I was flying on Porter Airlines and they had an info card about how similar the Bombardier (I still say DeHavilland) Dash 8 Q400s are to the Bombardier CSeries they have ordered.

There was a cool overlay photo to show relative sizes and shapes:

Dash 8 vs CSeries

Looking at that image, it got me wondering about the straight vs angled wing. Straight vs angled tails, etc.

I get that a jet is faster than a turbo prop.

Cseries cruise speeds are: Mach 0.78 (828 km/h, 447 kn, 514 mph)

Dash8 Q400 cruise speeds are: 414 mph (667 km/h) 360 knots

Those are pretty close and yet that is a pretty radical wing design change.

Is this just the history an old design (Dash 8) vs a very modern design?

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    Hint: M.60 is quite different from M.82. ;) – Qantas 94 Heavy Dec 24 '13 at 4:17
up vote 12 down vote accepted

Well, for starters, the speeds are actually pretty significantly different. The Cseries is almost 25% faster than the Q400. Swept airfoils are much more efficient at those higher speeds, as are jet engines.

The Q400 is also a wildly stretched version of the original DeHavilland Canada DHC-8 (Dash 8), which was itself based on the 4-engine DHC-7 (Dash 7) STOL airliner. DHC has a lot of history building short takeoff and landing aircraft with pretty extreme performance, and a lot of that colored the design of the Dash 8. What you see today in the Q400 is the result of about 70 years of backcountry aircraft design, applied to a commuter airliner.

The Cseries, on the other hand, is a clean-sheet Bombardier design meant to compete with the E-jets and the smallest 737s (and 717).

Hopefully that answers your questions!

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    For some more info on why swept wings tend to show up as you get above about mach 0.7 or so, check out the Wikipedia article on swept wings. You'll even find a reference there to a swept-wing transsonic propeller driven aircraft - the TU-95 which has a top speed around the same as the C series, – voretaq7 Dec 24 '13 at 4:19
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    Even more fun, the Tu-114 airliner was based on the Bear and could carry up to 225 passengers. They're pretty absurdly fast turboprops, and I think their engines are some of the most powerful in the world. – egid Dec 24 '13 at 4:22
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    As I recall they're also allegedly one of the loudest aircraft in operation - I've never had the pleasure of listening to one take off, but judging by the size of that propeller disk it would not surprise me if this were true. – voretaq7 Dec 24 '13 at 4:39
  • @voretaq7 Isn't the noise caused by the tips going supersonic rather than the size per se? (Sure, longer blades mean that the tips will go supersonic at lower rpm.) – David Richerby Jun 7 '14 at 8:53
  • @DavidRicherby Yup - and that's the reason large blades are potentially noisier (it's harder to balance "efficient RPM for the airfoil" with "please don't be making sonic booms"). You also get a larger "whoosh" as each blade goes by (moving comparatively more air than a smaller-diameter prop at the same RPM), but i's the noise off the tips that tends to dominate. – voretaq7 Jun 8 '14 at 3:42

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