May 3, 2010
“Avanti” (May 2010)
Randy Witt with friends and family right after our Avanti flight:
OK so X-Plane biz-manager Randy Witt knows a guy that flies his own Piaggio Avanti. (Google it) Built in Genoa, Italy (hometown of Sergio Santagada, the Godfather of X-Plane aircraft and art resources), the Avanti is a pretty fast propeller airplane. When I say ‘pretty fast’, I actually mean 400 miles per hour. That is faster than some Cessna Citation JETS. There are some Citation Jets that have about the same size and performance as the Avanti, but they use TWICE the fuel to get it. In other words: The Avanti gives jet performance on HALF the fuel as a jet. How does it do it? With propellers, which are more efficient than a jets below 450 mph or so.
In other words, if you are going slower than about 450 mph or so, you will use less fuel to do it in a prop than a jet… the Avanti is proof of this.
OK now for the long technical description of both why:
the prop is better than the jet for small planes,
the prop is better than the jet below 450 miles per hour,
and therefore, ultimately, why the Piaggio Avanti is better than any light-jet in it’s size and speed range: (this is an excerpt from an email I sent to a few friends when I cancelled the Laminar Research X-1 Cavallo light-jet project ( http://www.x-plane.com/x1/x1.html ) because it’s range simply would not have been good enough).
ok i think i am canceling the x-1 project (as it stands, with a single jet), and here is why:
FUNDAMENTAL REASONS SMALL JET WILL >>>NEVER<<< BE AS GOOD AS A BIG JET:
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?
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.
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?
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:
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
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
these are 4 FUNDAMENTAL LAWS that keep a VLJ from becoming a reality.
OK so now you see why small, slow jets will not work.
So, how DO you move a very small handful of people at 400 miles per hour? Simple: A turboprop. That nice big prop gets a nice big handful of air, and accelerates it only a little bit. THAT is what gives good propulsive efficiency. And the result is the Avanti, and it is absolutely amazing (and a challenge!) to fly.
All right so when you sit in the cockpit you are looking at a pretty big, heavy-looking, cluttered array of gauges and radios… the system is much less powerful, and much less user-friendly, than a G-1000 system, for sure. Note the panel in the pic below: There is no synthetic vision, no XM-weather, no moving map (!!!!!!!!), no selection of instrument approaches for single-button-press autopilot-guided instrument approaches… the system is really ancient… really very limited, much like the interior of a steam locomotive from the 1800’s: Cluttered with dials and knobs, a new one added hap-hazardly each time a new function needed an indicator or control. You will notice from this shot that the layout is totally overwhelmed by the silly splattering of gauges that results from not having an EFIS system. Observe the 6 nav-com radios, for example: The plane has 6 radios, and without an EFIS, it must have the displays, knobs, and controls for each of them, in hardware, taking up panel space all the time. As a result, a good portion of the panel is taken up with just radios, leaving no space for really good stuff like a weather-display! Observe the tall column of annunciator lights in the center of the panel. In an EFIS system, one area of the EFIS could simply annunciate any problem with the plane. But, with hardware like we see here, they built a physical annunciator for each thing that could possibly go wrong with the plane… so you have a tall stack of them, with only NONE OR MAYBE ONE OR TWO of them ever lit up… the others almost always sitting their dark, doing nothing but taking up space… a real waste of panel space! Note the artificial horizon: It has no synthetic vision, so it does not give nearly the situational awareness that it could.
So why do we have such a terrible interface? Why the lack of moving map? Lack of good GPS-controlled autopilot for full approaches? Lack of XM-weather? teeny-tiny traffic display that you can hardly even see from the pilot’s side? Panel covered with warning lights that will almost never activate and radios that include DME or ADF that are almost never used? The answer lies in the way the plane is OPERATED. Every Avanti in the world is operated IFR, every moment of flight, under air traffic control and supervision for every moment, flying airways as cleared by ATC.
Moving map? Who needs it? ATC guides us or puts us on an airway, and we follow it with a simple CDI.
Traffic avoidance? Who needs it? ATC gives traffic alerts and keeps us clear.
XM-Weather? Who needs it? We fly IFR so we do not care if it is foggy or raining, and ATC guides us around the bad storms.
Much of the situational awareness that I want in my G-1000 so that I can fly VFR, safely, alone, any place I like, becomes much less needed in the Avanti because every flight is pre-planned and then flown IFR with constant ATC supervision. This makes the old panel adequate for the needs of the airplane, even though a new G-1000 would provide much, much greater situational awareness and flexibility.
OK so panel let-down aside, let’s talk about the AIRPLANE.
Startup is typical PT-6: Engage the starters and igniters, then introduce the fuel AFTER the engine is humming along at at least 13%, and make sure the temps don’t go over redline when the fuel lights up. There is almost no noise or vibration at all during this process: There is only the slightest, quietest, barely-perceptible whine from way back behind you as you watch the numbers on the cockpit gauges. Once the engines are running, which basically only determined by the gauges in the absence of any sound, you taxi on out to the runway, the only sound being the radios, or the thumping of the nosewheel on cracks in the pavement. At power-application, you are pushed back amazingly firmly in your seat as the plane rockets forwards. The power-to-weight ratio of the Avanti is the same as a car that weighs 3,000 lb and has 500 hp. This is Ferrari F-430 Scuderia territory, with a twist: The props push just as hard at 200 mph as they do at zero. The plane pushes you back in your seat, and KEEPS pushing just about as hard as you come through 200 mph.
There is basically no noise, of any sort, while this is happening.
When you push the throttle forwards, you are pushed firmly back in your seat, and you just BARELY hear a tiny buzzing of angry bees way back behind you. The engine makes no noise that can be discerned, and the prop makes only a little, quiet, far-away buzzing sound.
The plane hops right up to 250 mph or so and then we pull the throttles back and within what seems like 5 minutes we have travelled 50 miles to our destination, a little airport in the middle of Kansas for a stop-and-go. Flying the approach in the Avanti is downright scary for me. The plane has an approach speed of about 120 knots, or abut 140 miles per hour. Compared to what I am used to, this is FAST.
Let’s do some math: This plane can operate from a 3,000 ft runway.
Approach speed as 120 knots.
That is 1 mile every 30 seconds.
The runway, at 3,000 feet, is half a mile long.
On approach, you are covering a mile every 30 seconds, and the runway is half a mile long.
YOU WILL EAT THE ENTIRE RUNWAY IN 15 SECONDS AT APPROACH SPEED.
I believe that this means that an aircraft-carrier mentality seems quite necessary: You need to be on-speed, on-glidepath, on-course, and ride the path right down to the runway, with no time for fiddling around with niceties like a flair. Regardless of my feelings, however, our pilot takes the controls from me on short final, rounds out in a quick flair, and gets a very smooth landing in the first 25% of the runway, brings in about half of his reverse thrust, and gets the plane stopped with 25% of the runway still remaining! Needless to say, this all has to happen in about 15 seconds. Touch-down in the Avanti is a bit surreal: You cannot hear the engines or see the wing or the nose or the prop or really anything at all except runway racing past the huge windshield. As you perform your brief flare, you hear a little ‘dweeep… dweeep.. dweeep…’ that is the stall warning, and let the plane touch down in a pretty flat attitude, with only minimal flare. There is still no noise at all other than the stall warning, but when the nosewheel touches you get a bit of bumping and rolling sound from the nosewheel.. but that is about it. You then engage the reverse, which flips the big prop blades (all 10 of them) into reverse, acting as big strong speedbrakes that push the air right back where it came from, slowing the plane vary strongly without touching the carbon-ceramic brakes, which apparently run about $15,000 each or so to replace.
Anyhoo, it was really interesting to fly this plane that operates in the IFR system with the speed and range and payload of a light jet, but on HALF the fuel. The equipment that makes this possible (old-style panel, propellers, small wings with high stall-speeds) provide interesting challenges in flight, but reward you with the smoothest, quietest ride of any airplane, and a fuel bill half of what the jets pay.
OK here is a little table comparing the Avanti to 2 jets.
The Mustang carries the same fuel, to go less than half as far, slower.
The CJ-4 goes about as far and fast… but takes over TWICE the fuel.
The turboprop is clearly the more efficient choice. The prop also gives better low-end thrust, as you see indicated by the shorter take-off distances.