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Why are these configurations not observed often?

The only time I see propellers are on straight wings or on the nose, while jet engines are always on swept wings or the tail. Why?

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closed as too broad by jwzumwalt, jwenting, SMS von der Tann, CGCampbell, fooot Jul 2 '18 at 14:49

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ Welcome to SE... As a new member I suggest you reword your question or narrow the scope of the question or you will see it soon closed. Neither assumption is true - for example consider pusher propeller airplanes and the A-10 Warthog ground attack fighter, L-39, or T-33 trainer. $\endgroup$ – jwzumwalt Jul 1 '18 at 14:03
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You can find propellers on swept wing aircraft, such as the TU-95, or more recently, the Airbus A400M.

One turbofan powered aircraft that has straight wings is the Cessna Citation

No, it's not common, due largely to the speed at which the aircraft normally operates. Turboprops tend to operate at a lower speed than turbojets or turbofans, where the benefits of swept wings (better performance when approaching Mach 1) aren't evident, while the benefits of straight wings (better performance at lower speeds) comes in handy on the shorter airfields that smaller turboprops operate from. Bump the cruising speed up to Mach .8+, and the swept wing is the optimal design.

In the case of both the TU95 and A400M, they cruise at higher speeds than most turboprop aircraft, hence the employment of a sweep to the wings.

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"You always see" is perhaps due to you not seeing a wide enough variety of airplanes. For instance, here's an example of a plane that has both props and jets on a moderately-swept wing: https://en.wikipedia.org/wiki/Convair_B-36_Peacemaker Here are a couple of other examples (of many) of jets with straight wings: https://theaviationist.com/2014/06/17/scorpion-new-livery/ https://en.wikipedia.org/wiki/Lockheed_F-104_Starfighter

@John K's answer covers the technical reasons for swept wings. There's also another factor that I'd think is pretty obvious: if you put props on steeply-swept wings, you need to get the prop far enough out in front of the wing so that it doesn't hit it, which creates structural problem. But here's an example of a single-engine propellor plane with swept wings: http://thanlont.blogspot.com/2011/04/bell-l-39-wing-sweep-evaluation.html Then there's the Beechcraft Starship, which has part of the wing swept: https://en.wikipedia.org/wiki/Beechcraft_Starship (Note the pusher propellors: it's hard to see how tractor props could be mounted on the same wing.)

Many more examples of both sorts can be found by spending a few minutes with a search engine.

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    $\begingroup$ Better example of a straight wing jet is the F-84 Thunderstreak or the Gloster Meteor. $\endgroup$ – jwenting Jul 2 '18 at 10:55
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Swept wings are for cruising close to the speed of sound (all the sweep does is make the effective chord of the airfoil longer so it's ratio of chord to thickness is higher - a finer profile you might say - and this delays the shock wave formation a little bit).

Propeller tips become supersonic way below the speed that wings start to become transonic, limiting the forward speed the propeller can operate at, so you simply can't go fast enough with a propeller to enjoy a significant benefit from wing sweep. The Russians did the sweep+propellers thing with airplanes like the TU-95, but in the West it wasn't considered worth the trouble.

So in a nutshell: Tubojets/turbofans go on the wings and tail/fuselage because that's the most efficient place for them, and they're associated with swept wings because they can operate fast enough to benefit from sweep; propellers are on the wings and nose because that's the most efficient place for them, and the wings are straight because there's no point in sweeping them (and dealing with the downsides of sweep) when the prop can't go fast enough for it to be worthwhile.

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  • $\begingroup$ What about the F-14 Tom Cat that has jet engines with straight wings which can be swept back in mid-air? I understand the bit about efficiency and drag, but what exactly is the benefit of having this "switchable" wing type? Is it just to conserve fuel? $\endgroup$ – Samid Jul 1 '18 at 23:51
  • $\begingroup$ @Samid, that is a question on its own. You can ask it separately, but I'm sure you'll find a ready answer if you search. (Such design is called "variable sweep"). Very basically, remember that aircraft sometimes need to fly fast, and sometimes slow (e.g. for landing)... $\endgroup$ – Zeus Jul 2 '18 at 0:18
  • $\begingroup$ @Samid One design purpose for the Tomcat's sweep is to land on the aircraft carrier at a slower approach speed. (which was a very desirable design feature at the time). $\endgroup$ – KorvinStarmast Jul 2 '18 at 13:10
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Propellers are more efficient, but they only work well when their tip speed is still subsonic, which limits the maximum speed of propeller-driven planes.

Jet engines on the other hand are inefficient at low speed, but their efficiency increases at higher speeds, and they can be designed for much higher flight speeds.

Straight wing is more efficient, but once the flow over it exceeds speed of sound—and the increase in speed over the wing means it happens about M0.70–0.75 (70–75% of speed of sound)—its drag increases significantly.

Swept wing delays this effect, because only the component of airflow velocity perpendicular to the wing causes it. That way swept wings remain efficient near, and if sufficiently swept even above, speed of sound.

Therefore the slower aircraft have propellers and straight wings and the faster ones have jet engines and swept wings, while the other two combinations don't make much sense.

Note that there are a few aircraft with jet engines and straight wings, either because they are not yet fast enough to need swept wings (e.g. the Cessna Citation I and II or the early fighters) or because they have supersonic wings (e.g. the F-104). And the fastest propeller-driven planes do have (moderately) swept wings (e.g. the Tu-95).

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  • $\begingroup$ Thanks for explaining. I know there's a few examples but do these configurations confer any advantage, i.e. swept wings on Tu-95? Basically what I'm asking is, why do they bother with these configurations when they know props are better suited on straight wings and jets on swept wings? $\endgroup$ – Samid Jul 1 '18 at 23:55
  • $\begingroup$ @Samid, sometimes you just can't do (or know) better. Tu-95, for example, was built at a time when jet engines were not mature yet, and Tupolev deemed them particularly unsuitable for a long-range aircraft because they were so thirsty. Yet the bomber had to be fast, at least in its maximum speed, so it was designed as such, including a swept wing. And it had to be built NOW, being at the peak of the Cold War: no time to wait for new engines. (At the same time, Americans put 8 jet engines on their new B-52, and the history has shown it was worth the risk). $\endgroup$ – Zeus Jul 2 '18 at 0:31
  • $\begingroup$ @Zeus: But the Americans probably had better jet engines than the Soviets. $\endgroup$ – jamesqf Jul 2 '18 at 19:20
  • $\begingroup$ @jamesqf, quite possibly (although early practical Soviet engines were replicas and modifications of the good British ones). But still, for everyone, after WWII, props were a mature technology, whereas jets were new. (BTW, J57 was a development of a turboprop). $\endgroup$ – Zeus Jul 3 '18 at 0:23

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