102

You want to reduce weight by removing a few kilos of air, and adding an equal, if not greater, amount of kilos of pumps needed to reflate the tires (see David Richerby's answer for details), not to speak the additional costs incurred for periodically checking the correct functionality of said pumps, that would be marked a "safety critical" (because ...


73

Yes it is correct that helicopters use more fuel when hovering: the engine needs to apply more power to overcome drag. Here is a graph of the engine power required for different airspeeds, from J. Gordon Leishman, Principles Of Helicopter Aerodynamics: The line for total power goes down between 0 - 70 kts with increasing airspeed, this is caused by the line ...


72

Yes, propellers have problems at high speed, but if done right, they still have an advantage over turbofans at speeds up to Mach 0.8. Look at the inner engine gondolas of the Tu-95: They are elongated and thicker aft of the trailing edge. This was done to stow the landing gear in them, but also to area-rule the aircraft. The Tu-95 applies knowledge that was ...


49

That scenario only makes sense if your airplane stays at cruising altitude: although taxi and takeoff does use up fuel it's really the ascent to cruise that takes the most. You aren't really going to be able to drop cargo accurately from cruising altitude, so you'll have to descend pretty low, then you'll need to climb up again, and that would suck up lots ...


47

Some do (or have in the past) but very high altitudes present their own issues. Historically the Concorde cruised anywhere from FL550 to FL600 and was actually allowed to climb and descend at its discretion up there since they were well clear of any traffic. However the increase in pressure differential on the airframe as well as supersonic flight meant the ...


46

Constraints Different applications have different constraints: Aviation: very light weight, highly reliable Marine: very high endurance Automotive: moderately light weight, responsive Motorcycle: very light weight, very compact, very responsive Different technology ages yield different solutions due to additional constraints, always limited by the then ...


45

There is an easy way to get an idea of what magnitude the savings are: The Breguet equation can answer this. You first calculate a reference aircraft and then start all over with one that weighs 1% less. The difference in fuel consumption shows how much fuel is saved per flight. Our reference design is the A320 which has an L/D of 18 and we let it fly a ...


41

From the B-29's commander training manual, page 69: The range loss at high altitudes is caused almost entirely by the rich mixtures required at high powers and the larger cowl flap openings needed to cool the engines at high altitudes. In general, there is no loss in range up to 15,000 feet and the range losses at higher altitudes occur almost entirely ...


40

Yes, it is correct, if the helicopter doesn’t fly too fast. A helicopter will produce the necessary lift most efficiently at a moderate forward speed. In a hover all the airflow which is available for lift creation must be generated by the rotation of the main rotor. This means that a small amount of air must be accelerated by a lot. If the helicopter adds ...


37

Some military transport aircraft can indeed adjust tire pressure from the cockpit for soft field operations. The Antonov An-22 would be one example. A central tire inflation system is a standard feature on Soviet military trucks as well, and also used on some US trucks. That the feature was removed on later versions of the An-22 should tell you something ...


36

You are right, more blades are bad for efficiency (follow the link for the definition). Ideally, a propeller should have only one blade. Every additional blade will cause disturbances which interfere with the flow on the other blades. When engine power increases, the propeller disc area should also grow, but this growth is limited by the resulting speed of ...


34

Along with the pump (and such) involved, it should be noted that inflating the tires of a large aircraft is a fairly non-trivial undertaking from a safety point of view. In particular, when you're inflating a tire you normally put it into a tire cage, like this: Photo Credit: Martins Industries. That's not the only variety, but you get the idea. It's ...


34

Looking purely at operational cost of the aircraft, yes. You save time, burn less fuel, don't have to pay for the landing etc. But dropping the cargo makes the cargo more expensive. You have to provide parachutes (and return them after use, inspect them etc). You have to combine cargo into parachute loads. You have to package the cargo for a hard landing, ...


33

In some contexts, the economics depends on how special the aircraft you're refueling is. Consider something like an AWACS mission. You have this very complex specialized aircraft, with a large, highly trained, crew in addition to the flight crew. If you didn't refuel that during the mission, it might need to spend a lot of its active hours being ...


31

Not storms, but there is a concept called "Pressure Pattern Flying" where you plan routing to stay in favourable circulation around Highs and Lows, to the extent that deviations to follow the circulation flow and stay in tailwinds can get you there sooner than going straight. To take advantage of this you need to be going fast enough and with the ...


28

First to the issue of cost-effectiveness: If fighter or strike aircraft would need to take more fuel with them, they would have to be larger and more fuel-hungry themselves. This is hard to quantify, but with air refuelling you can cap the size at a much smaller value and still be sure that all future range and endurance requirements can be met. Next, ...


28

Aviation engines run at near max RPM through out the flight. A car on the other hand doesn't use the full RPM spectrum except in bursts.1 If a car engine was utilized the same way an aviation engine is, it won't last long. So an aviation engine is sturdier, heavier, and weaker (hp) for the same displacement, but also provides higher torque (big cylinders)....


28

Your reasoning is correct if engine mass is not important. Ships use huge engines, because increasing the number of cylinders beyond 8 will have diminishing returns in terms of smoothing out the torque ripples, and bigger cylinders help to increase efficiency. But aircraft need to keep the mass of the engine down. Wartsila-Sulzer RTA96-C turbocharged two-...


28

You simply wouldn't save any weight. One of Goodyear's largest commercial aviation tyres is the 54x21.0–23 Flight Leader,* used on Airbus A330s and A340s. That is, outside diameter 54 in, width 21 in, rim diameter 23 in (137x53x48 cm). Assuming the tyre has rectangular cross-section, each tyre has a gross volume of about 0.7m3. ...


27

Yeah, I'm not a physics student, but I work on Black Hawks. If you conceptualise a helicopter as just a main rotor disc producing lift, then Peter Kampf's answer about mass-flow through the rotor disc is the greatest factor. (Remember the disc is tilted forward as the helicopter moves forward). However, your question actually asked why do they burn less ...


26

The efficiency issues are as follows. For a fixed amount of mass flow rate through a work-extracting thermodynamic process, one large engine is more efficient than two smaller ones because the pair of smaller engines present more interior surface area relative to the mass flow through them than the single larger engine. The more wetted area inside the ...


26

Your question already contains the answer. As you say kinetic energy is proportional to velocity squared, so it is easier to accelerate air from 0 to 100 m/s than from 100 to 200 m/s. The same is true for the air flowing through the propeller disk. Even if we replace the propeller by a black box, or better a black disk, which simply adds a bit of pressure ...


25

If you ever get the chance, do try a winch start in a glider. It is, to say the least, an interesting experience. I do not believe it would be accepted by the general public. So, that is one more reason, it would scare the paying passengers.


25

As a French who grew up in the Caribbean, I can tell that flights are getting longer. A flight between France and my island that took 7.5 hours 15 years ago is almost 9 hours long today. This post published by the MIT’s School of Engineering says (emphasis mine): Specified cruising speeds for commercial airliners today range between about 480 and 510 ...


24

First of all, keep in mind that the Philippines has one of the shortest average populations on Earth. Their average passenger weighs a lot less than an average American or European, since weight scales roughly with the cube of height. So, 436 Filipinos don't weigh nearly as much as 436, say, Americans or Europeans, assuming average populations from each ...


24

The main advantage of landing is that the plane can then carry another cargo on the return journey. Flying an empty plane back home is extremely inefficient and halves the range of the plane. Air dropping might make sense for a large number of relatively small but urgent packages with lots of destinations along a route, but even then the plane would be ...


23

One % of takeoff weight is huge. On a B747 with MTOW of 300 tons, that is a weight saving of 3,000 kg, weight that needs not be lugged around for the couple of decades that the aircraft will be flying. At the aircraft factory where I used to work, there were Idea Boxes everywhere where one could deposit suggestions for weight savings, every kg saved adds to ...


23

I won't prove it's not possible, but I'd would be surprised it is. I'll develop the reasons why and provide the physical framework you can use to compute a solution to have, with some effort, a final answer. The glider case cannot be scaled up to an airliner It works with a glider for two reasons: The height to be reached before the glider is able to gain ...


22

It's not a question of saving money, or emergencies. The reason militaries use mid-air refueling is so they can reach anywhere in the world. Aircraft have a limited range, without mid-air refueling they would have to have enough fuel to get to and from their area of operations back to a base. This would limit where military aircraft (or espionage aircraft) ...


21

Thermal efficiencies are very rarely quoted for aviation gas turbines. The metrics of interest are specific fuel consumption, and power to weight ratio. While a higher thermal efficiency will increase these, SFC and thrust/weight are performance terms that are easier to comprehend, and describe the performance in terms that can directly be used in ...


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