Thrust and Speed
Thread Starter
Join Date: Jul 2008
Location: Sydney, Australia
Posts: 37
Likes: 0
Received 0 Likes
on
0 Posts
Heard something interesting in my A/C engineering lecture today that was
"If you wish to increase speed of an aircraft you do not increase thrust, you must move the stick instead, aircraft speed is controlled by the stick due to angle of incidence". this hardly made sense to me as i would consider a stall to be a situation in which most thrust is required as a/c speed is lowest and during landing you would not require full power to descend. can someone explain this to me more clearly.
Cheers
"If you wish to increase speed of an aircraft you do not increase thrust, you must move the stick instead, aircraft speed is controlled by the stick due to angle of incidence". this hardly made sense to me as i would consider a stall to be a situation in which most thrust is required as a/c speed is lowest and during landing you would not require full power to descend. can someone explain this to me more clearly.
Cheers
Join Date: Jun 2006
Location: Europe
Posts: 716
Likes: 0
Received 0 Likes
on
0 Posts
This doesn't make any sense to me either. For one; the flight regime is not specified, secondly, the angle of incidence is (normally) fixed.
He might have meant something like this (my italic inserts): "If you wish to increase speed of an aircraft andyou do not want to, or are able to increase thrust, you must move the stick forward instead.
That last portion doesn't make any sense to me. The angle of incidence is the angle between the chord line of the wing and the fuselage/aircraft longitudinal axis, and has nothing to do with speed. He may be confusing this with angle of attack, though; since drag varies with angle of attack, so does the speed (or, more correctly, longitudinal acceleration). Also, with a change of angle of attack, everything else being the same, a change in load factor will occur. This will normally lend itself to the exchange between potential and kinetic energy, ie altitude and speed.
Some prefer to control the aircraft speed with pitch rather than power (thrust) at low speed (the "back side of the power curve") due to the natural speed instability at these speeds. That might also be where he's going.
edit: reading your question again, I see that you might be asking about power and pitch in a stall. If that is the case, your statement makes more sense. When an aircraft stalls, yes, it is important to set full thrust as soon as possible, but very few aircraft will be able to "power" themselves out of the stall. The first reaction, and the most vital action, is to lower the nose (really, lower the angle of attack) to regain lift. Power application is only vital to recover from a low-speed (and possibly low-altitude) regime to normal flight, but power in itself does not get you out of the stall (unless you're flying some kind of rocket!)
He might have meant something like this (my italic inserts): "If you wish to increase speed of an aircraft andyou do not want to, or are able to increase thrust, you must move the stick forward instead.
That last portion doesn't make any sense to me. The angle of incidence is the angle between the chord line of the wing and the fuselage/aircraft longitudinal axis, and has nothing to do with speed. He may be confusing this with angle of attack, though; since drag varies with angle of attack, so does the speed (or, more correctly, longitudinal acceleration). Also, with a change of angle of attack, everything else being the same, a change in load factor will occur. This will normally lend itself to the exchange between potential and kinetic energy, ie altitude and speed.
Some prefer to control the aircraft speed with pitch rather than power (thrust) at low speed (the "back side of the power curve") due to the natural speed instability at these speeds. That might also be where he's going.
edit: reading your question again, I see that you might be asking about power and pitch in a stall. If that is the case, your statement makes more sense. When an aircraft stalls, yes, it is important to set full thrust as soon as possible, but very few aircraft will be able to "power" themselves out of the stall. The first reaction, and the most vital action, is to lower the nose (really, lower the angle of attack) to regain lift. Power application is only vital to recover from a low-speed (and possibly low-altitude) regime to normal flight, but power in itself does not get you out of the stall (unless you're flying some kind of rocket!)
Join Date: Mar 2005
Location: Uh... Where was I?
Posts: 1,338
Likes: 0
Received 0 Likes
on
0 Posts
If that is said during a stall, ok. Although all you need is reducing angle of attack (not incidence, probably that is what he wanted to say).
Stall occurs at an AoA. Push the stick, you unstall the wings. Of course you will loose altitude. The time to add power is usually after you lower the nose, so the effects of the propeller and others do not put you in trouble before you manage to unstall the wings.
If it is the old wrong assumption that speed is controlled by pitch, and not power, well... It is wrong except during climb or descents at fixed power, where it is totally correct. In level flight, or in an ILS, speed is controlled with power and flight path with pitch.
For instance. To maintain altitude you pull or push the stick as necessary, while maintaining speed advancing or retarding the power levers as required. Altitude increasing? push the nose a bit. Speed decreasing? add some power.
If you are studiying aircraft engineering and are interested in how pilots fly an airplane there are very nice and simple books for private pilots.
Stall occurs at an AoA. Push the stick, you unstall the wings. Of course you will loose altitude. The time to add power is usually after you lower the nose, so the effects of the propeller and others do not put you in trouble before you manage to unstall the wings.
If it is the old wrong assumption that speed is controlled by pitch, and not power, well... It is wrong except during climb or descents at fixed power, where it is totally correct. In level flight, or in an ILS, speed is controlled with power and flight path with pitch.
For instance. To maintain altitude you pull or push the stick as necessary, while maintaining speed advancing or retarding the power levers as required. Altitude increasing? push the nose a bit. Speed decreasing? add some power.
If you are studiying aircraft engineering and are interested in how pilots fly an airplane there are very nice and simple books for private pilots.
Is this a pilot conversion course? Why didn't the whole class tell the instructor that he/she was talking rubbish? Ask your instructor the fundamental principle behind an autothrottle system - if the speed's low, it pushes the power up. It doesn't push the nose down.
If you're an effo:
"Always remember and forever take heed; right hand for glidepath and left hand for speed!".
One of the most fundamental rules of flying.
If you're an effo:
"Always remember and forever take heed; right hand for glidepath and left hand for speed!".
One of the most fundamental rules of flying.
Last edited by Capn Bloggs; 9th Mar 2010 at 22:58.
Join Date: Feb 2007
Location: Expatsylvania
Posts: 179
Likes: 0
Received 0 Likes
on
0 Posts
I've been hearing this old chestnut since Day One of groundschool. "Pitch controlls airspeed, power controls altitude." Never understood why people keep saying that. It's the coordinated application of both. Is that concept so hard?
Join Date: Jan 2007
Location: Vermont
Age: 67
Posts: 200
Likes: 0
Received 0 Likes
on
0 Posts
Bfisk hits the nail on the head...it depends on where you are with respect to the power curve. This argument is almost older than the Wright Brothers...years ago, I had a running feud with the chief at the college program I instructed at over it. "Aerodynamics for Naval Aviators" is adamant that pitch controls airspeed, and that makes sense when you are operating close to, or behind, the power curve (say during a carrier approach). But when I get slow in my 757 I sure ain't thinkin' of pitch, because I am well ahead of the power curve.
The problem arises most apparently in light aircraft, because many of them have little power margin. The power-control-airspeed approach works, but only within a small band of the approach speed range. If you get just a bit slow, there isn't enough power to get you back as quickly as your instructor might like. Naturally, this class of airplane is the one we are using to teach primary students, who have only an early comprehension of the issue. Thus, my preference was to use pitch, because it was more consistent, and it was guaranteed to be primary when the engine quit. But this should be accompanied by a thorough discussion of the relationships and an understanding that primary speed control would change from pitch to power as soon as you matriculate into a larger airplane.
The problem arises most apparently in light aircraft, because many of them have little power margin. The power-control-airspeed approach works, but only within a small band of the approach speed range. If you get just a bit slow, there isn't enough power to get you back as quickly as your instructor might like. Naturally, this class of airplane is the one we are using to teach primary students, who have only an early comprehension of the issue. Thus, my preference was to use pitch, because it was more consistent, and it was guaranteed to be primary when the engine quit. But this should be accompanied by a thorough discussion of the relationships and an understanding that primary speed control would change from pitch to power as soon as you matriculate into a larger airplane.
Join Date: Jul 2006
Location: USA
Posts: 451
Likes: 0
Received 0 Likes
on
0 Posts
A lot of bullXXXX out there....
GF is one million percent correct. All information contrary to that is total
BULLXXXX.
It's that simple....
There are a number of fine publications....many come from the U.S. Navy...that will confirm Galaxy Flyer's post.
Fly safe,
PantLoad
BULLXXXX.
It's that simple....
There are a number of fine publications....many come from the U.S. Navy...that will confirm Galaxy Flyer's post.
Fly safe,
PantLoad
Join Date: May 2000
Location: Seattle
Posts: 3,196
Likes: 0
Received 0 Likes
on
0 Posts
It depends quite a bit on the airplane design...
Many airplanes are speed-stable. If you increase thrust without changing trim, it will likely settle in a climb at or near the trimmed speed. This is especially true with underslung engines, where the thrust vector is well below the pitch axis.
Many airplanes are speed-stable. If you increase thrust without changing trim, it will likely settle in a climb at or near the trimmed speed. This is especially true with underslung engines, where the thrust vector is well below the pitch axis.
Join Date: Mar 2010
Location: Oklahoma USA
Posts: 34
Likes: 0
Received 0 Likes
on
0 Posts
Thrust and Speed.
Which came first the chicken or the egg?
Pitch Power Altitude Speed.
What controls what?
Well - think of these things...
Some military jet fighters can accelerate to a speed in excess of Mach 1 in a vertical climb...
As a flight instructor I teach my primary students the following. (in low power general aviation training aircraft)
Cruise flight - Pitch controls Altitude. You select your cruise speed with power. (small changes in pitch produce a large change in lift and very little change in drag at high speed)
Slow flight - Pitch controls airspeed, power controls altitude. ( a small change in pitch makes a large change in drag and very little change in lift therefore making the speed change)
They learn it that way and can succesfully operate an aircraft usssing that information....
If I am flying with an experienced pilot, and I ask them to do slow flight during recurrent training or transition to a new aircraft, and usually in a higher performance aircraft, I remind them to adjust pitch and power as necesssary to maintain airspeed and altitude. What a cop out!! I do not tell them how to accomplish slow flight unless they are having trouble. It is surprising how many pilots forget how to precisely control airpeed and altitude especially in higher performance aircraft. If they can make it work acceptably, I do not say anything... Many pilots get into the bad habit of using flaps and gear to control their speed and forget how it really works.
In reality.... Airspeed and altitude, power and pitch are all interrelated.
If we assume that airspeed is controled by power, and altitude is controled by pitch, then what happens in slow flight? Try getting up to lift off speed without adding full power on the runway, and try lifting off without increasing angle of attack or pitching up...
In slow flight, if we want to increase speed, we can lower the nose. (Pitch controls airspeed) When we lower the nose and decrease the drag, of the wing and airframe, the speed begins to increase but at the same time the aircraft begins to descend as there was a decreasse in lift due to the angle of attach change. We notice the descent and add power to compensate. (Power controls altitude).
Again in the low flight senario above, if we want to increase speed and we add power, if we have enough available power, the speed will begin to increase. As the speed increases more lift is produced causing a climb. We notice the climb and slightly lower the nose to compenate. (Pitch controls altitude).. The problem is in a small aircraft the available power may not be sufficient to accelerate the aircraft very much so lowering the nose and decreasing the drag has a more immediate effect.
So in conclusion..
I recommend that flight intructors teach their primary students using small low power aircraft how to control airspeed and altitude as I stated at the top of this post... Cruise and slow flight so not operate the same.
I recommend that flight instructors teach and permit their students to discover that airspeed is controlled by power and altitude is controled by pitch at all times when tranitioning to higher power high performance aircraft. That is one of the most important concepts to learn when transitioning form simple training aircraft into a high performance aircraft.
However, if a pilot can control airspeed and altitude succesfully there is no real reason to force them to adopt any particular method nor does their method need to be changed.
My 2 cents worth
Bill
Pitch Power Altitude Speed.
What controls what?
Well - think of these things...
Some military jet fighters can accelerate to a speed in excess of Mach 1 in a vertical climb...
As a flight instructor I teach my primary students the following. (in low power general aviation training aircraft)
Cruise flight - Pitch controls Altitude. You select your cruise speed with power. (small changes in pitch produce a large change in lift and very little change in drag at high speed)
Slow flight - Pitch controls airspeed, power controls altitude. ( a small change in pitch makes a large change in drag and very little change in lift therefore making the speed change)
They learn it that way and can succesfully operate an aircraft usssing that information....
If I am flying with an experienced pilot, and I ask them to do slow flight during recurrent training or transition to a new aircraft, and usually in a higher performance aircraft, I remind them to adjust pitch and power as necesssary to maintain airspeed and altitude. What a cop out!! I do not tell them how to accomplish slow flight unless they are having trouble. It is surprising how many pilots forget how to precisely control airpeed and altitude especially in higher performance aircraft. If they can make it work acceptably, I do not say anything... Many pilots get into the bad habit of using flaps and gear to control their speed and forget how it really works.
In reality.... Airspeed and altitude, power and pitch are all interrelated.
If we assume that airspeed is controled by power, and altitude is controled by pitch, then what happens in slow flight? Try getting up to lift off speed without adding full power on the runway, and try lifting off without increasing angle of attack or pitching up...
In slow flight, if we want to increase speed, we can lower the nose. (Pitch controls airspeed) When we lower the nose and decrease the drag, of the wing and airframe, the speed begins to increase but at the same time the aircraft begins to descend as there was a decreasse in lift due to the angle of attach change. We notice the descent and add power to compensate. (Power controls altitude).
Again in the low flight senario above, if we want to increase speed and we add power, if we have enough available power, the speed will begin to increase. As the speed increases more lift is produced causing a climb. We notice the climb and slightly lower the nose to compenate. (Pitch controls altitude).. The problem is in a small aircraft the available power may not be sufficient to accelerate the aircraft very much so lowering the nose and decreasing the drag has a more immediate effect.
So in conclusion..
I recommend that flight intructors teach their primary students using small low power aircraft how to control airspeed and altitude as I stated at the top of this post... Cruise and slow flight so not operate the same.
I recommend that flight instructors teach and permit their students to discover that airspeed is controlled by power and altitude is controled by pitch at all times when tranitioning to higher power high performance aircraft. That is one of the most important concepts to learn when transitioning form simple training aircraft into a high performance aircraft.
However, if a pilot can control airspeed and altitude succesfully there is no real reason to force them to adopt any particular method nor does their method need to be changed.
My 2 cents worth
Bill
As a flight instructor I teach my primary students the following. (in low power general aviation training aircraft)
Cruise flight - Pitch controls Altitude. You select your cruise speed with power. (small changes in pitch produce a large change in lift and very little change in drag at high speed)
Slow flight - Pitch controls airspeed, power controls altitude. ( a small change in pitch makes a large change in drag and very little change in lift therefore making the speed change)
They learn it that way and can succesfully operate an aircraft usssing that information....
Cruise flight - Pitch controls Altitude. You select your cruise speed with power. (small changes in pitch produce a large change in lift and very little change in drag at high speed)
Slow flight - Pitch controls airspeed, power controls altitude. ( a small change in pitch makes a large change in drag and very little change in lift therefore making the speed change)
They learn it that way and can succesfully operate an aircraft usssing that information....
).While it may work on a C150 because lighties are so speed-stable, it absolutely DOES NOT work on a jet on final, the most critical time of the flight where changes are required almost instantly.
Low-time pilots should not be taught to control the aeroplane via the secondary effects of controls, if for no other reason that when they fly something bigger, the technique doesn't work. While they are coupling effects so that the nose may come up when the thrust is increased, the primary method of changing the speed must be with the throttles (or speedbrake). Likewise, flight path change must be made with the elevator. Any speed changes are then sorted by changing thrust.
There is a case where, in the flare, the angle of attack/nose attitude is so high that jamming the thrust on may have a cushioning effect, but that is not the norm down final.
My analogy is flying down an ILS (or letting the AP do it). If you get low on the slope, you pull the nose up, just as the AP does. You don't put up the thrust and wait for the secondary effect of controls to get you back up onto the GS. Carrier pilots maybe, but not normal aircraft.
Similarly, speed. If you get slow, you definitely do not lower the nose! The speed will never increase satisfactorily; all that will happen is your descent rate will increase.
Join Date: Jul 2006
Location: USA
Posts: 451
Likes: 0
Received 0 Likes
on
0 Posts
What a great discussion....
Carrier aircraft are definitely not real aircraft. They're top secret and fly
quite differently from conventional aircraft.
I stand corrected....
Fly safe,
PantLoad
Aerodynamics for Naval Aviators
(If you really want to know how an airplane flies.....)
quite differently from conventional aircraft.
I stand corrected....
Fly safe,
PantLoad
Aerodynamics for Naval Aviators
(If you really want to know how an airplane flies.....)
Join Date: Sep 1998
Location: wherever
Age: 55
Posts: 1,616
Likes: 0
Received 0 Likes
on
0 Posts
Bernuli vs Newton
Pitch vs thrust
It's a wonder we fly at all!!!
There is only one way to accelerate allong the current flight path and that is to increase thrust/drag ratio.
Pitch vs thrust
It's a wonder we fly at all!!!
There is only one way to accelerate allong the current flight path and that is to increase thrust/drag ratio.
Join Date: Jul 2001
Location: East of West and North of South
Posts: 549
Likes: 0
Received 0 Likes
on
0 Posts
If you are too high on the G/S (or want to descend etc), you reduce thrust.
If the plane is in trim, it will keep the speed
Vice versa if you are too low / want to climb.
Hence pitch controls speed and thrust altitude
These are all constant speed scenarios. If you want to go down you need to remove energy (thrust) from the equation - it's as simple as that.
However, if you want to change the speed you use thrust to change speed and pitch / trim to maintain altitude. But this is not a constant system or stabile environment (and not the way an e.g. an ILS is supposed to be flow).
If the plane is in trim, it will keep the speed
Vice versa if you are too low / want to climb.
Hence pitch controls speed and thrust altitude
These are all constant speed scenarios. If you want to go down you need to remove energy (thrust) from the equation - it's as simple as that.
However, if you want to change the speed you use thrust to change speed and pitch / trim to maintain altitude. But this is not a constant system or stabile environment (and not the way an e.g. an ILS is supposed to be flow).
Pantload,
I wasn't trying to be smart. The reason I said
is because my understanding is that carrier technique is (was?) to do what the original poster mentioned: low on slope, push up the power to get back on it ie hold a constant AOA and control slope via the secondary effect of power; maybe has a more useful effect because of the body angle/thrust vector.
Kramer,
You are flying the aeroplane via the secondary effects of controls. Works well in a bugsmasher, not well in a jet in my experience because the effect doesn't happen fast enough to correct the error that prompted the correction. Each to his own, tho.
I wasn't trying to be smart. The reason I said
Carrier pilots maybe, but not normal aircraft.
Kramer,
These are all constant speed scenarios. If you want to go down you need to remove energy (thrust) from the equation - it's as simple as that.
Join Date: Jun 2009
Location: US
Posts: 497
Likes: 0
Received 0 Likes
on
0 Posts
Flying opposite of how an autopilot/autothrottle works doesn't seem logical. Speed gets low the autothrottle increases power. Autopilot gets high it lowers the nose. It doesn't matter if it is in altitude hold or following a glide slope.
Join Date: Jul 2001
Location: East of West and North of South
Posts: 549
Likes: 0
Received 0 Likes
on
0 Posts
Capn Bloggs
You are flying the aeroplane via the secondary effects of controls.
You are flying the aeroplane via the secondary effects of controls.
Elevator on the other hand have no secondary effect (not causing any motion around other axis).