Getting slow on final

[tgrayson]

I am out of the 'pitch vs. power' debate for now

Actually, you just jumped back into it with both feet. You cannot separate that discussion from the one you just embarked on.

First, a few fundamental principles:

1) Lift = Weight, in unaccelerated flight (level flight, climbs, descents)
2) Descents are due to thrust being less than drag
3) Climbs are due to thrust being greater than drag
4) Level flight occurs when thrust = drag.

Nothing can be understood about aircraft flight unless you accept those principles and use them as a basis for any further reasoning.

Using the above, let’s take a look at one of your statements:

Best glide speed is where lift and drag are equal.

Since we know that Lift = Weight, based on principle #1, it follows, from your statement, that you would agree (incorrectly) that drag = weight. Max Gross weight on a C152 is 1670 lbs, so you are saying that the drag on a C152 at best glide (60 knots) is 1670 lbs. Taking principle #4 above, we deduce that for level flight, we need thrust = drag, therefore, we need 1670 lbs of thrust.

If a C152 were capable of 1670 lbs of thrust, it could hang vertically from its propeller. Does this seem likely? That would take over 300 HP. Instead, the drag on a C152 is on the order of 175 lbs, assuming an airspeed of 60 knots and descent rate of 600 fpm.

Perhaps you were confusing the best glide speed with the fact that parasite drag = induced drag? Or it occurs where L/D ratio is highest?


Applying the fundamental principles also makes statements such as the following suspicious:

So, when you are above the best glide speed, and want to decrease sink rate, increase AoA, (pitch up) and the excess speed will transfer into lift and decrease sink rate.

This seems to imply that when we’re in a descent, we need more lift to stop it. Using principle #2, however, we know that what we need is more thrust or less drag. And principle #1 indicates that our quest for more lift is hopeless anyway, since it will equal weight in unaccelerated flight.

When you pull back on the yoke, you will get a slight temporary increase it in lift, but unless that increase in AOA results in less drag, you will continue going down after a brief balloon. This reduction in drag only happens on the front side of the thrust curve. You'll get the brief balloon on both sides of the thrust curve.

The short field approach speed published in the POH for a 152 is 54. Way too fast for a real short field. Should be 1.3, but that's what Cessna puts down for liability these days,

All your airspeed calculations are off because you’re using indicated airspeeds instead of calibrated airspeeds. You can’t do that.

If you start with a calibrated airspeed for Vs0, 43 knots and multiply that by 1.3, you’ll get 55 KCAS. To convert that to an indicated airspeed, refer to the charts in the performance section on the POH. At that airspeed, IAS is about 1 knot lower than CAS, which would make the approach speed 54 KIAS, exactly what the book recommends.

 

[nosehair]

Actually, you just jumped back into it with both feet.

God ! I did, didn't I?

First, a few fundamental principles:

2) Descents are due to thrust being less than drag
3) Climbs are due to thrust being greater than drag

hmmm...hafta think about that one...
I think that might be true in terms of changing thrust from level unaccelerated flight, but what about when you just push the nose down some. You haven't changed thrust (significantly) and you are descending....
Still thinking on that one...

All your airspeed calculations are off because you’re using indicated airspeeds instead of calibrated airspeeds. You can’t do that.

If you start with a calibrated airspeed for Vs0, 43 knots and multiply that by 1.3, you’ll get 55 KCAS. To convert that to an indicated airspeed, refer to the charts in the performance section on the POH. At that airspeed, IAS is about 1 knot lower than CAS, which would make the approach speed 54 KIAS, exactly what the book recommends.

OK, this I didn't know - or I forgot many years ago. However, the published stall speeds are at gross weight, and you can experiment with the airplane and see that you can pitch up in slow flight at about 55 and climb slightly untill the IAS comes down to something less than 50, then it starts to sink.

When I started flying, there were no 'short field' approach speeds in POH's, and it was expected to use 1.2 x Vso, and no greater than 1.3. The standard technique to determine actual indicated stall speed by stalling the airplane in current weight and landing configuration and multiplying by 1.2 as a minimum speed and 1.3 as a maximum speed.

The point here is that using the book stall speeds is like using the book fuel consumption rates. Yours may vary. Use the actual speeds in your airplane to determine your actual critical speeds. This is, of course, in light airplanes where the difference in IAS and CAS is but a few knots.

In any case, tha actual approach speeds used by most pilots will be slightly above the max L/D curve, giving additional lift with an increase in AoA. If power is not added, then speed (thrust) will decrease with drag, but the resultant descent path will not increase unless the drag increases (speed decreases) below the max L/D. As long as speed is at or above max L/D an increase in AoA (pitch up) will decrease descent path/angle.

 

[tgrayson]

what about when you just push the nose down some. You haven't changed thrust (significantly) and you are descending....
Still thinking on that one...

Good, I'm glad you're thinking about it, rather than rejecting it. There is a chance for an "ah ha" moment.

I don't have a good thrust required curve drawn, but I do have some nice power curves and they're similar. Let me show you one:

What the green curve shows is the power requirements for level flight at every airspeed.

The purple curve shows the power available at every airspeed. You can move the purple curve up and down by moving the throttle.

Level flight is only achieved when power requirements = power available. Right now, the chart shows the aircraft flying at an airspeed where the orange line is. At this airspeed, the power requirements are greater than power available and the aircraft will be descending. If you push the nose down, the orange curve will move to the right and it will be descending faster.

If you pull back on the yoke, the orange line will move left and power available will equal power required and level flight will result.

Important point: climbs and descents happen via the yoke because the airspeed change slides you back and forth along the power required curve.

If you slide the airspeed back enough, you get your max climb rate:

This is Vy. If you slow below that, you will get a decreased climb rate. Flying these airspeeds is known as flying "behind the power curve", or region of reverse command.

 

[MSNFlyer]

tgrayson,

Excellent graphs and explanation. I will definitely incorporate these into my lift, drag, thrust, weight discussions. They break it down to easy to understand fundamentals.

It is easy to fall into the thinking of nose down equals descent and not think of the other factors involved.

 

[tgrayson]

Excellent graphs and explanation. I will definitely incorporate these into my lift, drag, thrust, weight discussions. They break it down to easy to understand fundamentals.

I'm just pleased you're interested. These are really powerful tools, once you get comfortable sliding the curves up and down, and moving left and right along the curves as airspeed changes. Explains quite a lot!

 

[nosehair]

Excellent graphs showing the forces of which you are speaking. I am not in debate over 'pitch vs. power' on an aerodynamic level. The chart shows that if you want to maintain a consistantly constant airspeed and altitude (or constant angle descent path), each moment there is pressure on the yoke, there should also be pressure on the throttle.

The debate is on the instructional technique of breaking down the 'coordination' of throttle and elevator into the two seperate elements of throttle and elevator; pitch vs. power.

In the Airplane Flying Handbook, on the introduction of Turns, in Chapter 4, there is an aerodynamic explanation of the horizontal lift component which explains how this force turns the airplane. page 4-4. Then, at the end of this aerodynamic explanation, the last sentence says: "It is the horizontal component of lift that actually turns the airplane - not the rudder."

Then, the next paragraph explains adverse aileron yaw and explains the need for rudder, as in an afterthought, which is how most students apply rudder. They twist the wheel and then, because they feel the slip, or see the ball, they apply rudder to 'coordinate'.

This habit is mainly because they already have been trained to turn a wheel in their car, but also because the aerodynamic explanation is mis-leading. Yes, the paragraph explanation says "use coordinated rudder and aileron", but there is no emphasis on 'coordination'. The average student will not focus on that one word 'coordination' because it is not a part of a student pilot's actual experience. The instructor is supposed to provide that emphasis.

I do that by explaining the aerodynamic cause of a turn, but I tell them that it will take practice at achieving this simultanous application - this exactly precise application of pressures to rudder and yoke at the exact precise same moment, and I know that most students who have been driving a car will instinctively apply aileron pressure first. So I tell them to start off knowingly pushing the rudder first, then try to apply aileron pressure to 'coordinate' the turn. The objective is to coordinate both pressures simultaneously. Eventually, the student learns to do this much more quickly and effectively than just saying "keep it coordinated", or "watch the ball".

But there is something much more important than coordinated turns that the student is learning, on a subtle level: the rudder controls the left-right motion of the nose, and the aileron controls the bank.

And that is what you want your student to have control of when he starts trying to line up with the runway for a landing. No matter what else is happening, push the rudder to keep the nose aligned and twist the yoke to control bank - whether you are inputing aileron control for drift or for wind correction, the yoke conrols bank and the rudder controls where the nose is pointing or moving, regardless of what you are doing with the aileron - they are seperate controls with seperate functions.

You don't attempt to 'cordinate' rudder and aileron when landing, only when in cruise, climb or descent.

I use this same technique in breaking down the coordination of pitch and power.

If we consistently made power changes for cruise flight, and if we want to make constant little power changes on an approach, I can see your point of using pitch for airspeed control. Pitch changes for airspeed control will be easier to keep the airspeed constant, but you will have a constantly changing glidepath, even when you apply throttle pressure exactly when you apply elevator pressure.

The aerodynamic fact is, keeping a constant approach angle and constant airspeed is only possible in absolutely smooth air. In normally turbulant air, pitch changes to control airspeed changes caused by windshear will result in immediate glidepath changes, which can be corrected by power changes to regain glidepath. Since you can't 'see' an airspeed change outside, you are relying on the instrument to tell you of a change, then after the change, you apply yoke pressure to correct this instrument change, then realize a power change is necessary because this pitch change is also causing an altitude change. The normal or slow student takes too much time to process all this information which is also changing even as he processes it. By the time he inputs controls that should have been done one or two seconds ago, the new situation requires a new set of control inputs. He/She gets very frustrated. Aerodynamic charts be damned.

Just as in my 'coordinated turn' example, I teach coordinated use of throttle and elevator for approaches, but I start them off with the same control inputs that we use in all other phases of flight, except when in the region of reverse command, which is not a normal approach. I keep it consistent with pointing the nose towards a spot on the runway. Point the nose as if you have a laser gun mounted on the cowling in front of you and be aggresive with elevator to keep the laser beam pointed towards this spot and make throttle changes to keep the required airspeed.

You can 'see' a glidepath change looking outside, and students need to learn to see glidepath changes and respond immediately with yoke pressures, followed with throttle changes, if necessary, by looking inside at the airpeed. Most students who use the power to altitude technique don't have really good glidepath control. They see airspeed increasing due to windshear and pitch up which causes very much increased altitude, and a second later reduce power to compensate for the 'too high on final' condition. If the student is trained to pitch towards a spot, and keep the nose down when encountering a sudden gust of airspeed, the approach path is not compromised, and airspeed is still as well in control with power as it is with pitch.

Just like the coordinated turn technique, the student soon learns to accurately control both glidepath and airspeed to a precise landing spot.


I'm kind of an Idiot Savant. You know, The Rainman.

I am very good at teaching very, very basic control fundamentals; not so hot on deep aerodynamics. Oh, I get it, but I don't try to make every student really get it on the theoretical side, but I like finding a way of getting the student apply and control the basic fundamentals:

What contol in your hands controls:

The left-Right Motion of the nose.
The Up-Down Motion of the nose.
The Bank.
The Speed.

These flight controls need to be a part of your body just like your hands and feet.

Under normal conditions, we usually coordinate two or more to effect a smooth change, but not always, and in high stress knee-jerk response mode each control has it's own function.

 

[tgrayson]

Most students who use the power to altitude technique don't have really good glidepath control.

Neither technique will provide superior results over the other, because that depends on pilot proficiency. If a pilot corrects each small deviation, you won’t even be able to tell which technique he’s using. I suspect you're seeing what is called a "confirmation bias".

I do think it’s a poor idea to teach a student that there are separate techniques for flying an approach on the front side vs. the backside of the power curve. The pilot flying the backside of the curve didn't necessarily intend to get there and may not be prepared to switch mindsets. And why should he have to? The yoke controlling airspeed always works, because it’s a technically correct explanation.

Instructors, in my opinion, should strive to convey accurate information to their students. Telling them that the throttle sometimes controls airspeed and sometimes doesn't is confusing and incorrect, and will handicap their ability to absorb more advanced aerodynamic concepts.


In summary:

:

 

[Ian_J]

And this is why everyone should fly helicopters at some point. In one, that the yoke (cyclic) controls airspeed is fundamental from day one.

Anywho... great discussion here. I've learned a lot.

 

[tgrayson]

Anywho... great discussion here. I've learned a lot.

You have a very good attitude....I'd send a student your way in a heartbeat.

I'd love to learn to fly helos...that would motivate me to actually read the books I have on helicopter aerodynamics. Seems pretty pricey, though. Someday....

 

[nosehair]

I'd love to learn to fly helos...that would motivate me to actually read the books I have on helicopter aerodynamics. Someday....

Helos are fun...I taught helo instruments for the Army for awhile, too.

However, the same kind of misunderstanding abounds in helo flying, also.

It doesn't show up as much amongst helicopter pilots, because 99% of them are vfr only, and maintaing a specific approach airspeed is not in their mindset. But maintaining rotor RPM is !!

To a helicopter pilot Rotor RPM is the airplane pilot's equvilent of airspeed. The rotor is the wing. The speed of the rotor through the air is what provides the lift. What is keeping this speed? Power.

As the helicopter pilot approaches, he maintains rotor speed with power, and his approach path with collective, which is angle of attack of the rotor blade. Normally, he will reduce forward speed by coordinating cyclic and collective which is adjusting angle of attack of the rotor blades.

On instrument approaches, the forward airspeed is maintained the same speed throughout the approach until visual contact. This is most like an airplane, and the technique varies from one instructor to another, like in airplanes, but I found pitching to control VSI and powering to control forward airspeed and rotor RPM was best.

 

[tgrayson]

This is most like an airplane, and the technique varies from one instructor to another, like in airplanes

I was sorta thinking that ChinookDriver might have spoken too soon.

Anyway, I'm out of my element here. I'll let y'all discuss it, and I'll watch.

 

[FlyEDouglas]

Something that also needs to be considered is the aircraft type and type of approach. Perhaps the best answer to this entire debate is "it all depends."

I'm currently working with a student in an older model mooney based on a short grass strip. We actually covered pitch/power coordination and the "region of reverse command" in our last ground lesson... to apply them in the air in a few days. The approach and landing that this student will normally be performing is a combo short/soft field landing. 1.3 Vso(Vso=62mph) for the approach speed is 80mph (damn mooney for the mph instead of knots). Best glide for this airplanes is 104 mph. (Someone earlier in this thread stated that best glide is 1.3 Vso... we all have brain farts. In my experience it has tended to be between Vx and Vy, usually closer to Vy.) The field that the aircraft is based on provides between 2,700 and 2,100 feet of landing distance (depeding on which direction you land.) Therefore airspeed control is extremely critical when landing at this field. The student is not instrument rated (so no need for long-winded instrument approaches requiring a slightly faster speed.) And Vlo/Vle is 120mph (only 16mph above best glide.) There will not be many instances where this student will need to be on final going more than 104mph. Especially since this particular student is a creature of habit.

Sorry I became slightly long-winded there but the point of this post is that theories and techniques are wonderful. But it all depends on the situation, student, airplane, day, blah blah blah. Sure I'm going to teach my student the difference between being on the front and back side of the power curve, but I'm also going to emphasize that most of the landing approaches to be made will be performed on the back side of the power curve, and how to act accordingly if an approach is to be made above 104mph.

 

[Ian_J]

I was sorta thinking that ChinookDriver might have spoken too soon.

Nope, because...

As the helicopter pilot approaches, he maintains rotor speed with power, and his approach path with collective, which is angle of attack of the rotor blade. Normally, he will reduce forward speed by coordinating cyclic and collective which is adjusting angle of attack of the rotor blades.

When did you instruct at Rucker? Was it in the TH-55, 67 or UH-1? The reason I ask is because the only time I've had to manually control rotor RPM is during an auto rotation (TH67, OH58 and CH47) or during a hydraulics failure (TH67, OH58). In otherwards, thanks to the governor, the rotor rpm takes care of itself. (Unless of course you are doing something radical which exceeds the limits of the governor!)

On instrument approaches, the forward airspeed is maintained the same speed throughout the approach until visual contact. This is most like an airplane, and the technique varies from one instructor to another, like in airplanes, but I found pitching to control VSI and powering to control forward airspeed and rotor RPM was best.

I know I probably don't have the experience you do, but I've always found that cyclic for airspeed and collective for altitude works best, specifically because maintaining a certain pitch attitude will guarantee your airspeed will be close to where you want it. Saves time on the crosscheck (if we're talking instrument flight here.) Plus, it's a positive habit transfer to how you'd shoot a VFR approach. Not saying your wrong by any means, just stating my preference.

Maybe one day we can have a knockdown drag out argument over LTE, transverse flow, retreating blade stall, ETL, settling with power, or a myriad of other fun stuff!

And for all instructors learning from this thread, I refer you to my signature for all you really need to know.