Flying The Jet Exec

I spent the last several days providing flight instruction to two students in PA in a new Jet Exec built by Steve Walton. Steve has built 2 Jet Execs so far and I have had the privilege of providing the initial and subsequent flight training to the proud new owners in each of them.

One of the first things that I discovered about the flight characteristics of the of the Jet Exec is that the original rigging must be changed to account for the different weight and CG of the helicopter. If one is converting an existing Rotorway that has previously flown, the rigging may be quite different following the JE conversion.

Normally when I instruct in a new helicopter we remove all of the panels and covers off the helicopter from the fuel tanks back to the tail. This allows us easy viewing access to monitor for oil and coolant leaks, belt tensions, and any needed adjustments. Since this Jet Exec conversion had only 5 hours on it we removed all of the cowling from the rear portion of the engine compartment as shown in the photo.

The hover testing went well and we needed to only make a few rigging adjustments to get the helicopter airworthy. Jake buckled into the pilots position and I occupied the flight instructor seat.
I found that the following sequence worked best for starting the turbine. After the normal control checks were completed and with the throttle in the fully closed position, we turned on the master and instrument switches, turned the starter lockout key to the start position, and after announcing “CLEAR”, with one hand on the governor switch (off position) engaged the starter. At around 11% rpm the burner ignites with a low rumble and when that sound is heard the governor switch is engaged.

The pilot keeps his fingers on the governor switch throughout the start sequence so that it can be rapidly switched to the off position should a hot start occur. It is crucial to monitor the turbine outlet temperature(TOT) gauge during the start procedure because this gauge will indicate that a hot start is occurring by a rapid rise in the TOT. If a hot start is allowed to continue the turbine can burn itself up in a matter of seconds.

On Jake’s ship we found that it worked best if the starter button is held in the engaged position up to around 56% rpm before releasing the button. Idle on this particular engine was set at 52% and if we released the starter button near that speed, the governor would give a surge of fuel and we would hear a very unsettling sound that experts have explained as compressor stall. If we allowed the starter to spool the turbine up to 56% prior to releasing it, the governor would slightly reduce the fuel input and the engine would settle into a very smooth idle. The engine that most are using on the Jet Exec does not have a bleed air line to help relieve the excessive back pressure on the compressor turbine if too much fuel is introduced into the burner can at one time so advancing the throttle must be slow and smooth.

After the normal checks the throttle was brought up to 102% and then rolled off enough to get needle separation as we checked the sprague clutch for disengagement. If the throttle was rolled back to 102% too quickly we would get that annoying bark from the engine as the governor would dump too much fuel into the burner and the compressor would again experience compressor stall. This is one area that I feel needs to be addressed by the Jet Exec community. More on that later in this article.

Once the engine was running smoothly at 102% it easily picked us up into a 2 foot hover. The power of these little turbines is surprising! I am sure that it is capable of performing a vertical takeoff to 100 ft. AGL with no effort but that is not a safe or recommended maneuver in any experimental helicopter so we did not test that theory. The Jet Exec has very good tail rotor authority compared to it’s piston powered lineage. (Now several years later I experienced a control lockup covered under HINTS AND TIPS section of this web site. The Jet Exec that it happened on went straight up at least 100 feet before I was able to reduce the throttle enough to settle it back to the surface. This was not a fault with the Jet Exec design, see the write up for a full explanation).

After several hours of hovering trials, we decided that it was time to go to altitude and see how Jake's Jet Exec handled in forward flight. We were in for a surprise!

I discovered that the forward flight aerodynamics of the Jet Exec without it’s cowlings installed were extremely unfavorable, to put it mildly. The Jet Exec needs the cowling on during forward flight or the helicopter encounters dramatic porpoising and yaw oscillations that can really get your attention and under gusty conditions could be very dangerous. Take a look at the photo of Jake’s ship (under GRADUATES) and notice how much engine and transmission are hanging up there in the helicopters slip stream. There is a lot of surface for the wind to interact with, and interact it did.

The airport in PA that we were training at is on a knoll bordered by canyons on each side. Since the wind was a 90 degree crosswind to the traffic pattern we experienced dramatic up drafts as well as down drafts during our flight around the pattern. The winds were gusty and flying the pattern initially required abundant fore and aft cyclic movement to keep the helicopter from pitching up or diving. Remembering that unloading your main rotor system and experiencing a low G condition can be deadly, it took a lot of discipline to s-l-o-w-l-y and smoothly apply opposite cyclic to keep us upright.
When an updraft pitches the helicopter nose up 60 degrees the first impulse (especially for a fixed wing pilot) is to rapidly push forward on the cyclic to bring the nose level. Flight training and discipline teach us to slowly and smoothly apply enough forward cyclic to regain pitch control without unloading the rotor system. Remember that the helicopter is most stable in an autorotation so if you experience dramatic gusts that cause you to question the safety of the flight, slow down to the best maneuvering speed, use smooth control inputs, and enter an auto and land if you feel that the safe outcome of the flight is in question. Unloading the semi-rigid rotor on the Rotorway helicopter can be fatal. You need to get the proper training to avoid becoming a victim of Low G.

After experiencing that dramatic nose up and then nose down pitching, we thought it prudent to land and reassess the situation. We decided to install all of the cowling and take it up again to see if that made a difference. The next flight was perfect. The gusts no longer threw the little helicopter into aerial gymnastics and it just flew like a dream. I now know that the cowling makes a dramatic improvement in the stability of the helicopter and the Jet Exec should not be flown without it installed. There are several after-market engine cowls available that should improve the aerodynamics even more.

The wind that day was stronger than we prefer for flight training but the Jet Exec has abundant power to handle stronger winds due to the dramatic increase of available power and tail rotor authority. We made hovering pedal turns both to the right and left in winds gusting to 25mph without ever losing main rotor RPM or tail rotor authority. In fact, we only used about 50% of the available anti-torque pedal travel in right turns into the wind.

This doesn’t mean that if you have a Jet Exec you should go out into a heavy or gusty wind and start doing pedal turns. Flight experience in any helicopter is your safety margin. If you have little experience, be very conservative in determining your personal flight envelope. This is why flight instructors limit the students flights to conditions that are safely within the students abilities at the time. As the student gains experience, the instructor will increase the flight envelope that the student can safely operate his helicopter within.

That being said, while we were hovering back to the hanger at the conclusion of our flight training, a gust of wind hit the helicopter from the right side with such force that we experienced VRS (Vortex Ring State) of the tail rotor. VRS is what the FAA now calls what helicopter pilots know as Settling with Power. I had never experienced tail rotor VRS before, nor do I plan on doing so in the future.

The main rotor on the Rotorway helicopters turns to the right as viewed from the pilots position. Remember that for every action there is an equal and opposite reaction. The torque of the engine and drive train cause the nose of the helicopter to turn to the left. To counteract this torque we apply right anti-torque pedal which applies thrust to the left at the end of the tail boom by changing the tail rotor pitch. To produce the tail rotor thrust to the left the tail rotor side wash is to the right. One great thing about the Jet Exec is that it has enough power that we never lost any main rotor RPM at anytime during our flights. There is enough power available to get you out of trouble, should you need it.

As we hovered past a large hanger a very strong gust of wind hit the side of the helicopter sending the tail rotor side wash back into the tail rotor. Once this happened we momentarily lost all anti-torque control and the nose of the helicopter rapidly yawed to the left until we had turned about 90 degrees. At that point I was able to stop the unanticipated yaw of the helicopter and continue back to the hanger. Always be ready for the unexpected because when it happens, you don’t have time to think about it, only to react. We had decided that the winds were strong enough that we would not continue altitude flight that day, we knew that the lee side of the hangers would most likely be turbulent and were expecting turbulence. We were not anticipating tail rotor VRS but when it happened, we were able to handle it due to flight experience. The more you fly your helicopter the better you become at it as long as you had proper initial training so that you did not develop any bad flying habits.

After instructing in these two beautiful Jet Execs I have developed some opinions and concerns that need to be addressed by those that are going to be flying them. My first concern is the tendency of the engine to go into compressor stall, if that is what it is actually doing. I am not a turbine expert and I am sure that others with much more experience will weigh in on this issue. I am bringing this up from the perspective of my experience to hopefully generate dialog and a solution to what I perceive as a potential problem.

During the engine run up the throttle must be advanced slowly. With each increase of the throttle the governor increases the amount of fuel entering the burner chamber. If the throttle is advanced a bit too fast the engine falters and “barks” very loudly indicating that something is not right. When I brought this issue up on the msn groups rotorway fun web site, several fellows with high turbine time replied that the turbine had experienced “compressor stall”. They also explained that most turbines have a bleed air line that allows excessive pressures that build up in the combustion chamber to bleed off so that the compressor will not stall against that back pressure. The T62 does not have this feature as it was designed to be operated at one continuous rpm under constant load, I am told.

In the Jet Exec the T62 runs very well at the designed flight rpm. It is when the throttle input is changed that the governor can supply too much fuel for the combustion chamber to handle. This brings up a concern for practicing autos in the Jet Exec. When the auto is entered the throttle is rolled off slightly to allow for a small needle separation to insure that the sprague has disengaged. I noticed that when this occurred the Jet Exec’s engine governor would hunt for the proper rpm causing the nose of the helicopter to yaw right-left-right as we descended.

At the bottom of the auto, you initiate your flare, level out and roll the throttle back on. Will the governor apply too much fuel to the combustion chamber? Will the compressor stall just when you need the engine to produce sufficient power to maintain a hover? With the engine and transmission up high on the frame the center of mass is higher on the Jet Exec than on the standard Exec. The standard Exec already has a tendency to pitch forward and experience a dynamic rollover if a touchdown auto is not executed perfectly. It would seem that the Jet Exec would be even more unforgiving should the engine not come back on line when you need it.

I am sure that these issues are being addressed and solutions will be found as proper procedures are developed for operation of the Jet Exec. Just be sure that you invest the money on an engine cowl and don’t fly at altitude without all of the cowling installed.

Orv Neisingh Helicopter CFI