We have made at least 10 slow to medium speed taxi tests on grass in the last six months. We were unable to control aircraft forward stability over 20 MPH. At first we thought that it was the pilot's inexperience even though the pilot has 600 hours in a tail dragger aircraft. We tried to analyze how we could correct our problem.
One reason we knew was due to the rough terrain, 2nd reason was not locking the tail wheel. We felt that if we continued taxiing the way we were, it would result in damage to the aircraft. It was almost impossible to control the aircraft during ground testing.
Dec 4th.

We decided to remove the oil cooler screens under each wing, thinking this might help in stabilizing the aircraft.
Again taxied on grass, this time engaged the locking pin, we found that the aircraft handled better but still erratic.
After commencing the test and within a few hundred feet the aircraft again became unstable. Inspecting the locking pin and discovered the pin looked like a pretzel.
We disengaged the pin and continued taxing for the next 10 minutes without any results, shortly returning to the hangar.
Dec. 5th
Phil our mechanic removed tail covering its supports with included the complete tail wheel spring and tail assembly.
We replaced the cold rolled steel locking pin, with a ½ inch 2140 harden steel pin. Checking the alignment that worked fine.
Dec. 8th.
With difficulty we arranged to have our full staff of ground people on hand, and our 109 advisor. The town of Brookhaven had its fire truck standing by.This time we decided to use the hard surface runway. The chase and video vehicles were flanking the aircraft, staying a few feet behind. We were optimistic as the aircraft started to move.
Made a few medium speed runs, "the aircraft seemed to handle better. The pilot said, "he felt confident in controlling the aircraft" and was given approval for a hi-speed run. One of the jobs for chase vehicle was reading the ground speeds off and advising the pilot if the aircraft moved off the center line of the runway. (Which it did on occasion). Video was being taken on each run, we have some great shots. We were told during the war the 109 took off from open fields, they didn't have any runways to follow, all the pilot needed was to keep the power up.
At times the plane ground speed was past 40 MPH. With the weight and power factor we figured take off speed to be around 65-MPH.
The pilot was told a number of times that the tail wheel was bouncing, as if she wanted to come up, she wanted to fly.
In the chase vehicle, Luftwaffe pilot Gottfried Dulles who flew the 109 during the war was instructing the pilot.
This plane is a prototype of a 109, no one knows for sure how she'll react. We decided to hold the control stick forward of center, in order to raise the tail.
Each time we came to the end of the runway we had to unlock the tail turn make a 180 and re-lock after the plane was straight.
The wind shifted, this time the plane's straight into the wind, tail locked, chase vehicle reading off the speeds 35, 40, 45, ground speed now at 50 MPH the tail came up 2 feet for a few seconds. The pilot said, the plane was tracking straight with rudder pressure, the aircraft moved to the left, the torque from the prop was pushing it.
Unable to stop the aircraft in time she did a small ground loop, after inspections, there wasn't any damage, except to the new locking pin. (She bent again)
Dec. 12th to the 23rd.
We removed the tail assembly, and made up a longer reinforced brace to hold the locking pin. We replaced the ½" pin with a larger 5/8" 2140 steel pin. The new support brace that was welded cause the wheel to go off center. We had to shim and relined and balance the tail wheel.
Jan. 14th to 17th. Phil and Al changed the toe of the main gear from toe in to straight, hopping this will kept the plane straight.
Jan. 21 & 22:

Canopy: (trimmed safety cables AN 4 bolt). Installed wind insulation around the glass canopy supports on the right bottom right rear-side and top.
We fueled the 109 with 35 gal 100LL.
Charging battery and checked brake fluid.

Will kept you informed.
Our next test is for the week of the 28th, weather permitting.

Al Rubenbauer