14
$\begingroup$

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?

$\endgroup$
  • 6
    $\begingroup$ Downvoters, please explain why you downvoted so that the question can be improved $\endgroup$ – Manu H Feb 4 at 16:52
  • 19
    $\begingroup$ @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). $\endgroup$ – Zeiss Ikon Feb 4 at 17:12
  • 2
    $\begingroup$ @MikeSowsun thus it can be improved by pointing out the false assumption so that the OP can clarify its mind and precise the question. $\endgroup$ – Manu H Feb 4 at 17:20
  • 4
    $\begingroup$ @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. $\endgroup$ – jamesqf Feb 5 at 4:08
  • 2
    $\begingroup$ 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. $\endgroup$ – Robert DiGiovanni Feb 5 at 4:15
30
$\begingroup$

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.

| improve this answer | |
$\endgroup$
  • 1
    $\begingroup$ 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 ... $\endgroup$ – Shawn Feb 5 at 16:25
  • 1
    $\begingroup$ ... 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. $\endgroup$ – Shawn Feb 5 at 16:28
21
$\begingroup$

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.

| improve this answer | |
$\endgroup$
  • 3
    $\begingroup$ 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. $\endgroup$ – DJClayworth Feb 4 at 21:18
  • 4
    $\begingroup$ 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. $\endgroup$ – Robert DiGiovanni Feb 5 at 3:30
  • 5
    $\begingroup$ @DJClayworth: I would bet that the rowing record was set in a racing shell 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.) $\endgroup$ – jamesqf Feb 5 at 4:47
  • 2
    $\begingroup$ @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. $\endgroup$ – Chris H Feb 5 at 13:43
  • 2
    $\begingroup$ 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. $\endgroup$ – Zeiss Ikon Feb 5 at 14:40
10
$\begingroup$

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.

| improve this answer | |
$\endgroup$
  • $\begingroup$ Or even: “why can't an old Dodge Dart not keep up with a Top-Fuel prototype on the dragstrip”? $\endgroup$ – leftaroundabout Feb 5 at 12:44
  • $\begingroup$ 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. $\endgroup$ – user46196 Feb 7 at 6:41
  • $\begingroup$ 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. $\endgroup$ – user46196 Feb 7 at 6:43
  • $\begingroup$ Dude, you’re missing the point. $\endgroup$ – Carlo Felicione Feb 7 at 8:50
3
$\begingroup$

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.

| improve this answer | |
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy