Vx vs. Vy

[Bustamove]

Axioms work

We've all probably heard it before, but there are three useless things to a pilot:

- Fuel left in the truck
- Runway behind you
- Altitude above you

IMHO once I'm in a steady climb any usable runway is already behind me so I'll try to shift as much altitude as I can from above me to below me. That means Vy once clear of obstacles.

Just a repackage of the excellent posts above.

 

[tarp]

Timp - good one.

Eric - I wanted to take your understanding just a little further.

Vx to Excess thrust - you have that concept. What is the thrust overcoming? The usual suspect = drag. What kind of drag? Well we haven't changed the "smoothness" of the airplane, so it must be induced drag. We are denying our powerplant high quality dense air to produce power so technically we are losing some "excess" thrust. To try to arrest the amount of drag, we can lower the angle of attack on the wings. If we lower the angle of attack, we decrease the amount of induced drag and increase airspeed, but we also reduce our climb. ANA refers to the curve shift because you are changing the dynamics by changing the angle of attack. The bad thing is that we have to keep lowering the nose to try to provide the "excess" thrust buffer.

Vy to Excess power - power overcomes gravity over a function of time. With Vy we are trying to achieve greatest climb rate as fast as possible. Again the airplane runs out of power with altitude. At sea level, we have the best power to weight ratio. As we climb, we are going to have to lift the nose using the airfoil to make up for loss of "excess" power but we can only go so far before we begin to see a less than maximum rate of climb. The final bad thing is the advent of a stall - that is you can't keep pulling the nose up indefinitely. Sooner or later you are going to have to lower the nose to keep from stalling.

Ergo, Vx and Vy are coming together and the plane can no longer physically climb. Try to change either factor and you either stall and lose altitude or push the nose down and lose altitude. Absolute ceiling defined.

 

[ksu_aviator]

Another reason to climb at Vy that I didn't see yet, is to get enough altitude to return to the airport.

I was taught, that if you get to 800 ft you can easily turn into the wind and make the runway. Of course, with practice you can do it at 500 or 400 feet. I personally use 500.

Some tips:
1. Always turn into the crosswind (helps to keep you over the runway)

2. Prebrief the return...My brief is "If we have an engine fire or failure with remaining runway we will return to the runway. If its less than 500 ft than we will land straight ahead or within 30 deg. of the runway. If we are over 500 ft then we will make a right (left) turn and return to the runwa."

3. If the engine fails immediately put in full flaps, pitch down for your best glide speed with flaps and bank 30-45 deg. Be carefull to have enough airspeed as to not stall.

This is a safe manuever if you have practiced it a few times...start by performing the manuever at a safe altitude in the practice area, just do descending right and left turns with full flaps and at 45 deg AOB. Note your altitude loss. Then move back to the airport and start by "failing" the engine at 1000 ft straight off the runway. Then 800, 700 and on down to 500 feet. Don't go any lower than you are comfortable with. And don't do the maneuver in real life at an altitude that you couldn't easily do it at in practice.

 

[172driver]

Timp and Tarp,

Thank you... well explained. I understood Vy decreasing and absolute ceiling but not Vx increasing. Your explanations were great. Thanks for taking the time.

 

[Eric]

Thanks

Thanks guys for the explanations. I'm a graph and numbers kind of guy. Maybe if I walk around talking to myself with my eyes gazed skywards a little longer, it will hit me.

This link also helped http://142.26.194.131/aerodynamics1/Performance/Page10.html

 

[Checks]

"3. If the engine fails immediately put in full flaps, pitch down for your best glide speed with flaps and bank 30-45 deg. Be carefull to have enough airspeed as to not stall."

KSU,

Could you explain your reasoning for this? I personally wouldnt put in full flaps unless I had the landing point made. Full flaps is going to just add a lot of drag.

 

[ksu_aviator]

With full flaps you slow down the aircraft and shorten your turning radius so that it is easier to make the tight turn to the runway.

 

[Flightjock30]

This Vx vs. Vy debate is a coincidence because I received the July 2002 issue for AOPA Pilot magazine and Barry Schiff wrote an article about the decision of turning back to a runway after an engine failure in a single. He is qouted as saying "Although initial climb at the best-angle-of-climb airspeed (Vx) results in more altitude over the departure end of the runway than when using the best-rate-of-climb airspeed (Vy), pilots should recognize that an engine failure and delayed action at Vx result in a more rapid speed bleed that places the aircraft in greater danger of stalling. Furthermore, the transition from such a nose-high attitude to a gliding attitude requires lowering the nose aggressively, an action that to initially fill the windshield with rapidly rising terrain. This can startle even those prepared for such a low altitude phenomenon. Although a climb at Vy can reduce the likelihood of a return to the runway, the additional airspeed it provides might be more desirable."

As for my two cents. Even if you have enough altitude to turn around and make it back to the runway have you considered another aircraft that is just lifting off behind you? As you roll out of a turn facing the departure runway downwind you may be face to face with another plane that just lifted off. He won't be able to see you at a climbing pitch attitude most likely. At a busy airport this could happen and you may not have time to inform ATC of your failure! So of course the use of good aeronautical decision making is very important in a return back to the airport after a power loss.

 

[172driver]

A CFI at our school just got put on probation for making a low altitude return to the airport. He was @ 400'AGL, partial power loss to 1200 RPM, C172, nothing but trees ahead. He made the runway, actually overshot by 5 ft into the grass before stopping, nothing damaged.

We always brief landing straight ahead below 1000' thus the disciplinary action. The point is...those in authority frown upon low altitude returns in any scenario. If something is broken or injured, you are in big trouble.

He did a good job getting back. Who knows what would've happened had he elected to land in the trees. However, it could've been one of those all too common stall, spin, crash, die accidents. My two cents...perform it as you brief it.

 

[dmspilot00]

Sometimes climbing at Vx can be good insofar as getting through an initial cloud layer, low lying band of weather (beware icing)etc after takeoff, at least this is what I was taught once upon a time.

I don't see how that is possible, when it will take you a lot longer to climb above a cloud layer (or to any altitude) at Vx. Vy is the speed that gives you the fastest rate of climb.