Bjorn’s Corner: Sustainable Air Transport. Part 16. Thrust generation

April 22, 2022, ©. Leeham News: Last week, we examined propulsion system alternatives and their principal advantages and disadvantages. Now we go deeper into these alternatives.

All propulsion systems for aircraft use a propulsion device like a propeller or a fan to generate forward thrust. We use this article to understand how these work and their characteristics before we go into how we create the shaft power to drive them.

Figure 1. The propulsive efficiency as a function of speed for different thrust generating concepts. Source: Aircraft propellers, is there a future? MPDI document.

How to generate forward thrust for aircraft

We generate thrust to drive an aircraft forwards by accelerating surrounding air in a backward jet. Air has a mass of 1.2kg/m3 at ground level, half at turboprop cruise altitudes, and about one-third at jet cruise altitudes. So if we push the air backward at speed higher than our forward speed, we generate a thrust in the other direction through Newton’s Force = Mass * Acceleration. We can also write this in its momentum form:

           Force = Mass flow * Increase in speed of flow through propeller/fan

 Let’s call the increase in speed of the mass flow the Overspeed we give the air mass. So our Thrust then equals Mass flow * Overspeed.

The above means we can generate thrust either by having a low mass flow of air that we give a high Overspeed (a jet engine does this) or a high mass flow of air that we give a low Overspeed (a helicopter/VTOL rotor or aircraft propeller does this).

The forward thrust is the same in both cases, but the power we need to deliver to the propeller/fan shaft to give the mass flow its Overspeed will vary:

A thrust generated by giving a high mass flow a low Overspeed requires less power than one providing a small mass flow a high Overspeed.

On the other hand, as the aircraft speed increases, the Overspeed, which is the difference between the airspeed after passing the propeller/fan and the speed of the air hitting the propeller/fan, reduces. As Thrust = Mass flow * Overspeed, our thrust decreases.

Different thrust devices for different speeds

So helicopter or VTOL rotors, the propeller, the open rotor/fan, and the turbofan/jet engine all work in the same way. It also means they all have these characteristics. Different industries have different names for the parameters to achieve these characteristics:


For helicopters/VTOLs, we talk disc loading. As the surrounding air’s speed is zero at the hover and the force needed for hover equals the vehicle’s weight, it’s defined as vehicle weight divided by the total rotor area.

A low disc loading equals high mass flow and low Overspeed, i.e., a high propulsive efficiency. A Volocopter or Joby VTOL has much lower disc loading than the Lilium VTOL, Figure 2; thus, these need less power in the hover for the same weight.

Figure 2. The Volocopter (left), Joby (center), and Lilium VTOL (right) all generate thrust in different ways. Source: Leeham Co. and the OEMs.

As these fly forward, the Overspeed differences will affect the thrust decline of their propulsion systems.

Propeller aircraft

Propeller aircraft accelerate a large mass flow to a modest Overspeed. Thus, they are very efficient at low speed, but as the speed increases, the Overspeed decreases, and the forward speed eats into the thrust generation and, therefore, efficiency (Turboprop Figure 1). Turboprops seldom fly faster than Mach 0.6/350kts.

Propeller aircraft designers talk about thrust-, power-, and torque- coefficients. These all have the propeller RPM and swept area as parts, thus indirectly describing mass flow and Overspeed physics.


The above physics is indirectly described by ByPass Ratio, BPR.  ByPass Ratio is defined as the amount of air that bypasses the core compared with the air that passes the core.

As BPR increases, the mass flow increases, and the Overspeed decreases. The Overspeed, which defines the engine’s characteristics, is called Specific Thrust in Turbofan industry speak.

As the technical capability to do large diameter fans and nacelles improve, we have gone from medium ByPass Ratio turbofans (around five) to high BPR engines (BPR 10-12, HBPR Turbofan, Figure 1). We can expect this to reach 15 in the next generation, as composite fans and nacelles can handle larger fans.

Open rotor/fan

An open rotor is a turbofan without nacelle. As the turbofan nacelle has flow deswirling stator blades in the bypass stream behind the fan, open rotors have contra-rotating fans to achieve the same effect.

General Electric found it could simplify the scheme to a single rotor followed by a fixed deswirling stage. When both stages used variable pitch blades, the engine achieved the same function with reduced complexity. It is now industrialized together with SAFRAN as the CFM RISE engine, Figure 3.

Figure 3. The CFM RISE open fan engine concept. Source: CFM.

The solution is called Open Fan. By keeping the fan diameter modest and increasing the speed gain through the fan, the sensitivity to forward speed for the concept is reduced.

CFM says the speed range of the Open Fan engine is compatible with single-aisle cruise speeds. Keeping the fan diameter down also reduces the installation complexity of the Open Fan compared to larger diameter open rotor concepts.


The thrust concepts can be combined with any shaft power system

It’s important to understand that these methods to generate thrust, whether a large diameter rotor, a propeller, an Open Fan, or a nacelle enclosed fan, can be used with any scheme to generate the shaft power to drive these devices.

We can have a piston engine, gas turbine, or electric motor to generate the shaft power we need. As you design an air vehicle, you combine the chosen power system with a suitable thrust generating device. What we combine depends on what characteristics we want for our project. In effect, we combine mass flow and Overspeed to suit our vehicle.

Here are some additional effects to consider when we chose mass flows and Overspeeds:

  • To move a lot of air (high mass flow), we need large diameter rotors. These will be efficient for low-speed flight but will hamper us as we speed up.
  • A large rotor also generates noise from fast tip speeds if we drive the rotor at a high RPM. A nacelle can dampen this noise. Thus the low noise footprint of a high bypass turbofan and the enclosed fans of the Lilium VTOL (despite these having high tip speed fans).
  • High Overspeed concepts are noisy. The sound from an air jet is directly proportional to the Overspeed (the turbulent mixing of the flow with surrounding air). Low noise vehicles thus work with low Overspeeds of the engine jet.
  • If we have a high mass flow/low Overspeed system, we will have increased efficiency at low speed. Our take-off (normal or hover), climb, and a modest speed cruise will be efficient.
  • On the other hand, this setup is not suited for high-speed flight. An SST propulsion system must be designed with a low bypass ratio for the cruise, or if we fly over Mach 2, none at all. Otherwise, our power to thrust ratio decreases dramatically.
  • These contradictory requirements lead to variable cycle engine requirements for vehicles covering large speed ranges (fighter jets, SSTs). A variable cycle engine has low Overspeed at low speed, then increases the Overspeed as the aircraft speed increases. It does this via a variable ByPass Ratio, BPR.

25 Comments on “Bjorn’s Corner: Sustainable Air Transport. Part 16. Thrust generation

  1. Compared to the previous eighties open rotors with contra-rotating pusher fans the CFM open fan concept means:
    – n0 blade tip shockwave interactions (NOISE)
    – lighter and less complicated gear box
    – more efficient gearbox / engine cooling
    – no engine pylon wake hitting the fan (NOISE)
    Not an engine expert but I have a lot more confidence in this open fan approach.

    Modern fans and intelligent situational FADEC could further reduce noise levels significantly.

    • You loose some benefits you have with turbofans as the engine inlet slows down the air speed and increase pressure for the fan and booster intake, you might need an anti-ice system on both fan and the variable exit guide vanes. You loose the nacelle noise damping, you loose the traditional thrust reverser function and the exit nozzle to fine tune the fan exit speed, you loose the fan containment function and might need to set hard times on your fan blade set. You might need variable exit guide vanes. Still you gain in reduced mass and cost for the nacelle, you increase the bypass ratio even more than the fan size. So it becomes a trade-off as you reach the same cruising speeds and meet noise certification requirements with a lighter/cheaper powerplant installation with high bypass ratio.

      • Theres still a core turbine engine with a hot exhaust and current
        civil turbo fans dont use variable area exhaust duct ( they have been developed but not used as they add cost and maintenance load).

        It seems from some comments that the long use of turbo-props which have a turbine core and a geared propeller hasnt happened. Reverse thrust? Theres a solution for that

        • Current large tubofans have carefully dimentioned core nozzle exit area and fan exit areas aft the Thrust reverser or in the CNA “Common nozzle area”. The open rotor will only have the core one but will most likely not have pivoting fan blades, those blades/props works as thrust reversers on turboprops. So the airframer needs to decide how compensate lack of fan thrust revereser. Like using nose wheel brake (like on any car, modern truck or JAS39 fighter) or speed spoliers (like Fooker F-28) I doouble the movable fan exit guide vanes can do much braking action only supported on the i.d. and you want to avoid having core engine thrust reversers. I vote for nose wheel brakes..

          • -In the case of electric motor driven ducted fans, open rotors and propellers no variable pitch is needed for reverse thrust nor thrust reversers.
            -The motors can be placed into generator mode creating high drag as the fans become wind turbines. The electricity generated can could be be dumped into the PEM fuel cells which are actually reversible and become electrolysers and will generate hydrogen or they might be dumped into the de-icing system or any kind of resistive heating element. The fans can actually even of course be rotated in reverse to generate reverse thrust.

  2. Is it fair to say that a propeller blade or fan blade possesses a lift to drag ratio, exactly like a fixed wing? And when parts of the blade go transsonic, then the ratio is severely reduced due to wave drag?

    Personally, I don’t mind if regional and sub-regional aircraft cruise at a slower speed than, say, transcontinental aircraft. The cruise portion of flight is relatively short anyways.

    • Yes, except L/D varies substantially from hub to tip because of the rotational velocity (hence varying blade angle & chord). That sounds correct, with the tip being the first to go transonic of course.

      • You’d introduce supercritical profile sections.
        ( not looked, state of the art today already? )
        math theory (conformal mapping) would give you tool access to solving this.

    • “The cruise portion of flight is relatively short anyways.’

      Doesnt work like that, remember they dont climb so high either.

      just looking at ATR flights from flightaware and getting the flight profile a 1hr 40 min flight of an Air NZ route the climb to 5500m cruise altitude seems to be around 10% of the flight time. I can imagine a jet climb is shorter , maybe around 5% or less for similar flight time .

  3. -It would say then that purpose of the variable area nozzles on the EDF “Electric Ducted Fans” on LiliumJet are to reduce the over speed during hover. I have also noticed variable area intakes being developed for EDF.
    – I refer to them as EDF because this is the term used in the remote controlled hobby aircraft industry which initially developed these. Electric Jet is also correct.
    -If there is substantial heat to be rejected from a fuel cell or battery cooling the heat could provide inlet deicing as well as generate some thrust by heating the lining of the fan duct.
    -If an electric open rotor is developed it will be easy to use a contra rotating rotor pair by having two coaxial electric motors, one of which has a hollow shaft. These are already in existence.
    -A great opportunity will be power and thrust vectoring for yaw and maybe pitch and roll control.
    -If an electric propellor or open rotor is given not only variable pitch but cyclic pitch control(like a helicopter) it will be possible to generate pitch and yaw control. If a coaxial contra rotating prop is used varying the relative rotor speeds will allow roll control can be generated. (Even without power). We could this replace the entire empennage with elevator and rudder as well as stabiliser and tail fine removed.

    • Wind Turbines have individual electrical control of blade pitch. The power transfer can be contactless inductive rather than over a slip ring. In the case of a single stage open rotor with varabmr pitch open stair it will be possible to generate pitch, yaw and roll forces even with the motor of using the stator blades.

    • Regarding the use of ejected heat: heat transfer normally implies some sort of refrigerant/liquid system to transfer heat from one place to another. Taking the existing safety/certification requirements into account, because of which you have to consider things such as zonal safety analysis or particular risk analysis (per ARP 4741), this could be a very heavy an complex system
      Very interested to see technically feasible concepts complying with CS25 regulations!
      (Note: not saying that it cannot be done, just genuinely interested)

      • The exhaust of a fuel cell carries away very little waste heat. There is an electrolyte circulating between the electrodes with fuel on one side and oxidiser on the other. The temperature needs to be kept below about 80C. Material compatibility issues limit the use of copper. Where to get rid of the heat? Radiator, leading edge, engine cowling? A heat pump might be useful for the final part of cooling.

    • Looking at the LiliumJet configuration, lifting 5t above the earth with the fan space I see, means a really powerful, speedy, noisy airstreams. Enough to damage landing spots, creating noise levels that make ear protection for passengers required. Please factor in the normal restrictions for anyone in the neighborhood or flightpath of a LiliumJet.

      And there are solid safety / emergency requirements that will have more influence than the great visual promotions shown to us suggest.

      No breakthrough, technological, legal miracles there, unfortunately. And, as we know, ignoring means bankruptcy sooner or later.

      • -Lilium will be very quiet, its is designed to be much quieter than a helicopter, and will not suffer from vertiport ground erosion problems.
        -Ground erosion problems are associated with 1960s vertical lift turbojets and hot exhaust gases in the first generation of jet VTOL eg VJ101A/B. Most of this problem has been solved with the use of turbofans and their much slower and ‘cooler’ exhaust eg F35 and harrier. An electric fan with a variable area nozzle has no hot exhaust and even lower exhaust velocity than a high BPR turbofan.
        -Lilium Jet in operation will only ever land on Vertiports with a sealed surface of concrete, asphalt or maybe rubber. There is video of Liliums test bed landing on grass field (German Pasture) and the grass and underlying soil is not disturbed. In reality it’s never going to be operated like this.
        -In terms of sound Lilim Jets target is 60dB the worst estimates based on video suggest that LiliumJet producing about 65dB-68dB allowing for the larger models. For reference 70dB is the sound of a shower or dishwasher. 60dB is a quiet home conversation. This is without the anechoic liners taken from turbofan practice that typically remove 5dB of sound. There will be acoustic liners, acoustic traps and helmolholz resonators. The variable area nozzles will slow the air flow velocity to below that of a turbofan fan and of course there is no noise from the turbine core itself and its hot fast jet.
        The transition from forward flight to landing will probably take 12 at most 18 seconds. Because of the boundary layer suction effect Lilum must have a coefficient of lift in excess of 4.5-5 and they say the can fly extremely efficiently in this transition mode (and therefore will be very quiet on apporach)

        • CV22 ( turboshaft exhaust)
          F35 (rear movable nozzle)
          Harrier ( rear set of rotatable nozzles) :
          all have persistent issues with hot exhaust erosion effects.

  4. Here’s the source for Figure 1: The figure is on page 5 of the source document.

    That figure has the propulsive efficiency at Mach 0.8 to be about 0.8 for high bypass turbofans and 0.6 for propfans. But if you look at the Open Rotor Engine Aeroacoustic Technology Final Report that GE submitted to the FAA in 2013 (, its figure on page 9 has the M0.8 propulsive efficiency at 0.96 for open rotor and about 0.85 for the “NextGen narrowbody” engine (presumably the CFM LEAP and PW geared turbofan). If you look at the net efficiency on page 28, the net efficiency is still 0.86 at M0.80, and at M0.85 the efficiency declines but is still at 0.80.

    So the figure in this column drastically underestimates open rotor performance at higher speeds.

    • YES, CFD analysis/design together with crafted carbon blades and FEM analysis works well in skillful hands to increase cruise speed and efficiency while keeping noise within limits. There has been developments since the Tu-95 prop system (amazing for its time…)

    • It seems very sensible to me to employ aero-acoustic blade design, as opposed to aero-only blade design (Figure 17 in the referenced FAA report). Particulary so, in my opinion, when the efficiency penalty for the aero-acoustic design can be easily recovered by slightly lowering the cruise speed.

  5. The RISE has not been industrialized. Its not even been prototyped.

    Its a concept.

    My take is is CFM getting government money so they can build a Turbofan core and get the expertise of a geared system.

    With the geared aspect is far more of a Turboprop than anything. A variant at best.

      • GE and P&W most likely have new core engines in the military sector that would work fine, but putting these engine cores into commercial engines risks giving some of the technology to China/Russia. So the LEAP engine core is advanced enough to drive the RISE engine, maybe scaled up 10-12% to get 40k thrust (the big fan increases T-O thrust and climb performance up to speeds indicated in Bjorns Fig 1. due to higher efficiency.)

  6. Ultra-light liquid hydrogen tanks promise to make jet fuel obsolete

    A revolutionary cryogenic tank design promises to radically boost the range of hydrogen-powered aircraft – to the point where clean, fuel-cell airliners could fly up to four times farther than comparable planes running on today’s dirty jet fuel.

    Weight is the enemy of all things aerospace – indeed, hydrogen’s superior energy storage per weight is what makes it such an attractive alternative to lithium batteries in the aviation world. We’ve written before about HyPoint’s turbo air-cooled fuel cell technology, but its key differentiator in the aviation market is its enormous power density compared with traditional fuel cells. For its high power output, it’s extremely lightweight.

    Now, it seems HyPoint has found a similarly-minded partner that’s making similar claims on the fuel storage side. Tennessee company Gloyer-Taylor Laboratories (GTL) has been working for many years now on developing ultra-lightweight cryogenic tanks made from graphite fiber composites, among other materials.

    GTL claims it’s built and tested several cryogenic tanks demonstrating an enormous 75 percent mass reduction as compared with “state-of-the-art aerospace cryotanks (metal or composite).” The company says they’ve tested leak-tight, even through several cryo-thermal pressure cycles, and that these tanks are at a Technology Readiness Level (TRL) of 6+, where TRL 6 represents a technology that’s been verified at a beta prototype level in an operational environment.

    This kind of weight reduction makes an enormous difference when you’re dealing with a fuel like liquid hydrogen, which weighs so little in its own right. To put this in context, ZeroAvia’s Val Miftakhov told us in 2020 that for a typical compressed-gas hydrogen tank, the typical mass fraction (how much the fuel contributes to the weight of a full tank) was only 10-11 percent. Every kilogram of hydrogen, in other words, needs about 9 kg of tank hauling it about.

    Liquid hydrogen, said Miftakhov at the time, could conceivably allow hydrogen planes to beat regular kerosene jets on range. “Even at a 30-percent mass fraction, which is relatively achievable in liquid hydrogen storage, you’d have the utility of a hydrogen system higher than a jet fuel system on a per-kilogram basis,” he said.

    GTL claims the 2.4-m-long, 1.2-m-diameter (7.9-ft-long, 3.9-ft-diameter) cryotank pictured at the top of this article weighs just 12 kg (26.5 lb). With a skirt and “vacuum dewar shell” added, the total weight is 67 kg (148 lb). And it can hold over 150 kg (331 lb) of hydrogen. That’s a mass fraction of nearly 70 percent, leaving plenty of spare weight for cryo-cooling gear, pumps and whatnot even while maintaining a total system mass fraction over 50 percent.

    If it does what it says on the tin, this promises to be massively disruptive. At a mass fraction of over 50 percent, HyPoint says it will enable clean aircraft to fly four times as far as a comparable aircraft running on jet fuel, while cutting operating costs by an estimated 50 percent on a dollar-per-passenger-mile basis – and completely eliminating carbon emissions.

    HyPoint Dramatically Extends Zero-Emission Hydrogen Flight Range With New Ultralight Liquid Hydrogen Fuel Tanks

    HyPoint, the company developing zero carbon-emission turbo air-cooled hydrogen fuel cell systems for aviation and urban air mobility, today announced a partnership with Gloyer-Taylor Laboratories (GTL), an aerospace engineering research and development company, to integrate GTL’s advanced carbon composite “BHL™ Cryotank” liquid hydrogen fuel tanks with HyPoint’s fuel cell system. BHL™ Cryotanks have demonstrated a 75% mass reduction compared to existing state-of-the-art aerospace cryotanks (metal or composite), enabling hydrogen aircraft and eVTOL makers to store as much as 10 times more liquid hydrogen fuel without adding mass. As a result, aircraft can travel longer distances without refueling. HyPoint also announced that former GE Aviation and Rolls-Royce engineering executive Umeed Ashtiani has joined the company to lead the company’s system engineering team and oversee the implementation of the tank technology.

    • I dont think its ever seriously suggested that airliners use high pressure compressed hydrogen and the LH2 at a very low temperature was the obvious pathway.

      The claim of a mass fraction of 70% seems to be highly ambitious compared to the fraction of 35% in Bjorns articles ( using similar materials). Im thinking they are using computer models and maybe not including the system hardware to ‘hit their numbers’

      • Several makers such are proposing conversions of ATR42/74 and Q300, D328 with gaseous compressed hydrogen. Compressed hydrogen works well for flights under 400km (which turns out to be the bulk of flights for this class of aircraft)
        Universal Hydrogen
        I suspect these aircraft may be able to convert over to cryogenic hydrogen when systems are developed though I would expect that to be so far down the track purpose built aircraft may be more attractive.

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