I have your Vx right here

[zaitcev]

I went up with an instructor to complete a WINGS activity dealing with takeoffs and landings, and boy, did we do all the dumb stuff I never got to do when I was a student. Back then, I was flying at a respectful distance away from the edges of the envelope. The best part was when he asked me to do a short field takeoff, and then I overcooked it by just a tiny bit, possibly by the altitude differential in Vx (field is at 5800 ft). I managed to climb to about 50 ft, but could not keep it together and settled back on the runway despite my best efforts. Good thing I didn't raise the gear. Woo-hoo. Very glad not to learn this lesson at an actual short field with obstacles.

 

[bbchien]

Practice is good, eh?

 

[Lawreston]

Short Field? A video:

Original Russian IL 76 fully loaded take-off I don't suppose "short field take off" is covered in the flight manual........... A fully loaded Russian IL-76 cargo plane........ payload: 1 million pounds!!!! Listen to the "controllers" in the tower who are Australian: Ya gotta love it "The Vodka Burner" as the Aussies call it, literally uses every inch of runway....... WATCH THE WHOLE VIDEO... (shot from the tower) You can HEAR THE CONVERSATION IN THE TOWER. They are incredulous -- that it makes it....

HR

 

[Jaybird180]

I went up with an instructor to complete a WINGS activity dealing with takeoffs and landings, and boy, did we do all the dumb stuff I never got to do when I was a student. Back then, I was flying at a respectful distance away from the edges of the envelope. The best part was when he asked me to do a short field takeoff, and then I overcooked it by just a tiny bit, possibly by the altitude differential in Vx (field is at 5800 ft). I managed to climb to about 50 ft, but could not keep it together and settled back on the runway despite my best efforts. Good thing I didn't raise the gear. Woo-hoo. Very glad not to learn this lesson at an actual short field with obstacles.

Can you re-explain this in english please? I don't understand why you couldn't depart unless the airplane's absolute ceiling was 5850 feet.

 

[Henning]

Can you re-explain this in english please? I don't understand why you couldn't depart unless the airplane's absolute ceiling was 5850 feet.

He over rotated it onto the backside of the power curve and Vx so far he couldn't climb without lowering the nose.

 

[Jaybird180]

So at 5800 MSL, there is no Vx because you don't have available power?

 

[alfadog]

Short Field? A video:

Original Russian IL 76 fully loaded take-off I don't suppose "short field take off" is covered in the flight manual........... A fully loaded Russian IL-76 cargo plane........ payload: 1 million pounds!!!! Listen to the "controllers" in the tower who are Australian: Ya gotta love it "The Vodka Burner" as the Aussies call it, literally uses every inch of runway....... WATCH THE WHOLE VIDEO... (shot from the tower) You can HEAR THE CONVERSATION IN THE TOWER. They are incredulous -- that it makes it....

HR

1. "The vodka-burner is rolling."
2. "We have Smirnoff."
1. "Don't do that, I'll shake the camera."

LOL

 

[airguy]

So at 5800 MSL, there is no Vx because you don't have available power?

If you get the airplane slower than Vx you are on the backside of the power curve - your drag is too high and increasing angle of attack will only stall the aircraft. The only way to fly out of the backside of the power curve is to push the nose down, or add more power. If you are already at full power you'll need JATO bottles.

 

[alfadog]

I think he means he did not use the correct Vx for his altitude. Though something does not sound right. Did you have it leaned correctly?

http://www.faa.gov/library/manuals/aviation/pilot_handbook/media/PHAK - Chapter 10.pdf

"An increase in altitude also will increase the power required and decrease the power available. Therefore, the climb performance of an aircraft diminishes with altitude. The speeds for maximum rate of climb, maximum angle of climb, and maximum and minimum level flight airspeeds vary with altitude. As altitude is increased, these various speeds finally converge at the absolute ceiling of the aircraft. At the absolute ceiling, there is no excess of power and only one speed will allow steady, level flight."

 

[Jaybird180]

Why would someone acciden-tentionally want to climb at Higher AoA than Vx?

What is the purpose of the exercise?

 

[alfadog]

Why would someone acciden-tentionally want to climb at Higher AoA than Vx?

What is the purpose of the exercise?

On his blog, he describes what may have been the same takeoff in a manner that makes more sense to me than what he wrote here

"Also, at some point I took off prematurely and, despite my best efforts, ended settling down on the runway. Good thing I did not settle into bushes beyond, and that gear was still down. The lesson is: never force an airplane to fly if she does not want to. That aside, I bounced gently and kept it in the ground effect to gain the missing 5 knots that allowed us to climb."

I really could not see a climbout at even sea-level Vx that made it to 50' not continuing, even if only at 100 fpm so I guess he was well below Vx.

 

[denverpilot]

Hey Pete. Don't pull so hard!

Back side of the power curve is why my STOL kit is great for *getting off the ground short* but it's all drag. You're utilizing the excess power of the mighty O-470 to drag yourself aloft.

Begin pulling at 39 knots indicated, lift off at 43(.5) knots indicated, clear obstacles and then *accelerate* to 65 knots. If you can't accelerate you're going to have a Bad Day(TM).

You're quite literally stuck in ground effect and looking for lower terrain to push the nose down and speed up. You probably will also need to have the seat cushion surgically removed from your ass.

If you then want maximum climb after departure (and not to burn up your cylinders!) Vy is at 77 knots at sea level, dropping to 74 knots at 10,000' DA.

Trash the engine at this point and you're landing straight ahead into whatever's in the way. Hopefully the lower speed will mean it won't hurt as much.

The relevant checklist pages are below.

Also note a STOL landing requires *power* to arrest sink rate. Elevator alone won't cut it. If a go-around is required, you need to accelerate back up to 65 knots from 43. Which means you need even more power than it took to arrest the sink rate.

Remember, push forward houses get bigger, pull back houses get smaller.

Pull back harder, houses get bigger faster.

 

[zaitcev]

What Mr. Alfadog said. Also, although the book Vx in Arrow is 85 mph, this airplane just would not climb at all until it hits about 90 (although it would if flaps are up), and coaxing it into accelerating that much with 2 notches takes a very long time. Yes, prop full forward and properly leaned.

Honestly I question the wisdom and propriety of the official short-field procedure (section 7). Piper suggest tucking gear up immediately and I noticed that getting rid of flaps instead helps way more. The manual also says "and continue climb while accelerating to best rate of climb speed, 100 MPH". But this is just nonsense: the airplane will not accelerate with flaps down as I demonstrated.

BTW, the runway 22 at KAEG is 7400 ft, elevation 5800 ft. We scheduled an afternoon on purpose to catch good crosswinds. My calculated DA was 7900 ft.

 

[Henning]

So at 5800 MSL, there is no Vx because you don't have available power?

No, there is always a Vx, although at a certain high & IIRC, immaterial to this discussion altitude, ( My recollection on this may be faulty and the issue may arise at certain high altitude airports) it either merges or swaps with Vy,; I forget the exact rules surrounding that event.

The problem is if you are nose high behind the power curve and below Vx as if you over rotate your way through a bounce out of ground effect and then unable to provide the energy required to keep flying at those levels of induced drag.

The best way to fly out of it is shove the nose down with full power applied and immediately retracting to flaps 20*-25* (pretty much you most 'lift' efficient flap setting, beyond that you start needing a lot more power applied to see any further gain from the extra lift provided by the extra camber and sometimes chord of the flap enhanced section of the wing as you apply further flap.)

This is not where I want to be if I am low on energy trying to claw my way through 10 kts gain of flying speed. In the 45 vertical feet which if I push hard on the nose and unload the wing getting rid of that huge burden of induced drag, I find that gravity alone will have accelerated me over 20 kts in the first second leaving one with the ability to then level out and enter a shallow climb. This can be helped or hindered by going 'positive rate-gear up' on take off by the design of the landing gear. Check your POH/AFM it will always contain the detailed configurations for best general performance available as shown during flight testing. That's part of what we pay for when we buy certificated airplanes, that data that was collected and the information and procedures developed from it.

 

[Mtns2Skies]

No, there is always a Vx, although at a certain high & IIRC, immaterial to this discussion altitude, ( My recollection on this may be faulty and the issue may arise at certain high altitude airports) it either merges or swaps with Vy,; I forget the exact rules surrounding that event.

As altitude increases Vx increases and Vy decreses. When they meet it is your absolute ceiling.

 

[Jaybird180]

As altitude increases Vx increases and Vy decreses. When they meet it is your absolute ceiling.

My DPE explained the math behind this but I was too saturated to catch it. Wanna give it a whirl?

 

[dtuuri]

My DPE explained the math behind this but I was too saturated to catch it. Wanna give it a whirl?

Imagine the power required vs. power available graph at sea level. Vx is tangent to the curve, drawn from the origin. [EDIT: Disregard, thinking of wrong graph. See next post.] At higher altitudes, the power available decreases, so the curve sinks like a setting sun, moving the tangential point up-speed toward Vy, which is shrinking in the opposite direction like a fish trying to survive in a tidal pool.

Don't need no math, just a vacation.

dtuuri

 

[Jaybird180]

A picture is worth 1000's of words
http://home.pcisys.net/~aghorash/Why_Vx_and_Vy_Change_With_Altitude.pdf

Thanks

 

[dtuuri]

A picture is worth 1000's of words
http://home.pcisys.net/~aghorash/Why_Vx_and_Vy_Change_With_Altitude.pdf

Thanks

You got quite a bargain: Four pictures and 1,412 words.

dtuuri

 

[rottydaddy]

Why would someone acciden-tentionally want to climb at Higher AoA than Vx?

I don't think anybody wants to; they just end up doing it.
I think the most likely and most common cause would be to simply pull back on the stick or yoke too much without realizing it, while being psyched out by the challenge of demonstrating their shortest roll and steepest climb. But I guess it's also possible to make a crazy leap of logic and faith and try to do something you are pretty sure the airplane wouldn't normally do.

"Overcooking" is a pretty accurate analogy, I think.

The purpose of trying to do a max-performance short-field takeoff is pretty obvious: to practice doing it for when you actually need to. But if you don't practice enough, you won't really know what the airplane wants to do down in the corners of the envelope at any given time, and it could ruin your demo.

 

[rottydaddy]

Trash the engine at this point and you're landing straight ahead into whatever's in the way. Hopefully the lower speed will mean it won't hurt as much.

That's the ultimate ace-in-the-hole for any true STOL airplane- even if you don't make it out, you'll do better than the faster guys.
Slow can be very good. I know that if I suddenly lose my tow in either glider I fly at the worst moment- like, at 100 feet with no more runway and nothing but trees and power lines within gliding distance- I can just soldier on ahead, and even maneuver a little, at less than 40mph and well under 200 fpm down.