ThePledge
50cc
In the course of trying to dial in my little OS 33 I noticed in almost every example on the forums, a rear carb engine changing tune with orientation/gforces in flight the orientation corresponds with the position of the diaphragm plate. Outside of unbaffled yaks I'm not sure I found an example of a rear carb engine with an orientation problem that doesn't follow the position of the diaphragm plate. I'm not saying airflow doesn't cause or exacerbate tuning changes in the air, but if your rear carb engine is richer with the diaphragm plate facing up and leaner with it facing down it's likely gravity/Gforces on the diaphragm which are driving the issue. Especially if the tuning changes are almost instant.
It's gravity and Gforces acting on the diaphragm plate in the vast majority of cases for rear carbs. Carbs on twins mounted below the engine are somewhat immune to gravity/Gforces and susceptible to airflow because they point forward into the stream of air coming off the prop and are at their richest with the airframe in a vertical upline or when accelerating forward. Even if this orientation causes mixture changes in flight they happen to correspond to what the engine needs anyway so they may even run better this way.
E.g. DA 50s with the diaphragm plate facing up are at their richest position tuned on the ground and will go slightly lean in knife edge and a little more lean inverted and leaner still in a blender. Flip the carb around and they are at their leanest state of tune on the ground and will go rich in other orientations. Read the forums and you'll find people with the plate up have to tune it rich to keep it from dying in a blender and people with the plate down who tune it rich on the ground kill the engine in things like inverted elevators because it loads up. This relationship pops up with a bunch of different engines from lots of brands.
The fix is generally higher pop off pressure (stronger spring under the lever) to make the lever less susceptible to gravity/Gforces. To relate this to tuning a good practice would be to make note of the position of diaphragm plate and then test in flight for differential in tune between orientations and maneuvers where the plate is facing up or down and exposed to positive or negative G forces. If there's a difference in tune than apply that knowledge in your tuning strategy. Playing around with pop off pressure may fix or reduce the difference in tune with orientation, but just understanding and being able to predict where the plane will go lean or rich makes tuning easier.
E.g. if the plate faces up on the ground the tune will be at its richest when you tune it on the ground so it may need to be left slightly rich to account for it going lean in inverted maneuvers.
If the plate faces down it is at its leanest on the ground and should be tuned a little more aggressively otherwise it will go too rich inverted and load up at low throttle inverted.
If the plate faces sideways on the ground the tune won't change upright or inverted, but will be richer/leaner in opposite knife edge flight or hard rudder turns.
Based on the orientation of your diaphragm plate you can predict what maneuvers will kill the engine and incorporate that into a final tuning test. For the OS 33 with the carb facing up that would be things like full throttle inverted pulls into inverted uplines and blenders at various throttle settings.
It's gravity and Gforces acting on the diaphragm plate in the vast majority of cases for rear carbs. Carbs on twins mounted below the engine are somewhat immune to gravity/Gforces and susceptible to airflow because they point forward into the stream of air coming off the prop and are at their richest with the airframe in a vertical upline or when accelerating forward. Even if this orientation causes mixture changes in flight they happen to correspond to what the engine needs anyway so they may even run better this way.
E.g. DA 50s with the diaphragm plate facing up are at their richest position tuned on the ground and will go slightly lean in knife edge and a little more lean inverted and leaner still in a blender. Flip the carb around and they are at their leanest state of tune on the ground and will go rich in other orientations. Read the forums and you'll find people with the plate up have to tune it rich to keep it from dying in a blender and people with the plate down who tune it rich on the ground kill the engine in things like inverted elevators because it loads up. This relationship pops up with a bunch of different engines from lots of brands.
The fix is generally higher pop off pressure (stronger spring under the lever) to make the lever less susceptible to gravity/Gforces. To relate this to tuning a good practice would be to make note of the position of diaphragm plate and then test in flight for differential in tune between orientations and maneuvers where the plate is facing up or down and exposed to positive or negative G forces. If there's a difference in tune than apply that knowledge in your tuning strategy. Playing around with pop off pressure may fix or reduce the difference in tune with orientation, but just understanding and being able to predict where the plane will go lean or rich makes tuning easier.
E.g. if the plate faces up on the ground the tune will be at its richest when you tune it on the ground so it may need to be left slightly rich to account for it going lean in inverted maneuvers.
If the plate faces down it is at its leanest on the ground and should be tuned a little more aggressively otherwise it will go too rich inverted and load up at low throttle inverted.
If the plate faces sideways on the ground the tune won't change upright or inverted, but will be richer/leaner in opposite knife edge flight or hard rudder turns.
Based on the orientation of your diaphragm plate you can predict what maneuvers will kill the engine and incorporate that into a final tuning test. For the OS 33 with the carb facing up that would be things like full throttle inverted pulls into inverted uplines and blenders at various throttle settings.
