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Rotating Stretched A/Cs  
User currently offlineTurbulence From Spain, joined Nov 1999, 963 posts, RR: 20
Posted (13 years 7 months 1 week 15 hours ago) and read 1177 times:

I just read a very interesting article on stretched aircrafts. I had never thought of this, although after having read it, it is less than obvious, and even although I had already read before that the 767-400 has taller leg landing gears than other 767s do. (I don't know if 6-wheel boogies or not, but I think they are the ones of the 777). When I first read this, I thought it was because of the bigger engines (due to bigger power needed since it is quite heavier), but the truth is that stretched airplanes have less margin for... ROTATION!!!, since the tail is longer...

My question goes to pilots here of stretched models. As far as I know, 767-400 is actually more powerful than "regular" 767s, but 321 is fitted instead with more complexe flaps and slats for increasing hyper-sustemptation at smaller angle of attack.

Pilots of 757-300, 737-900, 321, 330-300, 777-300: what are the differences you must observe between these ones and the 757-200, 737-500, 321 & 319, 330-200 and 777-200?

Thanks in advance.
Best turbulences.

13 replies: All unread, jump to last
 
User currently offlineXFSUgimpLB41X From United States of America, joined Aug 2000, 4200 posts, RR: 37
Reply 1, posted (13 years 7 months 1 week 9 hours ago) and read 1059 times:

I dont fly them, their rotation rate is just a bit slower. Pretty much all jets rotate to such a high angle their tail would strike if they didnt lift off the ground during rotation. I believe the biggest difference comes in when landing..the flare is quite limited i know on the 777-300.


Chicks dig winglets.
User currently offlineJG From United States of America, joined Jun 2005, 0 posts, RR: 1
Reply 2, posted (13 years 7 months 6 days 11 hours ago) and read 1045 times:

Turbulence,

Exceed these pitch values in the A320 series and you will have to practice your penmanship in the aircraft log:
Aircraft/ at touchdown/ gear struts compressed
A319/ 15.5/ 13.5
A320/ 13.5/ 11.5
A321/ 11.0/ 9.5

You also asked about aircraft power:
The A320 series that I fly are powered by CFM 56-5B's
319- 23500 lbs Thrust
320- 27000
321- 34000

At takeoff, suggested rotation rate is the same for all three ~ 3 degrees/sec.

At landing, standard technique for all three again with the text stating that a concerted flare is not required or desirable. A slight increase in pitch (1-2 degrees) to check the rate of descent is all that Airbus says is necessary to produce a consistant touchdown point and protect against tailstrike and excessive float.

So far, they have been right. I hope this helps.

Regards,
JG


User currently offlineTurbulence From Spain, joined Nov 1999, 963 posts, RR: 20
Reply 3, posted (13 years 7 months 1 day 17 hours ago) and read 980 times:

Thank you both. But I was not referring to the rate of rotation, as much as the final (or initial?) angle of rotation itself.
I mean: of course the climbing angle is much higher than the initial take-off angle, but apart of that, the difference is that when the wheels loose contact with the asphalt, and taking JG's example, a 319 can have rotated until 15.5º, (which might not be strictly necessary), but the 321 can have rotated only 11º. This, added to the fact that it is 55,000 lbs. heavier, tells us that it needs a significantly faster Vr or that the hyper-sustemptators are quiet more complex.

Are truly the Vr different for 319s and 321s? (or for 762s and 764s, or for 342s and 345s, or for 772s and 773s, etc.

Thanks again

Best turbulences


User currently offlineXFSUgimpLB41X From United States of America, joined Aug 2000, 4200 posts, RR: 37
Reply 4, posted (13 years 7 months 1 day 16 hours ago) and read 963 times:

They climb out at the same 15-20 degree angle that most airliners do... just they have to be more careful in getting up to that angle.

Most aircraft are off the ground by the time 5-10 degrees in rotation is reached.



Chicks dig winglets.
User currently offlineJG From United States of America, joined Jun 2005, 0 posts, RR: 1
Reply 5, posted (13 years 7 months 1 day 15 hours ago) and read 971 times:

Turbulence

Vr for the 319 and 321 are almost the same and in many cases the Vr for the 321 is less. The 321 uses a double slotted flap, as they have different wings there will not be a valid comparison.

320 Vr speeds are generally faster than the 319.
Target pitch for takeoff in all three is about 15 nose up. Using an appropriate rate of rotation as mentioned in my previous post will insure lift off prior to the pitch attitudes for tailstrike as outlined in the same post. And will allow a smooth continuous rotation to the target climb pitch attitude. There should NOT be a need to rotate to and initial attitude, pause, then continue to the target climb pitch as I inferred from your second post.

There is no operational difference amoung the A320 family. There is , however, an operational "awareness" of tailstrike potential amoung the family members. Beyond that I probably can not add more to the discussion.

Regards
JG


User currently offlineOldman From United States of America, joined Sep 2006, 0 posts, RR: 0
Reply 6, posted (13 years 7 months 1 day 14 hours ago) and read 962 times:

JG, I think you have covered that quite nicely! Nothing more needs to be added here Smile

User currently offlineAJ From Australia, joined Nov 1999, 2391 posts, RR: 24
Reply 7, posted (13 years 7 months 1 day 9 hours ago) and read 947 times:

Just a note, aircraft that are restricted by their fuselage length on rotation are known as geometry limited. On the B744 the performance charts generate speeds and body angles for departure. The flight director will not exceed 15 degrees nose up until the aircraft is airborne, then will rise to hold V2+10. If you watch the aircraft take off you will note a second 'rotation' once the aircraft is clear of the ground. Very geometry limited aircraft will tend to have a tailskid of some sort.
The best example of an aircraft's production being limited by geometric limitations is the race between the B707 and the DC-8. The B707 was designed with squat undercarriage, that meant the -300 series was as far as it could be stretched without a major redesign. The DC-8, however, had tall undercarriage legs, allowing for the -60/70 stretches and a more flexible range of aircraft.  Smile/happy/getting dizzy


User currently offlineTurbulence From Spain, joined Nov 1999, 963 posts, RR: 20
Reply 8, posted (13 years 7 months 21 hours ago) and read 923 times:

These have been great.

Specially to JG, but thanks a lot to all of you.

Happy flights.

Best turbulences


User currently offlineCX flyboy From Hong Kong, joined Dec 1999, 6603 posts, RR: 55
Reply 9, posted (13 years 7 months 19 hours ago) and read 923 times:

We rotate at 3degrees/sec to 12.5 degrees nose-up in our 747-400s. A tailstrike in the 744 is actually not that easy to do, as long as you are not doing it on purpose!!!

We continuously rotate to 12.5 degrees and do not increase it after airborne, and we do not rotate a little and wait for a climb to initiate before further rotating. One continuous pull-back to 12.5 degrees. However, the amount of pressure needed to be applied to the control column in the same until around 10 degrees, as which point a little effort is needed to continue to 12.5, so that could give the impression of a second rotation, especially if the pilot let's it sit at 10 degrees pitch up a second or two before continuing to 12.5 degrees.


User currently offlineDakotaaviator From , joined Dec 1969, posts, RR:
Reply 10, posted (13 years 7 months 12 hours ago) and read 908 times:

"hyper-sustemptators"????

Could someone please elaborate on:
- what this/these are?
- what they do?
- how they function?
- the effect on A/C performance?

Thanks,
~DakotaAviator


User currently offlineCdfMXTech From United States of America, joined Jul 2000, 1341 posts, RR: 26
Reply 11, posted (13 years 7 months 11 hours ago) and read 906 times:

I'm not too sure that pilot's of the next generation 737s would notice a difference. The pitch system was upgraded on all Next Gens too improve rotation during takeoff. The system is even on the 737-700, but just for commonality. Called the FCNSE system. It involves a mechanism that changes the funtion of the elevator balance tab to an anti-balance tab to provide for pitch.
The 737-900 I'm told will also have a little bit of a taller gear.

I haven't been to school for the 757-300 yet...so I'm not sure how they compensated for this from a technical standpoint. I know that all the did on the 767-400 was change the gear. A taller gear compensated for the added length.


User currently offlineCdfMXTech From United States of America, joined Jul 2000, 1341 posts, RR: 26
Reply 12, posted (13 years 6 months 4 weeks 1 day 19 hours ago) and read 889 times:

I must correct myself.
The NG has two systems which assist it in takeoff pitch.

The FCNSE is controlled by the FCC which controls the Mach Trim actuator. When enabled, the actuator will change the neutral postion of the elevator to create a better pitch for TO.

The Elevator Tab Mech, converts the balance tab during flaps down and N1 > 18%.

Sorry


User currently offlineTurbulence From Spain, joined Nov 1999, 963 posts, RR: 20
Reply 13, posted (13 years 6 months 3 weeks 6 days 19 hours ago) and read 873 times:

To Dakotaaviator:

Since I posted the "word", I feel obliged to answer your question.
Hyper-sustemptators are the mechanisms that increase the wings sustemptation ability. They are the flaps (slats if they are on the attack edge of the wing)

But let's see some basic physics, first. Explained very simply.

Aircrafts keep flying because wings keep them up there.
The way it works is the wing having a curvature.
This curvature makes air flowing along the upper side to have a longer path, than the air flowing along the lower side.
It causes a lower pressure on the upper side, or higher pressure on the lower side of the wing (differential).
When this differential is bigger than the weight of the airplane, it lifts.

Off course, it depends, among others, on the air speed. The higher the speed, the higher the differential.

BUT:

The wings of modern jets are designed for efficiency at very high speeds (cruising speeds above 850 kms/h approx. 475 kts., or 530 mph), but unluckily it means that they are less efficient at lower speeds. Most of the airplanes have their lowest clean speed around 160 kts. Lowest clean speed means the slowest speed that can be maintained on the air without deploying any flaps or slats. Under this speed, sustemptation must be increased.
As said before, the sustemptation is generated by a differential of pressures. And increasing sustemptation can be done only by increasing this differential.

Luckily, this differential can be increased by different ways:

1.- Increasing speed
2.- Increasing wing surface
3.- Increasing wing curvature.

Of course, when take-off and landing, speed is not enough, but these devices mounted on the wings can increase both surface and curvature.
The ones in front go forward and the ones back go backwards. This way the wing surface is significantly increased.
But, once we have moving parts mounted on the wing, it is quite easy to give them not only horizontal motion, but a component of vertical, too. This way, when they go forward and back, they go some (quite) down, too, increasing, logically, the wing curvature.
Which results in significantly increased lift.

Ok. I think I answered 3 of your 4 questions. The fourth question, how do they affect A/C performance, is as follows:
When the hyper-sustemptators are deployed, the wing is not "clean". They increase air drag, and cause turbulences both under and behind the wing. Therefore they are reployed as soon as the airplane has overpassed 160 kias (knots indicated air speed), or deployed only when it is strictly necessary to maintain speeds below the "clean limit".
A little out of the exact topic, but this is also the reason why the T/Os and landings are made "against" the wind:
If the wind is 15 knots the opposite direction, the airplane will have reached the rotation speed (Vr) 15 knots sooner. Meaning:
If an airplane's Vr is 100 kts, and the wind is 15 kts, airplane will be suitable for rotation when its speed is going to be 85 kts ground speed. Or, otherwise said, when the airplane is stopped at the beginning of the runway waiting for T/O clearance, it is already having a "useful" speed of 15 kts. Or, if a 100-kt. wind could be continuous, the airplane could rotate and lift... WITHOUT MOVING!!!

Hope I helped.

Best turbulences


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