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Why Are All Bizjets Rear-Engine/T-Tail?  
User currently offlineRussianJet From Belgium, joined Jul 2007, 7703 posts, RR: 21
Posted (2 years 5 months 3 days 2 hours ago) and read 8311 times:
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Ok, may be a very basic question, but cards on the table - my knowledge of these things is not deep.

Seems to me that basically all small-medium bizjets are rear-engined with either a t-tail, or in Dassault's case a sort of 'cross' tail (which, btw, I think looks beautiful). What are the main reasons for this? I think a tiny jet with engines slung under the wing would look cool, but is such a thing feasible? Are there any examples in existence?

Is it principally a question of small wing needs to be a clean wing?

Grateful for any thoughts.


✈ Every strike of the hammer is a blow against the enemy. ✈
41 replies: All unread, showing first 25:
 
User currently offlineKPWMSpotter From United States of America, joined Dec 2006, 445 posts, RR: 2
Reply 1, posted (2 years 5 months 3 days 2 hours ago) and read 8298 times:

Quoting RussianJet (Thread starter):
I think a tiny jet with engines slung under the wing would look cool, but is such a thing feasible? Are there any examples in existence?

It may look cool, but think of how much taller the plane will need to be in order to fit a pair of engines under the wing. Landing gear is heavy - very heavy, the closer you can keep a jet to the ground, the better. Also, most business jets don't have the option of connecting to a jet bridge, keeping the plane low saves on the complexity of the airstairs that need to be built into the door. The T-Tail or cruciform tail configuration is a result of the tail mounted engines; you don't really want your horizontal stabilizer to be continuously in the wake of the engines and jetwash.

The real question is "why aren't more airliners rear-engined and T-Tails?" The answer there is more a function of fuselage length. The fuselage between the engines and the wings needs to be reinforced to carry the thrust and weight load of the engines. With short, stubby business jets that doesn't add up to much weight. A 757 or 737-sized airliner would need to be significantly reinforced. Douglas made the configuration work, but most large aircraft are better off with the engines on the wings, which are already the strongest part of the aircraft.

VFW-Fokker tried a "best of both worlds" approach with the engines on top of the wings on the VFW-614, Honda is now experimenting with the same thing with the Honda Jet. This keeps the airplane low, and the engine loads on the wing, but for some reason the configuration really hasn't caught on.

Hope this helps!



I reject your reality and substitute my own...
User currently onlinePihero From France, joined Jan 2005, 4445 posts, RR: 76
Reply 2, posted (2 years 5 months 3 days 2 hours ago) and read 8289 times:
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Two main advantages :
- The aircraft can sit lower, making shorter gear possible, needing shorter integrated airsteps, therefore saving quite a lot of weight.
The servicing is also mader easier, everything being at man-level.
- The wing is a lot cleaner, aerodynamically speaking

Add to these two :
-Generally speaking, the engines are protected from FOD
-The vertical stabilizer and the rudder could be designed smaller as the engines'thrust lines are close to the aircraft centerline ( an engine--out moment is smaller than on underwing-slung engines.



Contrail designer
User currently offlinevikkyvik From United States of America, joined Jul 2003, 10026 posts, RR: 26
Reply 3, posted (2 years 5 months 3 days 2 hours ago) and read 8282 times:
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Quoting RussianJet (Thread starter):
Is it principally a question of small wing needs to be a clean wing?

My major guess would be ground clearance. Not a whole lot of room under a bizjet wing to stow an engine.

Quoting RussianJet (Thread starter):
Are there any examples in existence?

Not a bizjet, but....


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Also found this thread:

Why No Wing-mounted Engines Small Jets (by Redcordes Feb 21 2007 in Tech Ops)



"Two and a Half Men" was filmed in front of a live ostrich.
User currently onlinePihero From France, joined Jan 2005, 4445 posts, RR: 76
Reply 4, posted (2 years 5 months 3 days 2 hours ago) and read 8277 times:
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Quoting KPWMSpotter (Reply 1):
Douglas made the configuration work,

Before Douglas, Sud Aviation pioneered the concept with the beautiful Caravelle.
Don't forget the 727, the Fokkers...etc...

The main problem of aft-mounted engines on an airliner is balance and as you said the ext-ra weight needed to reinforce the engine and the tailplane areas.



Contrail designer
User currently offlineCitationJet From United States of America, joined Mar 2003, 2438 posts, RR: 3
Reply 5, posted (2 years 5 months 3 days 1 hour ago) and read 8228 times:

Quoting RussianJet (Thread starter):
I think a tiny jet with engines slung under the wing would look cool, but is such a thing feasible?

Not all tiny biz jets have tiny engines. I don't think you would want to try and fit these big engines under the wings.

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Boeing Flown: 701,702,703;717;720;721,722;731,732,733,734,735,737,738,739;741,742,743,744,747SP;752,753;762,763;772,773.
User currently onlineKELPkid From United States of America, joined Nov 2005, 6383 posts, RR: 3
Reply 6, posted (2 years 5 months 3 days 1 hour ago) and read 8210 times:

Well, I can think of one notable exception...


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Of course, it does have the hometown advantage...  



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineRaginMav From United States of America, joined May 2004, 376 posts, RR: 1
Reply 7, posted (2 years 5 months 3 days 1 hour ago) and read 8184 times:

Quoting vikkyvik (Reply 3):
Not a bizjet, but....



Now we're onto something! A high-wing would allow for wing mounted engines, short landing gear, and would eliminate the annoying spar-carry-through protruding into the aircraft floor of some business aircraft.

The down fall may be headroom, as the spar would now be interfering with the ceiling structure.

Quoting KPWMSpotter (Reply 1):
Honda is now experimenting with the same thing with the Honda Jet. This keeps the airplane low, and the engine loads on the wing, but for some reason the configuration really hasn't caught on.



The canned answer is usually 'because they're ugly', although I don't entirely agree. For example, how different do these two competing deisngs really appear?


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User currently offlineRussianJet From Belgium, joined Jul 2007, 7703 posts, RR: 21
Reply 8, posted (2 years 5 months 3 days 1 hour ago) and read 8177 times:
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Great replies so far, thank you. Ground clearance is an obvious factor that I am kicking myself for not having considered, but then that is surely resolved with a higher wing, a la BAe-146 or indeed the Viking pictured in an earlier reply. Always thought the Viking looked great....but yes, not a bizjet of course.

Quoting CitationJet (Reply 5):
Not all tiny biz jets have tiny engines

Indeed not, but then - some do.

Quoting vikkyvik (Reply 3):
Why No Wing-mounted Engines Small Jets

Thank you for that, my friend.

The E170 is pretty small, with pretty small engines. Again, not a bizjet though of course.

How small can engines realistically get before they are just too inefficient? I can think of examples where small seems powerful, like an RR-powered A332.



✈ Every strike of the hammer is a blow against the enemy. ✈
User currently onlineKELPkid From United States of America, joined Nov 2005, 6383 posts, RR: 3
Reply 9, posted (2 years 5 months 3 days 1 hour ago) and read 8173 times:

Quoting CitationJet (Reply 5):
Not all tiny biz jets have tiny engines. I don't think you would want to try and fit these big engines under the wings.

The drag from the extra tall landing gear hanging down in the breeze would require higher power settings on approach. On the other hand, it would help your service ceiling certification requirements because it would probably get you down to altitude quicker on an emergency descent  Not to mention the need for built-in, retractable airstairs (a la early 737). But you'd have every lineman in the country mad at you, as refueling the Citation X would be more like refuelling a Merlin or Metro, a major pain in the arse.



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlinerwessel From United States of America, joined Jan 2007, 2351 posts, RR: 2
Reply 10, posted (2 years 5 months 2 days 23 hours ago) and read 8086 times:
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Quoting RussianJet (Reply 8):
Ground clearance is an obvious factor that I am kicking myself for not having considered, but then that is surely resolved with a higher wing,

Which leaves you a major problem of the wingbox passing through the occupied part of the cabin. Even the much larger (than a typical bizjet) BAe-146, has a quite noticeable protrusion into the cabin where the spars pass through. Then you either need to find room for the mains in the fuselage (since the really big Mach fairing is now going to be on top of the aircraft where you can't use it to hold stuff like wheels), or have really long gear legs.

Certainly not impossible to deal with, but a definite strike against the configuration.


User currently offlineRussianJet From Belgium, joined Jul 2007, 7703 posts, RR: 21
Reply 11, posted (2 years 5 months 2 days 23 hours ago) and read 8073 times:
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So basically we don't want a tiny plane on massive stilts, or a huge obstruction in the limited cabin space. Seems fairly sensible. On the other hand, 757-style big legs can look wicked.


✈ Every strike of the hammer is a blow against the enemy. ✈
User currently offlineC46 From United States of America, joined Feb 2011, 43 posts, RR: 0
Reply 12, posted (2 years 5 months 2 days 23 hours ago) and read 8048 times:

Wouldn't it also be quieter in the cabin with rear mounted engines which would be a good selling point for folks trying to be productive on a business trip?

User currently offlineAreopagus From United States of America, joined Sep 2001, 1369 posts, RR: 1
Reply 13, posted (2 years 5 months 2 days 22 hours ago) and read 8015 times:

You can get short landing gear on a high-wing aircraft if you either retract them into drag-producing sponsons like the military airlifters, or into cutouts in your nice otherwise-round pressure vessel. It appears to me that the BAe-146 does a little of both.

I had a window seat under the wing of a BAe-146. The view of the #2 engine was terrific. I didn't see much ground, though, and no sky. I suppose that kind of view is a hard sell to the kind of people who use bizjets.


User currently offlineStarlionblue From Greenland, joined Feb 2004, 17039 posts, RR: 66
Reply 14, posted (2 years 5 months 2 days 21 hours ago) and read 7967 times:

Quoting RaginMav (Reply 7):
Now we're onto something! A high-wing would allow for wing mounted engines, short landing gear, and would eliminate the annoying spar-carry-through protruding into the aircraft floor of some business aircraft.

The down fall may be headroom, as the spar would now be interfering with the ceiling structure.

Yes but:

Quoting C46 (Reply 12):
Wouldn't it also be quieter in the cabin with rear mounted engines which would be a good selling point for folks trying to be productive on a business trip?

Noise is an issue with a high wing and underslung engines.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlinebrenintw From Taiwan, joined Jul 2006, 1644 posts, RR: 1
Reply 15, posted (2 years 5 months 2 days 19 hours ago) and read 7894 times:

Quoting Starlionblue (Reply 14):
Noise is an issue with a high wing and underslung engines.

I was recently on a BAe-146 for the first time (JNB - Maun) and, even though I am a fairly experienced flier, I nearly freaked when the flaps were retracted after takeoff. It sounded like the pilot had suddenly fire-walled the throttles and those engines were about to spin themselves off the wing. I was having pictures of experiencing my first-ever emergency landing, with my extremely nervous-flier mother sitting next to me.

It took me a few seconds to realize that the sound was the flap mechanism.

If I was in a biz jet, I would certainly not want that kind of noise each time I take off or land.



I'm tired of the A vs. B sniping. Neither make planes that shed wings randomly!
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 79
Reply 16, posted (2 years 5 months 2 days 18 hours ago) and read 7862 times:

Quoting KPWMSpotter (Reply 1):
VFW-Fokker tried a "best of both worlds" approach with the engines on top of the wings on the VFW-614, Honda is now experimenting with the same thing with the Honda Jet. This keeps the airplane low, and the engine loads on the wing, but for some reason the configuration really hasn't caught on.

It's aerodynamically tricky; the upper surface of the wing is the most sensitive spot on the whole airplane. Trying to stick a strut there is not trivial.

Tom.


User currently offlineRussianJet From Belgium, joined Jul 2007, 7703 posts, RR: 21
Reply 17, posted (2 years 5 months 2 days 12 hours ago) and read 7726 times:
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How about an 'engine in wing' design like the TU-104? now that *would* look amazing on a small jet.


✈ Every strike of the hammer is a blow against the enemy. ✈
User currently offlineFabo From Slovakia, joined Aug 2005, 1219 posts, RR: 1
Reply 18, posted (2 years 5 months 2 days 11 hours ago) and read 7703 times:

Must be a maintenance nightmare. And I think aerodynamics are not that great either.


The light at the end of tunnel turn out to be a lighted sing saying NO EXIT
User currently offlineRussianJet From Belgium, joined Jul 2007, 7703 posts, RR: 21
Reply 19, posted (2 years 5 months 2 days 11 hours ago) and read 7701 times:
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Quoting Fabo (Reply 18):
And I think aerodynamics are not that great either

Worse than having struts on a small wing though? Maintenance yes, that's probably a given...



✈ Every strike of the hammer is a blow against the enemy. ✈
User currently offlineDashTrash From United States of America, joined Aug 2006, 1528 posts, RR: 2
Reply 20, posted (2 years 5 months 2 days 8 hours ago) and read 7634 times:

Quoting CitationJet (Reply 5):

Most turbofans used on bizjets use engines that have centrifugal flow compressors making the engines shorter than the axial flow AE3007. The PW307 is similar in thrust rating to the AE3007 but much shorter as a result of the centrifugal compressor. The CJ610 and JT12 are also axial flow engines and longer as opposed to a TFE731 which is similar in thrust to the JT12.

Quoting C46 (Reply 12):
Wouldn't it also be quieter in the cabin with rear mounted engines which would be a good selling point for folks trying to be productive on a business trip?

Modern bizjets are fairly quit inside. Many of the designs have the baggage compartment between the engines. In the X you can hear and feel a hum from the engines at idle, but that's about it. Very little engine noise in flight.

Earlier bizjets like the Lears and Sabreliner are fairly loud inside.


User currently offlineRussianJet From Belgium, joined Jul 2007, 7703 posts, RR: 21
Reply 21, posted (2 years 5 months 2 days 8 hours ago) and read 7629 times:
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Quoting DashTrash (Reply 20):
Earlier bizjets like the Lears and Sabreliner are fairly loud inside.

Always imagined a Jetstar would be ridiculously loud inside, was it so?



✈ Every strike of the hammer is a blow against the enemy. ✈
User currently offlinebj87 From Netherlands, joined Jun 2009, 448 posts, RR: 0
Reply 22, posted (2 years 5 months 2 days 7 hours ago) and read 7604 times:

Quoting RussianJet (Reply 17):
How about an 'engine in wing' design like the TU-104? now that *would* look amazing on a small jet.

I would love to see something like that! I think the DH Comet look would be even better but one would need a ridiculously thick wing to make that work on a bizjet so the TU-104 look will have to do.

Quoting Fabo (Reply 18):
Must be a maintenance nightmare. And I think aerodynamics are not that great either.

Safety might also become a factor. After all we are sticking a turbine between the fuel and the fuselage. One uncontained disc failure and you have one massive problem. That would probably mean additional reinforcements will be required on top of the reinforcement required to fit the engine in the wing in the first place. I am by no stretch of the imagination an aircraft expert but the weight penalty might be an issue.

Quoting brenintw (Reply 15):
I was recently on a BAe-146 for the first time (JNB - Maun) and, even though I am a fairly experienced flier, I nearly freaked when the flaps were retracted after takeoff.

You are not the only one with that problem. Let's just say the noise caught my attention on my one and only 146 flight. Btw the noise is such a special characteristic of the aircraft that the noise of flap motors is included in the professional flight simulators.


User currently offlineRussianJet From Belgium, joined Jul 2007, 7703 posts, RR: 21
Reply 23, posted (2 years 5 months 2 days 7 hours ago) and read 7601 times:
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Quoting bj87 (Reply 22):
but one would need a ridiculously thick wing to make that work on a bizjet

But would one? As I alluded to above, there are some good examples of seemingly small yet powerful engines, the reference for which was the RR-powered A332.



✈ Every strike of the hammer is a blow against the enemy. ✈
User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2556 posts, RR: 24
Reply 24, posted (2 years 5 months 2 days 6 hours ago) and read 7595 times:

In the Hondajet the engines are above and behind the wing, unlike the VFW 614. Thus they don't block the expensive view out of the cabin windows, produce less cabin noise and have much less interference with the wing uppersurface airflow. They are in roughly the same relative position as aft mounted engines on conventional biz-jets, but the mounting location is different, giving a structural advantage.

Quoting bj87 (Reply 22):
You are not the only one with that problem. Let's just say the noise caught my attention on my one and only 146 flight. Btw the noise is such a special characteristic of the aircraft that the noise of flap motors is included in the professional flight simulators.

I think it may well be an aerodynamic noise, as the TE slots close. It only happens during the last stage of flap retraction.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
25 DashTrash : Don't know. Didn't fly the Jetstar.
26 Viscount724 : I'm quite sure it's aerodynamic noise, not the flap mechanism. I've heard it dozens of times. Same noise on both flap retraction and extension.
27 JETSTAR : I think I will jump in here about the JetStar, to the contrary the airplane was fairly quiet in the cabin, the engines were way aft of the seating ar
28 boacvc10 : A "roadster" by any other name. Can anyone comment on its performance / flight experience as measured? Why would four engines have been considered fo
29 soon7x7 : The very nature of private jets is to be able to plane and deplane PAX wherever they operate without the need for jetways or drive up airstairs. The o
30 Post contains links and images KELPkid : Since it was designed to the same spec as the JetStar (a US Air Force request), I'm wondering if 4 engines was spelled out in the requrement? As we k
31 JETSTAR : Both the JetStar and the McDonnell 220 were designed to compete for an US Air Force contract for a light VIP jet, While the McDonnell 220 was built r
32 MD-90 : As far as bizjets are concerned don't forget that it's easier to manufacture a fuselage sitting on top of a wing (especially if one is carbon fiber an
33 jetstar : Both the Lockheed JetStar and the Gulfstream 2, both very early biz jets have the fuselage mounted above the wings. On the JetStar the fuselage sits
34 Post contains images tjwgrr : No rear mounted engines here: Granted, the Dork Jet wasn't really designed to be a biz-jet. ..
35 Post contains images KELPkid : Nor was it designed to be a jet It is a turboprop airframe w/2 pure jets slapped on it.
36 Western727 : Fascinating thread. May I add (speculation only): a desire to keep the aircraft height as low as possible to fit in smaller hangars. After all, wing-m
37 Post contains images Starlionblue : Or a jet.
38 MD-90 : I always thought it was a cute plane, though.
39 Fabo : I was supposed to work one. Then a sup switched me over to a 735 we had some seating or checkin system problems with. Dreaded day...
40 rampart : I'm thinking the majority of these bizjets have t-tails, a smaller proportion have the cruciform tail (you mention the Falcon, also the Jetstar, Corv
41 Chese : The cruciform tail is still fairly popular. Hawkers, Falcons, still many Citations. I like it in that there it makes working on the stab much easier a
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