Hot answers tagged

35

The white marks make it easy to see if the trim wheel is moving, which would be tricky if it was entirely black. But wouldn't the pilot always know if they was spinning the trim wheel? Remember that the autopilot can also adjust the trim, which might not be obvious to the pilot. The visual marks make it easy for the pilots to see what the autopilot is doing....


24

In this context, it would mean the aircraft is loaded in such a way that the Center of Gravity is too far forward or aft. That's actually not the way we usually use the word "trim" in aviation but it's what it means in this case.


22

Proper trim control technique involves exerting control pressure to attain the desired attitude then using the trim to alleviate the requirement of holding that pressure. There can be a temptation for pilots to make small adjustments using trim alone. Resist the temptation and use the proper procedure. We use the proper technique because it's the most ...


22

A small propeller driven plane like that will have a bit of adverse yaw, rolling to the left, because of propeller effects (P-Factor, slipstream, torque, etc) The trim tab's effect is minimal because it is very small and is close to the centerline of the plane. Any drag it creates causes a slight roll and yaw to the right, specifically to counteract the ...


19

Your friend is right. Elevator trim isn't setting a pitch, it is compensating for control forces on the elevator, which vary with speed. If you are trimmed for level flight at 100 kias and reduce power the plane will initially slow. As it slows the trim will cause a nose-down moment causing the plane to pitch down. Given no input, the plane will ...


19

All quotes in this answer are from the English translation of the BEA accident report. The elevator cable broke at "an altitude estimated at between three and four hundred feet",(p. 7) and there were eleven seconds between that time and the point of impact. If it had happened higher, the pilot would potentially have had time to figure out what happened and ...


17

Short answer: Yes Generally you are right. You are trimming the aircraft for a point on its polar, and that point is reached at a specific angle of attack. You say yourself we should neglect secondary effects like the propeller blast, and under this condition you are right. Trimming means to set the distribution of lift between wing and tail surface, and ...


15

From the Boeing 737 emergency procedures: After all attempts fail to free the stabilizer... Anticipate higher than normal elevator forces during approach and landing. The thrust reduction at flare will cause a nose down pitch. Note: Elevator control is sufficient to safely land the airplane regardless of stabilizer position. Note on ...


13

I might be mistaken, but it appears to me that you're missing an important piece on how trimming works. It's not a button you press to magically remove the control forces, but rather, you need to adjust the trim tabs (on most smaller aircraft at least, some aircraft, especially new large ones, have other mechanisms, but that's outside the 'basics' scope) to ...


13

Trim is used to zero the stick forces at the desired flight condition, and with the small, slow early airplanes, an adjustable spring somewhere along the control cables would do. When speed increased, trim tabs were introduced, because their trim force goes up and down with dynamic pressure, whereas the spring would always create the same force. This would ...


13

The reason the trim is only on one side is cost saving; one trim tab is cheaper to build and simpler (therefore cheaper) to maintain. As for the rolling motion yes, the trim tab will cause a slight rolling motion. Very, very slight to the point you won't detect it for the following reasons: The trim tab is small. Compare the surface area of the trim tab ...


11

Terminology first: Trim drag is the drag component added by adjusting the incidence of the horizontal tail rsp. the deflection angle of the elevator for trim. The increased parasitic drag of a larger tail surface itself is already part of the aircraft's zero-lift drag. Sizing criteria for tail surfaces The horizontal stabilizer surface must be dimensioned ...


11

Yes it is a viable way for roll control - aerodynamically that is. The mechanism you describe is a proper servo tab, where the aeroforces on the tab are fed back, and the tab acts like a lever on the main control surface. This was the way the post WW2 airliners were controlled, before they got too big and fast and only hydraulics could do it. However, as ...


10

The faster an aircraft is, the wider range of elevator deflections it needs. And so do aircraft with large change of weight (amount of fuel used) during flight. However larger elevator deflection causes higher drag. It is more efficient to move the whole horizontal stabilizer instead. Therefore jet aircraft generally have the forward part of the horizontal ...


10

Light aircraft and transport aircraft behave exactly the same regarding trimming for speed. Trim is for angle of attack, but power affects that trim and phugoid oscillation occurs in speed and pitch, because the feedback is second order. The Boeing FBW has no trouble mimicking anything. It simply translates the control forces and position exactly as a hydro-...


9

Just like normal trimming. However trim the fuel tank use aft fuel tank to achieve the trimming effect. Fuel is pumped between main tank and aft tank to shift CG around to a desire position during flight. Fuel tank trimming has additional benefit of keeping the airplane in "clean" configuration, creating less drag compare to normal trimming.


8

The main reason for having trim is to modify the forces felt by the pilot to relieve strain, or as you say, 'trim away' the constant forces. There are various reason for not using the trim as the primary control or have fixed trim positions: The forces on the stick and by extension the trim varies with the weight and balance of the aircraft. Though the ...


8

For the impatient reader: The answer is no. Let me explain it in detail. For this, it is helpful to simplify things as much as possible and then add the complications step by step so I can explain what each changes. The simplest layout uses a symmetric airfoil for both wing and tail and arranges both in the same plane and without a difference in incidence. ...


8

No, static longitudinal stability does not necessarily imply a download on the tail. Static longitudinal stability requires that the Centre of Gravity is in front of the Centre of Lift, indicated as n.p.$_{fixed}$ in the drawing. Only then will an increase in Angle of Attack d$\alpha$ result in an opposing pitching moment: if d$\alpha$ > 0 then $dC_N$ > 0, ...


8

Given your request to "answer with less smart words," it's hard to know what knowledge can be assumed, but I'll give it a try. Let's say you're hand-flying the airplane straight and level, and you find that the right wing keeps wanting to drop, which you prevent by using left-aileron pressure on the control yoke. You can "trim out" the need for that left ...


8

Boeing's patent (US4676460A) for the STS was filed on 1984-11-28, the same day of the 737 Classic (3/4/500 series) entering service. So since the addition of the bigger engines on the Classic, it's been there. Below is from a 737 flight manual of a company that had both the Classic and NG at one time, which highlights the differences between the two series....


7

The trim will behave differently with different loads, so you cannot use fixed trim settings on each flight, as each flight will be different from a load factor and center of gravity perspective. While my trim setting for a solo flight with half full tanks will make me us best climb, the same trim setting with 3 additional passengers and full tanks will ...


7

One reason for moving the entire surface for trimming is efficiency. Deflecting the elevators or trim tabs creates more drag than adjusting the whole surface. The control surfaces are more aerodynamic without trim tab deflection. The elevator is the main surface needed for trimming the aircraft. This is because things like fuel and payload will affect the ...


7

When changing wing incidence for roll control, it is important to have equal but opposite deflections on both sides. Otherwise you will add load factors to the results of your roll inputs. If you have some gearing which will couple the movements of the two wing halves, the idea works. But try to avoid a stall; stalling has a high potential to result in an ...


7

If the aircraft is trimmed, the flight controls are in a state where no force needs to be exerted in order to continue straight and level flight. Aerodynamic and gravitational moments about all three axes are nulled out. Changing the trim of the aircraft in pitch could happen in the following ways: a shift in centre of gravity, for instance by shifting ...


7

As per the A320 flight manual (§127.20 p3), the maximum permitted total pitch alteration in normal law depends upon speed, aircraft mass, CoG, and other factors: Pitch Attitude Limitation: Pitch attitude is limited to: 30º nose up in conf 0 to 3 (progressively reduced to 25º at low speed) 25º nose up in conf FULL (progressively reduced ...


7

The pitch trim setting is based on the angle of attack. The angle of attack is affected by indicated airspeed, center of gravity and any unbalanced forces. In a steady state aircraft where the airplanes forces are all balanced then we are left with airspeed and center of gravity. Under normal operations where fuel is not being transferred from an aft ...


6

The trim tab is like a control surface for the control surface. By changing camber at the rear end of the control surface, it creates a local force which changes the balance of forces such that the force-free deflection angle (sometimes called auswehwinkel after the German word for this angle) is shifted in a direction opposite to the deflection of the tab. ...


6

Yes, both are equal for small angle changes. Exceptions do apply, especially in transsonic flow. Both changing the stabilizer incidence and the elevator deflection will change the lift distribution between wing and tail surfaces and will trim the aircraft for a different angle of attack. Changing the incidence gives the empennage a new angle of attack while ...


6

The reason that large aircraft are often not exactly speed stable is that the line of thrust does not go through the centre of gravity of the aircraft. A change in thrust setting will therefore cause a change of pitch moment. In small propeller aircraft the thrust is usually acting through or near the centre of gravity and hence the aircraft are speed ...


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