Skymaster15L
16th Oct 2012, 08:38
Hello everyone-I could really use your expert help with some explanations here
While going thru the study phase to take my ATPL theory exams, apparently the chapter on power augmentation from the OAA manual really kicked my butt yesterday. It's either been really long since studying this material since my MEP training or it's being put in a different way making it slightly more difficult to grasp.
To begin with, the biggest confusion deals with super vs turbochargers. Now I know both of them share a common purpose (to increase weight of cylinder charge), only differing in the way they are driven. The amount of increase in weight of charge can either be so much as to increase sea level power (ground boosting) or to only maintain sea level power up to a certain altitude (altitude boosting aka turbo normalizing). The altitude at which both of these methods stop being effective is critical altitude or full throttle height.
Onwards, the chapter continued with an explanation of the turbo charger, after which it showed a graph comparing the performance of a turbo charged engine to a normally aspirated one. The graph showed that both of these engines start losing power from the moment they start climbing-I understand aspirated ones do, but I always remembered being shown comparison graphs btwn these 2 engines as one where the turbo charged one maintains a straight horizontal line of constant power until Crit. Alt. while the normally aspirated one has its normal downward slope.
When talking about superchargers (gear driven) the comparison graph shown by the book stated that again between two engines that are otherwise identical in power output, a supercharged one has less power at sea level due to loss of some BHP to drive the supercharger gearing-this makes sense. However, as the engine climbs, it actually develops MORE power with altitude, because according the book, the reduced back pressure and lower temp with altitude provide more efficient cylinder scavenging and better air density. I always knew that at altitude, decreased pressure has the predominated effect on lessening air density.
Now it seems, this train of thought is being inverted to say that these minor changes which in reality might lessen the rate of change of density drop, now provide the major effect for more engine horse power with altitude-theoretically making it possible to maintain altitude the higher you go, with no throttle at all. I have included both graphs for you to consult.
My questions are, does this make sense? Or, is this graph making certain assumptions that I haven't picked up on, but still carries the same meaning I am familiar with, put in a different way?
Also, with regards to altitude vs ground boosting, is there one type of equipment that is generally only associated with a certain type of boosting (supercharger for altitude, turbo for ground) or it is common to see either done by either. Finally, if an altitude boosted engine's role is to only maintain SL rated perf to a higher alt, does that mean it only has a max MAP reading of 30 inches on the gauge?
Any attempts to unconfuse me will be well appreciated; regards
http://s17.postimage.org/x715wylrz/DSC08397.jpg
http://s10.postimage.org/k8vlnbw6x/DSC08400.jpg
While going thru the study phase to take my ATPL theory exams, apparently the chapter on power augmentation from the OAA manual really kicked my butt yesterday. It's either been really long since studying this material since my MEP training or it's being put in a different way making it slightly more difficult to grasp.
To begin with, the biggest confusion deals with super vs turbochargers. Now I know both of them share a common purpose (to increase weight of cylinder charge), only differing in the way they are driven. The amount of increase in weight of charge can either be so much as to increase sea level power (ground boosting) or to only maintain sea level power up to a certain altitude (altitude boosting aka turbo normalizing). The altitude at which both of these methods stop being effective is critical altitude or full throttle height.
Onwards, the chapter continued with an explanation of the turbo charger, after which it showed a graph comparing the performance of a turbo charged engine to a normally aspirated one. The graph showed that both of these engines start losing power from the moment they start climbing-I understand aspirated ones do, but I always remembered being shown comparison graphs btwn these 2 engines as one where the turbo charged one maintains a straight horizontal line of constant power until Crit. Alt. while the normally aspirated one has its normal downward slope.
When talking about superchargers (gear driven) the comparison graph shown by the book stated that again between two engines that are otherwise identical in power output, a supercharged one has less power at sea level due to loss of some BHP to drive the supercharger gearing-this makes sense. However, as the engine climbs, it actually develops MORE power with altitude, because according the book, the reduced back pressure and lower temp with altitude provide more efficient cylinder scavenging and better air density. I always knew that at altitude, decreased pressure has the predominated effect on lessening air density.
Now it seems, this train of thought is being inverted to say that these minor changes which in reality might lessen the rate of change of density drop, now provide the major effect for more engine horse power with altitude-theoretically making it possible to maintain altitude the higher you go, with no throttle at all. I have included both graphs for you to consult.
My questions are, does this make sense? Or, is this graph making certain assumptions that I haven't picked up on, but still carries the same meaning I am familiar with, put in a different way?
Also, with regards to altitude vs ground boosting, is there one type of equipment that is generally only associated with a certain type of boosting (supercharger for altitude, turbo for ground) or it is common to see either done by either. Finally, if an altitude boosted engine's role is to only maintain SL rated perf to a higher alt, does that mean it only has a max MAP reading of 30 inches on the gauge?
Any attempts to unconfuse me will be well appreciated; regards
http://s17.postimage.org/x715wylrz/DSC08397.jpg
http://s10.postimage.org/k8vlnbw6x/DSC08400.jpg