Getting slow on final

[flyguy]

Reguardless, even if its not the exact definition of slow flight by the PTS, instructors should have their students practice flgiht at various airspeeds and various configurations from cruise all the way to slow flight. That would include approach and touchdown speed. And touchdown speed should be just above a stall - slow flight. And I never said always pitch for airspeed and always power for altitude, I said practice at the airspeed at which they will fly the approach and touchdown, so that they can learn the "proper control inputs required to maintain altitude and airspeed".

 

[tgrayson]

Is that last statement accurate?

No. Controlling airspeed never changes from the yoke. There are really two equivalent ways of defining region of reverse command:

1) Flight regime where more power is required to go slower, or
2) Flight regime where pulling on the yoke makes you go down rather than up (Or decreases your climb rate, etc.)

Isn't the region of reverse command any airspeed below L/D Max due to the increase in induced drag?

Depends on whether you're talking about the the drag curve or the power required curve. Most of the time, when we talk about the region of reverse command, we're talking about the power required curve. The point of minimum power is the point of least descent rate and is a slower speed than L/D Max. Theoretically, .76 * Vldmax.

If you're talking about the thrust required curve (drag curve), the point of least thrust required is L/D Max, and if you slow below that, your descent angle will increase. So there is effectively a region of reverse command here too.

I'm pretty sure normal final approach-to-landing speeds qualify, especially in the scenario that is the subject of this thread: "Getting slow on final."

Borderline for the drag curve (at proper approach speeds); check your POH for best glide with full flaps. And you'd probably need to get pretty close to the stall to get behind the power curve.

 

[tgrayson]

To demonstrate that AOA does control airspeed, consider the following:



We know that in unaccelerated flight, that

Lift = Weight​

​

This applies in level flight, steady climbs (angles < 15 degrees), steady descents (angles < 15 degrees). Given the lift formula,

L = (CL)qS = CL (1/2pV^2)S​

we can make the substitution of L = W, thusly

W = (CL)qS = CL (1/2pV^2)S​

Now we can solve for velocity

V = SQRT[2W/(p(CL)S)]​

(Yeah, I know that's hard to read. Best I can do with this technology.)

So we have a formula that gives us the airspeed of the aircraft. Which of the variables on the right side are controllable by the pilot?

• W = Weight? Nope, weight is fixed in the short-term, and not controllable by the pilot, unless fuel dumping is available.
• p (rho) = air density? Nope, fixed in the short-term, and not controllable by the pilot
• S = Wing area? Nope, not controllable by the pilot.

That leaves us with CL, which is the Coefficient of Lift, which is a direct function of Angle of Attack. We control the AOA by using the elevators.

Therefore, changes in AOA will produce changes in airspeed, because all the other variables are fixed.

 

[MSNFlyer]

I teach students to use both pitch and power for what they want in all flight conditions unless one variable is set. If your power is set--on takeoff for example--then pitch will set your airspeed. That way I am having them focus on a pitch attitude and then they can see what airspeed the aircraft settles on. I have found too many primary students will stare at the instruments if you try to make it too complicated.

Then we go out and do constant airspeed and then constant rate climbs and descents. This teaches them that you can vary pitch and power to get what you are looking for. Then I combine those into a 500 fpm descent at 90 kts with no flaps, 80 kts with 10, 70 kts with 20 etc. . .

After they get a better feeling for the way the aircraft reacts I have them do some pattern work and illustrate how you need to control the speed and descent rate. I have them get low and slow and then add power and pitch, then high and fast and reduce both then on glideslope and fast to reduce power and increase AOA and on GS and slow to increase power and reduce AOA.

Maybe it is just me but I don't believe in teaching primary students to ALWAYS do one thing or another. What happens if they are on final and an engine problem comes up? You can't really power up for altitude at that point can you?

 

[tgrayson]

Maybe it is just me but I don't believe in teaching primary students to ALWAYS do one thing or another. What happens if they are on final and an engine problem comes up? You can't really power up for altitude at that point can you?

Having only one proper response is a great asset, because it eliminates hesitation or confusion. For airspeed, this situation exists, because AOA always controls airspeed.

Altitude is slightly more complicated. It's excess thrust, not thrust alone, which controls altitude, and you change your excess thrust via the throttle or by changes in drag, such as with airspeed changes, flap settings, or gear extension. Even pilots with only a few hours experience intuitively understand this, so there's really no danger of someone becoming locked in thinking only the throttle works, unless his instructor tells him so.

I teach students to use both pitch and power for what they want in all flight conditions unless one variable is set. If your power is set--on takeoff for example--then pitch will set your airspeed.

It's true that you will end up adjusting both, but it's important to have a clear purpose in mind for each change you make. To say "when power is set, pitch controls your airspeed" is wrong on two counts. First, it's AOA that controls your airspeed, not pitch per se. And secondly, this doesn't change with the availability of power.

However, your pitch, which is the angle your longitudinal axis makes with the horizon, is determined by the quantity of excess thrust. With a fixed thrust setting, changing your AOA will change your drag and produce steeper or shallower climbs.

To tell a student that he must adjust pitch and power to get what he wants (and nothing more) is to say to them: "I don't really understand this relationship, you figure it out." It makes it a trial and error process. The goal of instruction is to help them interpret their observations and not let them form their own theories, which will probably be wrong.

 

[Dazzler]

To tell a student that he must adjust pitch and power to get what he wants (and nothing more) is to say to them: "I don't really understand this relationship, you figure it out." It makes it a trial and error process. The goal of instruction is to help them interpret their observations and not let them form their own theories, which will probably be wrong.

:yeahthat:

 

[MSNFlyer]

Maybe I am reading the last post incorrectly but are you saying that to control airspeed you ONLY set your angle of attack?

I completely agree in teaching the students the theory and physics behind altitude/airspeed but in this post the CFI was having trouble with students getting slow on final. You can have all the knowledge in the world but if you don't learn to recognize and "feel" if the aircraft is high, low, fast or slow you will have these problems. I was just suggesting he go out and get into these attitudes at altitude and have the student recover with both pitch and power to get a better feel for the airplane and how to control it.

 

[tgrayson]

you saying that to control airspeed you ONLY set your angle of attack?

Absolutely. A glider is no different in this respect from a C172 or a 747.

BUT: normally we have TWO requirements when we make an airspeed change. We often want to maintain our current altitude or descent rate or descent angle. This will require a thrust change, because the change in airspeed produces a change in drag.

If we're high and slow on final, reducing the AOA will fix both airspeed and descent angle...no thrust change needed.

If we're on glidepath, reducing the AOA will fix the airspeed problem, but we'll need a thrust change to stay on the glide path.

<<I was just suggesting he go out and get into these attitudes at altitude and have the student recover with both pitch and power to get a better feel for the airplane and how to control it.>>

And that's a fine thing, but you have to tell the student what to look for, rather than just say "make it happen."

I was with a new Commercial student once and I pointed out that he was 5 knots slow on final. He boosted the throttle a bit. The aircraft's glide path started to level out, but the airspeed didn't change. I pointed out again that he was slow and he boosted the throttle a little more. The glide path flattened out more, but the airspeed still didn't change. Then he noticed that we were very high and he pushed the yoke forward. FINALLY, the airspeed started to pick up.

Do you see what happened?

 

[MSNFlyer]

Looks like we are saying the same thing then. If this CFI students are getting slow on final they need to adjust both pitch and power depending on if they are slow and high, slow and low or slow and on glide slope. I believe in the original post just being slow was mentioned. I just wanted to emphasize that the student should be aware of exactly what your comm student should have known on final. You probably didn't think you needed to point that out to a comm student but that person obviously didn't have it engrained into his/her flying skills at that point.

 

[tgrayson]

Looks like we are saying the same thing then. If this CFI students are getting slow on final they need to adjust both pitch and power depending on if they are slow and high, slow and low or slow and on glide slope.

Yes. I just think an instructor needs to be more specific: push the yoke forward to gain airspeed, add thrust to maintain glideslope.

When I brought the issue to the attention of my student, he was a bit hostile. He thought the "what controls airspeed" was a matter of opinion, and that there wasn't a right answer.

Since he was an engineer, I went through the mathematical derivation of airspeed via the lift equation that I posted above. He shut up and never had a problem with the issue again.

As a side point (more to my student than to you), just because people argue about a particular issue doesn't mean that the answer is a matter of opinion, or that the truth lies somewhere in the middle.

 

[surreal1221]

Yes. I just think an instructor needs to be more specific: push the yoke forward to gain airspeed, add thrust to maintain glideslope.

Wow. . . that setence there has finally made it click in this primary student's mind. I understood why you pitched for airspeed, and power for altitude. . . but now. . . it makes sense, lol. I suppose it was the staying on the glideslope example that I needed.

thanks tgrayson!

 

[nightflyerDFW]

yeah, some CFI's get caught up in pitch & power but fail to give the student the bigger picture (ie. the glideslope). Oh well, off to find a new avatar.

 

[flyguy]

Since he was an engineer, I went through the mathematical derivation of airspeed via the lift equation that I posted above. He shut up and never had a problem with the issue again.

This is an excellent example of how a lesson should be tailored to the individual student. This type of explanation will work for some students and not for others. Excellent choice for explaining it to an engineer. Kudos! :nana2:

 

[E6BAV8R]

Remember that 'slow flight' is just above a stall. It is called 'area of reverse command' because at those low speeds pitch does control airspeed and power to the altitude. Reverse from normal, which is where you are on a normal approach. You are not in the area of reverse command so you pitch too the altitude or glide path angle and control the airspeed with throttle.

Is it me, or is this incorrect?

I guess I am failing to see how 'slow flight' is different from a "normal approach" to landing. Slow flight is still accomplished by using pitch for airspeed and power for altitude, as is a normal approach to landing.

Also in reverse command, pitch would have a direct effect on airspeed and rate of descent, correct?

 

[tgrayson]

Slow flight is still accomplished by using pitch for airspeed and power for altitude, as is a normal approach to landing.

Correct. (But substitute AOA for pitch, for bonus points. )

Also in reverse command, pitch would have a direct effect on airspeed and rate of descent, correct?

Yes. Except the effect of airspeed on rate of descent is opposite from "normal".

 

[Dazzler]

I'm waiting to hear opinions on Coanda and/or Magnus.
tgrayson? MSNflyer?

 

[tgrayson]

I'm waiting to hear opinions on Coanda and/or Magnus.
tgrayson? MSNflyer?

Maybe we need a Coanda section on this board?

 

[Dazzler]

Maybe we need a Coanda section on this board?

I just realised there's a Coanda thread elsewhere. Oops

 

[nosehair]

Is it me, or is this incorrect?

I guess I am failing to see how 'slow flight' is different from a "normal approach" to landing.

OK, I am out of the 'pitch vs. power' debate for now. I am addressing the 'slow flight' vs. 'normal approach' question here.

Generally speaking, we define 'slow flight' as being below best glide speed. Best glide speed is where lift and drag are equal. Increase lift with AoA (pitch) and drag increases more than lift, so the result is an increase in sink rate, so it is called 'area of reverse command'. When speed is above best glide, an increase in AoA (pitch) will increase lift with less increase in drag so the result is a decrease in sink rate.

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. When you are below best glide and want to decrease sink rate, add power (reverse command) to decrease sink rate.

Most normal apprach speeds are a little bit above best glide speed. You are not in the area of reverse command on a normal approach.

Best glide is normally at about 1.3 x stall speed. As an example, a Cessna 152 stalls at 35 with full flaps and at full weight. If you are less than full weight, the stall speed is less.

Using the 35 number x 1.3 = 45.5

If you are above this speed, you are not in the area of reverse command. You have sufficient speed to pitch up and gain altitude.

I see it in slow flight practice. Get a 152 slowed down to about 50 kts, and pitch up a little. It will climb. Get it down to 40 and pitch up a bit. It will not climb and may descend a bit. There is the area of reverse command. Not a big difference on little airplanes, but can be a big difference on large airplanes.

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, so even on a 'book' short field, we still have excess energy to fine tune the glideslope or apprach path with pitch.
Fine tune, I said. If you want to make little fine, fine adjustments with pitch, do it that way. If you want to make fine, fine adjustments with the throttle, do it that way. But even so, the power changes take a nano-second longer to cause the airplane's path to change than the pitch change does, therefore, the very finest approach path will be best controlled with pitch.

 

[MidlifeFlyer]

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

The published approach speed is in IAS. 1.3 Vs0 is based on CAS. I don't have a manual available, but take Vs0 in CAS, multiply that by 1.3. Then go to the CAS-IAS conversion table and see what the IAS should be. I think there's not as much of a buffer there, is there?