102
votes
Could airline tires be deflated in mid-flight to reduce weight?
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 ...
- 32.4k
79
votes
Accepted
Do helicopters use more fuel when hovering?
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. ...
- 61.3k
73
votes
Why is the Tu-95 so efficient despite having propellers that spin faster than the speed of sound?
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 ...
- 230k
48
votes
Accepted
Is it cheaper to drop cargo than to land it?
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 ...
- 52.9k
47
votes
Why increase the number of cylinders in an engine instead of increasing their volume?
Constraints
Different applications have different constraints:
Aviation: very light weight, highly reliable
Marine: very high endurance
Automotive: moderately light weight, responsive
Motorcycle: ...
- 10.7k
47
votes
Why then don't aircraft fly even higher, for even greater efficiency?
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 ...
- 100k
46
votes
Accepted
How efficient is a Scramjet?
A scramjet is more efficient than a rocket engine for the same two reasons that all air-breathing engines are more efficient than (chemical) rocket engines:
Chemical rocket engines need to carry both ...
- 5,700
45
votes
How much of an improvement would a 1% weight decrease on an airplane be to the industry?
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% ...
- 230k
42
votes
Do helicopters use more fuel when hovering?
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 ...
- 230k
40
votes
Accepted
What are the advantages of more than 4 propeller blades?
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 ...
- 230k
37
votes
Could airline tires be deflated in mid-flight to reduce weight?
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 ...
- 230k
34
votes
Could airline tires be deflated in mid-flight to reduce weight?
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 ...
- 3,157
33
votes
Is it cheaper to drop cargo than to land it?
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 ...
- 10.2k
31
votes
Can you ride a storm to save fuel?
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 ...
- 126k
28
votes
Why increase the number of cylinders in an engine instead of increasing their volume?
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 ...
- 230k
28
votes
Could airline tires be deflated in mid-flight to reduce weight?
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, ...
- 11.9k
28
votes
Do helicopters use more fuel when hovering?
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 ...
- 281
27
votes
Why are winches not used for towing commercial planes up?
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 ...
- 1,586
26
votes
Why are winches not used for towing commercial planes up?
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 ...
- 71k
26
votes
How does increasing the size of a jet engine make it more fuel efficient?
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 ...
- 20.7k
26
votes
Accepted
Why is thrust inverse to speed in piston engines?
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 ...
- 230k
25
votes
Did airlines fly their aircraft slower in response to oil prices in the 1970s?
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 ...
- 1,880
24
votes
Is it cheaper to drop cargo than to land it?
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 ...
- 3,353
24
votes
Why is a dedicated compressor more efficient than using bleed air to pressurize the cabin?
The other answer is on the right track, but doesn't emphasize the real difference (IMO).
Let's say it takes 100 hp (or kW if you prefer) to compress the amount of air you need for a given purpose. ...
- 7,319
23
votes
Accepted
Why the lack of faster piston-powered planes?
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 ...
- 230k
23
votes
How much of an improvement would a 1% weight decrease on an airplane be to the industry?
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....
- 61.3k
23
votes
Accepted
Which jet engines have the highest thermal efficiency?
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 ...
- 4,463
21
votes
Accepted
Are turboprops more efficient than piston engines (thrust per fuel consumption)?
No, piston engines are more efficient.
Their output is given in kilowatts or HP because this does not change much with speed, unlike the thrust of a propeller. By running a piston engine a few times ...
- 230k
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