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One Last Food Fight?


Bob - S50

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If this is covered elsewhere, please feel free to delete the topic.  Having read much of this  website, I would propose that we end the what controls the airspeed and what controls altitude debate.  Instead I would suggest we think in terms of energy.

The airplane has 3 types of energy: kinetic (airspeed), potential (altitude), and chemical (gas).

The yoke cannot create any energy and except during extreme maneuvering, what it really does is convert energy from one form to another.  That is airspeed to altitude or altitude to airspeed.  If we are in level unaccelerated flight and pull the nose up, climb a couple hundred feet and then push over to descend back to the original altitude, we should arrive back at that altitude with essentially the same airspeed.  No energy made or destroyed (within reason).  There may be some minor losses due to momentary drag increases while initiating the climb/descent/level off.  And there may be some minor energy increases because thrust will exceed drag at the lower airspeed (AOA) during the maneuver.

The throttle only controls energy.  The more power we use, the more energy we make.  As long as drag does not consume all that energy, we use the airplane to convert that energy into either airspeed or altitude.

So if we are on base and find ourselves high or fast, we have too much energy.  The only real way to get rid of that energy is to pull the power.  Drag then exceeds power and we get rid of energy.  In order to maintain airspeed we need to use the yoke to convert altitude to airspeed.  In order to maintain altitude, we need to use the yoke to convert airspeed to altitude.  So if we were high but on speed (or proper AOA as Mike would rightly point out), we then use pitch to maintain our airspeed while allowing the plane to descent to the proper height.  Or if we were on altitude, but fast, we once again pull power to reduce energy and then use pitch to maintain the proper glidepath while allowing the airspeed to decrease (AOA to increase).  So in both cases we pulled power first.  Then in one case pitch controlled airspeed while in the other it controlled altitude.

The same would happen if we were low or slow.  We need more energy so we add power and then use pitch to control either airspeed or altitude depending on which one needs adjusting.

The same thing works for instrument approaches.  If you are on course and on glideslope but fast, you have too much energy.  Pull power and use pitch to maintain the glideslope while allowing the plane to slow down.  Or if we are on speed but high, pull power and use pitch to maintain airspeed while allowing the plane to descend to the glideslope.

And in all cases, once we have the airspeed and altitude we want, we need to adjust the energy knob to maintain what we have.

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Anyone want to break out a physics book and apply conservation of energy equations....?

1) Getting a feel for how much energy needs to be lost to lose 1000’.

2) How to lose that exact amount of energy in the traffic pattern would explain a lot.

3) How much energy is dissipated with gear down, various flap positions, or speed brakes....

4) Other simplified methods of doing this...

  • Using 16” Of MP, generally keeps the energy in equilibrium at TPA.  No descent, no acceleration or deceleration....
  • Dropping MP 1” for each 100fpm descent.
  • Adjust to meet PIC requirements

5) Slowing, descending, and losing energy also includes the amount of weight you are carrying around...

Just trying to supply some technical fodder to the conversation...  the initial flight training I received left much of this out of the conversation...

Best regards,

-a-

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OK, I'll bite. 

I do love that online book, it is has fantastic explanations and diagrams.

You left out the variable of drag, though.  Along with airspeed and lift these are three variables, and that means any two can't be a simple direct relationship.  It would only be fair to say that airspeed and lift are inter-dependent

In other words, both pitch and power control both airspeed and lift.

For the purposes of instruction and procedures, it makes sense to simplify this when possible.  If you you're a little below glideslope, you don't want to muck around with adjusting two controls, but this is precisely what you do when you are a lot below glideslope--you increase power AND pitch up.

So for small changes, pick one or the other and stick with it.  It simplifies things and is close enough.  For some systems (turbine aircraft), the time it takes the engine to respond is much slower, so it makes sense to pick pitch controls for descent, but for our piston aircraft, they're pretty close so there is little advantage of one over the other.

But for big changes, you can and SHOULD use both.

TL,DR:  it's not a controversy because you can use either strategy most of the time (just not all the time).

Edited by jaylw314
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Disagree with this" So if we are on base and find ourselves high or fast, we have too much energy.  The only real way to get rid of that energy is to pull the power.  "

There are several options at this point.   Put more flaps in.   Turn uphill as you do in skiing, do a slip,

Since I do a decreasing energy landing instead of certain speeds there are all sorts of things to know to get rid of more energy.

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2 hours ago, carusoam said:

Anyone want to break out a physics book and apply conservation of energy equations....?

1) Getting a feel for how much energy needs to be lost to lose 1000’.

 

If you descend 100' from 100 mph with no drag, you will gain about 10 mph.  If you descend 100' from 75 mph, you will gain about 15 mph.  If you descend from hover, you will gain about 25 mph.

So figure descending a 1000' at 100 mph you'd need to be shedding 10 mph every 100'.

Not sure how much that helps...

ref. 2 x g x h = V1^2 - V0^2, where g is 10 m/s, h is the height in meters, and V1 and V0 are the start and ending velocities.  Note that it is not (V1-V0) ^2

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7 hours ago, Yetti said:

Disagree with this" So if we are on base and find ourselves high or fast, we have too much energy.  The only real way to get rid of that energy is to pull the power.  "

There are several options at this point.   Put more flaps in.   Turn uphill as you do in skiing, do a slip,

Since I do a decreasing energy landing instead of certain speeds there are all sorts of things to know to get rid of more energy.

I guess I should have qualified that statement further.  Assuming you do not change configuration and continue in coordinated flight...

Personally, I'm at full flaps before I start the base turn but that is technique only.  But if you are going to do a slip in the turn, just make sure it is a slip, not a skid.

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From what I remember of high school physics, all that gas in your tank would really be considered as potential energy. Yes, it is released through a chemical reaction but it’s still just potential energy. So if you’re at cruise power and want to go faster you can convert either altitude or gas (or both) to speed. Same mechanisms if you want to go slower. And yes, there are devices we have at our disposal as well for adding drag, etc.

I don’t think there’s a reason to simplify the debate about which controls altitude vs airspeed, etc. The answer is they all can have an effect and you have to learn to use them at the appropriate times.

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9 hours ago, 201er said:

No need to do over 30 degree turns in the pattern.  But I am using the U turn to land to get rid of excess energy.  That is why I call is a decreasing energy landing.    If you are full flaps on downwind at the threshold then you need to alot of energy to get to the runway.  Arriving at the threshold/downwind at the top of gear speed and then using that excess energy to get to the runway.

 

 

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