybe it is to keep the plane at its ceiling without any input required and one more possibility is that it may play a role in better stall recovery situations. Anyway, I may be wrong so can anyone tell me more about this design feature that I have noticed.
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A bit of nomenclature:
The fin is the fixed vertical bit. The wobbly bit behind it is the rudder.
There are various approaches to the stabiliser (the elevator is the big wobbly bit at the back of the stabiliser) but its primary function is always to provide negative feedback in pitch. In the simplest case it is a conventional aerofoil with a negative angle of incidence. If the angle of attack increases, the stabiliser generates positive lift and restores horizontality.
In the case of a large aircraft travelling at high speed the stabiliser is not operating in clean air but the turbulence of the fuselage, wing roots and engines, all at near-sonic speed, so its particular aerofoil has to be optimised for minimum drag at cruise configuration and may well turn out to look like an inverted wing. "T-tail" stabilisers look more like a normal wing and can be more aerodynamically efficient but have additional structural design and maintenance problems.
Airliners generally use an "all moving tail" where the stabiliser is also the elevator and trim (the tiny aerodynamic tab on the edge of a Cessna elevator) is done by electrically jacking the stabiliser incidence to hold altitude at cruise power - or to plough the plane into the deck in the case of a 737MAX.
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The fin is the fixed vertical bit. The wobbly bit behind it is the rudder.
I think the term fins refers to the vertical and horizontal sections as a whole just like the fins or flights on a dart the vertical fin consists of a vertical stabiliser and ruder the horizontal fins consist of a horizontal stabiliser and it is also the elevator consisting of two main parts low speed elevator the stabilizing horizontal fin and a separate elevator for high speed use as it is much smaller. Basically, stabilisers and fins are the same things. I believe you are correct in saying that stabilisers are the more common terminology for them. leaving the underbelly fitting as the fin that is if the aircraft has one.
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I believe that one of the limits is the engines. You have to compress the thin air so you can reach a combustible mixture. But when you compress the air, the air gets hotter. Eventually you have problems just because it gets too hot at the inlet/compressor and the air's nitrogen starts reacting with the oxygen. I believe nitrogen burns endothermically, so you're losing lots of energy at the compressor. That's really bad, by way of contrast the SR71 and Concorde recover almost all the energy from the inlet. I think that sets an altitude limit.
Can anyone shine some light on something that has fascinated me for a long time I have tried to research this but with no luck. OK, If you look at the horizontal fin/elevator on a 747 and on almost all large jet planes you will notice that the profile of the fin is like a wing only inverted.
That's completely right. Those tail wings push the tail downwards!!! Well observed!
The reason is because the aircraft has to be stable when moving through the air, so the centre of mass has to be as far forward as possible, so that the aircraft sort of shuttlecocks and keeps itself pointed into the airstream. In fact the CofM has to be ahead of the centre of lift. The centre of lift lines up pretty much with the wings, and the CofM has to be ahead of that. So if you think about it, the nose of the aircraft always wants to drop, and it should go into a screaming dive. This would be bad.
Instead, the empenage section has a wing (the horizontal stabilizer) which pushes the tail downwards, and balances out the torque from having the centre of mass ahead of the centre of lift. Because the tail is quite a long way back, it doesn't take a lot of downwards lift to balance the torque, and although it's not terribly efficient, it's not horribly inefficient.
Some 'tailless' aircraft designs, such as Concorde, don't do this, they sort of rely on just having a very long wing. Early tailless designs were horribly unstable, but newer ones are much better. Also, a lot of newer aircraft are actively stabilized, so that the centre of mass and the centre of lift are more or less inline and a computer is constantly fighting to keep the aircraft pointing forwards into the air stream using the control surfaces.
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Because the tail is quite a long way back, it doesn't take a lot of downwards lift to balance the torque, and although it's not terribly efficient, it's not horribly inefficient.
Thanks, Wolfekeeper for your reply. That is a very interesting point you make regarding the overall size of the stabilisers. If we look closely at the tail section on a 747SP we can see that the vertical and horizontal surfaces are much larger than on the standard 747 models this is due to the SP having a shorter body and needs more assistance to hold the aircraft in line as the torque on the fuselage is decreased with length with an outboard engine out we would need a larger ruder to correct with.