Paul Gleeson asks:
>Can anyone describe what a Lomchevak is. I have heard it several times and don't know what it is? <
That's easy! It's simply one of the most misunderstood families of maneuvers in the history of aerobatics. As solid evidence of this, I offer ALL of the previous posts to this thread!
If you REALLY want to understand Lomcovaks, find a copy of the book "Aerobatics" by the British champion aerobatics pilot Neil Williams, pub. by Airlife Publications, ISBN # 0 9504543 0 3 . Turn to page 189, the beginning of Chapter 22, "What is a lomcovak?" and begin reading. After you finish, go back to the beginning and read the entire book, you'll be glad you did. This is simply the best book on the general subject of aerobatics I've ever read!
A Lomcovak is NOT an inverted spin. It is NOT a non-precision maneuver. The pilot is NOT "just along for the ride". Recovery does NOT just occur at random without any control by the pilot. It is NOT a single maneuver, but rather an entire family of maneuvers, all of which are very precise and controlled from beginning to end IF they are properly done.
The key element in a Lomcovak is that the airplane's attitude and motion is controlled by four primary flight controls rather than the usual three. The fourth attitude control in this case is gyroscopic precession from the prop, controlled via the throttle. This is why it's extremely difficult (that's spelled i-m-p-o-s-s-i-b-l-e) to do a true Lomcovak with a model; in most cases our props simply aren't heavy enough to provide sufficent precession forces.
There are five main types of Lomcovaks. There are also variations within each type. All are performed under negative "G".
The one most folks are familiar with is the "Main" Lomcovak. This begins from an inverted climb. As airspeed decays to near zero, the pilot initiates something initially resembling an inverted snap roll, so that the aircraft has a rotation rate about the pitch, yaw and roll axes as it reaches zero speed. The aircraft performs three foward tumbles, each one at 45 degrees to the plane of the previous tumble. At the end of the third tumble, the aircraft recovers into a vertical dive.
The "Cap" Lomcovak begins like a hammerhead, but as the airplane rotates to the halfway point, with the fuselage horizontal and the wing vertical, a combination of gyroscopic precession (caused by the yawing rotation from the vertical entry) and down elevator cuases the airplane to perform a single forward tumble, with the plane of the tumble horizontal. As the plane returns to its initial position, throttle is closed and the aircraft yaws the rest of the way to a vertical downline for recovery. I haven't done a complete true Lomcovak myself, but I once did part of a positive "G" variation of this by accident in a DeHavilland Chipmunk, which is how I discovered that Chipmunks do not like to do hammerheads to the left! It's a really weird feeling the first time.
The "Positive Conic" and "Negative Conic" Lomcovaks describe a cone shape in the sky, with the airplane pointed upwards as it sweeps out the cone shape with its underside. The point of the cone is at the prop for the positive conic, and at the tail for the negative conic.
Finally, there is a version resembling the "Main" Lomcovak, but entered from knife edge rather than inverted flight. This one is particularly violent.
The pilot is near the center of rotation for most Lomcovaks. These maneuvers are very disorienting, but not generally too stressful in terms of "G" forces on the pilot. However, since Lomcovaks use the precession forces from the prop as one of the flight controls, as you might imagine, the forces on the prop, crankshaft, engine mounts and engine are extremely severe. The centrifugal forces on things like wing panels can also be surprisingly high, and usually totally different from what the engineers were thinking when the airframe was designed. It is prohibited in a number of aircraft, and results in severe life limits on the rotating components in a number of other aircraft.
Shortly after the maneuvers were invented by the Czechs, some of the top Russian pilots started trying them in their Yak 18's. Shortly after that, there were a series of prop, crankshaft and engine fractures on Yak 18's, including one where the entire engine was yanked off of the firewall by its roots! Right after that, the word went out from "upstairs" to the members of the Russian aerobatic team that anyone caught doing Lomcovaks in a Yak could expect his address to be changed to a gulag in Siberia IMMEDIATELY.
There are a number of other manuevers that also impose severe stress on props and engines. Snap rolls and flat spins are some of the worst. However, nothing can break a crankshaft quite so well as a good Lomcovak. One project I was involved in during my previous career (before I quit to go into the R/C model business full-time) was assisting development of a Kevlar-bladed prop for aerobatic aircraft that would have the strength to tolerate this sort of abuse, and low enough inertia to protect the engine as well. It was well received by the aerobatics enthusiasts, a number of whom had already been through the harrowing experience of landing an already sensitive and tailheavy aerobatic aircraft deadstick, with a few score pounds of aluminum missing from the nose.