Now let's get back to the NACA report so there is a better understanding of the issue.
We will look at a condition of flight essential to a dogfighter. The ability to make abrupt turns.
The pilot must be able to precisely control the amount of acceleration he loads on the aircraft. All aircraft performance depends on velocity. In order to get maximum performance out of the aircraft above maneuvering speed, Va, he needs to be able to make a 6 G turn and not exceed that load factor to prevent damage to the airframe. Below Va, the pilot needs to control the acceleration so that he does not stall the aircraft making the abrupt maneuver as well being able to maintain a maximum performance turn.
Doing that in an early Mark Spitfire was difficult and something only a skillful pilot could perform.
First the NACA report. Abrupt 180 degree turns were conducted at various entry speeds to gauge the level of control the pilot had in maintaining steady accelerations. The turns were also done to the stall point in order to gauge the behavior and amount of control.
"In turns at speeds high enough to prevent reaching maximum lift co-efficient" means turns above Va.
"By careful flying" a pilot can hold a steady acceleration. That agrees with the Operating Notes warning for the pilot to brace himself against the cockpit to get better control when making turns.
Now lets look at the measured results.
Here we see in a rapid left turn performed at 223 mph the test pilot is unable to hold constant acceleration on the airframe. Very small variations in stick movement and stick force changes of 1-3lbs results in large fluctuations in acceleration.
Next let's look at the pilots ability to control the accelerations in the pre-stall buffet.
Here we see the pilot was able to load the airframe to 5G's in 1 second to reach the pre-stall buffet 3 times. The smooth positive sloped portion of the curve represents the aircraft flying while accelerations are increasing. The top of the acceleration curve represents the pre-stall buffet. The bottom of the curve represents the stall point.
The take away is:
1. The large accelerations change for very little elevator movement.
2. The very rapid rate at which the pilot was able to load the airframe to 5G's.
3. The equally rapid rate at which the airframe unloaded down to 2G's when the pre-stall buffet was encountered. In 1 second, the aircraft went from 5G's to 2G's due to buffet losses. This means a rapid decay in turn rate resulted.
4. The violence of the pre-stall buffet combined with the longitudinal stability and control caused large fluctuations in the accelerations on the aircraft.
Last part of the NACA we will cover for today is the stick force travel. The amount of stick travel as measured by the NACA was not acceptable.
Next let's look at the opinion of Stability and Control Engineers on the Early Mark Spitfires.
Tomorrow I will post some of the plethora of references to this same issue of longitudinal instability as found in the Spitfire Mk I Operating Notes from July 1940. You will see the same references or similar to the same issue the NACA measured in the Spitfire Mk II Operating Notes.
There is no doubt that the Air Ministry was aware of the longitudinal instability of the early mark Spitfires.