V_x and performance takeoffs - POH discrepancy

[cgzro]

This is all pablum to 99% of everyone reading this,
but 99% of everyone reading this will NOT instinctively
do the right thing in the heat of the moment.

I've seen this a few times in draggy low powered training biplanes.
Especially at take off where there is a tendency to fly off three point which puts you right at stall more or less and needs to be corrected pronto.

[iflyforpie]

It's not the stall that concerns me.

What I worry about is people inadvertently entering
the back side of the power curve.

I think the problem is not too many people know about the back side of the power curve other than slow flight demonstrations. Even some instructors on here can't define it by specific V speeds outside of level flight. Vx for example is already on the back side of the power curve.

[Colonel Sanders]

Vx ... is already on the back side of the power curve.

I always thought it was at the very bottom of the power curve,
where there was maximum excess thrust?

[iflyforpie]

It's close. In fact it is very near the bottom. At absolute altitude it and Vy, minimum sink, best penetration, and endurance are all equal.

I guess it is how you identify the power curve. The power curve is a curve of excess power, right? And since power is energy over time, and we add potential energy as we climb, would not the excess power correspond directly to climb rate? That makes Vy right at the bottom (or top... I like to think of power as a positive thing rather than a negative one) of the power curve and any speed slower than Vy on the back side of the power curve.

[Colonel Sanders]

I shouldn't mention this, but I do short field approaches
(for real) ever so slightly on the back side of the power
curve. Not 'way up it, but ..

Roll out on long final, aircraft configured (gear and flaps)
for landing. Start with 1.3 x Vso. Then, decrease the
airspeed 5 mph at a time. Say from 70, to 65, then to
60, etc. This obviously requires the ability to precisely
maintain an airspeed.

Anyways, keep reducing the airspeed by 5 mph increments
until you feel the aircraft sink ever so slightly. You are now
on the back side of the power curve. Increase the RPM to
maintain that airspeed, which is your short field approach
reference airspeed.

You are now at the perfect approach speed for a real short
field, regardless of any weight, flap, etc variations.

If the airspeed decreases below your short field approach
reference airspeed - eg decreasing headwind, thank you PDW -
apply full power and lower the nose and regain airspeed
and level off and overshoot.

The key to a good short field approach is to minimize
kinetic energy by precisely flying the appropriate, SAFE
short field approach speed. Locking up the brakes,
flat spotting the tires, etc is just lipstick on a pig.

I am lucky enough to have a parallel grass runway,
next to the paved runway. It's 1000 feet long, but
has a ditch at the end of it which gets your attention.

From practice, I know that I can safely put any light
aircraft down on it, if I have just a breathe of headwind
on the sock, and 60 mph on short final. No brakes
required, actually. They don't do a whole lot on grass,
anyways.

With a breath of headwind and 60 mph, it's actually
not very hard to get stopped inside of 500 feet, which
I think is pretty good for a real short field landing.

[photofly]

That makes Vy right at the bottom (or top... I like to think of power as a positive thing rather than a negative one) of the power curve and any speed slower than Vy on the back side of the power curve.

Indeed - Vx is where the power curve is tangent to the line through the origin, and Vy is where the power-curve is flat, on top.

[photofly]

That makes Vy right at the bottom (or top... I like to think of power as a positive thing rather than a negative one) of the power curve and any speed slower than Vy on the back side of the power curve.

Indeed - Vx is where the power curve is tangent to the line through the origin, and Vy is where the power-curve is flat, on top.

[Colonel Sanders]

I always thought Vx was found at the "minimum power required"
point - the very bottom of the power curve - and that Vy was
found at the tangent to the origin point, where the airframe was
at it's most efficient (max L/D). So:

For maximum power, these speeds are Vx and Vy.

For minimum power, they are max endurance and best range.

For zero power, they are max time glide, and best distance glide.


Note that best distance glide speed is of course affected by
prop windmilling and wind. For example, the C172M has a Vy
of 88 mph but a best distance glide speed of 80 mph to reduce
the penalty of the prop windmilling, which takes a lot of energy.
Ever tried to spin a prop at that RPM, by hand?

[photofly]

Colonel,

I believe you've mixed up your "power on" and "power off" curves (your diagram shows the "power off" curve).

It's more intuitive to make the vertical axis a measure of "Engine Power minus drag", i.e. the excess power which is (ideally) converted 100% to rate of climb. Points below the horizontal axis represent a descent, points above, a climb, and then climb or descent gradient can be measured by the angle through the origin.

Scan.jpg (38.12 KiB) Viewed 2162 times

What your diagram indicates (correctly) is that the best angle of glide is slightly faster than the glide speed for slowest descent.
But the power-on curve shows that the best rate of climb is slightly faster than the best angle of climb, which is indeed on the back side of the curve.

In real life as you add engine power the drag curve slides sideways (a bit) and changes shape because it depends on aircraft attitude to the airflow etc, so Vy isn't exactly the same as the glide speed for slowest descent. In fact for the 172M best glide (distance) is 74MPH which is the same as Vx. Vy is actually 90MPH - the curve shifts to the right from zero to full power.

Your graphical treatment of a headwind is correct, and your comments rightly refer to gliding. As far as climbing into a headwind goes, you should actually climb slower than Vx, i.e. more on the backside of the curve, to get the best angle.

[photofly]

Alternatively, if you prefer to consider the drag curve rather than the excess-power curve then full-power climbs and glide descents on the same graph would look like this. You can see that for climbs and glides the point through which you draw the tangent is different in each case.

Scan 2.jpg (47.13 KiB) Viewed 2161 times

What you can't show very easily this way is that the power curve moves sideways in real life.

[LousyFisherman]

The real problems is soft/short field takeoffs are not taught realistically. Obstacles are not even dealt with in the PPL other than as an abstraction. Like many (the majority) of PPL's these days all my training was done on paved runways.Maybe not absolutely useless but close.

The summer after I received my PPL I went into to Vulcan, CFX6, to try grass for the first time. 3000 ft paved, 2100 ft grass. 70 ft trees at one end of the grass. Airport is about 3600 ft, 25 degree day, the plane, C150, is about 200 pounds under gross

After learning to taxi on grass, guess I should bank the aerilons when I turn at the ends, I did some takeoffs away from the trees. Oh you don't hold the stick all the way back, you pull it back just enough to lift the nosewheel. Since I was getting off in about 1600 ft I thought okay lets try over the trees. There is an escape to the left of the trees if things go sideways.

So, for my first takeoff over the trees I messed up the run and aborted. Second one, I did the standard takeoff run and immediately maintained a Vx climb. I cleared the trees by 10-20 feet and increased my heart rate by 100%. I landed looked at my math again and couldn't figure out why I was not 40-50 feet over the trees. Since I had the escape I started experimenting.

Once I realized that I should stay in ground effect for 1 to 2 seconds after rotating before climbing at halfway between Vx-Vy it was as though the obstacle did not exist. From what I have gathered here, I believe I received relatively good training and yet I had to unlearn just about everything taught to me about short/soft/obstacled takeoffs.

The other danger at Vulcan is that even after you announce which runway you are using most other pilots in the area just assume you are using the paved one (16/34). It is very good for teaching you to keep an eye out if you don't want to run over in the circuit.

YMMV
LF

[iflyforpie]

Photofly's diagrams also illustrate what happens when you go slower than Vy or minimum sink, ie the back side of the power curve. Reductions in airspeed lead to reductions in excess energy and reductions in climb rate (or increases in descent rate).

Contrast that with the front side of the power curve where reductions in airspeed result in increased climb rate.

The really tricky part is where the extra kinetic energy goes any time airspeed is reduced. Even on the back side of the power curve, the kinetic energy will still cause the aircraft to momentarily climb... often giving false hope to those who pull back at low speeds and low altitudes.

Vx is a good speed to use, because by definition it gives us more altitude per distance even though it is on the back side of the power curve, but any slower than that serves no useful purpose and greatly increases risk. Like I said before, I like being on the top or front side of the power curve when faced with an obstacle and pulling up over it. If I need to use Vx, I probably shouldn't have taken off.

A good study of a 'back side of the power curve' accident is Pinnacle Airlines 3701. An empty CRJ-200 that the pilots decided to get up to FL410. Unfortunately they selected Vertical Speed instead of Mach on the Autopilot so the plane was on the back side of the power curve close to its absolute altitude. It took several minutes for the airspeed to decay to the point the aircraft stalled and fell from the sky, with repeated attempts by the pilots to pull back causing both engines to flame out.

[Cat Driver]

I you are about to depart on a long over ocean trip with a significant fuel over load in a twin engine airplane what would the best cruise altitude be efficiency and safety wise?

The fine points discussion regarding VY and VX are great but many variables can affect the outcome.

The flight altitude question above has less variables.

[photofly]

I you are about to depart on a long over ocean trip with a significant fuel over load in a twin engine airplane what would the best cruise altitude be efficiency and safety wise?

If you're Russian, then you use both engines to get into ground effect then shut one down and fly the entire journey using half your available engines and at 20' altitude.

Like these guys:

ek.jpg (23.78 KiB) Viewed 2048 times

If the engine that you're using fails immediately feather it and pull up into a Vy climb, then a min-rate glide descent giving you time to fire up the other engine and carry on. If you're Bob Hoover you can probably do this while pouring a cup of coffee at the same time.

[Cat Driver]

Your answer did not address the question photofly.

Here it is again.

I you are about to depart on a long over ocean trip with a significant fuel over load in a twin engine airplane what would the best cruise altitude be efficiency and safety wise?

[photofly]

I thought I did .... 20'

[Cat Driver]

20 feet is far to difficult to hold for many many hours, just like climbing at exactly VX is because it is really not practical to expect to be able to fly at exact speeds.

So why did you choose 20 feet as your answer?

[photofly]

It was approximately the cruising altitude of that Ekranoplan. I'm looking forward to hearing your answer to your own question.

[Trematode]

Photofly,

Re: your drag curve diagram.

Either you are mixed up or I am. At the bottom of the curve, you get max L:D, best powered range, and best glide (angle, not slowest descent). On your diagram you've got the best glide for min sink at the bottom of your curve -- this should be at a slower speed than max L:D, which gives you the greatest distance covered over the ground (angle).

[Cat Driver]

The answer is really quite simple.

If you fly in the so called " T effect " ( ground effect ) you will get better range for the fuel burn.

On flights of around fifteen hours I generally would choose T effect knowing it had the downside of extending my drift down flight time.....T effect in the PBY was at or under 100 feet above the water and is not at all difficult to maintain...plus it was not as boring as flying at high altitude.

On the South Atlantic route Africa to South America the drift down time frame was around eight hours, after that we could maintain on one......drift down time was also extended due to lower fuel burn in T effect.

[photofly]

You didn't specify a flying boat... But I was on the right track with the ekranoplan. Would it change your answer to be in a land plane?

[photofly]

Photofly,

Re: your drag curve diagram.

Either you are mixed up or I am. At the bottom of the curve, you get max L:D, best powered range, and best glide (angle, not slowest descent). On your diagram you've got the best glide for min sink at the bottom of your curve -- this should be at a slower speed than max L:D, which gives you the greatest distance covered over the ground (angle).

The axes of the graph correspond to vertical and horizontal speed so your climb or glide angle is the same as the angle of the line through the origin. Best l/d isn't at the bottom of the graph, it's where the tangent line is. The bottom of the graph is min-drag, and gives you slowest rate of descent, best time endurance, and best rate of climb, depending on power.

[Cat Driver]

You didn't specify a flying boat... But I was on the right track with the ekranoplan. Would it change your answer to be in a land plane?

No, aerodynamics and physics remain the same.

Ground effect is a function of wing span and height above a surface.


(( You may find me to be irritating and anal about flight training and I guess I am.....))

......owning a flight school made me the way I am, I just never could wrap my mind around the way so many instructors spend so much time on issues that in the real world of flying are seldom needed.

One morning I arrived at the office around eight in the morning and two instructors started briefing their students for dual X/countries.

The phone rang and it was one of the people I flew a Turbo Commander for, he was on the way to the airport and wanted to be dropped off in Sitka Alaska......it took me about five minutes to check the winds aloft and the weather, do a mental calculation of the flight time and file a flight plan.

We dropped the boss off in Sitka and flew right back to Nanaimo......the instructors had not left yet on their X/countries....WTF do they talk about for all that time?

[Trematode]

Photofly -- I am very confused.

The main discrepancy I see in your graphs is that the relationship between the speeds along the horizontal access is not in the proper order

In a propellor driven air craft the relationship between speeds should be as follows (slowest to fastest):

1. Minimum power required/max endurance/minimum sink glide
2. Vx
3. Minimum drag/max range/best glide
4. Vy

Your graphs show Vx as slowest, followed by Vy and Minimum sink together -- the two aren't directly correlated like this -- and then best glide

[photofly]

No, aerodynamics and physics remain the same.

Sure, I get that, I just wondered if the ability to set down on the water made you more confident about cruising low. If you have an issue in a land aircraft at 100ft you don't get a lot of thinking time and you can't float it out. Not saying, just asking.