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787 Ground Clearance vs. Other Widebody Types?  
User currently offlineAviaponcho From France, joined Aug 2011, 613 posts, RR: 8
Posted (1 year 7 months 3 weeks 4 days 5 hours ago) and read 5125 times:

Hello everybody
My first thread here. I wanted to use the 787 6 wheel MLG thread, but it seems archived.
It's linked with that "scoop" that the 787-10 could have a 6 wheel MLG. That has been largely viewed as a growth opportunity move.
Can it be in fact linked with constraints on "take off angle" ?

Taken from the ACAPS for various aircraft, here are the height above ground at MTOW (and worst CG position)
Engine height above ground Forward Fuselage height above ground
A350-900 0.74 2.41
A330-300 0.69 2.1
A330-200 0.67 2.04
A330-200F 0.82 2.24
A340-600 0.52 1.94
A340-500 0.48 1.94
A340-300 1.25
A380-800 1.05 2.34
787-8 0.71 1.68
777-200LR 0.77
777-300ER 0.75

Fuselage ground clearance of the 787-8 is low, by far the lowest of all widebodies (and it's really the forward fuselage height above ground, not the wing fairing height). But due to the specific engine mounting on this airplane, the engine ground clearance is "standard" nothing to worry about.
So the legs of the 787-8 are rather short, but engines not that low.

But on takeoff, the angle of rotation is limited by tail strikes considerations. The lower the fuselage is, the lower the tail strike angle is

By the way I checked take off rotation angle on various operational documents

Model / take off / tail strike angle
777-200 / 8.5 / 12.1
777-300 basic / 7 / 8.9
777-300 ER / 8.5 / 10.0
787-8 / 8.2 / 11.2

So the "short 787-8" as already a lower tail strike angle than than shorter 777-200.
The 777-300ER is 10.2 m longer than the 777-200, but the relative increase in length is "only" 16%
The 787-10X will be at least 11.1 m longer than the 787-8, corresponding with a relative 20% increase in length
Under the assumption that the aft fuselage extension is +3 m from 787-8 to 787-9 and +5.5 from 787-8 to 787-10, I think that the tail strike angle can go as low as 8-8.5° for the 787-10.
So without any tricks, how can the 787-10 takeoff angle be better than that of 777-300 basic ?
We know that the 787 engines are already maxed out... and will end up a lot happier with a generous take off angle (lower thrust needed for a given taken length ?)
We also know that the 787-8 is a 227 t plane, and that the 787-9 is a lot heavier... (and was at the beginning to be the 7E7-9 @ 221 t)
I guess that Boeing need a trick for the 787-10, in order to have good field performances...


The trick can be a 6 MLG semi-levered, not necessarily sturdier BTW.
On the 777-300ER it permitted the same take off angle than that of the 777-200 series.

What do you think about that ? I can be totally wrong of course.

16 replies: All unread, jump to last
 
User currently offlineFlyDeltaJets From United States of America, joined Feb 2006, 1871 posts, RR: 2
Reply 1, posted (1 year 7 months 3 weeks 4 days 2 hours ago) and read 4641 times:
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Is the 787 lower than the 767-300? I don't have numbers but I had to bow my head to walk under a 767 forward of the main gear.


The only valid opinions are those based in facts
User currently offlineAviaponcho From France, joined Aug 2011, 613 posts, RR: 8
Reply 2, posted (1 year 7 months 3 weeks 4 days 1 hour ago) and read 4585 times:

Ok, so now it's in tech/ops. Just fine for me.
Thanks for the renaming...


@FlyDeltaJets, for what I remember, the double stretch 767-400ER has longer legs than 767-200 / 300

Isn't it ?


User currently offlineferpe From France, joined Nov 2010, 2803 posts, RR: 59
Reply 3, posted (1 year 7 months 3 weeks 3 days 18 hours ago) and read 4414 times:

Hi Poncho,

yes the 787 architecture uses a technique to mount the engines high in front of the wing that B started on the 777 and the 737, the limit is that they don't want important parts of the engine to be scraped if the frame has a wingtip go into ground. All this according to CM (who was on the project), I think it was discussed on the 350 thread some 2-3 months back.

The low LG on the 787 is good for a lot of things but not for doubly stretches for the very reason you gave. Observe that the 789 stretch was 10 frames, 5 before and 5 aft of the wing. The 787-10 stretch is 5 in front and 4 rear of the wing, go figure why. I am with you, for a double stretch B needs to do something about the rotation angle. The flaps architecture makes it pretty hard to move the Alfa-Pitch angle curve to the left. The flaps are simple dropped hinge and would need a complicated double flap arrangement like the 321 to get such an Alfa vs Pitch angle improvement, it would to some extent destroy the elegant, simple and effective TE flap structure, it would also complicate the cruise flap usage.

I think as you, the 6 MLG (which I think is plausible) is there to primarily allow more pitch angle at rotation and landing.

BTW thanks for all the research, Airliners does not have functional tabs therefore when you make a table put ... between the columns like

Entry....entry....entry

only way to get them aligned  .



Non French in France
User currently offlineAviaponcho From France, joined Aug 2011, 613 posts, RR: 8
Reply 4, posted (1 year 7 months 3 weeks 3 days 17 hours ago) and read 4390 times:

Hello Ferpe
Next time i'll the tabs in pictures... might be simplier
I'm near zero knowledge for aerodynamics... thank you for the insight


User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 5, posted (1 year 7 months 3 weeks 3 days 9 hours ago) and read 4221 times:

Quoting Aviaponcho (Thread starter):
So without any tricks, how can the 787-10 takeoff angle be better than that of 777-300 basic ?

Different wing incidence angle.

Tom.


User currently offlinerwessel From United States of America, joined Jan 2007, 2346 posts, RR: 2
Reply 6, posted (1 year 7 months 3 weeks 3 days 8 hours ago) and read 4199 times:
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Quoting tdscanuck (Reply 5):
Quoting Aviaponcho (Thread starter):
So without any tricks, how can the 787-10 takeoff angle be better than that of 777-300 basic ?

Different wing incidence angle.

Out of curiosity are you stating that that is, in fact, so, or you're listing one possible way the 787-10 might be less prone to tail strikes that otherwise expected.

And if the mounted incidence is higher, that would imply that the 787 cruises more nose down than the 777. Is that so?


User currently offlineAviaponcho From France, joined Aug 2011, 613 posts, RR: 8
Reply 7, posted (1 year 7 months 3 weeks 3 days 5 hours ago) and read 4123 times:

Quoting tdscanuck (Reply 5):

Different wing incidence angle.

Tom

Thank you Tom
So for aero dummies like me : 787 need less pitch than the 777 to achieve the same runway performances ?

Have a good day


User currently offlineStarlionblue From Greenland, joined Feb 2004, 17030 posts, RR: 67
Reply 8, posted (1 year 7 months 3 weeks 3 days 4 hours ago) and read 4100 times:

Quoting Aviaponcho (Reply 7):
787 need less pitch than the 777 to achieve the same runway performances ?

Yes, because the wing has a higher angle relative to the fuselage, thus achieving a higher pitch angle with the same fuselage angle.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineAviaponcho From France, joined Aug 2011, 613 posts, RR: 8
Reply 9, posted (1 year 7 months 3 weeks 3 days 4 hours ago) and read 4093 times:

All right, thanks Starlionblue

It remains to see if the 787-10 will be allright with the 787-9 MLG or not.
Ferpe's might tell us how much does the increased fuselage lengh cost on take off on the 787-10


User currently offlineFlyDeltaJets From United States of America, joined Feb 2006, 1871 posts, RR: 2
Reply 10, posted (1 year 7 months 3 weeks 3 days 4 hours ago) and read 4083 times:
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Quoting Aviaponcho (Reply 2):
for what I remember, the double stretch 767-400ER has longer legs than 767-200 / 300

Isn't it ?

The 767-400 is much higher than the other varients. It's rear height his comparable to the 777. They even share the same main gear tires.



The only valid opinions are those based in facts
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17030 posts, RR: 67
Reply 11, posted (1 year 7 months 3 weeks 3 days 3 hours ago) and read 4077 times:

Quoting Aviaponcho (Reply 9):

All right, thanks Starlionblue

It remains to see if the 787-10 will be allright with the 787-9 MLG or not.

I'm assuming Boeing did pay someone to do the math about 10 years ago. 



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineAviaponcho From France, joined Aug 2011, 613 posts, RR: 8
Reply 12, posted (1 year 7 months 3 weeks 3 days ago) and read 4015 times:

Yes on a 220-225t airplane, i'm sure !

User currently offlineferpe From France, joined Nov 2010, 2803 posts, RR: 59
Reply 13, posted (1 year 7 months 3 weeks 2 days 20 hours ago) and read 3933 times:

Quoting Aviaponcho (Reply 9):
Ferpe's might tell us how much does the increased fuselage lengh cost on take off on the 787-10

If you uprate the engines to 76klbf it should have the same start performance as the 789 (74klbf) or better after rotation.

I do not have the wing incidence but here it can be seen. Unfortunately I did not find a 777 picture from the same angle. As I remember 777 and 767 they fly with a nose up fuselage (the drink trolleys goes back faster then they arrive after start in Y  Wow!   ) this creates low pressure areas directly behind the nose and along the fuselage. As Tom has said earlier this lifting surface has lousy aspect ratio so better let the wing do the job, ie fly the wing with more incidence so the fuse goes more level. Also then helps with the rotation, here the wingbox of the 787, quite an angle:

http://i298.photobucket.com/albums/mm262/ferpe_bucket/787middlesectionwithtext.jpg


Re the shifting of the wing wind angle (Alfa angle ) and the rotation angle with the use of take off flaps, here what happens when an aircraft put down the flaps for take-off:

http://i298.photobucket.com/albums/mm262/ferpe_bucket/Take-offAlfa-Clchart_zps4c70ff48.jpg

The left diagram show that for a certain Alfa angle (ie certain rotation angle which is shown as Alfa limit line ) you get more lift with more flaps (flaps 3 equal to 15° flap setting). Now the drawback is you also get a worse L/D, so for the same L (roughly equal to your start weight) you need more thrust as your drag increases, price to pays for getting the lift at a lower rotation angle.

Now the 787-10 neither have lots of more power nor more rotation angle so even if the wing is sitting at a higher incidence things might have been fine for the 789 but I think it is tight for the 787-10.



Non French in France
User currently offlineViscount724 From Switzerland, joined Oct 2006, 25170 posts, RR: 22
Reply 14, posted (1 year 7 months 3 weeks 2 days 14 hours ago) and read 3815 times:

Quoting FlyDeltaJets (Reply 10):
The 767-400 is much higher than the other varients. It's rear height his comparable to the 777. They even share the same main gear tires.

Main gear legs on the 764 are 18 inches taller than the 763, and I assume the larger 50-inch tires (vs. 46 inch on the 763) adds another 2 inches. The nose down tilt of the 764 on the ground is quite noticeable compared to the 763.


User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 15, posted (1 year 7 months 3 weeks 2 days 6 hours ago) and read 3689 times:

Quoting rwessel (Reply 6):
Out of curiosity are you stating that that is, in fact, so, or you're listing one possible way the 787-10 might be less prone to tail strikes that otherwise expected.

I just meant that was a possible way to reduce required rotation angle...I don't know what the 787 incidence angle is relative to the 777 (or any other aircraft).

Quoting rwessel (Reply 6):
And if the mounted incidence is higher, that would imply that the 787 cruises more nose down than the 777. Is that so?

I don't believe so but we're talking differences of a degree or two in cruise, which is nearly invisible to passengers.

Quoting Aviaponcho (Reply 7):
So for aero dummies like me : 787 need less pitch than the 777 to achieve the same runway performances ?

Yes, if they alter the incidence angle.

Quoting ferpe (Reply 13):
I do not have the wing incidence but here it can be seen. Unfortunately I did not find a 777 picture from the same angle.

This helps but required rotation angle also depends on loft...the angle at the fuselage could be the same but, if the loft is different, the required rotation angle might not be.

Quoting ferpe (Reply 13):
As I remember 777 and 767 they fly with a nose up fuselage

Most airliners do. A tiny bit of up angle is helpful for stability and drag but you don't want very much. Using the fuselage to generate significant lift is a bad idea with a conventional fuselage (it works better for designed lifting bodies).

Tom.


User currently offlineAviaponcho From France, joined Aug 2011, 613 posts, RR: 8
Reply 16, posted (1 year 7 months 3 weeks 2 days 2 hours ago) and read 3644 times:

Thanks Ferpe,
Thanks Tom

I've got nothing more than that for the 777

http://subaru.com.au/news/fhi-celebr...-1000th-boeing-777-centre-wing-box

Not the good shooting angle I guess

Trent 1000 TEN is a 78 000 lbs capable engine...
http://www.rolls-royce.com/news/pres...ses/2012/120710_trent_1000_ten.jsp

The A350-800 is a twin 79 000 lb for 248/259 t MTOW with a larger wing (and a dragier cross section)

In fact, I'm just wondering how for from "the 787-10 is a simple riskless stretch" we are really with the 787-10...
Considering how the 7-E-7 evolve since 2004

I'll have to wait, rather than trying to guess


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