# Jet engine bypass air produces 85% of engine thrust? **URL:** \ **Category:** Factual Questions\ **Created:** [February 5, 2009, 5:16am UTC](https://boards.straightdope.com/t/jet-engine-bypass-air-produces-85-of-engine-thrust/484537 "2009-02-05T05:16:25Z")\ **Posts on this page:** 12\ **Page:** 2
**Author:** ![ElvisL1ves](https://avatars.discourse-cdn.com/v4/letter/e/5daacb/32.png) [@ElvisL1ves](https://boards.straightdope.com/u/ElvisL1ves)\ **Post date:** [February 6, 2009, 2:20pm UTC](https://boards.straightdope.com/t/jet-engine-bypass-air-produces-85-of-engine-thrust/484537/21 "2009-02-06T14:20:24Z")
[quote=“1920s Style “Death Ray”, post:18, topic:484537”] A small correction to myself. NL and NH are the low and high **pressure** stages . [/QUOTE] 'Sokay. The low and high pressure rotors normally have lower and higher speeds, too. The choice of nomenclature (N1/N2, NL/NH, NF/NG) depends on the manufacturer and sometimes the model. Each rotor has both a compressing section and a turbine section, each of which can have multiple rows (“stages”) of airfoils, or centrifugal impellers. The “100 %” figure often refers to the maximum rated power of an earlier model. Upon making and certifying changes that improve the engine’s performance, it’s just easier and clearer to raise the number than to rescale all the previous data. ---
**Author:** ![Icerigger](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/icerigger/32/432_2.png) [@Icerigger](https://boards.straightdope.com/u/Icerigger)\ **Post date:** [February 6, 2009, 2:26pm UTC](https://boards.straightdope.com/t/jet-engine-bypass-air-produces-85-of-engine-thrust/484537/22 "2009-02-06T14:26:14Z")
So a modern jet engine is essentially a turboprop, the huge fan is really the propeller and that provides most of the thrust. I often wondered why the air intakes on newer airliners were so large, compare the engines on a 707 Vs a 777. ---
**Author:** ![billfish678](https://avatars.discourse-cdn.com/v4/letter/b/7bcc69/32.png) [@billfish678](https://boards.straightdope.com/u/billfish678)\ **Post date:** [February 6, 2009, 2:29pm UTC](https://boards.straightdope.com/t/jet-engine-bypass-air-produces-85-of-engine-thrust/484537/23 "2009-02-06T14:29:08Z")
> [@puppygod](#): > > Forget helicopters with their couple of hours aloft. Now, Zeppelins can hover for weeks! That’s the future! 😃 > > Seriously though, your analogies are invalid. There are some tasks that helicopter can do better, there are some tasks that airliner do better. And it seems that some tasks can’t be done by either - hence people still are developing new constructions and test new ideas. Nothing wrong with that. > > Making “hovering car” sure is engineering challenge, and sure it’s not gonna be efficient enough to replace helicopters in what they do. But there are conceivable uses for them and, who knows, maybe even significant market niche. If they were anything other than toys, the military would be using them already. But instead, the’ve been going “hmmm, maybe” for 50 years. ---
**Author:** ![Bookkeeper](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bookkeeper/32/281_2.png) [@Bookkeeper](https://boards.straightdope.com/u/Bookkeeper)\ **Post date:** [February 6, 2009, 6:55pm UTC](https://boards.straightdope.com/t/jet-engine-bypass-air-produces-85-of-engine-thrust/484537/24 "2009-02-06T18:55:39Z")
> [@billfish678](#): > > If they were anything other than toys, the military would be using them already. > > But instead, the’ve been going “hmmm, maybe” for 50 years. I suspect that, for both military and civilian use, the real problem is designing a control system that doesn’t require the equivalent of Harrier jump-jet training for use, as well as getting a good enough fuel efficiency that you can get further than the corner before the on-board tank goes dry, and without devoting all of the payload other than the driver to fuel. I’d like to see some endurance figures for the Moller designs, for example. I’d also like to see what happens if the driver/pilot takes his hands off the controls for a moment to change a CD. :dubious: ---
**Author:** ![sailor](https://avatars.discourse-cdn.com/v4/letter/s/a587f6/32.png) [@sailor](https://boards.straightdope.com/u/sailor)\ **Post date:** [February 6, 2009, 7:07pm UTC](https://boards.straightdope.com/t/jet-engine-bypass-air-produces-85-of-engine-thrust/484537/25 "2009-02-06T19:07:19Z")
> [@Icerigger](#): > > So a modern jet engine is essentially a turboprop, the huge fan is really the propeller and that provides most of the thrust. I often wondered why the air intakes on newer airliners were so large, compare the engines on a 707 Vs a 777. A propeller, like a fan with no duct, also pushes the fluid outwards by centrifugal force, not only backwards. The duct improves the performance of the fan by keeping the fluid inside a narrower path. ---
**Author:** ![billfish678](https://avatars.discourse-cdn.com/v4/letter/b/7bcc69/32.png) [@billfish678](https://boards.straightdope.com/u/billfish678)\ **Post date:** [February 6, 2009, 7:16pm UTC](https://boards.straightdope.com/t/jet-engine-bypass-air-produces-85-of-engine-thrust/484537/26 "2009-02-06T19:16:39Z")
ANY (at least as far as I can think of) materials/engineering/power/efficiency/weight/construction/computer control/whatever “breakthrough” that makes hover cars and jet/rocket packs more “practical” will ALSO apply to normal things like aircraft and helicopters, making them STILL head and shoulders above these one trick ponies. Hover cars and jet packs will always be the red headed (and retarded) step children of the aerodynamics community. To reiterate what got this whole thing started, baring a Sci Fi level break through in power or anti gravity, throwing a small mass down fast (rockets, jet packs and hover cars) will always PALE in comparison to throwing a large mass down slow (helicopters, airplanes, gliders). Its simple physics. Quit hovering over my lawn. The noise and fumes are getting on my nerves. ---
**Author:** ![puppygod](https://avatars.discourse-cdn.com/v4/letter/p/ce7236/32.png) [@puppygod](https://boards.straightdope.com/u/puppygod)\ **Post date:** [February 6, 2009, 7:26pm UTC](https://boards.straightdope.com/t/jet-engine-bypass-air-produces-85-of-engine-thrust/484537/27 "2009-02-06T19:26:36Z")
Now, that’s just silly. Plane can’t hover. Helicopter has that huge-ass rotor that just wait to hit nearby flagpole, power grid or trees. Ducted fan vehicle can easily out-perform either in tight urban environment. It’s a fact. Not antygravity. Ducted fan. Sheesh. ---
**Author:** ![billfish678](https://avatars.discourse-cdn.com/v4/letter/b/7bcc69/32.png) [@billfish678](https://boards.straightdope.com/u/billfish678)\ **Post date:** [February 6, 2009, 7:42pm UTC](https://boards.straightdope.com/t/jet-engine-bypass-air-produces-85-of-engine-thrust/484537/28 "2009-02-06T19:42:56Z")
> [@puppygod](#): > > Now, that’s just silly. Plane can’t hover. Helicopter has that huge-ass rotor that just wait to hit nearby flagpole, power grid or trees. Ducted fan vehicle can easily out-perform either in tight urban environment. It’s a fact. > > Not antygravity. Ducted fan. Sheesh. When I see a ducted fan hover car flying in an urban canyon I’ll believe you. ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [February 6, 2009, 10:10pm UTC](https://boards.straightdope.com/t/jet-engine-bypass-air-produces-85-of-engine-thrust/484537/29 "2009-02-06T22:10:15Z")
> [@sailor](#): > > A propeller, like a fan with no duct, also pushes the fluid outwards by centrifugal force, not only backwards. The duct improves the performance of the fan by keeping the fluid inside a narrower path. If I’m not mistaken, another crucial point is that the duct keeps the fan from encountering air that’s moving supersonic relative to the tips of the blades. That’s what limits conventional propellers to airspeeds of around 400mph. ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [February 6, 2009, 10:41pm UTC](https://boards.straightdope.com/t/jet-engine-bypass-air-produces-85-of-engine-thrust/484537/30 "2009-02-06T22:41:44Z")
> [@Lumpy](#): > > That’s what limits conventional propellers to airspeeds of around 400mph. There are limitations, though a bit above 400 mph. Here’s a [Wiki article](http://en.wikipedia.org/wiki/Fastest_propeller-driven_aircraft) that mentions _Rare Bear_, a Reno racer that has done 528 mph with a reasonably normal (albeit large) propellor. ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [February 6, 2009, 11:16pm UTC](https://boards.straightdope.com/t/jet-engine-bypass-air-produces-85-of-engine-thrust/484537/31 "2009-02-06T23:16:33Z")
> [@Bookkeeper](#): > > I’d like to see some endurance figures for the Moller designs, for example. I’d also like to see what happens if the driver/pilot takes his hands off the controls for a moment to change a CD. :dubious: There’s no reason it can’t have computer assisted controls to maintain stability, don’t know what that would do to the cost though, or maybe it already has it. I’d like to see what happens when it has an engine failure. ---
**Author:** ![billfish678](https://avatars.discourse-cdn.com/v4/letter/b/7bcc69/32.png) [@billfish678](https://boards.straightdope.com/u/billfish678)\ **Post date:** [February 6, 2009, 11:53pm UTC](https://boards.straightdope.com/t/jet-engine-bypass-air-produces-85-of-engine-thrust/484537/32 "2009-02-06T23:53:39Z")
[quote=“1920s Style “Death Ray”, post:31, topic:484537”] There’s no reason it can’t have computer assisted controls to maintain stability, don’t know what that would do to the cost though, or maybe it already has it. I’d like to see what happens when it has an engine failure. [/QUOTE] Moller flees to a 3rd world country with lax banking laws and no extradition treaties 🙂 THATS what happens [Previous page](https://boards.straightdope.com/t/jet-engine-bypass-air-produces-85-of-engine-thrust/484537.md?page=1)