Vol. 5 November 2007
Velocity's Quarterly Whenever-We-Get-Around-To-It Newsletter
   
 
This is a ListBox

Velocity University
Scott Baker

Velocity University
Duane Swing

Another Interesting Comparison
Duane Swing & Ken Baker

Twin Dreams
Duane Swing

Flight Training
John Abraham

Are You Clear on the 51% Rule?
Duane Swing

Service Center Updates
Duane Swing

 

Twin Dreams
Duane Swing

We have been talking a lot around here about the design of the Twin Velocity aircraft. While spending some time driving on a long trip, I begin to envision just what the IDEAL twin might be. Safety is of prime concern regardless of the number of engines, but in the twin I came down to only two configurations that made any sense. The ideal twin would have to be in the push-pull configuration like the Cessna Skymaster, or would have to have a canard.I never liked the Skymaster so let me concentrate on why a canard twin makes a lot of sense.

One of the more demanding tasks a conventional twin engine airplane presents to the pilot is proper procedures when one of the fans stop turning. One reason the twin engine airplane is expensive to insure is due to that ugly thing called Vmc. To us laymen, a simple explanation of Vmc is the speed, below which a twin engine airplane with one engine feathered and non-operational, with gear & flaps up, will lose directional control with the operating engine running full throttle. Obviously the twin will want to turn toward the inoperative engine. To stop this, one must apply enough rudder to keep the airplane flying straight and level. (actually we would normally bank the airplane into the good engine about 5 degrees to help stop this turning tendency) As the airplane slows down, more and more rudder is needed to hold directional control. As the mountain ahead gets closer, we try to out-climb the mountain by pulling back on the yoke and our speed bleeds down more until we either make a controlled crash into the mountain or stall the airplane which will almost certainly result in a snap roll. If, in a canard airplane, we can limit the minimum speed with the canard stalling before we run out of rudder and the main wing continues flying, then we have solved the problem of Vmc and of the snap-roll. Directional control is easier in my ideal twin due to the engines being placed as close to the centerline of the airplane as possible. The only thing that separates one prop from the other is the vertical tail. We still may fly into the mountain in a controlled crash but not due to a stall/spin.

Let me now take you through two likely scenarios that will demonstrate the appeal of my IDEAL twin. The first will be a normal cross-country with everything working normally. The second will be one of those "when all hell breaks loose" flights.

After our pre-flight we climb into the cockpit and notice we have no mixture levers or prop controls. Starting is by setting idle throttle, flipping on the switch labeled left engine, and push the button labeled "left start." Once the engine is running, we duplicate this procedure for the right engine. Actually in my ideal airplane we will have a battery for each engine and you can start both at the same time if you like. Both the engines are controlled by something similar to a 10 year old automobile computerized engine control system. Once all the gauges come to life we can start our taxi. With both doors closed, we turn on the climate control system and set the temperature at 72 degrees. Since we will be climbing to 19,000 on this trip, we set the pressure controller for 19,000 and let the automatic system take over. Once we're at the take-off ramp we notice a light comes on - first on the left engine saying READY FOR TAKEOFF. This is soon duplicated for the right side. No need to do a run-up as both engines are being monitored by our on board "brain" and any temperature, pressure, ignition error or system that isn't in the green will not allow the READY lights to illuminate. We have no prop control as they are also controlled by the brain and automatically set for take-off RPM. Power is now applied to the two turbocharged engines and the props are automatically in synch through the computer brain. Once clear of the runway and 100 knots is reached, the landing gear AUTOMATICALLY retracts. No gear switch to mess with if you don't want to. Power back to 85% by looking at the digital display before you. The prop will automatically select the proper pitch for best power climb. Once we are at our assigned altitude, power is again reduced to cruise setting and read on the digital display. Since the engine has an automically controlled mixture system based on CHT's, EGT's and other critical engine information, no need to worry about any of this. As a matter of fact, this information can be transmitted to a ground station for constant monitoring while you cruise along. Also displayed will be the range (endurance) based on the current winds aloft, fuel on board, fuel burn and a reserve. Once we near our destination, power comes back to whatever we like. Louvers in front of the engine nacelles automatically close as needed to maintain correct cylinder temperatures to prevent any shock cooling. Once in the pattern, power is reduced to slow to 120 knots where the landing gear will automatically extend and lock. As power is further reduced, the propellers will automatically re-set to take-off RPM should a go-around be called for. The touchdown is cushioned by the air/oleo main and nose gear systems. It is now time for your passengers to congratulate you on your superior flying ability.

The next flight is one of those "wish I would have stayed home" days. Soon after take-off and climbing through 4,000 feet, the left engine suddenly quits (In an IDEAL twin, this would never happen of course. But let's be realistic here.) In my ideal twin, the pilot would simply push both throttles forward to 100 % power. If there were mountains just ahead, an emergency switch on the control wheel (stick) would be activated that would allow the turbos to increase to 120% power. Since the engines are automatically controlled, mixture would be increased to provide the additional fuel needed for the higher power. A thrust sensor would automatically feather the dead engine and a yaw damper would add whatever rudder was necessary to keep the nose pointed in the right direction. Once level at cruise, power would be reduced to high power cruise, probably about 75% to 85%. My ideal twin does not have a fuel selector, just fuel level sensors. As fuel is burned by the operating engine, fuel from the unused tank is automatically pumped into the tank that is feeding the operating engine thus maintaining even fuel on both sides. As we continue on to a suitable airport to land, the airplane suddenly banks hard and descends a couple hundred feet. This was caused by a sensor attached to the autopilot that reads the transponder of an approaching airplane and automatically flies my ideal twin to avoid a mid-air collision. Once the landing is complete, it's time for the mechanic to find the cause of the engine failure.

At this point I need to educate you on the only statistically accurate data available on what is called catastrophic engine failure in piston engines. Robinson helicopters have a firm policy that at the 2000 hour hobbs time, the helicopter receives a complete make-over by the factory. Included is a factory new engine to replace the old engine. Robinson has kept very good records of the catastrophic engine failures and found that the time between these failures was actually less than the time between failures of the turbine powered helicopters. I don't remember the numbers anymore but I believe it was something like one failure in 100,000 hours of operation. Remember, however, Robinson always replaces the engines with factory new, not overhauled. This isn't to say an overhauled engine is not going to reach 2000 hours, only that SOME overhauls are just not up to the high standards needed.

Well, that's it!! Think my ideal twin is impossible? Actually, everything I have mentioned above is possible NOW. It would never get FAA approval in a certified airplane but in an experimental it is not only possible, but has been done before. Never in the same airplane but in different airplanes all of this has been done successfully. Can we do it in our new Twin Velocity? I would like to incorporate some of these ideas if time and money permit. As a matter of fact, some of the things mentioned would work equally well on a single.

-Duane