Balancing Act: Maintaining the Sukhoi SU26M with Dynavibe DYNAMIC PROPELLER BALANCER

by rick volker

It had been a great start to a new air show season. After many years of Unlimited competition and air shows at the surface level, my Russian aerobatic plane (a mint Su-26M) and I had melded. The Sukhoi is known to have the highest stressed propeller blades of any piston aircraft, as the 400-horsepower engine is geared, allowing a 102-inch prop diameter with huge paddle-like blades. A steady diet of lomcováks and snap rolling maneuvers exacts a toll, requiring 200-hour propeller overhauls. I was enjoying the smoothness of the engine and prop eight hours after a complete prop overhaul when a postflight check revealed a horrifying crack on one of my MT-Propeller blades. I sent pictures to all parties, including MT. Some thought it was from foreign object debris (FOD). Others thought there may have been an internal crack after some old repair on the “exchange” blades of this prop. Regardless, it would need to be fixed immediately to salvage
​my airshow season. 

While a new blade was being shipped, I bought a spare used propeller with 250 hours TT on it. The spare prop exhibited more vibration than I was used to. Enter DynaVibe. I used this dynamic propeller balancer to improve the spare prop from .55 IPS to .02 IPS in three runs. The process is simple. Attach the vibration sensor and prop timing light according to the manual with supplied fixtures. Make an initial run. The unit automatically prescribes a solution about where to add weights on the spinner screws to start the balancing process. Rinse and repeat for two more runs and the result is a glass smooth prop. Then move the final weight solution to the spinner backplate and the job is done.

My damaged prop returned from MT with a new replacement blade. I installed it on the Sukhoi and used the DynaVibe unit to balance it to a hypersmooth .04 IPS on the ground. I jumped in for my next practice flight. Taxi and run up were buttery smooth. As soon as I was in flight, something was very off. As the speed increased, the prop vibration became horrible. I used the DynaVibe unit while in flight to show .57 IPS at cruise and take off power settings. I sent the vibration spectrum readout from the DynaVibe unit to MT for their examination. Here is part of the discussion:

“This "B" prop behaves as you would expect it. It is .02 IPS to .11 IPS on the
ground, depending on rpm. In flight, it is .04 IPS at low manifold pressure and
82% rpm at 100 mph to 140 mph. At full power take off, in climb it is .06 IPS and
in level flight it reaches a max of .13 IPS.

At max cruise 82% rpm climb, it is .11 IPS to .13 IPS.  As it accelerates in level
flight, it reaches .16 IPS as a high. When it is pulled back to pattern speeds, but
still at 82% rpm, it shows .04 IPS. It feels very smooth on climb at any power.
It still feels good at any power during level flight. Even in a dive over max level
speed, it does not feel rough. There is not much perceptible change at any
power, rpm, attitude or air speed. I am very happy with it! Thanks, DynaVibe! Thanks, MT-Propeller in DeLand!"

Contrast the above to the behavior of my "A" prop:
"The "A" Prop balances on the ground to .04 IPS. In flight, at full power climb at
100 mph, it is just acceptable, but in level flight, it goes to .54 IPS. I have seen
.57 IPS at times.

Max cruise power 82% rpm shows .45 IPS in level flight. If I dive at max cruise or
full power above level speed, IPS goes noticeably higher.

Low power 82 % rpm approach at 100 mph is .44 IPS. Very rough!”

My mechanic and I suspected the new blade was flexing in flight differently than the other two blades with the airflow of flight. MT did not immediately agree with this determination. They felt that my engine was likely the culprit. I was determined to prove that a blade mismatch was at fault. Luckily for me, I still had the backup prop.

We put this prop on and flew with the DynaVibe in flight for comparison, demonstrating that the backup prop was smooth in flight with very low IPS readings, regardless of air speed and power setting. MT now had to agree that a blade mismatch was the cause of in-flight vibration, despite their bench tests showing the new blade was within their spec. MT provided me with a new set of 3 matching blades. They were instantly much smoother. Again, I whipped out the DynaVibe and confirmed that blade mismatch was in fact the issue with their single blade replacement.

Without such a user-friendly and accurate prop dynamic balancer, I would not have been able to provide enough information to justify replacing and matching all 3 blades of my original propeller. The DynaVibe unit prompts information input and makes each step in the balancing process consistent and accurate. It includes a method to record all steps. The vibration sensor also provides information about any rotating mechanism on the airplane, allowing the identification of a change in balance of each component.

Now I have two perfectly balanced propellers while in flight. I can show that my supercharger, magneto drive, etc. is not contributing to a propeller problem, and identify if there is likely a change in those components in the future. The operation is so simple I was able to use it one-handed during flight operations to prove my case.

I would encourage any aircraft owner to use a DynaVibe prop balancing unit to make sure they are at the lowest vibration level possible for maximum component durability.