I hear folks call planes “prop-jets” a lot. I used to think this was a plane that had both propellers and jets, but I gather that this ain’t the case. Is it just the same thing as a turbo-prop? I mean, is it accepted to substitute “jet” for “turbine”?
I think of a jet as moving something by spitting hot mass out its tailpipe (insert dirty joke here) … sort of an air-breathing rocket.
In the absence of jets, does the exhaust from a propeller engine (turbo- or otherwize) significantly add to thrust?
My dad said that Bell JetRangers used exhaust for forward motion. Is this all it takes to be a jet helicopter? Are there any helicopters that have jets in the narrow sense? Like, separate “air-breathing kerosene-drinking rocket” engines which aren’t the same thang that turns the rotor?
Does a turbofan really operate on the same principle as a turboprop (i.e., spinning little slats which push air backwards, not blasing air back with regular explosions)? Somebody told me they were basically the same thing except the -fan is enclosed in the jet-looking-thing, while the -prop is exposed, which means it can’t spin as fast without some bad aerodynamic condition messing things up.
What is a non-turbo prop? I used to hear about reciprocating engines … are they obsolete? How about pistons? Rotary … in-line … V … I read about them in descriptions of WWII fighers but not much in modern planes.
Last question, mostly unrelated … if V engines (V6, V12, whatever) have so many advantages over straight ones (four-clunkers, etc.), do the straights have any advantages over the Vs? Weight? Cost? Not sounding like a tomato-vegetable juice blend?
He wasn’t denying that JetRangers use the ordinary cyclical blade-pitch controls to tilt their discs forward, he just said that exhaust helped push them forward a little.
First, the jet engine. I don’t want to get too esoteric, so I won’t get into pulse-jets or things that are technically jets but we call rockets. I’ll start with a basic turbojet.
A turbojet has a rotating disc of blades in front called a “turbine”. Behind that is a fixed disc of blades called “stators”. There is a shaft that connects a similar arrangement at the aft portion of the engine. Between these banks of rotating bits is the combustion chamber. An electric motor spins the compressor turbines. This compresses the air in the combustion chamber. When the turbine is spinning fast enough and the pressure in the combustion chamber is high enough, fuel is introduced as a mist into the chamber and ignited. It burns very rapidly, causing a lot of expanding gas. Since the path of least resistance lays to the rear, that’s the way it goes, providing a fine Newtonian reaction (i.e., thrust). Along the way it spins the other bank of turbines, which as you remember are tied to the front set with a shaft. This causes the front turbines to spin, which compresses the air in the combustion chamber, which allows the fuel to burn, which shoots the gasses out the back, spinning the turbine, which spind the compressor… Et cetera. Things can be attached to the shaft, such as generators and pumps.
So a turbojet uses thrust to propel an aircraft forward.
A turboprop engine has reduction gears on the shaft (which spins rather rapidly) and there is a propeller attached it. The jet engine provides the power and the propeller provides the thrust. There is some residual thrust in the jet exhaust as well; but almost all of the thrust is from the prop. Turboprop engines are lighter and more efficient than reciprocating engines; but their fuel flow is higher (offset by the higher speed of the aircraft) and jet engine tolerances must be much tighter than for recips. Therefore, they’re much more expensive.
A turboshaft engine is used in helicopters. Instead of a prop, the jet engine’s shaft is connected to the rotor system. Again, there can be some residual thrust. The McDonnel “NOTAR” has the jet exhaust offset to one side to help counteract torque (which is mostly taken care of by the “coanda effect” – basically Bernoulli’s Principal in action). Turboshafts can be very large; some naval vessals are powered by them.
I haven’t studied turbofans, but it looks like a lot of thrust comes from the back end of the jet, and a lot more comes from the big rotating turbine (fan) in front. Someone will correct me, I’m sure.
Reciprocating engines (piston-engines) are alive and well. The basic design dates back from the 1930s. Don’t look for too much innovation. Certifying a new engine type is a very costly endeavor. Porsche developed the PFM (Porsche Flugmotor) for aircraft, but it was not very profitable. There are aircraft flying around with V-8s, one or two of which are certified. But it’s a very slow process.
I think the reason we see boxer-4s and boxer-6s in airplanes is that liquid-cooled V-8s and straight-6s are a bit heavy. Fine if you want to make 2,000 h.p., but not very good when you’re looking at 160 h.p. So I think you’re basically right as to why we use the flat engines. They’re cheaper and lighter.
It seems that in general use “turbine” and “jet” have become synonymous - for example the Jet in JetRanger. Jetprop/propjet is as far as I can tell synonymous with turboprop.
The first turbojet (or ‘jet’) engines were just turbines spitting fire and exhaust out the tailpipe - your kerosene & air breathing rocket. Later came the turbofan in which some of the air passing the fan blades does not go into the compressor/turbine engine, but ‘bypasses’ the engine - thus the ‘bypass’ ratio is how much air bypasses the engine vs. how much air goes into the engine (or maybe vs. the total air, whatever.) However, even a turbofan uses the ‘rocket/jet’ portion of the engine for thrust. The fan portion of the turbofan greatly increases efficiency of the engine - i.e. less kerosene consumed per pound of thrust.
A turboprop is not quite like a turbofan because usually the ‘jet’ function of the turbine engine is not used to add to the thrust. On turboprops such as the KingAir the exhaust is this cute little pipe that sticks out the side of the engine and doesn’t look beefy enough to be a thrust source. If it were using the jet exhaust for propulsion, you could think of a turboprop as a really high bypass turbofan. I believe this is because so much more power is generated by the prop than could be generated by the jet that it doesn’t make sense to use it, but I’d love to hear from someone involved with turbine powerplants to clear up details. Also, turboprops can be run with reverse pitch on the props. Airplanes so equipped can taxi backwards, or use reverse thrust to slow down when landing. If the jet portion of the thrust was significant this would be more difficult. The reverse thrust on a turbofan is different and looks clunkier to my untrained eye - a big scoop shoots out to redirect the air to point forward.
Oh, also a turboprop in general will have some reduction gears to spin the prop slower than the turbine is spinning, because the bigger the ‘fan’ the slower it has to go to avoid coming apart/exceeding the speed of sound/whatever other bad things you can think of.
Basically for high efficiency turboprops are the way to go, but you can’t get the props up to the same speeds as the turbofans, so big jetplanes don’t use them.
I hope that explains the turboprop vs. turbofan vs. turbojet questions you had.
This should be pretty minimal, sort of the same way that turning the volume up on your radio makes your car slow down because of the extra load on the alternator. I.e., not much.
There are no helicopters I’m aware of that use a seperate power plant for propulsion. It is a conceivable design, but I think it would be inefficient - you’ve got to drive the rotors anyway, and you can’t make the helicopter go too fast or you’ll stall half of your rotors (airwolf notwithstanding), so what’s the point?
Oh, what a rich history there is here. A non-turbo prop would generally be a piston engine. There are some Wenkel rotary engines in use in homebuilt planes, I’m sure, but in aviation a ‘rotary engine’ usually refers to the great big ‘radial’ engines of WWI that were mounted on a fixed crankshaft, so the prop and the engine rotated in flight. This gave huge torque problems/benefits as was discussed in the “what’s the best WWI fighter” thread.
A radial engine is a piston engine (generally air-cooled) in which the pistons all point at a central point (the crankshaft) and are arrayed in a circle. These were very popular in WWII and earlier airplanes for reasons which escape me. Some aircraft such as IIRC the DC-6 had gargantuan piston radial engines - these were about the most powerful piston engines in the days before jets became popular. Some huge figure like 5000 HP each springs to mind, but I’m not going to ‘vintagewings.com’ right now due to time constraints.
Some WWII fighters used water cooled engines, the Mustang being an example that springs to mind. I think it was a V but I’m not sure, but it was 12 cylinders of purring power. Nothing else sounds like a P-51 flying by - if you have a chance to see one (are there still races in Reno?) I recommend it, just for the sound effects.
As for modern planes, most small airplanes use piston engines. Traditional aircraft engines are air cooled and low-tech, because it is hard to get the FAA to approve fancy schmansy new designs. Sure you can get better fuel economy with the 18 computer chips in your BMW, but when one of them goes wonky the worst that happens is you roll to a stop. Airplane piston engines are purposefully built simpler to avoid problems, and to maintain a design that was approved by the FAA.
Good question. I dunno about relative benefits of V vs. straight, but see above as to why traditional piston engines (e.g. Lycoming, Continental) have changed so little over the years. Also, I believe air-cooled is lighter and has less to break than water-cooled designs.
Many homebuilt/experimental aircraft use non-traditional engines, some use auto engines and some use 2-stroke engines like snowmobile engines - there is a popular manufacturer of snow mobile engines whose name escapes me right now.
The ‘Stang used the Allison-Packard contract-built version of the Rolls-Royce 27-litre V-12 “Merlin”. It was also used in the Brits’ Spitfire, and in pairs in our P-38 Lightning fighter and their DeHavilland Mosquito bomber (wasn’t it made of wood, for “stealth”?).
I could be wrong, but I think the Allison engine and the Rolls-Royce “Merlin” engine were different. The Mustang was originally equipped with an Allison engine and didn’t perform well at altitude. (BTW: The Mustang was originally designed and built for the RAF.) The Merlin engine made a world of difference and subsequent models of the P-51 used a lisence-built version of it (possible built by Allison, but I’m not sure). I think the P-38 used the Allison (as opposed to the Rolls) engine. I’d have to look it up.
The DeHaviland “Mosquito” was built of wood not for stealth, but for economy. IIRC, there was a shortage of such strategic materials as aluminum, so the Mosquito was built “on spec”. It turned out to be a very fast, maneuverable design… and it had a reduced radar signature to boot!
I think the reason the U.S. used radial engines was because they were in-vogue before the war. You could get more power out of a relatively compact design (WAG). I don’t know if the lack of liquid cooling parts was a consideration as far as reliability. Certainly we had V-12 designs: The P-38, P-39 and P-40 come immediately to mind. One thing to consider is that liquids are heavy. Less weight means more payload or more speed. “Speed is life” to a fighter pilot. Perhaps that was a factor? The P-51, being a later design, probably benefited from V-12 engines that were more powerful than the ones available for earlier designs. (IIRC, the Merlin was designed for the Spit; at least it seemed so in Spitfire starring Leslie Howard.)
There’s a beautifully-polished P-51D that flies out of Van Nuys (CA) airport.
Back to the turbine/piston thing. I don’t know how much your typical Pratt & Whitney PT-6 goes for, but $250,000 sounds familiar. A big boxer-6 Lycoming or Continental probably costs about 1/5 that. (I noticed that a factory-remanufactured 150 h.p. Lycoming O-360 goes for about $15,000.) Kerosene (or Jet-A) is less expensive than the more highly-refined and volotile 100 octane avgas. Turbines are more reliable than piston engines. Pilots tend to want more power, speed and payload. But all that comes at a price. There just isn’t a market for airplanes that expensive (for non-business pilots).
But what if you could get a turbine for the same price as a piston? Most turbines are too powerful for, say, a Cessna 172. If the engine manufacturers don’t make a smaller engine, a new airframe would have to be designed. Aircraft manufacturers tend to be very cautious. AA case in point: The venerable Cessna 172 you can buy today (at about $165,000 new) is not all that much different from the ones that were around about 40 years ago. Why go to all of the expense of designing a new, more efficient airframe when people are still buying the old design?
It’s a viscious circle. People buy old designs because Cessna, Piper, et al aren’t making new designs. The makers aren’t making new designs because it’s too expensive (and risky from a liability standpoint). So they see no reason to build very small turbine-powered aircraft. So the engine makers see no need to build small turbines. Which people aren’t buying because there are no airframes that need them. Things are picking up in the R&D department (at least according to the AOPA), but all of the articles seem to say, “Don’t hold your breath.”
Seriously though, you two have been very informative. Out of curiosity, what are your backgrounds? Engineers, hobbyists, private pilots, used to know a guy who flew, something like that?
I had no idea that a reciprocating engine and a piston engine were the same thing.
It sounds like a turbofan operates on two principles simultaneously. Neato.
I hadn’t realized that a helo turboshaft and a ship’s turbine engines were similar, but it makes sense I guess. If a ship has “steam turbines”, that means it’s oil-fired but not a diesel, right? Does this mean it has roughly the same thing as an early steam engine? Or is their some great divide between non-internal-combustion engines, one type being turbines, the other being … uhhhh … old?
And yes, thanks for being patient with my interminable list of questions.
My dad has flown all his life, and I’m the only male in my immediate family that hasn’t got his pilot’s license - it was too expensive when I got old enough.
I’ve flown all over in my dad’s Cherokee, and been to tons of airshows and air races and read tons of books.
Back semi-on-topic, Johnny LA might like this.
On the way back from Canada on a road trip in the Cherokee my dad and I stopped by an air shop that shall remain nameless. They had some piper malibus in the midst of conversion to turboprops - he’d take the engine off a king-air and slap it on the Malibu. The nose got 18" longer, and the thing looked really sleek. He had one that was flying, and claimed that if you bought a used airframe for 500000 he could upgrade it for 300000 and you’d have this kick-ass Malibu jet-prop for $800000. Retail for a new piston Malibu is about the same. (!)
Some quick stats off the top of my head:
Climb: 3000 fpm
Cruise: 270 kts @ 27000 feet
Runway needed: 700 feet
He had beta on the propeller so he didn’t have to touch the brakes.
If it ever gets close to certified, and I ever get close to rich, I’ve gotta have it.
Is this what they call a “gas turbine” engine, or is that something else still? I hear gas turbines have been used experimentally on trains and cars and some military vehicles (tanks?).
NASA has a program to develop two new General Aviation engines. One is a alternate fuel engine, and the other is a low-cost turbine. I think they intend to develop an engine that is cheap enough to put in a typical 4-place privately owned aircraft, but I can’t remember any other details.
Boris, I’m a private pilot with airplane and helicopter ratings. My father was a flight instructor, and spent 22 years in the FAA as a Flight Service Specialist. Before he was commissioned in the Navy, he was combat aircrew (before my time). My mother was a private pilot for a while, but she lost interest. She worked at airports for about 20 years. Her husband (my folks divorced when I was 8) was a pilot until he lost his medical. I grew up around airplanes.
Boris and scr4, I guess I should have differentiated between gas turbines and steam turbines. A gas turbine burns jet fuel (basically kerosene) internally to spin the turbines. A steam turbine has a boiler that supplies steam to turn the turbines. I believe the first steam turbines were coal-fired, but modern ones (developed early in the century) are oil-fired. Of course, nuclear vessels use fission to boil the water that spins the turbines. Larger vessels use steam turbines and a few smaller ones (the one that immediately comes to mind was the USS Tucumcari hydrofoil) use gas turbines. The M1-A1 tank also uses a gas turbine engine.
Douglips, 800 kilobucks and the wings don’t even go round and round like they’re supposed to? I dunno how good a deal that is! BTW: I never have to touch the brakes in the Schweizer 300!
dhanson, I knew someone was working on a small turbine, but I didn’t know it was NASA. It seems every time I read about it, it’s “not too far away”; but it never seems to come!
One more thing about the personal turbine market. Beechcraft (now a division of Raytheon) built the T-34C “Mentor” for the Navy (plus some T-34C-1s with hardpoints under the wings for attack roles for foreign – South American? – countries).
The T-34As and -Bs are very popular in the “warbird” market, and a few have been fitted with Allison turbine engines. Beech saw what they thought was a good thing and tried to market the T-34C (Lycoming PT-6 powered) to civilian pilots; but at $1.2 million, they never sold one. People with that kind of money would rather, it seems, pay $600,000 for a Beechcraft B-36 with 6 seats and a reciprocating engine. The T-34 is a two-seater, and basically a rich man’s toy. (BTW: The T-34 was developed from the venerable Beechcraft Bonanza. Basically it’s an aerobatically-certified Bonanza with a tandem cockpit.) Pilots who really want a turbine T-34 can buy an -A or -B model for about $200,000 and pay an additional $300,000 or $400,000 and get the Allison conversion. Still half the price of a new one from the factory.
With the new JPATS training aircraft (I think Raytheon/Beech is calling it the “Texan II”, based on a Swiss (?) design), there may be surplus T-34Cs on the market eventually. There was a flight restriction placed on T-34As and -Bs after one that was being used in a civilian “dogfighting school” broke up in flight (the wing spar failed). I don’t know the status of the flight restriction.
So, the cost of turbine aircraft is what’s keeping them out of the small aircraft market.
Incidentally, the SAIA Marchetti S.260 is the basic trainer for the Italian air force. This hot little recip. is also being used at civilian “Top Gun” schools, and doesn’t have the flight restriction that the T-34 has.
Can any of you aviator people tell me what types of turbine aircraft various jump planes are? I’m curious. In skydiving we (generally) refer to planes only as turbines or non (the nons usually being C182s, 206s, etc.) It makes such a difference that we differentiate drop zones that way, i.e. “Why would want to jump over there? Over here they have a full-time turbine.”
I’m guessing they’re turboprops, but some of them are so very different as far as takeoff length and climb rate, so I’m interested in the following:
B-90 King Air
DeHavilland Twin Otter/Super Otter
Cessna Grand Caravan
Super CASA
Skyvan
DC3
Are these all turboprops?
Also, why is there such a difference in runway length requirements? I seem to remember from a few years back that one of the above could take off on a little grass strip between cornfields - do you know which one it would have been?
Lastly - I’ve heard Otters aren’t being made any more. Are there any new twin turbines that you could see being used as jump craft in the future?
Sorry for the hijack, and thanks in advance for help.
All of the aircraft you mentioned are turboprops, with the exception of the DC-3. There are, however, DC-3s that have been modified by replacing the big radials with turbines.
Required runway lengths depend on a number of factors. Primarily (and all other things being equal), weight and thrust. A light plane with a lot of power will each rotation speed more quickly than a heavy plane with little power. More power means that the aircraft will also have a superior rate of climb (gain in altitude per minute) and angle of climb (gain in altitude per distance unit).
Some aircraft, notably the Helio “Courier”, were designed for high-performance takeoffs. These STOL (Short Take-Off and Land) machines are fairly light, have high-lift wings, and lots of power. This combination makes them good pack-horses. Maule makes piston-powered STOL aircraft exclusively. (You may have seen a Maule in a 1970s film – Cannonball Run(?) – where an airplane takes off from an intersection in a small town.) They also make a turboprop.
There are Otters and there are Twin Otters. The Otter was built in the 1950s and was powered by a radial engine. Most of them seem to be in Canada or Alaska, where they are often used as float planes or ski planes. The Twin Otter is a twin-turboprop design from about the 1960s. I hadn’t heard that it’s not being made. Twin Otters are papular in the arctic and in warmer climes are often used as skydibing platforms.
The reason people like to skydive from turboprops (and I’m guessing here, because I’ve not had the opportunity to jump) is probably because they get to altitude more quickly, or can attain a higher altitude than piston-powered aircraft. I’m sure the skydiving operators like them too because they can carry more (paying!) customers aloft per trip.
There are some really great animated .gifs there; the whole site is a treasure trove of information about, well, how stuff works. There’s also a link at the end of the turbine article to an extremely small, semi-inexpensive turbine.
Model aircraft were available for years with ducted-fan engines, basically piston engines with internal props. Now, true turbine engines are available.
Certain universities/companies are currently developing microturbines to power things like cell phones and laptops. These things would be about the size of two sugar cubes placed next to each other.
Re: v-engines vs straight engines
Okay, my knowledge here is imperfect, but:
One major reason for using v- or flat-configured engines is packaging. you can make the hood of a car much shorter if you have a v-8 under it than if you’re trying to wedge a straight eight into the same space. I’ve heard it repeated that straight-six engines have perfect primary and secondary balance. One generally encounters the BMW 3-series with a straight-six engine, and it is acclaimed for its smoothness.
Current Ducati motorcycle engines are V-twins, which some people insist on calling L-twins, because the cylinders (which hold the pistons) are at a ninety-degree angle to one another. 90-degree v-twins have perfect primary balance (in the plane in which the pistons move) but imperfect secondary balance (um, I’m not sure which plane that is.)
Ducatisti think the secondary vibrations are a goood thing.
Harley-Davidson also uses v-twin engines, but theirs are generally at a 72-degree angle. This produces perfect secondary balance but imperfect primary balance. This is one reason why Harleys vibrate with such amplitude.
Harley riders think the primary vibrations are a good thing.
Neither Ducati riders nor Harley riders think the other is a good thing.
Regarding four-cylinder engines (which are almost always in a straight configuration…there are exceptions, but I’m going to ignore them for now) they are known for being smooth in motorcycles and not-as-smooth-as-straight-sixes in the car world. I don’t have a clue about primary or secondary vibes on this one.
Straight twelve-cyclinder engines, an affliction confined largely to the British in the early part of this century, also have perfect primary and secondary balance as they are just two straight six-cylinder engines next to one another.
I don’t think there have been any 4-cylinder in-line aircraft engines since the 1920s or '30s. All current 4- or 6-cylinder aircraft engines are boxer configuration.
Yeah, every piston aircraft engine that I can think of in current production is horizontally opposed. There are a few V-8 auto engine conversions kicking around, but I can’t think of one that I’d be willing to fly behind.
Radial engines are very efficient. Having all the pistons connected to a short crank means a higher power to weight ratio, less vibration, and other good things. The only major drawback is frontal area. A big radial adds a lot of drag to the airframe, and results in a hog-nosed design that makes it hard to see out the front, especially when taxiing in a taildragger. WWII radial-engined fighters like the P-47 and the Corsair were really bad for this. You had to S-turn like mad when taxiing because you were completely blind.
“I don’t think there have been any 4-cylinder in-line aircraft engines since the 1920s or '30s. All current 4- or 6-cylinder aircraft engines are boxer configuration.”
Yeah, I forgot to specify that I was talking about terrestrial engines. The original post asked about the advantages and disadvantages of various engine v-configurations, so I was trying to address that.
BTW: I never have to touch the brakes in the Schweizer 300! 
