Sujoy Fri Oct 18, 2013 6:28 am
Deep Throat wrote:QUESTION : Why did Russia choose NOT to develop a Vertical Take Off version of the PAKFA ? They could have used this on the MISTRAL LPH as well ?
The number one problem in the aeronautical sense, with VTOL, is you’re trying to have it all, and in aviation, everything is a tradeoff.
Bear in mind that more often than not the VTOL aircrafts are actually used in STOL mode with thrust only partially vectored down and lift provided partially by aerodynamical lift and partially by downward thrust . Why ? Because for any VTOL design, weight is a critical consideration. The plane will lift only if the thrust-to-weight ratio is above 1 . Thus with a full fuel and weapons load, the plane is too heavy to lift.
Contrary to popular perception VTOL aircrafts behave differently from a helicopter. Unlike helos , torque is absent for a jet engine. The aircraft does not in itself develop a tendency to yaw when lifting off the ground. However, the roll stability is dramatically different in hover flight. One can think of a helicopter as the mass of the helicopter body hanging underneath the lifting rotor. Thus, when the body of the helicopter starts to roll, it has a tendency to swing like a pendulum underneath the rotor, but the roll doesn’t grow by itself. In contrast, the VTOL aircraft is a mass balanced upon a column of lifting thrust, so any roll tendency will not lead to a pendulum motion but will be self-reinforcing, and if it is not corrected will lead to an unstable condition.