Originally Posted by quorneng

jangodog
My own guess is that none of the changes you have made would be expected to stop it flying, although each would degrade performance a bit, but all together might be different.
I would imagine the centre burner unit is heavier that the motor cowl it replaces and iit is not as aerodynamic as the motor cone. In addition its lead is directly in the fan airflow.
The twin wheel undercarriage will almost certainly be heavier and will also add a bit of drag both extended or retracted..
The added triangle cowling looks ok unless it is in the narrowest part of the duct in which case it could be restricting the fan airflow.
The extra 'decoration' on the tail pipe will have added some weight and more importantly it is right at the back so could have effected the CofG.

The Viper is described as having an 'adequate' performance rather than being a 'rocket ship' so it is possible it doesn't take that much to have a significant effect.
Just my two penn’orth.