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Why can the Quest Kodiac and Cessna Caravan not exceed 200 knots?

PT6 from Pratt and Whitney in other implementations like Epic E1000 and Pilatus PC12 can cruise at over 300 knots.

The Kodiac and Caravan are advertised at only ~180 knots. Why is there such a difference?

Is the fixed landing gear the problem for such a limitation on speed?

Jpe61
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    Downvoters, please explain why you downvoted so that the question can be improved – Manu H Feb 04 '20 at 16:52
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    @MikeSowsun You shouldn't answer a question you think ought to be closed. Either close vote, or answer -- but not both. IMO, this question will educate the asker more if answered well (by removing the false assumption). – Zeiss Ikon Feb 04 '20 at 17:12
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    @MikeSowsun thus it can be improved by pointing out the false assumption so that the OP can clarify its mind and precise the question. – Manu H Feb 04 '20 at 17:20
  • @ZeissIkon i upvoted your comment as this is encouraged by the help center ("Answer well-asked questions") – Manu H Feb 04 '20 at 17:25
  • This is a classic thrust vs drag question, all the way down to canoe vs rowboat. You cannot be serious about closing this. – Robert DiGiovanni Feb 05 '20 at 03:23
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    @Robert DiGiovanni : Why not? If it's such a classic question, the answer ought to be (and is, at least to me) so blindingly obvious that it would be difficult to answer without embarassing the OP. – jamesqf Feb 05 '20 at 04:08
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    It is, but very fundamentally important to re-inforce, especially for the newer engineers. Work done on drag reduction (for the same thrust) made higher speeds and longer range possible, and is a great part of the aviation story. – Robert DiGiovanni Feb 05 '20 at 04:15
  • Welcome to aviation.SE! This is like asking why two cars with the same engine have different top speeds. You can probably see that different designs give different results because they have different body shapes, tires, weight, engine accessories etc. Why should any two vehicles (car, boat, aircraft) with the same engine perform identically? You might find that the [tour] is useful if you're new to the site. – Pondlife Feb 05 '20 at 04:24
  • @RobertDiGiovanni You're right and it's also important to note that, while many of us here are engineers, not everyone here is. This site is for aviation enthusiasts, too, and the answer to this question isn't necessarily as obvious to many of them as it is to those of us who are engineers, pilots, A&P mechanics, etc. – reirab Feb 05 '20 at 16:51
  • @reirab ...or even just model builder/fliers. I've never had hands on the controls or mechanics of a full size airplane, but I've built a bunch of models that flew well -- and as was noted by a character in Flight of the Phoenix, a model must be more carefully designed, because it has to fly without a pilot. – Zeiss Ikon Feb 06 '20 at 12:05

4 Answers4

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The P&W PT6 comes in many different varieties. The smallest PT have 500hp while the largest have 1700hp. It is not the "same engine" as you state in your question.

The standard Caravan has 675hp while the other aircraft you mention have 1,200hp. That alone can account for the major difference in performance. Fixed gear and struts also add to an increase in drag and a reduction in speed.

The bottom line is they are different aircraft, with different engines, designed for different roles. There is no reason to expect similar speed or performance.

Mike Sowsun
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    The PT6 is an AWESOME engine and has always been one of my favorites. One other thing to note is that even the exact same engine model can be configured for different output. On the Beech 1900, the engines were rated for a little under 1300 HP, but were capable of putting out 1500 HP. This is really a limit placed on the pilot's operation of the engine, but I don't know too many pilots who made a habit of keeping gauges on the wrong side of a red line. "Limiting" those engines like that allowed for the engine to keep producing that output at altitude, it extended the life of the ... – Shawn Feb 05 '20 at 16:25
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    ... engine, and best of all, if you had an emergency on takeoff, you still had another couple of hundred HP you could throw at problem. That aircraft could lose an engine on takeoff and still climb out at 500 feet/min. – Shawn Feb 05 '20 at 16:28
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Even if the installed engine version was identical (which, as @MikeSowsun noted, is not always the case), different types have different weights, drag profiles, and may have different propellers fitted -- all of which affect top level flight speed, as well as rate of climb, takeoff performance, maximum load capacity, range; the list goes on.

It's like asking why a canoe is faster than a rowboat -- they have the same engine (a single human), but that's where the similarity ends.

Zeiss Ikon
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    I know it has no effect on the answer, but a canoe is not faster than a rowboat. World record one man kayak for 1000m is 3:21 (and a true canoe is slower). World record for one man rowing is 6:30 for 2000m. – DJClayworth Feb 04 '20 at 21:18
  • @DJClayworth It's only fair to note a 2 km effort really should be just over twice a 1 km effort. – Criggie Feb 05 '20 at 00:43
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    This is a great play on viscous drag. Intuitively, the kayak/canoe would seem to be more "streamlined", but may have more surface area in the water, especially on the power stroke. But the advantage of the rowboat may be in thrust line efficiency. A kayak would yaw slightly with each paddle, 2 canoers slightly better in unison. Never saw a canoe beat a racing scull. The longer oar will work better at higher speeds. – Robert DiGiovanni Feb 05 '20 at 03:30
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    @DJClayworth: I would bet that the rowing record was set in a racing shell https://en.wikipedia.org/wiki/Racing_shell rather than a conventional rowboat. Which again just points up the importance of drag reduction. I would think that rowing shells with the same drag as a canoe/kayak might be faster because you're using both arms (and torso?) for power, while when I'm canoing, I basically use one arm for power, while the other just braces the top of the paddle. (I don't race canoes, though, so it's possible racers use a different technique.) – jamesqf Feb 05 '20 at 04:47
  • @DJClayworth taking into account time needed to speed up and slow down at start and finish, that still leaves the canoe as faster at maximum speed. It also doesn't mention the relative conditions for the record attempts. Wind, currents, wave action, etc. etc. all have effects on actual performance for a given attempt. – jwenting Feb 05 '20 at 06:20
  • @jwenting based on paddles per second the canoe is faster, but only over a shorter distance. As the paddlers tire, the longer, slower oar (high gear) will pass them. However, still remember the crew of the outrigger canoe from Hawaii 5 oh" (and the music). They may have the edge in rough surf. – Robert DiGiovanni Feb 05 '20 at 11:02
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    @jamesqf kayaking certainly uses the core muscles quite heavily; a hard effort in a canoe will too. Neither recruits the big muscles of the thighs as much as rowing with a sliding seat though. It should be possible to fit out the same hull as a solo racing scull, fixed seat rowing boat like a Thames Cutter, canoe, or kayak. That would be a good experiment, though a true racing shell might be a little unstable. – Chris H Feb 05 '20 at 13:43
  • @RobertDiGiovanni saying paddles/sec is a measure of speed is like saying propeller RPM is a measure of airspeed. – Chris H Feb 05 '20 at 13:45
  • In a further indication that drag is critical, Human-powered hydrofoils can beat records set in conventional watercraft. – Chris H Feb 05 '20 at 13:48
  • @Chris H paddles/sec is a measure of RPM, not speed! Paddling is low gear, Oars are higher gear (they are longer), lower RPM. – Robert DiGiovanni Feb 05 '20 at 13:53
  • @RobertDiGiovanni yes, so "the canoe is faster" doesn't follow. A more suitable conclusion would be "the canoe's engine spins faster" – Chris H Feb 05 '20 at 13:55
  • @Chris H my initial thought exactly, BUT, as a "low" gear, paddlers will have better acceleration, and will win shorter races. – Robert DiGiovanni Feb 05 '20 at 14:02
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    I was thinking of ordinary rowboats, not racing shells, and I wrote it that way because I did outrace a common rowboat in a canoe once at summer camp, over a quarter mile or so. Both Grumman aluminum craft, similar weight. – Zeiss Ikon Feb 05 '20 at 14:40
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That’s kind of like saying why can’t a Ford F150 Raptor keep up with a Ford GT sports car on a race track? Both have the same engine, don’t they?

It’s a different airframe designed around different performance criterion. Some aircraft eg a C208 are designed and optimized to operate from short, rough fields hauling cargo on short hauls. Others eg a Piaggio P.180 are designed for high speed cruise at high altitudes and extended endurance profiles.

Romeo_4808N
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  • Or even: “why can't an old Dodge Dart not keep up with a Top-Fuel prototype on the dragstrip”? – leftaroundabout Feb 05 '20 at 12:44
  • Sir, Airplane runs in a straight line. We are considering straight-line performance. The question is not on combating maneuvers, like on a race track. Its just straight-line performance. –  Feb 07 '20 at 06:41
  • SUV's don't cross over 150 MPH, because, they cannot be controlled over that speed, even by a skilled driver. Hence the performance is tuned accordingly. It doesnt mean, SUV's cannot go above 150MPH. Please give it a thought. You will find my view. –  Feb 07 '20 at 06:43
  • Dude, you’re missing the point. – Romeo_4808N Feb 07 '20 at 08:50
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When considering top speed vs power (thrust), drag is the deciding factor. Lower induced drag means higher top speed, but also less lifting ability and higher landing speed.

Retractable slats, flaps, and landing gear all serve to help increase top speed by reducing drag and to improve take-off/landing performance (particularly landing gear).

The Flyak is a good example of how drag reduction can improve top speed.

It's aerodynamic equivalents would include variable geometry wing concepts, as well as slats and flaps. Landing gear would not be needed here.

Robert DiGiovanni
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