By Vic Syracuse, EAA Lifetime 180848
This piece originally ran in Vic’s Checkpoints column in the September 2023 issue of EAA Sport Aviation magazine.
This month I’ll get back to the flight testing of the Hummingbird. Yes, the 40 hours required by the Phase I operating limitations have been completed, but I am learning that properly flight testing a helicopter is a whole different ballgame than flight testing an airplane.
Most of the flight testing of other aircraft I have built has been uneventful, except for the Prescott Pusher. After completion of Phase I, it was off to the races, going places, and having fun. Helicopters are much more complicated with lots of moving parts that need to be measured, tweaked, and adjusted to play nice with each other.
It’s still a lot of fun, and I remind myself that I built one to learn something new, and it is doing that quite well. Support from Vertical Aviation has been stellar and made it enjoyable as well.
The only things required on a fixed-wing aircraft propeller are to use the proper torque on the mounting bolts and check the blade track at the tips. Balancing the propeller is a nice touch and usually makes for a substantially smoother ride but is optional.
Helicopter blades are at the opposite end of the spectrum. While propeller blades stay in a fixed-pitch position throughout a revolution (yes, even constant-speed ones), helicopter blades are continually changing in pitch. In forward flight, for example, the retreating blade needs to create more lift across itself as the relative wind has decreased. It does this by assuming a higher angle of attack.
Then, as it continues its arc and becomes the forward-moving blade, the angle of attack needs to be decreased. This is all accomplished at the hub through many moving parts — which all require adjustments. At the same time, all the blades need to be tracking in the same plane.
Initial adjustments for proper blade tracking are done by adjusting the pitch control links at the hub. On the Hummingbird, this is accomplished by adding or subtracting washers. There are multiple ways to measure or see where the blades are tracking.
Brad Clark at Vertical Aviation let me borrow his Chadwick-Helmuth balancer, a time-proven tool for adjusting blade tracking on helicopters. Chadwick balancers are expensive, and the newer digital models are almost prohibitively expensive for an individual owner with one helicopter.
DynaVibe has created a less expensive version. I have been playing with that one recently. The primary difference that I see on the Chadwick is that one can “see” the blade track by using reflective angles on the blade tips versus waiting for trackers using an optical sensor to take a reading and then display the results.
There is one other method that uses LED lights on the blade tips. Unfortunately, LEDs aren’t usable during bright daylight conditions, and as you know, Phase I is day VFR only.
Prior to the first flight, the blade angles were carefully measured at the root and adjusted within one-tenth to two-tenths of a degree of each other. This is only a starting point. Once the blades are turning, they fly differently. Each blade is now flying through air disturbed by the preceding blade. Using a Chadwick requires two people in the cockpit, and I wasn’t willing to do that during initial flights. I felt there was too much risk. However, we did use it on the ground at rated rpm to get close.
After I had completed about eight hours of flight testing, I felt it was time to engage the help of another rated helicopter pilot to start doing the blade tracking. There are two components — adjusting for proper track in the hover, and then adjusting for proper track in forward flight.
Lots of short flights were done over multiple days. Hover, land, adjust. Repeat. Then fly, land, repeat. Each adjustment of one blade would also affect the other two blades. Forward flight needs to be at the same speed all the time, as changing the speed required different adjustments.
In the end, it is a compromise between the hover and forward speed. The result was noticeable from a smoothness standpoint inside the helicopter. People watching the videos commented about how much better the blade track looked.
However, there came a point when I just couldn’t seem to make any more progress. There was a noticeable shake in the cyclic control stick that was uncomfortable. There is one other component in the hub called a blade dampener that comes into play when the blades are turning. One is attached to each blade, and the primary purpose is to keep the blades in the proper lead/lag position as they traverse from one side to the other. They are oil-filled and adjusted at the factory. I rigged up a way to measure the speed of each blade as it traveled the full range of the dampener and noticed two of them were close in timing to each other, and one was substantially different.
This is where the support from Vertical Aviation came into play. Brad visited in person and brought some different dampeners. The transmission had a slight leak to it. Brad thought it would be a good time to change it out as well, so he brought a new transmission at the same time.
We installed the new transmission. While there was improvement, the shaking during startup was bad enough that hovering was out of the question. After many hours of measuring and trying some other solutions, Brad decided it was best to remove the rotor head, and he would take it back to his shop to thoroughly go over everything.
It turned out that two of my original blade dampeners were bad, and one he brought with him was also bad. He was back within a week.
Since the main blades were off, Brad took the time to make sure all the trailing edges were bent to the same angle using a special tool he had made. He also rebalanced the tail rotor blade. Once everything was reassembled, the difference was noticeable, and we were able to go flying and make good progress in short order.
I’ve made a lot of progress since then, but Brad is continuing to tweak another set of blade dampeners that we hope will be the final solution to the stick shake. Now that I am out of Phase I, I can also use the LED lights in the dark to have another visual comparison.
The other area I was focused on during Phase I was the engine, as I am the first one to use the Lycoming Thunderbolt installed in the Hummingbird. Reliability has never been in question, but performance numbers needed to be figured out, especially fuel burn for cross-country trips.
The injection system is an FM-200 from Airflow Performance, with which I am very familiar. Two of the features that I like are that the main jets are easy to change, and the injectors have removable inserts so you can balance fuel flow to the cylinders independently.
Most new engines run quite a bit hotter during the break-in period, and the helicopter is no exception. It’s even worse because the initial flights are done in a hover, which requires more power than forward flight, the lack of movement means less cooling airflow, and the helicopter is hovering in its own hot air.
The system came with a number 42 jet, which seemed lean to me. Sure enough, cylinder temps were touching 440-plus, and I didn’t like that.
Don Rivera, who founded and owns Airflow Performance, sent a few extra jets and nozzle inserts to try. Installing a number 38 jet (smaller numbers mean richer jets) lowered the temps to right around 400 degrees, which was great.
As I continued through the test program and the engine break-in showed lower temps, I slowly worked back through the jets and am currently running a 40.5. I’m thinking by 100 hours I could be back to a 41 or 41.5. The fuel flow is substantially different between the 38 and the 40.5 — about 3 gph less with the 40.5. Fuel burn now at 75-80 knots is around 20 gph. With the 57-gallon fuel tank, we get a solid two hours plus almost an hour’s reserve of fuel.
I know, some of you are cringing at the fuel burn versus speed. I do, too! But, it’s a helicopter. It’s not meant to go fast, and as my wife, Carol, says, “The view is awesome.”
Now I’m waiting on the blade dampeners to arrive this week. Brad is personally bringing them and will again help with the blade tracking. We are also going to try some different strength control system dampeners to help eliminate some of the stick shake, if necessary.
Next month I’ll continue to share some more on the flight testing progress. By the time you read this, I hope I will have met many of you in person at Oshkosh with the Hummingbird so you can see how much of a fun factor it has.
Vic Syracuse, EAA Lifetime 180848, is a commercial pilot, A&P/IA mechanic, designated airworthiness representative, and EAA flight advisor and technical counselor. He has built 11 aircraft and logged more than 10,000 hours in 74 different types. Vic founded Base Leg Aviation, has authored books on maintenance and pre-buy inspections, and posts videos weekly on his YouTube channel. He also volunteers as a Young Eagles pilot.