tilopa
Well-Known Member
Although this topic has been discussed before, I would like to revisit it with a slightly different focus, if only for the purpose of revealing its infeasibility.
The idea is to start with an airplane of a certain type and fit it with an engine and prop that are optimized at cruise, and artificially augment power for takeoff and climb. The type of aircraft most suitable would be those more streamlined for high altitude cruising, with characteristics such as low parasite, induced, and form drag. Aircraft such as the Lancair 320, long ez, and glasair would be examples. A significant point to emphasize about such aircraft is that for the majority of flight time (cruise) only roughly 50-60% of the engines power capacity (and by extension weight) is being used to maintain rated speed, that is at least if the prop was optimized for cruise.
So, if we look at the power requirements, and the optimal fixed pitch prop configuration, for cruise of a particular aircraft, could we not fit that aircraft with an engine that would be something like 40% less rated power and weight and still maintain our cruise performance. And by "optimal prop" I am not sure what that would be, perhaps it would be something like a 48 inch diameter high angle of attack pitched 4 blade prop rotating at 4200 rpm, just to throw out some numbers. Obviously such a engine-prop configuration would be disastrously inadequate for take off and climb. But what if for the relatively brief periods of takeoff and climb we significantly boost the engine to reach the given rpm by means of turbocharging, or electric engine driven supercharging, or with Nitrous oxide combined with CNG, which seems to be the most effective way, as the combined N2O-CNG reduces risk of detonation, and it is lighter, cheaper, and simpler, for short period use - but I'll defer to you experts on that. At takeoff, static thrust would have to be boosted significantly for the first 15 seconds or so, then as speed increased boost could be decreased. At cruise climb boost could be much reduced, and at cruise boost would be stopped.
Some actual numbers might make it easier to sink your teeth into. let us take a Long EZ, which would minimally use a 115 HP O-235 engine, which can cruise at around 150 MPH at 50% power (58 HP). Those statistics (which I pulled from the internet and may be inaccurate) do not include prop specifics. And at this point I would like to interject a question: what would be the optimal cruise prop for a Long EZ, generally speaking. I'm making two general assumptions about prop optimization at cruise: smaller disk diameter is better because of reduced drag and higher disk loading inefficiency is mitigated at higher speeds. And higher pitch angle blades (to a degree) are better. So, let's say we use a 48" 4-blade high pitch prop spinning at 4200 rpm, completely ridiculously unconventional, I know. What power would be required to spin that prop at 4200 rpm static thrust? Obviously more than the 115 HP O-235 with a 62" prop (or whatever). Would 130 HP be enough? let's say we use a Yamaha Genesis 130FI 1.0 Liter 4-stroke engine which produces about 65 HP at 4200 rpm, and 130 HP @ 8000 rpm. If we use N2O to boost the engine, could we increase the manifold pressure of that engine to produce 130 HP @ 4200 rpm for say 30 seconds without destroying the engine?
That is what my real question boils down to, despite all the preamble. What kind of stresses can an auto engine take even for short periods. And there is very little information/data to be found on the subject. It seems that when designing an engine, inertial forces are most significant, but when very high torque pressure is introduced the engine can breakdown very quickly, and without much predictability or warning.
These question are significant to me because if we can take an off-the-shelf, better-power-density, reliable, more efficient, and much cheaper engine and boost it for short periods at a given rpm (eliminating the need for a PSRU) we'll have better engines, better engine options, at a better price.
Thanks in advance for humoring me.
The idea is to start with an airplane of a certain type and fit it with an engine and prop that are optimized at cruise, and artificially augment power for takeoff and climb. The type of aircraft most suitable would be those more streamlined for high altitude cruising, with characteristics such as low parasite, induced, and form drag. Aircraft such as the Lancair 320, long ez, and glasair would be examples. A significant point to emphasize about such aircraft is that for the majority of flight time (cruise) only roughly 50-60% of the engines power capacity (and by extension weight) is being used to maintain rated speed, that is at least if the prop was optimized for cruise.
So, if we look at the power requirements, and the optimal fixed pitch prop configuration, for cruise of a particular aircraft, could we not fit that aircraft with an engine that would be something like 40% less rated power and weight and still maintain our cruise performance. And by "optimal prop" I am not sure what that would be, perhaps it would be something like a 48 inch diameter high angle of attack pitched 4 blade prop rotating at 4200 rpm, just to throw out some numbers. Obviously such a engine-prop configuration would be disastrously inadequate for take off and climb. But what if for the relatively brief periods of takeoff and climb we significantly boost the engine to reach the given rpm by means of turbocharging, or electric engine driven supercharging, or with Nitrous oxide combined with CNG, which seems to be the most effective way, as the combined N2O-CNG reduces risk of detonation, and it is lighter, cheaper, and simpler, for short period use - but I'll defer to you experts on that. At takeoff, static thrust would have to be boosted significantly for the first 15 seconds or so, then as speed increased boost could be decreased. At cruise climb boost could be much reduced, and at cruise boost would be stopped.
Some actual numbers might make it easier to sink your teeth into. let us take a Long EZ, which would minimally use a 115 HP O-235 engine, which can cruise at around 150 MPH at 50% power (58 HP). Those statistics (which I pulled from the internet and may be inaccurate) do not include prop specifics. And at this point I would like to interject a question: what would be the optimal cruise prop for a Long EZ, generally speaking. I'm making two general assumptions about prop optimization at cruise: smaller disk diameter is better because of reduced drag and higher disk loading inefficiency is mitigated at higher speeds. And higher pitch angle blades (to a degree) are better. So, let's say we use a 48" 4-blade high pitch prop spinning at 4200 rpm, completely ridiculously unconventional, I know. What power would be required to spin that prop at 4200 rpm static thrust? Obviously more than the 115 HP O-235 with a 62" prop (or whatever). Would 130 HP be enough? let's say we use a Yamaha Genesis 130FI 1.0 Liter 4-stroke engine which produces about 65 HP at 4200 rpm, and 130 HP @ 8000 rpm. If we use N2O to boost the engine, could we increase the manifold pressure of that engine to produce 130 HP @ 4200 rpm for say 30 seconds without destroying the engine?
That is what my real question boils down to, despite all the preamble. What kind of stresses can an auto engine take even for short periods. And there is very little information/data to be found on the subject. It seems that when designing an engine, inertial forces are most significant, but when very high torque pressure is introduced the engine can breakdown very quickly, and without much predictability or warning.
These question are significant to me because if we can take an off-the-shelf, better-power-density, reliable, more efficient, and much cheaper engine and boost it for short periods at a given rpm (eliminating the need for a PSRU) we'll have better engines, better engine options, at a better price.
Thanks in advance for humoring me.
