Question of the Week

Why can't aeroplanes fly faster?

Sun, 23rd Nov 2014

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Paul Jen asked:

Love your show; listen to it on podcast. Also thank you for answering a few of my questions in the past (survival time in sealed car, coughing while cleaning ear).

I was on an interminable flight from New York to Hong Kong a few days ago, and I don't understand why are Boeing and Airbus not working on increasing the speed of air travel? They have worked hard to improve the seat comfort, entertainment and food, but heck, if they just shortened the trip by 50% that would please me 500%. Are we going to be forever stuck in this air speed? What are the constraints of aerodynamics? Are noise rules the issue? Can a supersonic jet be ever built with "decent" fuel economy? Will we ever see another "Concorde" in our lifetime?



Sara -  This week, we buckle our seat belts ready for takeoff with this question from listener, Paul Jenn.

Paul -  I was on an interminable flight from New York to Hong Kong a few days ago and I don't understand why airplane manufacturers arenít working on increasing the speed of air travel.

Sara -  So, why can't we fly just a little bit faster?  Is it a case of better design and engineering or will we be stuck at this slow speed forever?  Well, weíre off to a flying start as Neil Scott, head

of engineering at Airbus shot back fast with this answer...

Neil -  In short, speed costs fuel and money.  If you're driving your car at 40-mile an hour and then accelerate to 80 miles an hour, what happens to fuel consumption?  It goes up.  The same is true on an aircraft.  The faster you go, the more fuel you burn.  Drag, which causes the increase of fuel burn, is actually proportional to the square of the speed [addendum: energy to overcome drag is proportional to the cube of the speed].  So, drag increases at a faster rate than increasing speed.  So, you can't just get there more quickly, you have to pay for it.  One of the biggest costs for an airline of course is fuel, therefore the likelihood of us designing a supersonic aircraft for major commercial flying is probably not going to happen anytime soon.

Sara -  Basically, more speed means more air resistance, means that cheap holiday flight you were planning on booking wonít be so cheap anymore.

Neil -  Another advantage of decreasing fuel burn of course, is there's less CO2 emissions, less nitrous oxide emissions and less noise, so we all win.

Sara -  Well, itís good to hear that we all benefit from flying slower, but is it a flight fancy to think I wonít be stuck in the air for 13 hours at a time as a jet setting grandma?

Neil -  At the moment, itís not possible to build a super efficient plane that's also super speedy and cost-efficient.  But with future innovations in propulsion systems, thatís engines, based for example on hydrogen or electricity, it might well be possible in the future.

Sara -  Thanks, Neil.  Maybe someday then.  In the meantime Paul, maybe try to get some shuteye on the redeye.  For next week, Matthew Boniface wrote in this yawn first of a question...

Matthew -  Why do I yawn and why do cats yawn and can I catch a yawn from a cat?

Sara -  What do you think?


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Law of diminishing returns applies to making aircraft go faster... RD, Tue, 17th Jun 2008

Yes, but this can be solved by flying higher, which is what Concorde did. I generally agree though, we have not got the technology yet to both speed up the flights AND make it cheaper. graham.d, Tue, 17th Jun 2008

The Boeing 7E7, which eventually emerged as the 787, was originally planned to be a highly area-ruled design, capable of flying at just below the speed of sound for the same fuel burn as the then current range of airliners.  However, this aspect of the design was eventually dropped in favour of flying at the same speeds as other airliners, but with reduced fuel costs, the main reason being that the airline businesses could charge the same price for the same journey time at a reduced fuel cost to themselves.  Reducing the journey time would have seen the same fuel costs - guess which option was most attractive to the airline businesses?

Modern jet airliners rely upon high-bypass turbofan engines to achieve their economy but these engines produce relatively low-speed exhaust thrust - only a relatively small proportion of the air drawn in by the fan is actually routed through the gas generator and burnt (which produces high-speed thrust) - the majority of the air is simply accelerated by the fan, just like a propeller, and bypasses the engine core entirely.

Supersonic aircraft need high-speed exhaust thrust engines, which means thrust from burnt fuel, and although the engines used in supersonic military jets now are bypass types, they have low bypass ratios.  The bypass air can help with subsonic cruise but can't contribute much at high speeds.

Both noise and complexity is an issue with supersonic capable engines.  Higher thrust speeds mean more noise, but as well as this, the air entering turbine type engines must do so subsonically.  This means that the engine nacelles have to incorporate additional mechanisms for slowing down the airflow before it gets to the engine compressors.  Depending on the type of nacelle, it's usually done with either a moving centerbody i.e. the cones you see in the engines of the MiG21, EE Lighting and SR-71, or internally with a series of movable ramps i.e. Concorde, F-15.

Flying supersonically is much more costly in terms of design, manufacture, maintenance and fuel - OK for the military but not for airlines who are primarily concerned with making a profit. LeeE, Tue, 17th Jun 2008

yeah i would imagine it being really expensive and polluting  and if its too fast it may give people heart attacks so there may be a speed limit for a aeroplane to be able to made up to benep, Tue, 17th Jun 2008

Aiplanes at present flying altitude have reached the maximum efficieny level. As can be seen be the abadoned Concord.

The hypersonic plane soon to be revealed will be a hybrid rocket/ jet plane and fly high above the atmostphere at enormuos speeds.

For example New York City To Sydney in little over six hours. Alan McDougall, Sun, 29th Jun 2008

One version of this "soon to be revealed" project died in 1988.
Getting a plane to that speed, then slowing it down again is horribly expensive in energy terms. Oil isn't getting cheaper at the moment. I don't expect to see this project happen any time soon. Bored chemist, Sun, 29th Jun 2008

Air travel gets slower and slower the time actually spent in the aircraft is often about half the of the actual journey time  syhprum, Tue, 25th Nov 2014

If you've time to spare, go by air.

I love flying, to the extent that I fly myself for any journey of more than 100 miles (or less if it involves crossing the London area) and less than 1000. What I resent is the 3 hour trip to Heathrow and a 4 hour "security" checkin for a 6 hour flight to Boston, followed by an hour waiting for immigration and then two days waiting for the suitcase: the best bit is sitting in the plane, eating, sleeping, or watching a movie, with no worries or frustrations at all. Flying to the Far East is great - a whole day in a plane, even if someone else is driving it! Until "they" improve the ground transit and handling facilities at airports, I see no point in flying any faster than Mach 0.9, and the British Isles are just beautiful at 5000 ft and 150 mph.

If I really need to talk urgently to someone in Australia, or on the Moon, I pick up the phone. Otherwise, enjoy the trip.  alancalverd, Tue, 25th Nov 2014

The next step may be sub-orbital rocket-assisted space hops. 
Yes, it will come.  Hopping from Britain to Australia in a few hours. 

Not that it will be cheap.  You may be spending a million dollars a ticket. 

How much is it worth to save an hour of travel time?  $100,000 per hour saved?  Even if you are sleeping most of that saved travel time?

Don't forget spending hours getting to the couple of space airports sparsely scattered around the globe. CliffordK, Tue, 25th Nov 2014

Even if we get suborbital flights that can reach anywhere on Earth in a few hours (most of which is spent getting the last 50km through the atmosphere), there is still the jet lag, which is sometimes quoted as 1 day elapsed time to fully recover from each hour of timezone change. evan_au, Wed, 26th Nov 2014

I suppose you could minimize your jet lag by flying from Britain to South Africa, or Australia to Korea.  Just choose your destinations carefully.

I'm not sure about the day per time-zone recovery.  However, what you do around the time of the international flight may be crucial to jet lag recovery.  In particular, one should try to start the destination sleep schedule while in flight, or perhaps even before leaving. 

So, with that in mind, the time in the air isn't entirely wasted, although not particularly productive either (full internet access would help some).

Just think how times have changed.  Not too many generations ago, and people were sailing across the Atlantic, and taking wagon trains across the Great Plains. CliffordK, Wed, 26th Nov 2014

I think airline pilots gain brownie points for saving fuel and often fly the aircraft much slower than the technology alows syhprum, Sat, 29th Nov 2014

The object of airline flying is to meet a schedule: missing a landing slot can be very expensive and screws up everyone else's day too. Private pilots and bush pilots on the other hand tend to fly for maximum range. The speed you fly, in either case, depends on the expected wind vector. Flying slowly is rarely the answer unless you have a hell of a tailwind. You can reduce your fuel consumption to "idle" but in the case of a small piston engine you may end up going backwards (it's quite good fun in a Piper Cub), and in the case of a jet the burn rate at stall speed can actually be higher than the cruise consumption.   

The difference between permitted takeoff and landing weights for an airliner is significant, so arriving overhead with much more fuel than planned (say due to an unexpected jet stream) can mean having to divert to burn off the excess.

Most airliners are optimised to cruise at Mach 0.9 or thereabouts.They won't survive supersonic flight and really don't like going much slower because to do so you need to deploy flaps and slats, which increase drag. alancalverd, Sat, 29th Nov 2014

The following isn't much help for crossing oceans, but on all other paths it would be better to use a system like the hyperloop - maglev capsules running through vacuum tubes on a narrow track. Faster than planes and environmentally friendly too. David Cooper, Sat, 29th Nov 2014

This sounds like the theory in the early days of motor vehicles: If you travel faster than 15 miles an hour, your brain will turn to jelly.
...Perhaps, on the roads of the day, it may have felt like that!

What matters are the G-Forces involved, and too much vibration can make you feel ill  .
But speed itself causes no problems - if you didn't look out the window, you wouldn't know how fast you were travelling. evan_au, Sat, 29th Nov 2014

High speed rail is already being used in many places around the world, except in the USA. 

Perhaps the future is a maglev rail in a vacuum, but that may be dangerous as traumatic decompression could be lethal.  And, of course, the infrastructure to build it would be extreme. 

The British have already put rail under the ocean...  well the channel, and I can imagine a Gibraltar-Spain tunnel or bridge will come in the future.  An underwater Atlantic or Pacific crossing is possible, but would be expensive, and would require some pretty extreme engineering to deal with pressures and safety margins. CliffordK, Sat, 29th Nov 2014

It's the wrong kind of high-speed rail: highly polluting, destructive and noisy. HS2 will have to be banned on environmental grounds soon after it's been built.

Decompression in a plane isn't great either, but it's rare, but in a tube with a vacuum in it you'd be able to let air back in in an emergency while you slow all the capsules to a halt over the course of half a minute or so, and that would make it fully survivable.

Narrow tubes for narrow capsules (no room for passengers who are highly overweight) shouldn't be a great challenge. Aeroplanes are more extreme.

The best bet would be to have something that floats. A thin tube for the maglev capsules to go through would be held higher than the tallest waves and would bridge between floating towers which would have their bulk kept deep underwater below the bottom of the deepest troughs. Alternatively, the tube could be kept at a deep level, perhaps a hundred metres below the surface. It may never happen at all though as there's little need for such a thing. It might be possible to go from London to New York via Siberia using a hyperloop system and get there faster than a plane which flies a direct path. David Cooper, Sun, 30th Nov 2014

A Moscow to Beijing and on to Alaska high speed rail line is already in the planning stage and will get built if wars don't intervene syhprum, Sun, 30th Nov 2014

Narrow tubes for narrow capsules (no room for passengers who are highly overweight) shouldn't be a great challenge. Aeroplanes are more extreme.

This statement is baffling. What infrastructure? The beauty of air travel is that all you need is a bit of flat grass (though concrete is nice) at each end and nothing but air in between. No problem of demolishing houses or destroying communities to connect a thousand destinations in any order: you take off, then fly in a great circle direct to wherever you are going. GPS, radio and radar are useful but the capital investment is negligible compared with railways and the flexibility of air travel is unbeatable. I recently flew from Cambridge to Swansea for a routine job, but literally as I got out of the plane I got a call to an emergency in Dublin. No new infrastructure, no need to re-route anyone else or wait for tomorrow's train - just get back in the plane and fly direct across the water.

The economics of HS2 are absurd. It would be cheaper, quicker, and have ten times the passenger capacity, to invest the money in aircraft to fly between Leeds, Manchester, Birmingham and London.  alancalverd, Mon, 1st Dec 2014

Narrow tubes for narrow capsules (no room for passengers who are highly overweight) shouldn't be a great challenge. Aeroplanes are more extreme.

This statement is baffling. What infrastructure?

I can see why you find it baffling, but I wasn't talking about the infrastructure when I said aeroplanes are more extreme. What's extreme about planes is that you have to tie yourself into a bomb and blast through lots of thick atmosphere before you get to altitude, and even there you've got masses of air to punch through, so the pollution involved is astronomical.

If everyone on the planet flew about the way that you do, the pollution would lead to a collapse of ecosystems all by itself, and then the mass-starvation of billions. Planes are not the solution - they are part of the problem and we need to get rid of them. Sending small capsules through narrow tubes would have a tiny impact and you wouldn't have to demolish much to make room for them. By having a network of them and giving capsules the ability to switch track without slowing down, journeys from any town to any other could be made in minutes with very little energy used, so it would be almost as good as a teleport. Capsules are the future of transport, not just for people but also for goods. We have idiots wanting the sky filled with angry buzzing as drones deliver things at high energy cost, but a network for sending capsules would enable people and goods to be sent from place to place for little energy cost and at much higher speed. That is where all our efforts should be going instead of building more roads, planes and high-speed trains. It all comes down to energy. For high speed travel you need maglev and a vacuum, and you'd have that for the intercity links. For lower speed travel to get to actual houses, wheels may be more appropriate, but it should be organised around the idea of capsules and packet switching, using extremely lightweight tracks (covered to keep the weather out) that can be build overhead to keep them away from existing infrastructure. That is where the future will inevitably take us, so it's a pity we have to watch so much of the planet being destroyed while the idiots at the top in politics insist on trying everything else first. David Cooper, Mon, 1st Dec 2014

The time has changed! Christophe B, Mon, 1st Dec 2014

I think a legitimate comparison is with a car. My current private aerial barge carries 4 people at 130 mph and does 20 miles per gallon, but then it's 40 years old and things were different way back then. The fossil energy required to make a car is about the same as it consumes in its lifetime, and at 15,000 miles per year I would have replaced the car at least 4 times during the life of the plane, and strewn about 30 sets of tyres across the country, not to mention the asbestos from the brake pads. I think we are just on to our second set of (very small) aircraft tyres, and all the debris has been deposited at least half a mile from civilisation .

Next plane (for delivery next year) will be a slightly slower single seater - most cars only have one occupant during the working day - that does 65 miles per gallon at 120 mph. Expected airframe life is about 30 years.

Because the plane flies in straight lines, flight distances over short trips are 20 - 40% less than road distances.  And because we don't slow down at intersections, my presence in the air isn't causing anyone else to decelerate and accelerate - most of the time we are all cruising at maximum efficiency. 

As for tying myself into a bomb, the big safety advantage of aviation is that you have 100% of the countryside in 3 dimensions to avoid others, instead of being confined to a narrow road with cyclists, animals and pedestrians. There used to be a sign over the exit gate at RAF Lakenheath "you are entering the most dangerous place in Britain".

Billions of people have managed to starve over the centuries without my help, or that of the Wright brothers. The solution to practically all the world's problems is to reduce the human population to a sustainable level everywhere. So it gives me great pleasure to quote your final sentence:

alancalverd, Tue, 2nd Dec 2014

Planes are now about as good as the best cars for pollution put out per mile, but there's greater potential for cars to improve. Planes also enable people to make lots of unnecessary trips that they simply wouldn't do by car, so the pollution goes up an up. There are a few people who do need to travel around more than the norm, of course (you may be one of them), and they can justify that travel as it is necessary and of benefit to all, but we need to eliminate unnecessary travel. We also need to free people up by eliminating unnecessary work so that they have more time to travel and can get places without needing to go so fast so that they can do it in much less polluting ways than using cars, planes or conventional high-speed trains. David Cooper, Tue, 2nd Dec 2014

Air travel is essential for environmentalists to attend conferences on pollution. Along with politicians, they have a neolithic intellect and are unable to use Skype. And if it were not for conferences on pollution, we wouldn't be able to exempt the major polluters (India and China) from abiding by the rules they make for everyone else. Apparently it is terribly important for people of limited brain power only to talk when the sun is within 60 degrees of the zenith, so they all have to be in the same place at the same time (with a break for lunch - the middle 30 degrees has some primitive religious significance). I've just been hearing about a 2-day conference of 5000 people in Sydney. Everyone went to dinner with their usual five friends and of course nothing was decided: 5000 environmentalists = 5000! (it's an astonishingly big number) arguments. You can't do that without aeroplanes.

I would be perfectly happy to run my business like a garage or GP surgery - if your x-ray machine doesn't work, or you have spilled radioactive material on the carpet, bring it to me and I'll prescribe some Prozac or explain that you can't get the parts for that model. But, alas, big machinery tends to be bolted to the floor and you certainly can't put a radioactive carpet in a plane, so Moses (or at least his descendant) has to go the mountain, preferably by yesterday.

It was  a stroke of genius to move the HQ of the NHS from London to Leeds, leaving the Dept of Health in Westminster. Obviously the highest-paid executives at each end are unable to use the telephone so they spend half of each working day on the train, and thus don't interfere with real work. A laptop full of speadsheets keeps them amused.

But I presume by "unnecessary trips" you mean holidays. WRONG! You can't have a holiday by Skype, or delegate skiing to the locals. A holiday is the one activity where you really, really must be there to do it, and the aeroplane is the cheapest and least offensive way of moving the unwashed across the planet. 

alancalverd, Tue, 2nd Dec 2014

Planes are essential so that environmentalists, politicians, and other highly paid saviours of the planet who are unable to use the telephone, can gather in one place in such numbers that they cannot make a decision.   

Not sure what you mean by "unnecessary travel", but I guess you mean holidays. WRONG!!!! A holiday cannot be had by Skype and you can't delegate skiing to the local staff - you absolutely have to be there. Business can mostly be done by phone or email.

Personally I'd love to run my business like a garage: if your x-ray machine breaks down or you spill radioactive material on the carpet, bring it to me and I'll tell you that you can't get the parts for that model any longer. Unfortunately big machines are bolted to the floor and it's not a good idea to move contamination around the country, so Moses (or at least this descendant) goes to the mouintain, and everyone wants it done yesterday.    alancalverd, Wed, 3rd Dec 2014

Cars also enable people to make lots of unnecessary trips that they simply wouldn't do by foot or horseback.  And there are many trips that people do in cars that would be better to do on foot or by bike. CliffordK, Wed, 3rd Dec 2014

Unnecessary air travel is only undertaken by primitive people like politicians and environmentalists, who are unable to use a telephone and cannot communicate unless they are all in the same place. So they travel to huge conferences involving too many people to actually make a decision, apart from exempting the worst polluters from complying with their wishes. I've just been hearing about a 2-day environment conference in Sydney involving 5000 delegates from around the world, who achieved absolutely nothing by being there. 

The only essential air travel is holiday traffic. You can't holiday by Skype, or delegate skiing: you absolutely have to be there yourself.  Almost everything else could be better done by phone or email. alancalverd, Wed, 3rd Dec 2014

Most conferences are a waste of time - there's no point in going anywhere to hear a speaker, and while small group discussions are more productive, they lend themselves well to video conferencing.

Holidays are a better excuse for travel, but if you have more time off you can travel slowly by more environmentally friendly means and see more of the world along the way. People would be able to have much more time off work if work was shared out more evenly. It's also better to go and live for a few years in a far-off country instead of flying out there repeatedly for short holidays. David Cooper, Thu, 4th Dec 2014

But if you lived there, it wouldn't be "far-off", nor a holiday. The essence of holidays is their transience. alancalverd, Sat, 6th Dec 2014

But if you lived there, it wouldn't be "far-off", nor a holiday. The essence of holidays is their transience.

A lot depends on what your vision is. If you want to live in a fair world where everyone has the same opportunities and a pleasant life, having everyone fly round the planet lots of times for short holidays is incompatible with that. You can only do it today because a rotten political system allows you to take more than your fair share. David Cooper, Sat, 6th Dec 2014


I recommend reading the whole thing though. PmbPhy, Sun, 7th Dec 2014

It's a good read and I think the most telling part is the fuel consumption panel. Concorde operated at 17 passenger miles per gallon, about a quarter of the efficiency of a small Cessna, whilst a 747 (or Airbus 380) flies at over 100 passenger miles per gallon, competitive with a diesel car and far less environmetal impact than road or rail travel.

Size matters! Technical turnround time is pretty much the same for all subsonic aircraft, and rather more for supersonics (they have to cool before they can be refuelled - or even to empty the toilet), so the economic limitation is how quickly you can load a passenger, and how many destinations can handle the aircraft (that's where the 4-seat Cessna 182 wins over just about anything - 40 pmpg and you can fit it with skis, floats, or tundra tyres, and operate from 300 yards of anything vaguely flat!). Given the facilities at most city airports, the economics of a 747 or 380 beat a supersonic every time.

Speed is remarkably unimportant. From carpark to carpark, the subsonic NY-LON journey actually takes 11 - 12 hours, of which about 7 is spent in the air. Reducing the air time to 3 hours doesn't make a lot of difference, and any glitch that adds an hour of ground time will ruin a day that is timed to such precision that it's worth spending 20 times the economy ticket price: is your presence on the ground really worth £2000 per hour? alancalverd, Sun, 7th Dec 2014

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