Breaking the "elevator for altitude" habit

[tgrayson]

I still don't see how any of this allows you to slow down enough to land if you fly in fast.

You simple level off. As I pointed out in the previous post, the fast airplane reaches the target altitude further out from the landing site than the slow one, so it can fly level for the time it takes to bleed off the speed.

Whether it's possible to do so really depends on whether or not the drag during the descent was truly the same or greater than the slow airplane. If you come in fast, but not fast enough to produce drag equal to the slow speed, you'd end up floating forever.

The drag curve on the fast side is parabolic too, but it's relatively flat near Vldmax speeds. For this to work, you've obviously got to be on the steep portion of the curve, where small change in airspeed produce large increases in drag.

 

[BillPerryCFI]

But that is exactly the point I was trying to address--the slower approach speed does very little, if anything, to steepen the descent. The flaps add drag...the slower speed does not.

This one I will have to disagree with you on. In order to maintain the same aiming point on the runway, or the same glidepath, a slower speed requires a larger angle of attack. The larger angle of attack in turn causes an increase in induced drag. While this may temporarily cause a decrease in the rate of descent, the overall result is an increase in the rate of descent.

This can be easily proven by maintaining a single pitch attitude with a given power setting and then reducing the power to idle while maintaining the same pitch attitude. Watch the VSI and you will see the result.

 

[jrh]

I dunno, I tend to think that slow speed with flaps and gear out will be draggier than highspeed low AoA, thus resulting in more energy lost, or are you talking clean config?

In the scenario you gave, both aircraft were losing 800 fpm.

A loss of 800 fpm is just another way of expressing a loss in potential energy.

It's like asking, "A small oven is set to 400 degrees. A large oven is set to 400 degrees. Which oven is hotter?" Neither is hotter. By definition, they're the same.

Such is the case with two aircraft losing altitude at the same rate.

 

[jrh]

This one I will have to disagree with you on. In order to maintain the same aiming point on the runway, or the same glidepath, a slower speed requires a larger angle of attack. The larger angle of attack in turn causes an increase in induced drag. While this may temporarily cause a decrease in the rate of descent, the overall result is an increase in the rate of descent.

The lower airspeed will have a larger amount of induced drag, but greatly reduced form and parasite drag. Thus, total drag will be about the same, or even lower, than the higher speed. Total drag is what we care about.

This can be easily proven by maintaining a single pitch attitude with a given power setting and then reducing the power to idle while maintaining the same pitch attitude. Watch the VSI and you will see the result.

That's not an accurate illustration. Pitch attitude does not directly correspond with angle of attack. Nor does pitch attitude directly correspond with the flight path, especially at high angles of attack.

 

[jrh]

I know most of you in this thread are career CFIs but don't you think y'all overcomplicate this stuff?

I wouldn't go this deep with most primary students. But it's important for CFIs to have a solid understanding of a topic before they teach it. And although we're talking about a lot of hypotheticals here, everything ties back to the real world, where a misunderstanding can at the least result in some wasted time and embarrassment, or at the worst, bent metal and injuries.

Discussions like this are why I'm of the opinion that instructing is more challenging than 90% of the jobs where one simply has to show up and fly from Point A to Point B. No offense to freight dawgs or anything

 

[shdw]

The lower airspeed will have a larger amount of induced drag, but greatly reduced form and parasite drag. Thus, total drag will be about the same, or even lower, than the higher speed. Total drag is what we care about.

Just to add graphical representation and a piece on min sink:
http://images.absoluteastronomy.com/images/encyclopediaimages/d/dr/drag_curve_2.jpg

Parasite drag is a result of the square of airspeed, at slower speeds the change in parasite drag is minimal, look at the curve. Minimum sink speed occurs approximately 5 knots below best glide and that speed corresponds with the lowest point on the drag curve anything slower or faster than that is adding drag.

As you can see from that curve any speed increase from minimum drag will yield approximately the same drag as an equal decrease in speed from the minimum drag speed. For instance, if min sink was 55 then 65 and 45 would have approximately the same total drag. Would it be exactly the same? I don't remember the math behind this curve to know if it is the same, I would assume it can't be perfectly the same, but certainly close.

Edit: removed what tgray pointed out was wrong.

 

[tgrayson]

min sink (minimum drag)

Vmin sink != Vmin drag

 

[shdw]

Vmin sink != Vmin drag

Gah L/D max/BG/min total drag woops

Let me make sure I have this clear for the graphical representation off of a power required curve, min sink would be the minimum power required and L/D max occurs where the tangent line meets the power required curve?

 

[tgrayson]

min sink would be the minimum power required and L/D max occurs where the tangent line meets the power required curve

Bingo.

 

[dasleben]

It's difficult to break because it does work...on the front side of the power curve. But I really don't think you want to break the habit completely; if you made throttle changes for every altitude deviation on a turbulent day, the throttle would have a continuous back and forth movement, which would be annoying.

It can also be dangerous depending on the aircraft type.

 

[jynxyjoe]

I've got into the habit of flying small cessna's and pipers with just rudder and elevator trim, can you help me break that?

 

[shdw]

I've got into the habit of flying small cessna's and pipers with just rudder and elevator trim, can you help me break that?

Amputate your legs and epoxy the trim wheel.

 

[jynxyjoe]

Amputate your legs and epoxy the trim wheel.

You're not playing fair.

 

[splash]

I don't slow down on every approach, but when I need to lose altitude in a short distance that seems to work better than diving for a target shorter than the runway, pitching up and bleeding off airspeed, configuring, and landing.

me too....and that diving for a target shorter than the runway can lead the student to blowing pass the white arch with the flaps fully extended. I'd tell him/her to make a short field landing out of this one. Then tell the student, "If you plan on touching down in the middle of the runway then you may want to just go around instead". For the primary area student, I'd say this is a good title.

 

[shdw]

As promised here in a C172P, it had 40 degrees of flaps but I attempted to only have them at 30 (who knows if the 40 year old indicator was right though).

At 40 knots the groundspeed was 39.56 with a descent rate of 600 FPM. With no wind that is 910 ft/nm, with 10 knots = 1218 ft/nm, with 15 knots = 1466 ft/nm, and finally 20 knots = 1841 ft/nm.

At 50 knots the groundspeed was 49.45 with a descent rate of 750 FPM. With no wind that is 910 ft/nm, with 10 knots = 1141 ft/nm, with 15 knots = 1306 ft/nm, and finally 20 knots = 1528 ft/nm.

At 85 knots the groundspeed was 83.52 with a descent rate of 1600 FPM. With no wind that is 1149 ft/nm, with 10 knots = 1306 ft/nm, with 15 knots = 1401 ft/nm, and finally 20 knots = 1511 ft/nm.


Well there you have it folks, it is interesting to me that the descent rate at the slowest speed was actually slower than the fastest speed. My guess is I could probably get another 5 knots or so slower but I doubt I would use a speed that slow in a landing situation, these were all done at altitudes of 2000-5000 feet.

Each number was taken after a 15 - 20 second period of stable descending flight held VIA natural horizon pitch, not instruments. It is easily seen that with even a little headwind the rate of descents are about even and it likely wouldn't make a difference which you chose. With no wind the fastest speed is obviously the best and with strong winds slower speeds seem more effective.


Tgray, I have to go back up to try out the top of the green arc. I did it today and was pegged at 2000 FPM so I need to go up with a stopwatch to figure this out. The descent angle appeared to be 40+ degrees to achieve that descent rate. When I get a chance I will grab some more numbers from other aircraft and post them up.

Enjoy.

 

[tgrayson]

With no wind the fastest speed is obviously the best and with strong winds slower speeds seem more effective.

Well done. Are your low end speeds in CAS or IAS? (I'm guessing the latter, since 40 knots is below vs0.)

 

[shdw]

Well done. Are your low end speeds in CAS or IAS? (I'm guessing the latter, since 40 knots is below vs0.)

Yes, IAS sorry. I honestly didn't even think to pull up the CAS speeds for this. It likely won't make a difference though since each will probably drop 1 or 2 knots but I can check the POH at work tomorrow.

 

[tgrayson]

Yes, IAS sorry. I honestly didn't even think to pull up the CAS speeds for this. It likely won't make a difference though since each will probably drop 1 or 2 knots but I can check the POH at work tomorrow.

Actually, the speeds may go up almost 10 knots.

 

[shdw]

Actually, the speeds may go up almost 10 knots.

Right you are, not by such high numbers but here. This is for a 172R I do not have my 172P POH here with me.

40 IAS = 47 CAS
50 IAS = 53 CAS
85 IAS = 84 CAS

So that would slightly alter each calculation, the conclusion would still be true but the effective winds are probably closer to 20 knots and greater instead of 15+ from the original conclusion.

So revised conclusion for this aircraft, under 20 knots headwind faster is the same/better than slower for rate of descent, above 20 knots slower is better. Of course above 20 knots turbulence will probably kill you if your slow but we are pilots, particulars don't matter right?

 

[splash]

Interesting shdw. For topic reasons now what speed do you find it comfortable to touch down at and how much runway do you have left at touch down on a high airspeed vs. low airspeed approach? The 172 is a big floater IMO.