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Fall_guy

Any Ideas Guys..........Nose Pitch under take-off

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Hi Guys,When I start to roll and reach 40-50 kts the Nose comes up and the NOSEgear wheel parts from the runway, and while still accelerating down the runway it pitches back down,......but dos'nt quite make contact with the runway again.ANY IDEAS??also, when deciding upon a missed approach, I'm aware that a pre determined altitude is normally set into the MOP as part of the approach/landing checklist, But if anyone knows could you please describe to me the actually procedure a pilot goes through,........................Does he fly manually? or Autopilot?..........does he press any buttons?.................Is this all taken care for by the FMS?.......what speed is normal for this procedure?any help or guidance would be greatly appreciated.Oh the aircraft I'm refering to is a Boeing 737NGThanks in advance guys.John

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make sure that the hst is removed. Make sure that you repair it in the option menu--just in case. Try another airport. Make sure that the joystick is calibrated and it is verified in the axis menu of FLY.Let us know after these tests....tony

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G'day John,Lets talk real aircraft for a moment.Boeings are by design aerodynamically nose heavy and sit on the ground on their wheels with a slight nose down attitude.The horizontal stabilizers are actually inverted airfoils (flat on top and curved underneath) The stabilizers are of variable incidence. Hinged at the rear and a jackscrew is used to raise or lower the front thus giving the variable incidence. ALL normal longitudinal TRIM is by the stabilizer. Lift is behind weight so the effect of lift/weight is to form an aerodynamic couple trying to pitch the aircraft nose down. Stabilizers are used to produce a down force in flight to create an opposing moment to counter/balance the aircraft in the pitching plane. The elevators are used to control the aircraft around this balanced condtion. Before flight the pilot has to set the amount of stab. nose down (aircraft nose up) depending on the aircraft weight and Centre of gravity. The stab trim scale on the console has a green band and within that band is the MAC (mean aerodynamic chord). The pilot must set the stab trim to the Aircraft centre of gravity on the MAC. This means that as the aircraft reaches V2 (rotation) speed the aircraft will be basically balanced and the elevator authority will be adequate to rotate the aircraft and climb. Imagine if you will setting the stab to zero angle of attack; on reaching V2 the pilot would haul back on the stick only to find the elevators didn't have the authority to rotate the aircraft - disaster.TAKE-OFF warning also monitors the stabilizer position (as well as several other things). If it's not in the green range the take off warning horn will sound. After take-off as the aircraft speed increases the stabilizer angle of attack must be decreased to maintain the downforce a constant to keep the aircraft in a balanced condition. @ cruise the stabilizer would be very close to zero angle of attack yet still keeping the downforce constant to balance the aircraft. There is no pilot operated elevator trim.The elevator is trimmed to a fixed mechanical schedule however as the stabilizer mooves. With stabilizer at full aircraft nose up the elevator neutral is 4 degrees up from faired and as the stabilizer mover to full aircraft nose down the elevator neutral moves to 2 degrees down from faired. This movement of the elevator neutral assures that a constant up and down elevator authority exists irrespective of the stabilizer position.Now your problem... I'm not a pilot but it doesn't seem unreasonable to me to have a slight pitching up moment with the application of take-off power, but I wouldn't expect it to get the wheels airborne. Up to approx. 60 kts nosewheel steering is the primary directional control. That would be a little difficult with the wheels off the ground.This fact alone make me wonder if the stab trim is set correctly. You may have far too much nose up trim set. Try reducing it slightly.Failing that it could be simply poor flight dynamic(hate that phrase) programing for large jet transport.This one's a long shot bit how's the calibration of your joystick?Cheers,Roger

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Hi Roger, TonyI've removed the HST file as Tony surgested, and my joystick and trims seem fine. Ive made quite alot of changes to the Engine files and Aircraft files in general so I suspected It may have been my own doing. HOWEVER, I downloaded Gorgio original Lufthansa 737 v2 and the same problem occurs.(Nose wheel departs from runway under power).Could someone be kind enough to try Gorgio's 737 from an exterior view, and verify the initial take-off roll.Thanks Roger for your above info (exceptional as ever). I'll try to understand the principals and procedures you described and see if i can build some logic into my 737NG files.Finally, can someone please explain the reading from the center of gravity window display. (from the main pulldown menu Fuel/Gravity). Whenever I change the fuel loadouts I always seem to end up with a fore + loadout.e.g. if I choose a 50% fuel load and select the show CofG window. It always appears to show more weight at the FORE.........................is this normal? and does this mean more FUEL(Weight) has been distributed towards the Nose of the Aircraft.Thanks for All you HelpJohn

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Also, a weak suspension may be contributing to the nose up behavior. I test flew the craft, and if I push the stick down the nose stays on the runway--no trim adjustments. If I trim down, the plane's nose stays down. see pic.tony

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Hi Tony,Thanks for that,..............What happens if you trim level and don't put forward pressure on the joystick, (does the nose rise then)?John

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John, make sure that the elevator trim is set in the "aircraftoption" menu. Set it a couple notches. In the aicraft, give it a nose down trim. You will not need any yoke down pressure if trim is configured correctly. Just before lift off, get ready to pull the stick all the way back, including a bit of trim. I've just tried it, and it works fine...of course, before takeoff, you should visually check to see that trim is properly set....tony

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G'day John,e.g. if I choose a 50% fuel load and select the show CofG window. It always appears to show more weight at the FORE.........................is this normal? and does this mean more FUEL(Weight) has been distributed towards the Nose of the Aircraft.The short answer is Yes!....Yes! :-)But then there's the long answer.. :-lolMany moons ago when I was a snork I was sitting in the cockpit at the end of a maintenance check on a B707 transfering fuel from the centre wing tank to the main tanks in the wings (just to check the tank access panels on the wing weren't leaking). Suddenly the cockpit door burst open and an engineer told me to stop immediately. The problem was that as the fuel was being transfered out to the wing tanks it was also being tranfered BACKWARDS due to SWEEPBACK of the wings. The centre of gravity was SLOOOWWLY moving aft, not detected by me in the cockpit but the aircraft was rotating around its MLG. The rudder had hit the tail docking and continued on. There was about a 12" gash in the trailing edge of the rudder. :-eek That was my introduction to the effects of fuel and sweepback on the centre of gravity.Fuel distribution has a great effect on the centre of gravity of swept wing aircraft.If you open the *.gas world file you will see that the fuel tank quantities AND tank POSITIONS RELATIVE TO THE EMPTY CENTRE OF GRAVITY are defined for your aircraft. Most aircraft model designers have just made an educated guess at that figure. In the *.vld files are the positions for crew, passengers and cargo. If you agree with these positions then change them to give the C of G you want. BUT BUT that will alter the flight handling characteristics and may require more changes to the *wng file. Its a juggling act with LOTS of trial and error. I don't know if anyone knows exactly how Fly! arrives with the C of G values as a % (of what). The empty C of G is defined and all varible loads are referenced to the empty C of G. Fly! has the data to calculate a mean chord ( NOT MAC ) so maybe it's a % of the mean chord. I'd also be very interest to hear from someone knowledgeable on the topic. Stick with it! It's fun :-)Cheers,Roger

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Hi John,Good to see you are making in roads in the 737..... I posted what may be an answer to you problem on the thread about the PDQ-3.Roger will probably contact you first, but if he doesn't take a look and see if my suggestoin helps.Keep up the hard work John!!! The end result is SATISFACTION Mate!!Kind regards.Jason

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