Thursday, October 23, 2008

Going in the Right Direction

The cover of the November, 2008 Popular Science magazine features a proposed design for an four passenger tilt wing vertical take off and landing (VTOL) aircraft. The design has unique features, including the use of an electric/piston engine hybrid power system, which leads to a lightweight craft. The story can also be seen on the Popular Science website.

I applaud all attempts to create vertical take off and landing airplanes for trying to take general aviation in the right direction. Airplanes should take and and land in the minimum amount of space, and runways and airports should be only for the really large jets. However, I do have some issues with this design by a British company called Falx. Many of these remarks are similar to those I made about the V-22 Osprey in other blog entries.

  • I believe that tilt wing (and tilt rotor) have a design flaw in that one of the propellers can get caught in its own turbulence as is comes in for a landing. This is known as vortex ring state.
  • The laterally disposed rotors present an asymmetrical lift situation, and any unequal lift from one or the other propellers can cause severe roll moments. It is essential to have centerline thrust
  • In total power failure or “running out of gas“, a tilt wing aircraft is a free falling body. It cannot use its wing for gliding flight to non disastrous landing, because the large propellers will impact the ground on landing and crash the craft. Again, neither can it autorotate its propellers like a helicopter, allowing a hard but survivable landing.
  • I believe the Falx tilt wing machine is grossly under-powered. A four place helicopter can lift 4 passengers with 150 h.p., because of it 40 foot diameter rotor. Thrust efficiency of rotors is directly proportional to the swept area of the rotors or propellers. To lift four people with two puny single rotating propellers of the tilt wing will require more than 1000 h.p. It is inconceivable that batteries and the proposed 104 HP engine can muster that power and be light enough to fly horizontally, long take off, much less vertically, or VTOL.
I'm not saying any of this to deter the Falx people or any other aviation inventors. I think it is good that others are pursuing the "Holy Grail" of general aviation, which is adding vertical flight to a fixed wing aircraft. I have spent many years - 55 of them in fact - exploring VTOL technology, and I really do think that the elements of my Arc Wing VTOL airplane are important to the proper design of any aircraft that will take off and land vertically. These elements include center line thrust, large dual-rotating propellers, the deflected slipstream approach to vertical flight, and the Arc Wing, which has unique lift. More about this craft can bee seen at our website and other blog entries.

In the 1940s and 50s, a lot of government sponsored research was performed on all sorts of approaches to vertical flight. In those pre space race days, innovation in aviation, including VTOL, received a lot of attention and funding. Many of the VTOL designs of those days had issues that limited their effectiveness, and others petered out when aviation funding was greatly decreased. Those extent of the approaches to VTOL can be seen in Micheal Hirschberg's exceptionally comprehensive VTOL wheel of misfortune.

No comments: