Ozexpat,
Well, thinking about it, a certain ammount of knowledge has to be assumed. The assumption is that, in a properly structured course, the student has the knowledge required to be sitting in front of you for their briefing. After all, you can't start every briefing by "In the beginning, there was darkness...."
But, as you say, enough about intructional technique and back to the topic at hand, which I see as a deeper look at PofF for interest, not necessity.
So, for Gonewest and anyone else interested:
Lets look at Kermodes model.
His aircraft is in S&L.
Thurst is increased.
The Pilot does not select a lower nose attitude, so the aircraft commences a climb.
He makes the simplification, stated later, that "Assuming that the path actually travelled by the aeroplane is in the same direction as the thrust..."
On that basis, it is not possible to enter a climb without seeing an increase in Lift to initiate it.
Argument:
Kermodes thrust is acting along the flight path. Therefore, when in S&L, Thrust is horizontal (to the world).
a) If all other forces were in balance, and
b) The pilot maintains the same attitude (granted, this requires control inputs),
c) assumption: ignore prop effects (imagine its a jet)
then initially only one thing will happen: the aircraft will accelerate horizontally. Hence speed will increase.
As speed has increases, this will effect lift production.
Thus far:
Pitch attitude unchanged.
Flight path unchanged.
therefore AOA is constant, along with everything else in the lift formula apart from speed (which has increased).
Hence lift is increased.
[Incidentally, drag will have increase as well, exactly in proportion, so the Total Reaction has also increased if you are using a 3 force model, but that is not of direct interest. The key point is that an extra upwards force component is now present that will start the aircraft climbing.]
Therfore, to initiate a climb in this manner, Lift has increased.
Yet Kermode clearly shows how, once established in a steady climb, Lift is actually less than it used to be in S&L.
No arguement from here!
How do we get to a from a stage where Lift is greater than weight to establish a climb to being less than weight to sustain one?
The answer lies in looking at the Angle Of Attack.
We've said the pilot is maintaining the same attitude, however the flight path is now sloping upwards. Hence AOA will be decreasing. The higher the climb rate, the more the AOA decreases until that effect on lift production overwhelms the effect of the increased speed. Eventually, lift will decrease (in line with Kermode) until the forces all balance out. At which point the flight path (along with AOA) remains constant.
Summary:
Conditions:
- Attitude constant.
- Thrust increased.
Results in:
- Speed increased.
- Lift increased.
- Climb Rate.
- AOA reduction leading to new equilibrium, as Kermode, with L<W
Alternatively, it is perfectly possible to climb,
sustainably, without increasing thrust. (proviso - unless you are already at ceiling, or already on the back of the drag curve, in which case a stall will result). The key thing is the necessity to get to a regime of greater
excess power. This is not the same as saying
more power.
Select a higher nose attitude by using the elevators. This increases AOA. At that moment, speed is unchanged, but the AOA increase makes more lift. Aircraft has unbalanced upwards force, so accelerates upwards. Since there is currently no excess power, kinetic energy must initially be traded to gain gravitational potential energy, so aircraft slows down.
But the speed decrease has the effect of reducing power required, so we now have some excess power with which to sustain climb, albeit with a lower airspeed.
Meanwhile, the AOA decrease described earlier, plus the speed recuction, combine via the lift formula to end up with a net reduction in lift to eventually be less than weight, i.a.w. Kermode steady climb analysis.
Summary:
Conditions:
- Attitude increased.
- Thrust constant.
Results in.
- Lift increase.
- Climb rate (initially zoom).
- Speed reduction.
- Power required decreased.
- Sustainable climb.
- AOA (& speed) reduction leading to new equilibrium, as Kermode, with L<W.
So we can see two methods by which a climb can be initiated, either increase thrust or increase pitch. Both of them involve an initial increase in lift to establish the climb, followed by a decrease as the new equilibrium is established.
Q.E.D.
In reality, we tend to do both for sustained climbs, i.e. increase thrust and select a higher pitch attitude. For temporary climbs (or descents), say to correct our cruising altitude, we tend to make a small power increase (decrease) and let everything else come out in the wash. The mechanism is, however, as shown.
Footnotes:
(1) Of course, a steady climb isn't really an equilibrium. If you fly a constant air speed in a climb, then your TAS is steadily increasing, so you are not technically in a state of uniform motion i.a.w. Newton. It may not seem like a big deal, but it can make a big difference in certain circumstances.
(2) As mentioned earlier, S&L isn't really a uniform state of motion either (due earth curvature), but it doesn't make much difference unless you fly very fast. Feeling keen? Try doing some sums for Concorde or an SR-71.
(3) In spite of the argument above, it
is possible to show how a a climb can be established without a transient lift increase. it just isn't the way average aircraft usually do it. Can anyone see the hole in Kermodes assumptions?
CPB