Why the lack of faster piston-powered planes?

Question

There aren't very many fast piston airplanes in production. By "fast" I mean, if you look back in the early to mid 20th century, before turbines won out, there were lots of piston airplanes in production that pushed the practical velocity limit of propeller-driven aircraft, cruising 360+ knots in military planes, sometimes as fast as 330 knots in commercial airliners. And this was with a terrible understanding of aerodynamics and piston engines compared to today.

Today, for both singles and twins, the fastest speeds around aren't much higher than 200 knots. ~240 knots in the case of the very fastest couple available airplanes like the Cessna 400 and fastest Mooney.

There are indications that with modern materials and engineering, much faster speeds can be achieved using piston engines already available. The Cobalt CO50 Valkyrie design claims to push a spacious 4 person cabin up to 260 knots with a single 350hp engine. The recently-certified Diamond DA-62 can move a spacious 7 passenger cabin at ~200 knots using only a pair of 180hp engines.

Why aren't there more faster piston airplanes available? Would the market not be interested in piston powered planes that could do 300 knots? Shouldn't you be able to make almost those speeds for a very small plane powered by a single commonplace 350hp engine? Shouldn't you be able to make those speeds in a more spacious plane with a pair of commonplace 350hp engines?

My understanding of the marketplace is that the #1 reason people choose piston is cost. And isn't the cost of operating piston engines, even two piston engines versus a single turboprop, much much lower? How much does a 350hp turbocharged piston engine cost brand new, 50k-60k dollars? So even buying a pair of them is 100k-120k dollars. And then how much does a single comparable turbine engine cost? ~800k dollars with proportionately higher rebuild costs per flight hour? Plus 20%+ higher SFC than a piston?

Basically, I see the Piper M600 listed at 2.8M USD, or the TBM930 listed at 3.9M USD, and I don't understand why it would be hard to achieve nearly the same performance for a fraction of the price using a pair of cheap piston engines. For example the Piper M350 has the same 6 passenger cabin as the M600 and also includes pressurization, and with a single 350hp engine makes over 210 knots on a very old airframe with a list price of under 1.2M USD. If you basically built the same airplane but reoptimized for twin engines and using modern materials and aerodynamics, shouldn't you be able to achieve near 300 knots by adding another piston engine? And shouldn't you be able to sell the resulting airplane for well under 2M USD and with a significantly advantaged SFC and therefore range and payload than the M600?

Answer 1

The power requirement of an airplane grows with the cube of speed. When you fly fast with an airplane which needs to comply with a set minimum speed imposed by regulations, your drag coefficient is nearly constant, so flying faster does two things to a propeller aircraft:

1. Drag goes up with dynamic pressure, which is proportional to speed squared
2. Thrust goes down with the inverse of speed.

Power is thrust times speed, so if the the 350 hp get you to 210 knots, doubling the installed power can only bring you to 264 knots. A turbine can still make limited use of the increased dynamic pressure by ram recovery, so here the thrust decrease over speed is not quite as bad as for piston engines.

Expressed in an equation, the power $P$ demand for a given speed $v$ is: $$P = \frac{D\cdot v}{\eta_{Prop}} = \frac{\rho\cdot v^2\cdot S_{ref}\cdot c_D\cdot v}{2\cdot \eta_{Prop}} \rightarrow v = \sqrt[\Large{3}{}]{\frac{2\cdot P\cdot\eta_{Prop}}{\rho\cdot S_{ref}\cdot c_D}}$$

If you want to fly fast with better efficiency, you need to increase the minimum speed - note that the landing speed of many fast piston aircraft was around 100 mph. Now you need a long runway, and for operation in bad weather or at night infrastructure for instrumented approaches, and you will end up at airports where turbine fuel is easy to get and cheap, but piston fuel will be hard to find and expensive.

As with all expensive machinery, you need to use it often to justify the expense. Now your fuel cost will factor in, and will make a piston engine rather unattractive.

Fast piston aircraft were only built while turbines were not yet available. As soon as turbines emerged, all fast piston designs were obsolete. And aerodynamics in the early 1940s were already very advanced; engineers back then built airplanes which would be impossible to design with the engineers of today. Back then, they got fully manually controlled airplanes into the air when today everyone would refuse to do it without hydraulic boosters at only half the flight speed. They could design cooling ducts which actually increased thrust, an art which is (almost) lost today.

Answer 2

A piston means a propeller, and a propeller is more efficient at lower altitude and much less efficient at higher altitudes (that's part of why there are variable pitch propellers ). But the converse applies to jets, which is why you don't really see jets cruising around at 5000 ft.

A piston engine is pretty complex and has quite a few moving parts that are moving in a somewhat violent fashion. A piston changes direction every stroke, and the combustion that drives each stroke is a different force and purpose than that of a jet engine. Those dynamics change a bit with the type of engine (a rotary engine is different than an inline engine), but it still requires a combustion to force a piston to push a propeller to drive the aircraft. The mechanics are much more complex than a jet engine (which essentially just brings air in, compresses it, lights it on fire to Bernoulli it, and uses that exhaust to turn the compressors [usually]). (I found some nifty engine animations at http://www.animatedengines.com)

Also, propellers and jets move an airplane differently. A propeller is no different than any other airfoil. Its primary mode of motion is by decreasing the pressure on one side, causing movement towards that lower pressure. A prop feels like a big fan, but most of its motion is pulling the aircraft. However, a jet functions by pushing the aircraft. Because of this difference, a propeller can reach its upper performance limitation at a slower speed than a jet will. Again the converse applies and a jet can reach its lower performance limit at a slower speed than a prop will. This allows a turbine engine to fly faster.

Lastly, a piston engine will likely weigh more than a comparable turbine engine. And in an airplane, weight makes a difference in pretty much everything.

Answer 3

If you want a fast propeller-driven aircraft, it's much easier to use a turboprop engine than a large piston engine.

• the turboprop is lighter and more compact
• it produces less vibration
• it produces less noise
• it's more reliable (the moving parts count drops from thousands to tens = less to go wrong)
• it's easier to build a high-performance turboprop. Late WW2 piston engines operated at the edge of what was possible at the time. They needed special high-octane fuel to prevent detonation (and even then, backfires were somewhat common on e.g. the Griffon).

Answer 4

I agree with your sentiment. We can effectively take general aviation into jet territory by using existing piston tech. There absolutely is an opportunity to capitalize on where the M350 leaves off.

IMO, it is not with a twin, but a more powerful (at altitude) and more efficient engine than the TIO-550. Making the M350/Malibu into a twin would give you a Piper Navajo, which for the cost of doubled operating expenses, you get no increase in speed and only two additional seats. The two nacelles kill a lot of the power benefit.

I'm convinced the answer is a diesel, possibly two stroke, with some serious forced induction. Displacement need only be enough for takeoff requirements, so a 550 ci engine or smaller may suffice for a six-seater; but with FI and two stroke, it could reach serious altitudes, speeds and savings.

Personally, I love the M350/Malibu because it is the only pressurized piston single (with club seating), cruising at almost 250mph at 20gph. Even an interim solution could be for a company to develop some STCs for a larger diesel conversion, with FI as icing on the cake. It may add weight, but it would cut the fuel consumption in half, and Jet-A is an additional savings. Taking some of the old Malibus and upgrading the interior, avionics and giving a diesel engine may cost $1M for a big package deal but it would make for a more desirable aircraft than a new M350.

The possibilities with forced induction are under appreciated. Aeromomentum Engines show what can be done with a small engine block.

As for people on here saying power requirements vary with the cube of the velocity - that does not add up. It should be the square, and the figures for the Piper M Series reflect that. 213kts(600hp/350hp)^1/2 = 278kts For max cruise, Piper advertises 213kts for the M350 and 274kts for the M600, so accounting for possible rounding in performance specs or maybe cruise altitudes, this is right on.

Anyway, rather than wasting the time of talented engineers on electric planes and flying cars (that would be impossible to keep airworthy or insure with fender-benders), we just need a certified FI diesel and then we can produce planes with jet performance for M350 prices and DA-50 operating costs... done.