Departures: Vx, Vy, or Zoom

[Jay Honeck]

In another thread, there has been a lively discussion over whether a zoom climb to pattern altitude is as safe (however safe is defined) as the more normal Vy climb out.

Here is the plane:

An RV-8A with a 180 HP Lycoming O-360, and a Hartzell constant speed prop.

Temperature: 91 degrees.

Procedure: Start stopwatch at brake release. Record time as altimeter hits 1000'.

Results:

Vy Climb at 100 knots, from brake release to 1000' took 59 seconds.

Zoom Climb. Break ground and accelerate to 120 knots in ground effect, with a +1G pull, took 61 seconds to 1000'.

(I didn't try a 140 knot version of this pull up. That will wait till next time.)

Vx Climb at 75 knots. Within ten seconds, I aborted this departure when the deck angle exceeded 45 degrees up. The feeling of impending doom so close to the ground had Mary muttering sweet nothings to me in the intercom.

So there you have it -- a COMPLETELY unexpected result. A "normal" (in a lower performance plane) Vy departure felt so unsafe that we aborted and dropped the nose to Vx.

A Vx departure would be quicker over an obstacle, and may be as fast to 1000', but the discomfort and lack of forward visibility made this option too unsafe for our tastes. YMMV.

Note: Original post edited to correct Vx versus Vy error.



Sent from my Samsung Galaxy S3...

 

[Jim Logajan]

Vx Climb at 100 knots, from brake release to 1000' took 59 seconds.
[...]
Vy Climb at 75 knots.

You have these reversed. Vx (best angle) is always less than Vy (best rate). They become equal at the service altitude.

So you've done the experiment at Vy vs zoom. Vy wins.

 

[Jim Logajan]

So, he went out, did some tests and learned something and had the guts to post it here...

Agreed. I think he did great by performing the experiment. I should have posted that sentiment earlier.

I try to treat my knowledge of the world as always provisional. Sometimes mistakes in important things can last decades. For example, in 1921 zoologist Theophilus Painter published his count of human chromosome pairs - 24. Others confirmed his count. He was held in some regard, so it is possible that any other number might have been dismissed by the counter as an error. After all, perhaps if someone else counted only 23 pairs it was likely they simply missed a pair.

It was not until 1956 that the count was corrected to 23 pairs.

Perhaps some here can learn as well. How many believed the difference would be under 5% in time to altitude? Not really that significant...

What he hasn't yet determined is how far down the runway 1000 ft is reached for each method. This is good info to know as well. With his info better known, he can then determine his tradeoffs vs risk, etc... for any particular takeoff situation.

In theory he could get approximate values using an app on his tablet computer that records GPS track over time. He'd need something similar to the CloudAhoy app available for iPads.

The one textbook on my shelf that goes into the theory of estimating values like Vx and Vy is "Aerodynamics, Aeronautics, and Flight Mechanics" by McCormick (not a great text, but adequate for my purposes when I first bought it years ago.)