Aviation Mythology and Misconceptions
Hi galaxy flyer, hawk37, and willl05,
See my argument below.
I don't think you can 'gain power' in a physical sense. You can gain energy and convert that energy into work. The rate of doing that work is a measure of the power.
If the myth had stated, 'there is no energy in a stationary aircraft' I would absolutely agree that it was nonsense. There is a massive amount of energy, just waiting to be converted into work and power!
I would say that work is being done on the air, not on the aircraft, as it is the air that is being accelerated, not the aircraft.
An interesting set of responses. It all comes down to semantics I suppose.
I was concentrating purely on the performance aspects of the whole aircraft, i.e. Power available and Power required. As we know, for all aircraft, power required against speed follows the classic curve that is similar in shape to the total drag curve.
For a jet aircraft, power available against speed is an almost straight line starting at the origin and shooting up and to the right. The faster one goes, the more power is produced (which is one reason that jet aircraft can fly at high TAS).
However, at zero TAS, there is zero power available; that was my point.
Of course there is considerable thrust available, as evinced by the exhaust stream. There is also electrical and hydraulic energy available from the engines, accumulators and battery, but I don't see how this relates to the 'myth'. After all, a passenger in the cabin may switch on a torch/flashlight using the chemical energy in that device's battery, but that hardly matters when one is discussing the performance-related power produced by the aircraft.
The aircraft cannot be producing any performance-related power when stationary, as there is no power available, due to the zero TAS. As soon as you release the brakes, the thrust accelerates the aircraft and the power immediately increases, in direct proportion to the speed (assuming constant thrust).
I fully accept that that this is a narrow interpretation of the word 'power' but I thought that this was what the original 'myth' was addressing. Sorry if I got that wrong. Another issue is the casual interchange of the words 'power', 'energy' and 'thrust' in everyday speech. It doesn't cause any difficulties at all, until someone comes up with the statement in the myth! Then some more accurate definitions are required.
We really need someone with a better understanding of physics than mine to put this one to bed!
Eckhard
Assuming the generators and hydraulics are operational, there is power on a stopped jet. Small point
Heat energy, and kinetic energy (exhaust) come to mind.
Power plants use turbines to produce electrical power, however of course, like the stopped aircraft, the power plants are not moving.
I'm sure the quote/myth is really meant to mean "there is no power (ie energy) gained by the aircraft" (except possibly warmer brakes).
However I've seen it referred to as "the aircraft is producing no power", clearly incorrect.
Power plants use turbines to produce electrical power, however of course, like the stopped aircraft, the power plants are not moving.
I'm sure the quote/myth is really meant to mean "there is no power (ie energy) gained by the aircraft" (except possibly warmer brakes).
However I've seen it referred to as "the aircraft is producing no power", clearly incorrect.
If the myth had stated, 'there is no energy in a stationary aircraft' I would absolutely agree that it was nonsense. There is a massive amount of energy, just waiting to be converted into work and power!
If you were to stand behind a running engine, you could test if really no work is produced.
An interesting set of responses. It all comes down to semantics I suppose.
I was concentrating purely on the performance aspects of the whole aircraft, i.e. Power available and Power required. As we know, for all aircraft, power required against speed follows the classic curve that is similar in shape to the total drag curve.
For a jet aircraft, power available against speed is an almost straight line starting at the origin and shooting up and to the right. The faster one goes, the more power is produced (which is one reason that jet aircraft can fly at high TAS).
However, at zero TAS, there is zero power available; that was my point.
Of course there is considerable thrust available, as evinced by the exhaust stream. There is also electrical and hydraulic energy available from the engines, accumulators and battery, but I don't see how this relates to the 'myth'. After all, a passenger in the cabin may switch on a torch/flashlight using the chemical energy in that device's battery, but that hardly matters when one is discussing the performance-related power produced by the aircraft.
The aircraft cannot be producing any performance-related power when stationary, as there is no power available, due to the zero TAS. As soon as you release the brakes, the thrust accelerates the aircraft and the power immediately increases, in direct proportion to the speed (assuming constant thrust).
I fully accept that that this is a narrow interpretation of the word 'power' but I thought that this was what the original 'myth' was addressing. Sorry if I got that wrong. Another issue is the casual interchange of the words 'power', 'energy' and 'thrust' in everyday speech. It doesn't cause any difficulties at all, until someone comes up with the statement in the myth! Then some more accurate definitions are required.
We really need someone with a better understanding of physics than mine to put this one to bed!
Eckhard
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However, at zero TAS, there is zero power available; that was my point.
What about a 10 kt headwind, then, Eck? perhaps you mean 'zero GS' (think conveyor belt?)
Hi BOAC
No, I think I mean TAS. An aircraft flying at 100kts TAS into a 100kt headwind requires exactly the same power as an aircraft passing it in the opposite direction at 100kts TAS with a 100kt tailwind.
If an aircraft is parked with the engines shut down and the park brake set, is there any need for any 'power'? What about a 10kt tailwind? Does this require some sort of 'reverse power' to resist?
The fact that that aircraft remains stationary comes down to the friction between the tyres and the runway.
BTW, what's an Oozlum bird?
What about a 10 kt headwind, then, Eck? perhaps you mean 'zero GS' (think conveyor belt?)
If an aircraft is parked with the engines shut down and the park brake set, is there any need for any 'power'? What about a 10kt tailwind? Does this require some sort of 'reverse power' to resist?
The fact that that aircraft remains stationary comes down to the friction between the tyres and the runway.
BTW, what's an Oozlum bird?
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BTW, what's an Oozlum bird?
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downwind turn
Mechta Silverstrata
Quite right about climbing/descending through appropriate wind gradient
Indeed add local patches of accelerated wind above and around any hill or ridge particularly in 'stable' conditions and turning analysis gets very difficult
(There are two relationships we try to isolate in discussion, Aircraft to Air , and Aircraft to Earth , which are however linked by Air to Earth)
.
I think Lyman hit on something when he said Time came into it.
During the Turn(not straight upwind or downwind)the aircraft has to be accelerated + or - with respect to the ground in order to maintain flying speed. This takes time ,and has to be paid for with engine power or gravity(potential energy= height]
If you take the extreme case of an instant turn (on the spot) you can arrive at a minus airspeed.....So??? (Dont ask, Ive been there done that )
Consider further the interesting situation you were watching from the balloon,
Perfect circle ? Constant G?? Constant angle of bank???....I dont think so
These effects most noticeable where windspeed is a large compared to aircraft airspeed
I wonder how a Jumbo fares doing a 180 downwind turn in a 200 mph wind .
Quite right about climbing/descending through appropriate wind gradient
Indeed add local patches of accelerated wind above and around any hill or ridge particularly in 'stable' conditions and turning analysis gets very difficult
(There are two relationships we try to isolate in discussion, Aircraft to Air , and Aircraft to Earth , which are however linked by Air to Earth)
.
I think Lyman hit on something when he said Time came into it.
During the Turn(not straight upwind or downwind)the aircraft has to be accelerated + or - with respect to the ground in order to maintain flying speed. This takes time ,and has to be paid for with engine power or gravity(potential energy= height]
If you take the extreme case of an instant turn (on the spot) you can arrive at a minus airspeed.....So??? (Dont ask, Ive been there done that )
Consider further the interesting situation you were watching from the balloon,
Perfect circle ? Constant G?? Constant angle of bank???....I dont think so
These effects most noticeable where windspeed is a large compared to aircraft airspeed
I wonder how a Jumbo fares doing a 180 downwind turn in a 200 mph wind .
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Originally Posted by Dozy
:
Originally Posted by fireflybob
How about airline pilots are millionaires and all they do is sit there and push buttons all day because flying now is "automatic"?.
Originally Posted by fireflybob
How about airline pilots are millionaires and all they do is sit there and push buttons all day because flying now is "automatic"?.
Regards,
Ihg
Two quick points about hang gliders
Typical flying speed is of the order of 20 kt. Landing into a 20kt wind you can if you get it right, just step out of the air. But flying at 40kt groundspeed downwind at 200 feet is truly scary - one reason I gave up.
If you leave it very late to turn upwind the wingtip closer to the ground sees more airspeed than the other wingtip (windshear - because the tailwind is less very close to the ground), which makes it hard or impossible to turn - because the wingtip closer to the ground is experiencing more lift, which is just what you don't want when trying to turn. You are then committed to a downwind landing. Generally only a problem if you turn below 50 ft agl. Not a problem for airline pilots I trust.
If you leave it very late to turn upwind the wingtip closer to the ground sees more airspeed than the other wingtip (windshear - because the tailwind is less very close to the ground), which makes it hard or impossible to turn - because the wingtip closer to the ground is experiencing more lift, which is just what you don't want when trying to turn. You are then committed to a downwind landing. Generally only a problem if you turn below 50 ft agl. Not a problem for airline pilots I trust.
Last edited by 911slf; 5th Nov 2011 at 13:57.
During the Turn(not straight upwind or downwind)the aircraft has to be accelerated + or - with respect to the ground in order to maintain flying speed.
This takes time ,and has to be paid for with engine power or gravity(potential energy= height]
If you take the extreme case of an instant turn (on the spot) you can arrive at a minus airspeed.....So??? (Dont ask, Ive been there done that )
Consider further the interesting situation you were watching from the balloon,
Perfect circle ? Constant G?? Constant angle of bank???....I dont think so
Perfect circle ? Constant G?? Constant angle of bank???....I dont think so
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somebody..... make.... the... sufferring... stop.....
And yet, I too keep on reading on... must be a masochistic streak.
Please tell me more about those dangerous "downwind turns"...
Hi wily flier
Never done a 180 turn but maybe about 45 degrees with a 150kt wind from the front quarter which after the turn becomes a tailwind from the rear quarter.
What happens? Well, as you'd expect, the groundspeed changes fairly quickly during the turn as the headwind component changes to a tailwind component but the airspeed stays absolutely steady at around 300 kts indicated.
In a holding pattern, the highest wind speed I've seen is about 65 kts. Here of course, I've done 180 turns and the same applies. There is no change in the IAS (about 220 kts). In the case where the wind is aligned with the hold axis, the groundspeed varies between about 185 and 305. (TAS about 240 kts).
So to answer your question; 'Very well, thank you!'
Eckhard
I wonder how a Jumbo fares doing a 180 downwind turn in a 200 mph wind .
What happens? Well, as you'd expect, the groundspeed changes fairly quickly during the turn as the headwind component changes to a tailwind component but the airspeed stays absolutely steady at around 300 kts indicated.
In a holding pattern, the highest wind speed I've seen is about 65 kts. Here of course, I've done 180 turns and the same applies. There is no change in the IAS (about 220 kts). In the case where the wind is aligned with the hold axis, the groundspeed varies between about 185 and 305. (TAS about 240 kts).
So to answer your question; 'Very well, thank you!'
Eckhard
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>>Please tell me more about those dangerous "downwind turns".
As before, it is the wind gradient that is dangerous, rather than the turn. Its like taking off in calm conditions and climbing up into a 50kt tailwind. The instantaneous effect of the wind gradient on your airspeed can be catastrophic.
Likewise turning into wind onto final, in a long-wing glider, in a region with a strong wind gradient. The upper wing can have more airflow than the lower, resulting in a turning force greater than the ailerons can counter. And since landing on the wingtip is not the approved arrival technique, the landing becomes rather exciting. (The solution is more airspeed.)
All useful things that a modern ATPL will never learn or directly experience. But knowing the flight sensations of such manouvres may well prove vital if you are - well - plummeting down at 10,000 ft per min over the Atlantic in a large jet with your nose pitched 15 degrees up, and not understanding the situation.
.
.
As before, it is the wind gradient that is dangerous, rather than the turn. Its like taking off in calm conditions and climbing up into a 50kt tailwind. The instantaneous effect of the wind gradient on your airspeed can be catastrophic.
Likewise turning into wind onto final, in a long-wing glider, in a region with a strong wind gradient. The upper wing can have more airflow than the lower, resulting in a turning force greater than the ailerons can counter. And since landing on the wingtip is not the approved arrival technique, the landing becomes rather exciting. (The solution is more airspeed.)
All useful things that a modern ATPL will never learn or directly experience. But knowing the flight sensations of such manouvres may well prove vital if you are - well - plummeting down at 10,000 ft per min over the Atlantic in a large jet with your nose pitched 15 degrees up, and not understanding the situation.
.
.
silverstrata
Very well put. I too have seen large airspeed changes when climbing/descending into wind shear in the 747. Wave activity can also produce large thrust and/or airspeed changes.
I guess we never see those effects that you describe close to the ground as our bank angle is necessarily small, no large changes of heading are made below about 1000ft (OK, except at JFK on the Canarsie and other circling approaches) and our approch speed is in the order of 160 kts.
Very well put. I too have seen large airspeed changes when climbing/descending into wind shear in the 747. Wave activity can also produce large thrust and/or airspeed changes.
I guess we never see those effects that you describe close to the ground as our bank angle is necessarily small, no large changes of heading are made below about 1000ft (OK, except at JFK on the Canarsie and other circling approaches) and our approch speed is in the order of 160 kts.
I KNEW mentioning down-wind turn would catch SOMEONE!!
Wily, have you got the message that turning in a constant wind has no different effect on the aircraft than turning in still air?
The aircraft has no idea what the velocity of the ait it is in is in relation to the ground.
Trust us!!
(P.s.- I have done just as you suggested- held in 150kts of wind. No change in airspeed up vs downwind- really!!)
Wily, have you got the message that turning in a constant wind has no different effect on the aircraft than turning in still air?
The aircraft has no idea what the velocity of the ait it is in is in relation to the ground.
Trust us!!
(P.s.- I have done just as you suggested- held in 150kts of wind. No change in airspeed up vs downwind- really!!)
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downwind turn
wizifoz
No auto throttle or power changes at all, during the 1 minute of turn?
If yes, then its still an open question, neither of us can tell how much due to banked turn and how much due to the effect I am supporting
Well it happens with a hang glider , changes of bank and G as you go round too.
As I implied before, analysis is masked by the impact of various different packets of windspeed as you go round. It is not a problem of 'the ground looks a bit faster', its to do with the need to energise acceleration and momentum against inertia during the turn downwind, and then turning the excess speed back into height during the turn into wind.
Not discernible with big powerful aircraft. very discernible with lack of power and light aircraft
Different question once running straight downwind, then the normal wind gradient helps you flatten/stretch the glideslope,as you descend; trouble is its harder to roll into the turn back, and you may not have the height to do it
Please dont mind me , its surprising my strange concept has not killed me in the last 72 years
PS it occurs to me you might get a different view of what happens if you watch a helicopter or auto gyro and see how those wings go up and down on the way round!
No auto throttle or power changes at all, during the 1 minute of turn?
If yes, then its still an open question, neither of us can tell how much due to banked turn and how much due to the effect I am supporting
Well it happens with a hang glider , changes of bank and G as you go round too.
As I implied before, analysis is masked by the impact of various different packets of windspeed as you go round. It is not a problem of 'the ground looks a bit faster', its to do with the need to energise acceleration and momentum against inertia during the turn downwind, and then turning the excess speed back into height during the turn into wind.
Not discernible with big powerful aircraft. very discernible with lack of power and light aircraft
Different question once running straight downwind, then the normal wind gradient helps you flatten/stretch the glideslope,as you descend; trouble is its harder to roll into the turn back, and you may not have the height to do it
Please dont mind me , its surprising my strange concept has not killed me in the last 72 years
PS it occurs to me you might get a different view of what happens if you watch a helicopter or auto gyro and see how those wings go up and down on the way round!
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it occurs to me you might get a different view of what happens if you watch a helicopter or auto gyro and see how those wings go up and down on the way round
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assymetry in rotor
Brian,
Just that in some teeter helos and most autogyros the advancing blade tend to rise and the retreating blade drops, thus looks canted from front or rear,you probably cant see that from where you sit heh heh
Just that in some teeter helos and most autogyros the advancing blade tend to rise and the retreating blade drops, thus looks canted from front or rear,you probably cant see that from where you sit heh heh
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Wily,
And would this be related to airspeed or ground speed?
Wiz,
I am going to hire a team of ninjas to hunt you down. Either that or just commit ritual suicide myself.
pb
Just that in some teeter helos and most autogyros the advancing blade tend to rise and the retreating blade drops, thus looks canted from front or rear,you probably cant see that from where you sit heh heh
Wiz,
I KNEW mentioning down-wind turn would catch SOMEONE!!
pb