Breaking the "elevator for altitude" habit

[jrh]

The angle of attack to lift curve is not linear at slow airspeeds, so theoretically there should be an very very high AoA that gives you the same lift that a very low AoA. The problem with diving and driving in on the approach is that you might not be able to slow down enough to land in some airplanes, thus negating the effectiveness of being on glideslope, depending on how high you are.

Hmmm...interesting point.

I think the physics support the diving and the driving approach most of the time...at least in most "draggier" aircraft, on calm wind days.

In a clean configuration, or with slicker aircraft in general, you might be right. It might take so much speed at the top end to equal the drag from the slow end that you'd have to exceed red line.

As a comparison, when I fly jumpers in a C-182 I descend by pitching for 160-170 mph, clean configuration, power slightly above idle. I come down at about 2500 fpm.

To equal the same angle through the air at 55 mph (one third the speed), I'd have to descend at a little over 800 fpm (one third the rate of descent)--since 182s love to fly, even at the extreme low end of the spectrum, I don't think I could get it to "mush" at 800 fpm.

However, throw wind in to the equation and things begin to look different. Say I have a 30 knot headwind during the descent. That drops the ground speed to nearly a hover, making the "slowly mush down" option look a lot more attractive.

Ahhh...as with most things in flying, I guess the answer is, "It depends."

 

[MidlifeFlyer]

Do you happen to have any info on how much drag is actually added to the aircraft by flying Vx?

I wasn't talking about flying Vx. I was talking about flying short field configuration and noting that the speed for that is pretty close to Vx.

My understanding is that the slower speed used on a short field approach is to minimize floating during the roundout and flare, not to add drag to the plane.

It also utilizes full flaps and the slower approach speed to steepen the descent.

I don't buy the "mush" idea and agree with your limitation on it as too close to a stall. Short field configuration and slips are normal maneuvers. If you really need to "mush" to get down, go around and try again.

Never mind the engineering. You can try it and see.

 

[tgrayson]

The angle of attack to lift curve is not linear at slow airspeeds, so theoretically there should be an very very high AoA that gives you the same lift that a very low AoA.

What's relevant is the drag curve, not the lift curve. However, the non-linearity of the lift curve is hinting that you are experiencing some separation on the airfoil, which will result in increasing drag. On the drag curve, you will see it deviating from its normal parabolic shape. While interesting, it doesn't really change the nature of the discussion.

Getting slow enough to having airflow separation begin seems unsafe, IMO, and probably still inferior to diving at high speeds.

The problem with diving and driving in on the approach is that you might not be able to slow down enough to land in some airplanes, thus negating the effectiveness of being on glideslope, depending on how high you are.

It's definitely possible, because you're losing more energy per mile than with any other technique. Energy = altitude = airspeed.

 

[ppragman]

What's relevant is the drag curve, not the lift curve. However, the non-linearity of the lift curve is hinting that you are experiencing some separation on the airfoil, which will result in increasing drag. On the drag curve, you will see it deviating from it's normal parabolic shape. While interesting, it doesn't really change the nature of the discussion.

Getting slow enough to having airflow separation begin seems unsafe, IMO, and probably still inferior to diving at high speeds.

It's definitely possible, because you're losing more energy per mile than with any other technique. Energy = altitude = airspeed.

If you're descending at 800fpm with airspeed high (== energy), how is that losing more energy than descending at 800fpm with low airspeed?

 

[jrh]

It also utilizes full flaps and the slower approach speed to steepen the descent.

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.

Never mind the engineering. You can try it and see.

Indeed, I have!

 

[jrh]

If you're descending at 800fpm with airspeed high (== energy), how is that losing more energy than descending at 800fpm with low airspeed?

It's the same, actually.

Your *total* energy (potential + kinetic) will be lower at the lower airspeed. However, your *rate of energy lost* will be the same, because you're losing potential energy at the same rate in either scenario, while your kinetic energy stays the same as well.

 

[tgrayson]

If you're descending at 800fpm with airspeed high (== energy), how is that losing more energy than descending at 800fpm with low airspeed?

The aircraft that accelerates to a high airspeed is actually converting one form of energy (altitude) to another (airspeed). The aircraft that slows to the stall is also converting one form of energy (airspeed) into another (altitude). Once the conversion is complete, neither aircraft is worse off than the other because they have the same total energy.

What matters after that is how much energy they lose per nautical mile. Assuming the same drag, both airplanes will descend at the same angle, but the faster airplane began its descent earlier. It will arrive at altitude further away from the runway than the slower airplane, but it'll be fast. By the time it reaches the point where the slower airplane reaches altitude, they'll be at the same altitude and same airspeed, since they started off with the same energy and burned it at the same rate.

 

[shdw]

With full flaps, the recommended glide speed in a C172 is about 60 knots; any speed slower (or faster) than that is steeper, by definition. Since we only have about 10 knots to spare on the slow side, you clearly have an ability to lose a lot more altitude by flying at the top of the white arc, rather than the bottom. In a dire situation, you might ought to be clean and fly at the top of the green arc. Combining any of the above with a max performance slip (with S-turns) can lose a phenomenal amount of altitude.

Unfortunately the guy that wrote stick and rudder and Mr Rich Stowell both disagree with you here and I tend to agree with them. The descent angles at 5 knots above stall and the top of the white arc in a 172RG are nearly identical, the only difference being at the slow speed you have nothing left when you are at your landing point, the fast speed is another story.

Both authors claim that that this is a more optimal way of loosing altitude if high on final. I won't argue any further though, but instead I will head on out to do some testing. Ill post numbers for rate of descent at 85 full flaps and 5 knots above stall with full flaps, the rest is trig. Hopefully I can do this tuesday, if not I have to wait till the weekend.

P.S. If anyone else decides to run the numbers here do it at altitude and be sure the aircraft has been stable for 10 or more seconds. I will try to get them for both a 172/152

 

[ppragman]

It's the same, actually.

Your *total* energy (potential + kinetic) will be lower at the lower airspeed. However, your *rate of energy lost* will be the same, because you're losing potential energy at the same rate in either scenario, while your kinetic energy stays the same as well.

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?

 

[tgrayson]

Ill post numbers for rate of descent at 85 full flaps and 5 knots above stall with full flaps,

Try a top of the green arc descent, too.

 

[shdw]

Try a top of the green arc descent, too.

Ok but I will do this one with out flaps in the interest of not ripping anything off, thats cool right?

 

[deadstick]

I am thoroughly confused. :banghead:

For the KE/PE argument (high on final), I am assuming one of the PE sources is the power available. So if the 800fpm descent is at a lower IAS, then that aircraft has more PE than the 800fpm/high IAS airplane. Right? The KE values differ due to the physical speed of the aircraft.

I use the "slow down to go down" technique and rationalize it this way: I need more time to get down to the proper approach path. Looking at the 800fpm scenarios (high IAS v. low IAS), I need to reduce my closure rate (lateral distance) to get the time to get down.

Thinking vectors and trig, if one side (vertical) of the triangle is 800 and the hypotenuse is the IAS, the angles cannot be the same if the opposite side meets the adjacent side (high/low speeds equal at 800) at a right angle. Clear as mud? Just like the 3 deg g/s correlation to ground speed. The faster you are going the faster you have to descend to maintain it. Now that angle is variable because the rate of descent is fixed.

 

[tgrayson]

Ok but I will do this one with out flaps in the interest of not ripping anything off, thats cool right?

But of course.

 

[shdw]

I am thoroughly confused. :banghead:

For the KE/PE argument (high on final), I am assuming one of the PE sources is the power available. So if the 800fpm descent is at a lower IAS, then that aircraft has more PE than the 800fpm/high IAS airplane. Right? The KE values differ due to the physical speed of the aircraft.

When discussing aircraft energy the engine is usually left out of the equation especially as most of the detailed discussions of KE/PE are related to gliders.

The bucket:
| . . . . |
| . . . . |


| . . . . |
|__ . __|

Edit: ignore the dots, they are there for spacing out the lines.

Draw something like this on a piece of paper. You will see there is spacing on the side of the bucket and this will be used. On the top draw a faucet and imagine that is power available. On the bottom is a drain hole, that is power required. Put a stick in the middle, like a joystick so that a pull back would angle the stick towards the right side of the bucket and on the right side write PE on the left side KE. On the right side hole where PE is, lift can be put as an increase of PE and sink as a decrease of PE. On the left side for KE headwind will decrease KE and tailwind will increase.

Tilting the stick back should cause water to fill on the PE side of the bucket and pushing it forward should result in water shifting to the KE side of the bucket. I find it a great visual tool for understanding energy.

I use the "slow down to go down" technique and rationalize it this way: I need more time to get down to the proper approach path. Looking at the 800fpm scenarios (high IAS v. low IAS), I need to reduce my closure rate (lateral distance) to get the time to get down.

Every airplane will be different and finding out which is best can be easily figured out with simple performance data.

Thinking vectors and trig, if one side (vertical) of the triangle is 800 and the hypotenuse is the IAS, the angles cannot be the same if the opposite side meets the adjacent side (high/low speeds equal at 800) at a right angle. Clear as mud? Just like the 3 deg g/s correlation to ground speed. The faster you are going the faster you have to descend to maintain it. Now that angle is variable because the rate of descent is fixed.

A diving aircraft at Vfe will likely have a faster descent rate than that just above stall speed. In my tests on the RG, not mathematical so I only take them with a grain of salt, which were done with about 10 knots headwind found that slowing up gave about the same descent angle as speeding up. Do this math with real numbers and then it will be clear as mud, right now it is made up so it is really pretty cloudy.

 

[tgrayson]

For the KE/PE argument (high on final), I am assuming one of the PE sources is the power available.

Nope. If you're trying to descend steeply, power is the last thing you want. Power adds energy.

So if the 800fpm descent is at a lower IAS, then that aircraft has more PE than the 800fpm/high IAS airplane.

There cannot be two 800 fpm aircraft. The faster aircraft must have a much higher descent rate if it is to achieve the same or greater descent angle, as you note with your trig comments.

 

[mikecweb]

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

 

[deadstick]

Nope. If you're trying to descend steeply, power is the last thing you want. Power adds energy.



There cannot be two 800 fpm aircraft. The faster aircraft must have a much higher descent rate if it is to achieve the same or greater descent angle, as you note with your trig comments.

I agree that it is the last thing you need, but it is available.

I agree with 2 different rates of descent. I couldn't figure out how somebody was maintaining 800 fpm in both scenarios.

I default to my original statement: I am thoroughly confused. :banghead:

Good discussion, though. Over complicating? Maybe (the bucket made my brain hurt), but chalk it up to a little Sunday night PD (Professional Development). The summer tv lineup stinks!

 

[splash]

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

fo sure.

just stall it to the runway and recover quickly.

 

[shdw]

fo sure.

just stall it to the runway and recover quickly.

Inverted makes the show much more spectacular.

The bucket is easy when it is drawn out right, hard to do it and try to explain it on here, much easier on paper.

Unless you fully understand how an aspect of flight occurs that you intend to teach you risk giving your student false information, so no I don't think this is over complicated in the least.

 

[ppragman]

I still don't see how any of this allows you to slow down enough to land if you fly in fast. Less say you have to go from 4000' to the a sealevel airport in 10miles. And let's say, for the sake of argument, that you're going down a box canyon, so you cannot do S-Turns, how do you slow down enough with the dive and drive method if you're that steep? You can always dive to lose altitude, what's the most efficient way is somewhat irrelevant in my eyes, how do you slow down if you've got to come screaming out of the sky like that? Slow down to go down works if you've got to come down in a short period of time. 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. I think we're arguing about different things here.