Some mind food on Light Jet acft design..
FUNDAMENTAL REASONS SMALL JET WILL >>>NEVER<<< BE AS GOOD AS A BIG JET:
1:
let's start with an airbus a-380.. a huge plane that goes maybe 8,000 miles at mach 0.85 or so.
it weighs 1.2 million pounds and has length and breadth of 250 ft
now let's say we want to build a small jet.. a plane that is 1/10th the size.
this plane will have length and wingspan of 25 ft.
this light jet plane is
1/10 as long
1/10 as tall
1/10 as wide
that should give a plane that is one one-thousandth the volume
that should give a plane that is one one-thousandth the weight
that should give a plane that has one one-thousandth the thrust (to push one one-thousandth the weight)
so, we have a plane that is like an airbus a-380, but with one one-thousandth the weight and thrust and fuel-burn, right?
WRONG!!!!!!!!!!!!!!!!!!!!!!!!
the frontal area and wetted area of our airplane is ONE ONE-HUNDREDTH THAT OF THE AIRBUS, NOT ONE ONE-THOUSANDTH!!!!!!!!
scale the airbus down by 10x and you have one one-hundredth the frontal and wetted area, not one-one-thousandth!
so, your new scaled-down plane has one-one-thousandth the thrust, but one-one-hundredth the (parasite) DRAG!!!!!
so we have TEN TIMES THE DRAG PER UNIT THRUST!!!!!
again: scale down an a380 by a factor of 10, and you have ten times the parasite drag per unit thrust, all else being equal!!!!!
speed goes with square root of drag, so we should expect to fly at a speed fraction of the square root of 10, or about one THIRD the speed.
if we have identical endurance (one-one-thousandth the thrust, one one-thousandth the fuel, would give the same endurance!) but one third the speed, we will clearly have ne third the range.
so, if we managed to do everything as well as an airbus a-380 scaled down, we would still only have one THIRD the speed and range!!!!!!!!!!!!!!!!!!!
this rules makes a very light jet impossible, since very light jets need to have near-airliner performance to perform like jets.
2:
it gets worse.
jets do well because they have a high bypass ratio... teeny little turbines spinning at huge rpm driving giant, slow-turning fans
these teeny fast turbines give huge compression efficiency, these high bypass ratio fans give huge propulsive efficiency.. so we just scale it down, right?
WRONG!
if the turbine or compressor tolerance is 0.02" for blade-radius on the airbus, and we scale the engine down by 10x in every dimension, our part tolerance is STILL 0.02" since that is the best part we can make... that is now TEN TIMES THE ERROR on an engine that is one-tenth the size. in other words, the smaller engine has ten times the losses due to manufacturing tolerances. this means that you can NOT have a big fan with a small turbine.. the losses due to imperfections in the geometry of the engine are TEN TIMES GREATER, so the turbine can not be one tenth the size in each direction... that turbine is too small to work efficiently! remember, A FEW GRAINS OF SAND GOING INTO THIS ENGINE WOULD BE THE EQUIVALENT OF THROWING BAGS OF GRAVEL INTO THE ENGINE OF THE A-380.
so a small engine can NOT be as good as a big one, because the manufacturing tolerances become 10x as large, so the turbine must be larger, so the bypass ratio must be lower. (a larger turbine is by definition a lower bypass ratio, if the total air going thru the engine is held constant)
ok, so, our very light jet that is a scaled down airliner goes 1/3 the speed of an airliner,
going 1/3 the distance,
and actually does WORSE than the above because the the bypass ratio is lower because the turbine cannot be that small.
so now we are down to 250 knots or so.. maybe 300 knots if we put a bigger engine on and sacrifice even more range.
guess who flies alongside us if we are flying in a jet at 300 mph?
this brings us to our NEXT 2 fundamentals:
FUNDAMENTAL REASONS A 300-MPH JET WILL >>>NEVER<<< BE AS GOOD AS A 300-MPH PROP:
1:
the thrust we get from air is the momentum-change: amount of air we grab times how much we accelerate it
the fuel flow we put into the air is the kinetic energy: amount of air we grab times how much we accelerate it SQUARED
therefore, for any propulsion system to be efficient, it must take a LOT of air and accelerate it a LITTLE.
thus, all else being equal, the HUGE prop of a lancair is inherently more efficient than the tiny compressor of a mini-jet
2:
an internal-combustion recip engine gets the same compression ratio no matter how fast it turns. set the throttle to idle, take-off, cruise, descent, approach, or holding-pattern... it makes no difference: if the compression ratio of the engine is 7:1, you will get that compression ratio at all power settings: 7:1... the compression ratio is realized no matter how fast or slow the engine is turning... the piston still covers the same sapce in the cylinder, regardless of speed.
the JET engine, though, must turn at 100% rpm to get it's designed compression.. if the jet turns 1% less rpm than redline, compression is lost, and efficiency with it... the compression is caused by the dynamic pressure on the blades... 1% less speed on the blades is 2% less compression across them, with the resulting loss in efficiency. you can only run a jet on-design at 100% rpm... any speed less and the efficiency falls apart... no surprise that going to low power settings still involves huge fuel-flow... a jet engine at low power is losing compression! a jet engine at low power is like a recip engine that is losing compression and needs to have it's pistons replaced!!!!!!!!!!
so there you have it. 4 fundamental laws of physics that prove that a VLJ can't work:
-a plane that is 1/10th the size has 1/1000 the weight and thrust, but 1/100 the parasite drag, so will go about 1/3 the speed, all else being equal
-a plane that is 1/10th the size will have 10 times the manufacturing error in size-ratio, resulting in a larger turbine and therefore lower bypass ratio
so the small jet cannot go as fast as the a big jet, so we are down to 300 mph, so comparing to props:
-a plane with a jet takes a smaller bite of air than a plane with a prop, so cannot have the same propulsive efficiency
-a plane with a jet cannot run at lower power settings for much of the flight, like a prop can, without huge losses, because the compression ratio i only maintained at 100% rpm
Austin Meyer