# Expain aircraft engines & propellers??? **URL:** \ **Category:** Factual Questions\ **Created:** [March 29, 2014, 3:05am UTC](https://boards.straightdope.com/t/expain-aircraft-engines-propellers/684813 "2014-03-29T03:05:08Z")\ **Posts on this page:** 6\ **Page:** 2
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [March 30, 2014, 12:38pm UTC](https://boards.straightdope.com/t/expain-aircraft-engines-propellers/684813/21 "2014-03-30T12:38:06Z")
> [@Raguleader](#): > > one post-war development with the advent of the Jet engine was the Turboprop: A jet engine that used the energy produced to spin a propeller, rather than trying to propel the aircraft with raw jet thrust. It is, I’m told, a very efficient design, more fuel efficient than a jet engine Modern jet engines gain efficiency by means of a high bypass ratio (ratio of air that passes around the engine core to that which is involved in combustion). Such engines can be thought of as “ducted turboprops”. As you’d expect, they tend to be of [substantial diameter](http://en.wikipedia.org/wiki/File:Boeing_787_first_flight.jpg). ---
**Author:** ![Senegoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/senegoid/32/6606_2.png) [@Senegoid](https://boards.straightdope.com/u/Senegoid)\ **Post date:** [March 31, 2014, 1:03am UTC](https://boards.straightdope.com/t/expain-aircraft-engines-propellers/684813/22 "2014-03-31T01:03:52Z")
> [@Raguleader](#): > > As others have said, you’re going to need to impart a shit-ton\* of torque . . . > > \< . . . \> > > \*Please pardon the highly technical engineering jargon. It’s an SI unit, I assure you. ??? I would think the SI system would use the shit-kg instead. 🙂 ---
**Author:** ![Raguleader](https://avatars.discourse-cdn.com/v4/letter/r/258eb7/32.png) [@Raguleader](https://boards.straightdope.com/u/Raguleader)\ **Post date:** [March 31, 2014, 9:28am UTC](https://boards.straightdope.com/t/expain-aircraft-engines-propellers/684813/23 "2014-03-31T09:28:52Z")
> [@Senegoid](#): > > ??? I would think the SI system would use the shit-kg instead. 🙂 No no no, you’re thinking of Metric. SI is really weird, and possibly French. ---
**Author:** ![Hypno-Toad](https://avatars.discourse-cdn.com/v4/letter/h/b2d939/32.png) [@Hypno-Toad](https://boards.straightdope.com/u/Hypno-Toad)\ **Post date:** [March 31, 2014, 5:03pm UTC](https://boards.straightdope.com/t/expain-aircraft-engines-propellers/684813/24 "2014-03-31T17:03:10Z")
> [@Raguleader](#): > > In addition to air resistance, you also have to consider the inertia of the propeller itself. Some examples were pretty freaking big ([compare the propeller on the F4U Corsair to the guy standing next to the plane](https://upload.wikimedia.org/wikipedia/commons/8/85/F4U-1_NACA_1943.jpeg)). As others have said, you’re going to need to impart a shit-ton\* of torque on that propeller to get it spinning and keep it that way, especially if you need to change pitch or RPM rapidly as a fighter pilot might find reason to do in combat (or in just trying to put the thing down on an aircraft carrier without crashing spectacularly into the sea). It’s funny, because the whole reason the Corsair has that gull-wing shape was to get clearance for the massive prop. The prop had to be big in order to use the huge quantity of horsepower that engine put out. Sure, you can use a smaller prop and spin it faster. But like others have said, that only works to a certain point. Once a certain prop speed is reached, the air no longer flows over its surface in a smooth laminar fashion and breaks up into vortices and other turbulence. The prop is no longer imparting the engines power to the air as thrust. It’s just chopping up the air. A bigger prop has more surface area so it can transfer more energy to the air than a smaller prop at the same speed. ---
**Author:** ![Raguleader](https://avatars.discourse-cdn.com/v4/letter/r/258eb7/32.png) [@Raguleader](https://boards.straightdope.com/u/Raguleader)\ **Post date:** [March 31, 2014, 9:50pm UTC](https://boards.straightdope.com/t/expain-aircraft-engines-propellers/684813/25 "2014-03-31T21:50:12Z")
> [@Hypno-Toad](#): > > It’s funny, because the whole reason the Corsair has that gull-wing shape was to get clearance for the massive prop. The prop had to be big in order to use the huge quantity of horsepower that engine put out. Sure, you can use a smaller prop and spin it faster. But like others have said, that only works to a certain point. Once a certain prop speed is reached, the air no longer flows over its surface in a smooth laminar fashion and breaks up into vortices and other turbulence. The prop is no longer imparting the engines power to the air as thrust. It’s just chopping up the air. A bigger prop has more surface area so it can transfer more energy to the air than a smaller prop at the same speed. Also, if the prop blades break the sound barrier (as was an issue with the truly weird XF-84H Thunderscreech), they do what everything else does while breaking the sound barrier: Make a sonic boom as they pass. Except unlike an airplane which does the boom and flies off into the next state, the propeller blades are remaining in roughly the same spot, a few feet in front of the pilot, the whole time. Yeah, hearing loss was a problem for people working with/around/on the same base as the XF-84H. ---
**Author:** ![Polar\_Iceman](https://avatars.discourse-cdn.com/v4/letter/p/f9ae1b/32.png) [@Polar\_Iceman](https://boards.straightdope.com/u/Polar_Iceman)\ **Post date:** [April 1, 2014, 3:09am UTC](https://boards.straightdope.com/t/expain-aircraft-engines-propellers/684813/26 "2014-04-01T03:09:37Z")
> [@Napier](#): > > You are generally on more or less the right path but we can tidy up quite a few things in here. It would take zero power to keep either of your propellers spinning in a perfect vacuum (ignoring some very small effects like power loss through your framework for vibration). However, it would take some energy to bring the propeller up to speed, so the lawnmower might take a while longer to accelerate it. With less power, the lawnmower would require more time to produce that much energy. But, the propeller would give you back that energy while it is slowing down. > > It doesn’t matter much that the air is viscous. Viscosity tells you how quickly a fluid converts mechanical energy into heat energy by friction. The other effect is how dense the air is (how much inertia a volume of it has at a given velocity), which converts propeller mechanical energy into kinetic energy in the air (which is what you’re actually trying to do). The ratio of the inertial effect to the viscous effect is called the Reynolds number, and it’s high for airplane propellers. It’s equal to the size times the speed times the density divided by the viscosity, and should be a few million for an airplane propeller. We call this situation “inviscid” because it’s practically without viscous effect. > > You say " for every unit of m accelerated by the propeller, there is an equivalent unit of thrust". You have the dimensions inconsistent here, because mass accelerated by the propeller happens over some time, whereas thrust is instantaneous. Thrust is force, which is instantaneous. What you are missing is the idea of “impulse”, which is the product of force and time. To get a certain amount of thrust, you have to be accelerating some quantity of mass per time at some level of acceleration. > > You also say “to move a mass of an aircraft M at acceleration a, it requires a displacement of air of mass M”. However, you could accelerate less mass at a greater acceleration, or vice versa, to do the same job of accelerating the aircraft. There’s no particular need to make the masses the same. > > So, you were sort of on the right track, but this is a bit more accurate. Thanks for the clarifications. I get it now. [Previous page](https://boards.straightdope.com/t/expain-aircraft-engines-propellers/684813.md?page=1)