Limitations of Our Weapons System in a Pursuit Curve
To accurately predict the lead requirement (prediction angle) any sight requires the following basic information: range and target angular velocity. In the disturbed sight, range is provided by the ranging radar and target angular velocity information is acquired by tracking the target aircraft. Since the pilot is forced to turn his aircraft at a certain rate, this rate of turn is a representation of target motion and is directly proportional to target speed. The angular velocity of the attacking aircraft is fed into the sight through gyro action. This information is then married to the input from the radar and the proper prediction angle is computed by placing an electrical restraint (stiffness current) on the gyros. At long ranges, little restraint is applied, while at short ranges, great restraint is applied. In other words, the disturbed sight is directly tied to the responses of the pilot and to the dynamics of the aircraft when computing for target motion.
In an attack where the fighter velocity is less than twice that of the target, the following occurs:
- The rate of turn necessary to track the target diminishes as the range decreases.
- The stiffness current to the gyros in the sight increases as the range decreases.
Because of these two conditions, a pilot closing for the attack experiences a sensation that the pipper is drifting in front of the target. To correct for this, he relaxes back-pressure to reduce his turn rate. This, in turn, creates a lower rate input to the sight gyros, which causes the pipper to be repositioned to match the new turn rate. So, once again, the pilot must change is turn rate to reposition his tracking index, which again repositions his pipper and so on. Yet, for the attacker to have the proper prediction angle, he must continually make these corrections and also tract the target one-half to two-thirds the time of flight of the bullet, to allow for sight solution time. As we can see, a pilot may nullify this effect of chasing the pipper by firing at lower G. However, if he does, he will be forced to fire at longer range. His sight will have even less restraint and his target will be smaller in perspective. If he fires at higher G (shorter range) his sight will have more restraint, but the prediction angle will continuously change in greater magnitude. As a result, the attacker has very little choice as to the type of pass he may execute. He is forced into a narrow attack cone to avoid either extreme.
To solve this problem of G bleed-off, we must refer back to the mechanics of the pursuit curve. You will recall that if an attacker’s velocity is twice or more than twice that of a target, there will be an increase in rate of turn. In attacks of this nature, a different situation exist.
- The rate of turn necessary to track a target increases as the range decreases.
- The stiffness current to the sight gyros increases as the rate decreases.
In an attack where these conditions exist, the pipper tends to remain on the target because the forward drift of the pipper is cancelled by the demand for greater G as range diminishes. In this attack, as the prediction angle is changing less in magnitude, the pilot has a more stable tracking index, and consequently, is more able to track for the required time to achieve sight solution. Naturally, this would be the ideal attack for us to perform; however, this is impossible, because the disproportionate increase in fighter and target velocities has not only pushed the firing range out, but has also created conditions where the sight prediction angle is continuously changing in considerable magnitude. These conditions seriously impair our ability to destroy a non-maneuvering target with a gun attack.