Since the rotor has to both lift and propel the aircraft, do helicopters burn a lot of fuel compared to conventional aircraft? How far could a Cessna and a helicopter of similar weight travel on the same fuel load?
A Cessna 172 and a Robinson R44 are about the same weight, about 2500lbs. The C172 has a usuable fuel load of 53gals and the R44 carries 50.1 gals. The C172 has a range of 575NM and the R44 350NM. That works out as;
575/53 = 10.84 NM per gallon.
350/50.1 = 6.98 NM per gallon.
This isn’t an entirely fair comparison as the R44 has a bigger engine and the C172 cruises at a faster speed, but they still have similar performance and carry a similar payload.
But on a purely equal weight carried over same distance divided by fuel used, the fixed wing plane wins. Of course a helicopter has over advantages that a fixed wing aircraft can’t match.
A helicopor at hover burns less fuel than, say, a harrier jet doing the same.
However, nothing that flies will ever be as “fuel efficient” as something of similar weight that only rolls on the ground for the simple reason that in addition to being propelled horizontally aircraft also have to expend energy to couter-act gravity.
The Robinsom R-22 and a Cessna 172 share the same engine. (Well, virtually the same. Each is certified for its own use, and there are minor differences. Suffice it to say that they both use Lycoming O-320s.) A C-172 will carry four people along at about 120 kts. An R-22 has a V[sub]ne[/sub] of 102 kts. and is most efficient at 84 kts. A C-172 has a range of about four hours on about 40 gallons of fuel. (This is based on the 1970 C-172 dad used to have.) An R-22 carries about 30 gallons, and you should think about refueling after a couple of hours. (Interestingly, the R-22 POH does not have gallons per hour figures.)
So a C-172 goes half-again as fast, carries twice the number of people, and burns less fuel while doing it. I haven’t been flying recently. I’m barely making enough to pay bills; let alone fly. But last time I looked, a C-172 rented for about $75/hr. and the R-22 rented for about $180/hr. Clearly the fixed-wing is more economical. And remember the TBOs. A Robinson needs it’s rotor blades replaced every 2,000 hours. A Cessna’s wings will last the life of the airframe. Helicopters are maintenance intensive.
And then there are aerodynamics. As I mentioned, the R-22 has a V[/sub]ne[/sub] of 102 knots. This is because of “retreating blade stall”. It works like this: Let’s say that a rotor blade needs to pass through the air at (arbitrary number) 200 knots in order to sustain lift. Let’s say the rotors normally spin at 300 knots at the tips. So far, so good. Now let’s fly forward at 100 knots. Now the advancing blade is passing through the air at 400 knots at the tips. The retreating blade is passing through the air at 200 knots. If the forward speed increases, the speed of the retreating blade on our hypothetical helicopter falls below the 200 knots we’ve said it needs to sustain flight. It stalls, and unless the pilot catches it in time (before it stalls) the ship and passengers are doomed. Retreating blade stall is also why as altitude increases, airplanes fly faster and helicopters must fly slower. A Cessna (or any other fixed-wing) can increase its ground speed by flying higher in the “thinner air” and cover more miles per gallon of fuel. (This, of course, is in a “standard day, no wind” situation.)
While I definitely see where you’re coming from, this generalization makes certain broad assumptions about flight that lean heavily towards current aviation, rather than “what is possible”. I think that you are considerably overstating the case when you say “will ever be”. Arguably we’ve already passed that point
The fuel economy of most planes in actual use is affected by many more factors than ground-huggers, yet planes are more fuel efficient for many uses. The hoary old 747 with a seating capacity of up to 568 burns 36,000 gallons for a 10 hour flight at 550-600 mph or roughly 100 passenger-miles per gallon, which exceeds the actual mixed terrain mileage of any road vehicle, and approaches the point to point fuel efficiency of a train (Railways generally take a longer actual route to go around unfavorable terrain, and their efficiency doesn’t account for “fuel costs” of building and maintaining rails, bridges and tunnels.) 142 economy cars, each with 4 passengers, will consume far more fuel driving from NY to LA than a single 747.
Anything on a ballistic or semi-ballistic path (e.g. upcoming scramjets or other hypersonics) could have surprising fuel economy on long flights, even after heavy fuel consumption at take off, if that were the primary goal. Even a Cessna 172 (glide ratio of 20:1 IIRC from the old days) might exceed the fuel economy of a car of the same weight, if it made the most efficient climb to altitude, followed by an unpowered glide.
Of course, there are aircraft with glide ratios of 1:80 or 1:100 or more. Most of these are actually meant to be operated primarily in unpowered mode (e.g. gliders) For such craft, the fuel consumption of take off and climb to altitude could be almost negligible, considering the “free” glide of 16-20 miles per 100ft of initial altitude.
Almost 20 years ago, a powered airplane (the Voyager) flew around the world at an average speed of 115 mph on under 480 gallons of fuel. Even after detouring around a storm over Brazil, and losing 5000ft of altitude over Baja Mexico, this 2250 lb aircraft, carrying two passengers (and sleeping accommodations) averaged over 50 mpg, in a real world, mixed terrain test.
An unfair comparison? I don’t think so. We’ve spent many billions of dollars more on researching and improving automotive fuel economy than aviation fuel economy (as important as aviation economy is, the automobile industry is larger, and CAFE was a national priority). The Rutans designed and built the Voyager on a relative shoestring budget. Yet no petroleum-fueled ground-hugger, past or present, could maintain 50 mpg over 25,000 miles of often rugged terrain (or whatever longer route might be required to stay on suitably clement roads) with absolutely no en route maintanance, even if we ignore the those pesky oceans.
Bottom line: the Voyager did it with 1980s tech; no ground vehicle has even come close. If anything, a 2-passenger 2250-lb experimental high economy car with triple digit mpg would be a less practical vehicle for realistic long distance, mixed terrain use. Those cars take big mileage hits for even ordinary traffic or moderate hills vs. test track or highway cruise.
If you want an unfair comparison, I’d suggest lighter than air craft (balloon, blimps dirigibles), but I don’t know anything about their fuel economy in actual use (e.g. against unfavorable winds, etc.)
A helicopter must use engine power to run the rotors AND a tail rotor to counter the main rotor’s torque.
An airplane’s engine sends all of its power to the prop delivering pure thrust.
I’d say a helicopter with counter-rotating rotors (aka egg-beaters) would have moderate economy gains over traditional helos.
IIRC, the C-172 has a glide ratio closer to 10:1. In any case, I’d like to see some figures to back up your statement. (I don’t have my books handy, else I’d do it myself.) I think that the fuel burned to get to a given altitude will offset the savings from gliding down from it. I stand ready to be corrected, of course.
In any case, helicopters are special purpose vehicles – like SUVs. They do what they do well, but at the cost of fuel efficiency.
Good post, but you’ve lost me on this point. Are you requiring ground vehicles to be driven off-road for comparison purposes? Because my ancient diesel Rabbit still gets ~43mpg (city or highway, makes bugger all difference), and seats 4, for ~170 passenger miles to the gallon. I can make a lot of detours on that margin, and fuel costs of building roads distributed across the total number of trips that can be made on them are very small. And there are newer vehicles that are somewhat superior to my old beater in this regard.
Aren’t newer airliners doing a lot better in the fuel-efficiency dept, though? I thought that things like the 777 and the latest Airbuses soundly trounced the old 747 in that dept.
Comparing aircraft to cars is not entirely useful. A car will get more miles per gallon than a Cessna 172 or most other light aircraft. (I’m not getting into airliner efficiency, since I don’t know enough on that subject.)
Let’s say a C-172 burns 10 gallons per hour with the throttle all the way forward, and makes a 120 mile statute mile trip in an hour. (Aircraft “mileage” is stated in gallons per hour, not miles per gallon.) That comes out to 12 mpg or 48 seat-mpg. My Jeep gets 20 mpg if I drive it carefully. If I were able to drive a straight line (i.e., there are no curves in the road) then at an average speed of 60 mph it would take me two hours to cover 120 miles and I’d burn six gallons of fuel. So six gallons vs. 10 gallons. But roads are not straight. Depending on the road, the distance to drive may be significantly greater than the straight-line distance and the fuel burned will be greater; thus narrowing the difference between flying and driving. How much is your time worth? $20/hour? When you take time into consideration, flying a light aircraft can “cost” less than driving. (Indeed, when you take factors like driving to a major airport, baggage check and/or screening, waiting to board, etc., flying a 130 mph Cessna might actually take less time than flying on a 525 mph jet. I think the “break even point” is something something like a 400-mile trip.)
Airplanes are a lot more fuel efficient over distance, but they’re a pain to hoist from ;).
If we want to stretch the definition of what flies, then the Voyager spacecraft have an average fuel consumption of 30,000 miles per gallon and rising. Then again, though, they also currently have a max acceleration equivalent to 0 to 60 in twelve hours, so maybe it’s not worth bragging about.
Although, yes, I was generalizing I think that my statement will stand for a long time.
To some extent, you’re comparing apples and oranges. Or maybe apples and kumquats. Compare a hybrid sedan to a sailboat the hybrid won’t look good in comparison, but you can’t sail down main street.
A helicoptor IS the “most efficient” machine if you need to hover mid-air, or for certain construction jobs (in downtown Chicago, for instance, new radio and TV antennas are invariably hoisted to the top of skyscrapers by chopper), or for many airborne ambulance applications. Saying a car (or boat, or moped) gets better fuel economy is ridiculous - a car can’t do at all some of things a helicoptor does well.
But that only works over long distances, with a full load. Drop the passenger list to 1/2 capacity and suddenly it’s not so economical - that’s why I said weight, not passenger capacity. Where you save with the jumbo jet is over long distances with huge cargo loads because they don’t have to detour around terrain - if trains could go straight-line I think they’d still exceed the efficiency from a fuel/weight standpoint. They would never be able to beat the *speed * of a jet (barring some hypothetical future breakthrough that hugely reduces the rail’s friction), and it’s in speed that aircraft have some of their greatest advantages
Well, yeah, but that’s not how people fly them in the real world. Unless you’re setting a record or proving a point it’s not going to happen that way. Max fuel economy in a C172 would require flying somewhere around 75-80 knots - but people cruise in them at 100-120 knots. I could really reduce the fuel burn by finding a thermal and using that boost up to altitude, too, but who bothers to do that outside of (again) proving a point or some bizarre circumstances.
I could probably get extraordinary gas mileage out of my car, too, if I figured out the speed where the engine is most efficient with the least amount of air resistance - but I’m willing to bet it’s not a speed anyone normally drives at, probably something like 20 mph or something (if someone does want to figure that out, please step into the conversation). Coasting down a mountain in my car? Turn the engine off, gravity gives me all the power I need. Probably is, I live on the Great Plains - no mountains for a couple hundred miles.
Uh-huh - and you have to have them towed to altitude, usually (except for motorgliders). And they’re small, with very limited payload. They have very limited uses. A bicycle is more efficient than a car in many ways, but we don’t use them tow 50 ton cargos.
Wah…? Check your figures, dude. It was 7,011 lbs of fuel, that’s over 1,100 gallons of avgas by my calculations.
Sleeping accomodations? May I suggest the book Voyager by Jeana Yeager, Dick Rutan, and Phil Patton? There’s a lot to be said for listening to the folks who have actually been there. There was a seat for the active pilot. The other “accomodation” was a slightly padded spot next to the active pilot. Dick Rutan wasn’t even able to stretch full length in it, or turn around. Jeana could - but she’s about a foot shorter than him. No passengers - both occupants were pilots and both had duties throughout the flight. Neither got much sleep.
The Voyager was designed to do just one thing - fly around the world without refueling. Everything else - including the comfort of the occupants - was sacrificed towards that end. But most aircraft are multi-purpose, and thus lack absolute efficiency in any given task.
And what makes you think that automobile engine improvements haven’t had an effect on airplane engines? Many of the digital engine control technologies that enable cars to get 30-40 mpg (even without being hybrids) are used to improve the engine performance (including fuel economy) of airplanes. Newer models of two and four seat airplanes come with fuel injection instead of carbeurators, computerized engine control for maximum efficiency, and other tweaks and improvements.
Now, jets are a different matter - turbine engines are quite different than piston, and there’s less room for overlap.
On the other hand, some of the technologies developed for aviation, such lighter, stronger metal alloys and composites, are also part of what has improved automobiles over the years.
Point 1 - the Voyager burned much more fuel than you think it did. Over 1100 gallons.
Point 2 - there was a LOT of en route maintenance required, even aside from the chore of pumping fuel from one fuel tank to another. Read the book the builder/pilots wrote.
And no one - including the Rutans - have repeated the feat since. Why? Because the second trip would cost either an order of magnitude more, or be just as uncomfortable and risky as the first. There’s a reason Burt Rutan is building spaceships these days instead of round-the-world-on-one-fill-up airplanes.
I don’t feel like digging up my info on the golden age of dirigibles right now (I’m getting ready for a trip of my own - you most likely won’t see me on the board this coming week), but all aircraft are subject to “unfavorable winds”. Go by airspped in a hypothetical no-wind situation. Anyhow - the Hindenburg and the Graf Zepplin both could clock about 80 mph. Fuel wise, they were FAR more efficient at ocean crossings than anything else ever devised. But weather and lighter-than-air craft don’t mix well. Most of the dirigibles from the early 20th Century crashed during storms.
Which sort of gets back to my original point - efficent compared to what? What are you trying to accomplish, here? Speed? Nothing moves you faster than an aircraft. Mileage per gallon? I dunno - railroad? Across the ocean, with minimal fuel? - oh, definiately a sailboat. Across the ocean, without having to worry about weather? How about a nuclear sub?
Right tool for the job and all that.