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Need For Rotation  
User currently offlinekaramara From Switzerland, joined Mar 2013, 9 posts, RR: 0
Posted (1 year 8 months 2 weeks 4 hours ago) and read 3951 times:

What exactly would happen if a pilot doesn't rotate the nose up when takeoff speed is reached? Would the aircraft eventually lift off all gears at the same time if the runway was long enough? What is the need for nose up other than a high angle of attack

36 replies: All unread, showing first 25:
 
User currently onlinevikkyvik From United States of America, joined Jul 2003, 10332 posts, RR: 26
Reply 1, posted (1 year 8 months 2 weeks 3 hours ago) and read 3919 times:

Quoting karamara (Thread starter):
What is the need for nose up other than a high angle of attack

That would pretty much be the reason right there. Well, that and you generally use a nose-up attitude to climb, with climb rate generally being governed by excess thrust (that is, you have more thrust than you need to fly level, so you point some of it upward to push the airplane up).

If there was no rotation, you'd have to achieve a much higher speed to enable liftoff. Many airplanes probably would rotate of their own volition at some point, just due to the increased moment. If I remember correctly, if you increase thrust while in level flight, the airplane will tend to increase attitude and start climbing.

Not sure if all that is true for neutrally stable / statically unstable aircraft.



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User currently offlinePapaChuck From United States of America, joined Aug 2010, 136 posts, RR: 0
Reply 2, posted (1 year 8 months 2 weeks 3 hours ago) and read 3918 times:

It depends on the aircraft. An aircraft like a Cessna 172, properly trimmed, will tend to lift off in ground effect and climb away by itself once it gains enough speed. A tailwheel aircraft, if the tail is held on the ground, will naturally want to take off since the wing will be at an optimum angle of attack.

However, a big jet like a 747 would be a different story. An airplane like that requires a few degrees nose up to fly even at cruising speeds. Kept level, it wouldn't matter how fast you were going. If you can't achieve the required angle of attack, you're not going to fly. I suppose it would be possible to get airborne in ground effect, but not before you exceed your tire speed limits. Over-speeding and shredding all your tires before getting up in ground effect would introduce a whole new set of problems.

PC



In-trail spacing is a team effort.
User currently offlinetimz From United States of America, joined Sep 1999, 6902 posts, RR: 7
Reply 3, posted (1 year 8 months 2 weeks 3 hours ago) and read 3887 times:

You could attach the wing to the fuselage at the right angle for the aircraft to lift off the runway pointed horizontally like a B-52-- then when you land and bring the nosewheel down to the runway the wing is still trying to lift the main wheels off the ground. Which disimproves braking.

User currently onlinevikkyvik From United States of America, joined Jul 2003, 10332 posts, RR: 26
Reply 4, posted (1 year 8 months 2 weeks ago) and read 3797 times:

Quoting PapaChuck (Reply 2):
Over-speeding and shredding all your tires before getting up in ground effect would introduce a whole new set of problems.

And, of course, very quickly running out of runway.  



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User currently onlineN243NW From United States of America, joined Jul 2003, 1640 posts, RR: 20
Reply 5, posted (1 year 8 months 1 week 6 days 23 hours ago) and read 3770 times:

I would imagine that at extreme nose-up stabilizer settings, most large jets would become much more prone to "self-rotation" than with the stabilizer in the normal takeoff range. However, couple such an extreme nose-up trim with a lot of thrust and the aircraft will likely return to earth even more dramatically than it left.  

[Edited 2013-04-08 20:48:52]


B-52s don't take off. They scare the ground away.
User currently offlineBreninTW From Taiwan, joined Jul 2006, 1719 posts, RR: 1
Reply 6, posted (1 year 8 months 1 week 6 days 23 hours ago) and read 3768 times:

Quoting karamara (Thread starter):
What exactly would happen if a pilot doesn't rotate the nose up when takeoff speed is reached? Would the aircraft eventually lift off all gears at the same time if the runway was long enough? What is the need for nose up other than a high angle of attack

Isn't that essentially what happens with low clearance aircraft such as the C-130 and AN124? The aircraft don't really have room to rotate without smacking the tail.

Of course, once off the ground, they do rotate to improve climb.



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User currently offlineStarlionblue From Greenland, joined Feb 2004, 17172 posts, RR: 66
Reply 7, posted (1 year 8 months 1 week 6 days 23 hours ago) and read 3766 times:

Quoting N243NW (Reply 5):

I would imagine that at extreme nose-up stabilizer settings, most large jets would become much more prone to "self-rotation" than with the stabilizer in the normal takeoff range. However, couple such an extreme nose-up trim with a lot of thrust and the aircraft will likely return to earth even more dramatically than it left the earth.

Like this? http://www.youtube.com/watch?v=OcuBW2nX9bY. I know, I know, that was due to CG.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineSAAFNAV From South Africa, joined Mar 2010, 287 posts, RR: 1
Reply 8, posted (1 year 8 months 1 week 6 days 22 hours ago) and read 3737 times:

Quoting BreninTW (Reply 6):

Isn't that essentially what happens with low clearance aircraft such as the C-130 and AN124? The aircraft don't really have room to rotate without smacking the tail.

You do positively rotate with a C130. Some of the longer ones are prone to tail strike, but you still have to rotate.

IIRC, the nose wheel is 139kts, so that would be the first limitation you run into.



On-board Direction Consultant
User currently offlineKELPkid From United States of America, joined Nov 2005, 6428 posts, RR: 3
Reply 9, posted (1 year 8 months 1 week 6 days 22 hours ago) and read 3715 times:

Some planes (the DeHavilland Beaver comes to mind) just seem to be able to levitate up with the nose in a level attitude...  


Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineRoseflyer From United States of America, joined Feb 2004, 9810 posts, RR: 52
Reply 10, posted (1 year 8 months 1 week 6 days 15 hours ago) and read 3619 times:

It really depends on the plane. A 737 would never rotate if the pilot did not put in elevator input unless the plane is trimmed for a climb. With no trim, a 737 generates a pitch down attitude on the ground. This is common on large transport aircraft.


If you have never designed an airplane part before, let the real designers do the work!
User currently offlineAesma From France, joined Nov 2009, 6925 posts, RR: 12
Reply 11, posted (1 year 8 months 1 week 6 days 11 hours ago) and read 3537 times:

I started flying on MS flight simulator and with the Cessna 172 it takes off by itself, so I wondered for a long time about rotation, especially after seeing the A340-600 at the Paris Air Show the time it made an incredible take-off with the pilot pulling the stick full aft.

Quoting Roseflyer (Reply 10):
It really depends on the plane. A 737 would never rotate if the pilot did not put in elevator input unless the plane is trimmed for a climb. With no trim, a 737 generates a pitch down attitude on the ground. This is common on large transport aircraft.

I'm guessing the 330/340 family would be the same since they have a notorious nose-down attitude on the ground. In fact I asked here if they had to change the fly by wire parameters for the A330F since it has the nose jacked up.



New Technology is the name we give to stuff that doesn't work yet. Douglas Adams
User currently offlineCosmicCruiser From United States of America, joined Feb 2005, 2255 posts, RR: 15
Reply 12, posted (1 year 8 months 1 week 6 days 11 hours ago) and read 3534 times:

There was a Jetstar crash on T/O in '76 at Midway that resulted in 4 fatalities due to lack of nose up trim set. The jet went thru the field boundary fence. Witnesses said the nose wheel came off the ground slightly twice before leaving the runway.

User currently offlinefreeze3192 From United States of America, joined Oct 2006, 170 posts, RR: 0
Reply 13, posted (1 year 8 months 1 week 6 days 9 hours ago) and read 3491 times:

Quoting KELPkid (Reply 9):
Some planes (the DeHavilland Beaver comes to mind) just seem to be able to levitate up with the nose in a level attitude...

We call a Flaps 15 takeoff in the Dash 8 a levitation takeoff. Really pronounced on the -300. I



"A passenger bets his life that his pilot is a worthy heir to an ancient tradition of excellence and professionalism."
User currently offlineKELPkid From United States of America, joined Nov 2005, 6428 posts, RR: 3
Reply 14, posted (1 year 8 months 1 week 6 days 6 hours ago) and read 3416 times:

Quoting Roseflyer (Reply 10):
It really depends on the plane. A 737 would never rotate if the pilot did not put in elevator input unless the plane is trimmed for a climb. With no trim, a 737 generates a pitch down attitude on the ground. This is common on large transport aircraft.

I wonder how the 737 would behave with the CG at the aftmost acceptable position    I'm sure CG plays into this, as most GA planes that I've flown are rather "eager" to get airborne when your CG is aft...   Whether or not the 737 could be dispatched this way is another matter entirely...  



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineKELPkid From United States of America, joined Nov 2005, 6428 posts, RR: 3
Reply 15, posted (1 year 8 months 1 week 6 days 6 hours ago) and read 3414 times:

Quoting freeze3192 (Reply 13):
We call a Flaps 15 takeoff in the Dash 8 a levitation takeoff. Really pronounced on the -300. I

Well, it is a distant cousin of the Beaver, wouldn't be suprised if some of the same designers had a hand in the design of the DHC-8-300 and the Beaver...   When did Bombardier take over? I don't think that happened until at least the Q400...



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlinehivue From United States of America, joined Feb 2013, 1113 posts, RR: 0
Reply 16, posted (1 year 8 months 1 week 5 days 7 hours ago) and read 3143 times:

Quoting karamara (Thread starter):
What is the need for nose up other than a high angle of attack

My (non-expert) understanding is that no airfoil will generate aerodynamic lift without a positive angle of attack. The aircraft is rotated specifically to set up sufficient positive AOA.


User currently offlineStarlionblue From Greenland, joined Feb 2004, 17172 posts, RR: 66
Reply 17, posted (1 year 8 months 1 week 5 days 3 hours ago) and read 3063 times:

Quoting hivue (Reply 16):
My (non-expert) understanding is that no airfoil will generate aerodynamic lift without a positive angle of attack. The aircraft is rotated specifically to set up sufficient positive AOA.

Correct. Without positive AoA, no lift. However if we want to nitpick this is only valid for a completely symmetrical airfoil. Given the shape of an airliner airfoil (camber etc) you get lift even with zero AoA if you define AoA as airflow relative to the mean chord line. I believe the term is "effective AoA", meaning you take into account the shape of the airfoil. You do need positive effective AoA.

If the wings are mounted at an incidence that gives the wing a sufficient positive AoA while rolling on the ground with the nosewheel touching the asphalt, no need to rotate. The B-52 is an example as mentioned above.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlinehivue From United States of America, joined Feb 2013, 1113 posts, RR: 0
Reply 18, posted (1 year 8 months 1 week 5 days 3 hours ago) and read 3056 times:

Quoting Starlionblue (Reply 17):
If the wings are mounted at an incidence that gives the wing a sufficient positive AoA while rolling on the ground with the nosewheel touching the asphalt, no need to rotate. The B-52 is an example as mentioned above.

I believe that the B-17's wings were arranged so that the airplane could take off and land 3-point whereas most tail-draggers needed to lift the tail off first to avoid a stall.


User currently offlineRoseflyer From United States of America, joined Feb 2004, 9810 posts, RR: 52
Reply 19, posted (1 year 8 months 1 week 5 days 1 hour ago) and read 3026 times:

Quoting KELPkid (Reply 14):
Quoting Roseflyer (Reply 10):
It really depends on the plane. A 737 would never rotate if the pilot did not put in elevator input unless the plane is trimmed for a climb. With no trim, a 737 generates a pitch down attitude on the ground. This is common on large transport aircraft.

I wonder how the 737 would behave with the CG at the aftmost acceptable position I'm sure CG plays into this, as most GA planes that I've flown are rather "eager" to get airborne when your CG is aft... Whether or not the 737 could be dispatched this way is another matter entirely...


There is insufficient lift because of the nose down attitude. Flaps and leading edge slats give some lift, but not enough to get the airplane off the ground with the 2-4 degrees of nose down attitude. The nose down attitude is useful because it shortens braking distance.



If you have never designed an airplane part before, let the real designers do the work!
User currently offline7BOEING7 From United States of America, joined Oct 2012, 1750 posts, RR: 13
Reply 20, posted (1 year 8 months 1 week 4 days 23 hours ago) and read 2970 times:

Quoting Roseflyer (Reply 10):
It really depends on the plane. A 737 would never rotate if the pilot did not put in elevator input unless the plane is trimmed for a climb. With no trim, a 737 generates a pitch down attitude on the ground. This is common on large transport aircraft.
Quoting KELPkid (Reply 14):
I wonder how the 737 would behave with the CG at the aftmost acceptable position I'm sure CG plays into this, as most GA planes that I've flown are rather "eager" to get airborne when your CG is aft... Whether or not the 737 could be dispatched this way is another matter entirely...
Quoting Roseflyer (Reply 19):
There is insufficient lift because of the nose down attitude. Flaps and leading edge slats give some lift, but not enough to get the airplane off the ground with the 2-4 degrees of nose down attitude. The nose down attitude is useful because it shortens braking distance.

Once upon a time on a run back to Anchorage from some runway out west in Alaska, as we sped down the runway in a 737-200 belonging to a long defunct airline, the nose came off without any pull on the yoke about 10 kts below our planned rotation speed. Why? The stabilizer trim was set correctly for the weight and balance we were given but the actual CG was a lot farther aft due to a lot more weight in the aft cargo area. If the stab trim is either set to far aft or forward for the "actual" CG the 737 will either rotate on its own or take a lot higher force to get the airplane off the runway.


User currently onlinevikkyvik From United States of America, joined Jul 2003, 10332 posts, RR: 26
Reply 21, posted (1 year 8 months 1 week 4 days 22 hours ago) and read 2967 times:

Quoting Starlionblue (Reply 17):
However if we want to nitpick this is only valid for a completely symmetrical airfoil.

I wouldn't call that a nitpick at all. Given that most airliners, at least, have nonsymmetrical airfoils (and I'd guess the majority of current airplanes), it's a completely valid and important point.

I don't personally recall hearing "effective AoA", but you could be right on that.

Quoting Roseflyer (Reply 19):
There is insufficient lift because of the nose down attitude. Flaps and leading edge slats give some lift, but not enough to get the airplane off the ground with the 2-4 degrees of nose down attitude.

That may be true, but I believe the question is whether it would rotate on its own, given an aft CG, but with stab trim set for a center or forward CG. At least, that's how I read it.



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User currently offlineRoseflyer From United States of America, joined Feb 2004, 9810 posts, RR: 52
Reply 22, posted (1 year 8 months 1 week 4 days 14 hours ago) and read 2825 times:

Quoting vikkyvik (Reply 21):
That may be true, but I believe the question is whether it would rotate on its own, given an aft CG, but with stab trim set for a center or forward CG. At least, that's how I read it.

Good question. If the airplane is not trimmed correctly for its CG then I don't know what would happen.



If you have never designed an airplane part before, let the real designers do the work!
User currently offlinehivue From United States of America, joined Feb 2013, 1113 posts, RR: 0
Reply 23, posted (1 year 8 months 1 week 4 days 13 hours ago) and read 2799 times:

Quoting vikkyvik (Reply 21):
Quoting Starlionblue (Reply 17):
However if we want to nitpick this is only valid for a completely symmetrical airfoil.

I wouldn't call that a nitpick at all. Given that most airliners, at least, have nonsymmetrical airfoils (and I'd guess the majority of current airplanes), it's a completely valid and important point.

This is getting into the area of how airfoils work to keep a plane in the air, a topic that is complex at best and has been discussed many times in these forums.

See the following:
Airbus A330 Nose Down Attitude (by A380Heavy Feb 13 2010 in Civil Aviation)?threadid=4711515&searchid=4715141&s=aerodynamic+lift#ID4715141

-- particularly tdscanuck's reply 35.


User currently offlineBoeingGuy From United States of America, joined Dec 2010, 3258 posts, RR: 7
Reply 24, posted (1 year 8 months 1 week 4 days 12 hours ago) and read 2770 times:

Quoting Roseflyer (Reply 22):
Good question. If the airplane is not trimmed correctly for its CG then I don't know what would happen.

Boeing airplanes will give the crew a Takeoff Configuration Warning when the thrust levers advance to takeoff power if the trim is set outside of the correct range for its current CG and weight.


25 vikkyvik : Not sure what you mean. Nothing I wrote is at all in conflict with what Tom stated in that thread.
26 7BOEING7 : If the airplane is not trimmed correctly for its actual CG, either due to an incorrect weight and balance or an incorrect stabilizer trim setting (wi
27 hivue : I didn't say that it necessarily was. However, if what you and Stationblue are saying is that "nonsymmetrical" airfoils can generate sufficient lift
28 vikkyvik : I think you're misunderstanding what Tom said, or what we're saying. In that thread, Tom said: If you have a wing at it's Cl=0 angle of attack, it do
29 hivue : Thanks. Perhaps I did, as I am certainly no aeronautical engineer. I will maintain, as I did in reply 16, that no airfoil will generate lift (or, at
30 JETSTAR : I can tell you that on the Lockheed JetStar, we considered the trim setting one of the killer items, it was so critical that it was on the checklist
31 Post contains images vikkyvik : You are wrong about that. That is true for symmetrical airfoils only. Nonsymmetrical (unsymmetrical, whatever) airfoils will still generate lift at 0
32 Post contains images Starlionblue : As vikkyvik points out, you are wrong. It is a matter of definitions. AoA is defined as the angle between airflow and the mean chord line. Note in th
33 hivue : Thanks much for the info, and the diagram. So just to make sure I have this straight, if the free stream airflow is running parallel to the mean chor
34 Roseflyer : No airplane that takes off with flaps has 0 degrees AOA. Flap change the chord line. There is a nose down attitude on many airplanes, but just to cla
35 Starlionblue : Correct.
36 vikkyvik : Given an extremely long runway, and no tire speed limit, yes, your airplane may lift off the ground with enough speed. But that also depends on your
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