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Thread: Trim for Speed and Performance

  1. #1

    Default Trim for Speed and Performance

    Everything you ever wanted to know about trimming your aircraft.

    An aircraft, any aircraft, is designed to fly most of the time in a cruise configuration with cruise power settings. So when an aircraft is designed it is designed to take into account the effects of engine torque and propeller slipstream. Just a refresher, engine torque is the force that wants to turn the airplane in the opposite direction that the engine and propeller are turning. Most American airplanes have the engine turning the prop to the right which makes the airplane want to roll to the left. Propeller slipstream is the corkscrew effect of the airflow caused by the propeller moving massive amounts of air back over the airplane. Viewing the plane from the rear, visualize the swirl of air moving back and striking the left side of the horizontal stabilizer. It is the same as having a hand pushing on the left side of the tail and forcing it to the right which in turn forces the nose of the airplane to the left.

    Now if an airplane is to spend the bulk of itís time at cruise with the appropriate power setting, the aircraft has to be designed to compensate for these two ďleft turning tendenciesĒ. To counter engine torque the engine is mounted somewhat off center and the wings are attached to the fuselage at different angels. These wing attach angles are called the Ďangle of incidenceí. I donít know what the different angles of incidence are, but in order to keep the aircraft from rolling to the left in flight, the left wing is attached at a higher angle of incidence than the right wing. Thus the left wing when flying straight and level is producing more left than the right. To compensate for the propeller slipstream the horizontal stabilizer is attached at an angle to the left of center just enough to keep the airplane from yawing to the left. Theoretically, when flying in this configuration the aircraft trim settings should be at neutral.

    Because we are not always flying in cruise the airplane designer has given us a way to control those adverse effects caused by engine torque and propeller slipstream. These little devices are called Ďtrim tabsí and are located on at least one aileron, the elevator, and the rudder. Trim tabs are actually small airfoils and are a part of the trailing edge of each control surface. For example, when looking at the rudder from the rear and the rudder trim tab is defected to the right, it changes the shape of the rudder and forces the rudder to the left by changing the rudder airfoil. Once again, looking at the elevator from the rear, and the elevator trim is deflected down it changes the airfoil of the elevator causing it to rise.

    Any aircraft's before takeoff check list will require a trim setting that is appropriate for that aircraft. For example the P-51,s before takeoff checklist calls for the rudder trim to be set 6 degrees right, the elevator and aileron trims both set at 0 degrees. Because airspeed has a direct effect on control effectiveness it goes without saying that slower airspeeds will required the greatest degree of trim settings. Now, how do you tell if the trim settings are correct or not? After takeoff, you what to set a climb speed of 170 mph and climb to 15,000 ft. In order to tell if you have the elevator trim set correctly let go of the controls. If the nose drops and the airplane begins to descend, then you do not have enough up elevator trim dialed in. Increase up elevator trim until you can let go of the stick and have the nose stay in the attitude that will give you your 170 mph climb. As long as the wings stay level the aileron trim is correct. The rudder trim is the only trim that has an instrument that will tell you if it is set correctly. The ball in the needle and ball instrument will either be centered or out to the left or right. Adjust the trim in the direction of the ball and bring it into the center to keep the airplane trimmed for straight flight. Now remember, your climb speed is less than your cruse speed so the built in corrections that compensate for torque and propeller slipstream havenít quite got enough force on them to completely compensate. Thatís why you have trim tabs, to trim out the control forces that you are placing on the stick to keep it in the attitude you want. And remember, any stick force causes an increase in drag and we all know what drag does to your airspeed.

    So now, you have reached your assigned altitude of 15,000 ft and itís time to level off. If you push the stick forward, you will in fact level off, but you will have to continue to hold forward pressure on the stick or when you take your hand off the nose will rise and the airplane will try to once again attain the climb attitude that you previously had it trimmed for. Also, when you push the nose forward to level off you will notice the ball in the needle and ball instrument will move from the center to the left. Now that your airspeed is increasing the designed compensation is starting to take effect and you must readjust your trims or the combination of designed compensation and rudder trim will over compensate and the airplane will want to turn to the right. So in order to have your aircraft properly trimmed and producing the least amount of drag you should be able to take your hands off the controls and the airplane will fly straight and level with the ball centered. If that isnít the case, readjust the trims until you are trimmed for hands off straight and level. You are now getting the best performance from your aircraft because you are not applying any control forces that in turn produce drag that in turn slows you down.

    Try a test or two to see just what effect these trim settings have on your airplane. Trim for straight and level flight with the ball centered. You should be able to let go of the stick and not gain or lose altitude and you should also have the ball centered so you are flying straight. Now push the stick forward and start down hill. You will notice three things happen with regards to the trim of the airplane. First you will have to continue to hold forward pressure on the stick in order to continue downhill. Secondly, you will notice the ball will move to the left, and thirdly you will notice the airplane will have a tendency to roll right. Thatís because all those designed compensation features are now over compensating, and in order for you to continue in the descent you will have to apply control forces to correct for the over compensation.

    With the nature of a dogfight and the dynamics of speed and altitude changes, it would be impossible to re-trim for ever occasion. You should however, be aware of the basics changes associated with trim settings and in the heat of battle you can apply control pressures as needed to stay in trim as much a possible.

    Here is how I fly the Sim with regards to trim settings and how to use them as effectively as possible.

    You should have the neutral position of each trim mapped. For example, if you are using settings on your controls, place them in the neutral trim position and using the mapped setting in the game synchronize them so the trim settings on your aircraft are set to zero degrees. Now takeoff and adjust trims as necessary to achieve your climb with the airplane trimmed to hands off as best as possible. Before engaging in combat briefly establish a straight and level attitude with a reasonably high power setting. Trim for hands off and this will give you a neutral configuration prior to entering combat. With this configuration you know that when you dive on an enemy you will have to hold forward control pressure and left rudder to keep the ball centered in order to not only fly straight, but to shoot straight. As your speed changes so does your trim settings, but just keep in mind that slow means right rudder and fast means left rudder. Now you want to disengage with the enemy and beat a fast retreat, you dive down and head for home. Your airspeed increases and the trims are all out of adjustment. If you are looking for maximum forward speed, now is the time to bring your aircraft back into trim. Make whatever adjustments are necessary so you donít have to apply any control pressure whatsoever. Remember, any pressure on the controls produce drag which costs you speed.

    Because of the great speed differential and the great amount of horsepower associated with these fighters, the trim settings are constantly changing in order to fly as efficiently as possible. Always try to start with a trimmed airplane, then use control forces as necessary to stay in trim when engaging the enemy. A trimmed airplane is a fast airplane.

    Capt. Ramblin' Jack
    352nd VFG


    "The most important branch of aviation is pursuit, which fights for and gains control of the air"
    U.S. Brigadier General William Mitchell

  2. #2
    Senior Member clayp's Avatar
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    Default Re: Trim for Speed and Performance

    Good stuff RJ...

  3. #3

    Default Re: Trim for Speed and Performance

    Thanks, RJ. Very clear, concise and helpful. Now to apply it.

  4. #4
    Member O'Donovan's Avatar
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    Quote Originally Posted by ramblin_jack View Post
    Everything you ever wanted to know about trimming your aircraft.

    *snip*

    And remember, any stick force causes an increase in drag and we all know what drag does to your airspeed.

    *snip*

    So in order to have your aircraft properly trimmed and producing the least amount of drag you should be able to take your hands off the controls and the airplane will fly straight and level with the ball centered. If that isn’t the case, readjust the trims until you are trimmed for hands off straight and level. You are now getting the best performance from your aircraft because you are not applying any control forces that in turn produce drag that in turn slows you down.

    *snip*

    Remember, any pressure on the controls produce drag which costs you speed.

    Capt. Ramblin' Jack
    352nd VFG
    As a non-pilot I will take exception to this, as will most pilots who have to trim aircraft, I would imagine. Basic aerodynamics teaches that to change direction one has to change airflow. The same amount of airflow will have to be displaced to keep a plane straight and level, whether it's stick and pedal pressure keeping the control surfaces slightly off-center or the trim tabs being set to stick out from their control surfaces. By definition, they would have to produce the same amount of drag. The trim tabs simply keep the pilot from having to do the work. Theoretically, the LEAST amount of drag would be with the trim tabs set to 0, as that way there would be no time where the trim tabs and control surfaces would be applying opposite force...both creating drag and cancelling each other out.

    Imagine flying for a few hours with constant stick and pedal pressure, to keep the plane flying level. When finally reaching combat, the pilot would be too fatigued to respond properly. However, I have read a story from a WWII pilot, where he mentioned that some pilots overtrimmed their aircraft when entering combat, so the sticks would push into their hands with a couple pounds of pressure, rather than flopping around like a dead fish in a neutral configuration. He said it was easier to hold the stick when it was not trying to pull itself out of his hand so much, during maneuvers.

    Just my $.02.



    -Irish
    Last edited by O'Donovan; 09-07-2014 at 02:09 PM.

  5. #5

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    Your plane always has a "trim" state. The trim that allows you to fly hands free offers the "least" drag. If you have to add any sort of "extra" control force to maintain your flight state (eg: level flight, slight climb etc.) you are adding "more" drag to your aircraft making it less efficient. You are working against your trim, that is not efficient.

  6. #6
    Member O'Donovan's Avatar
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    Quote Originally Posted by Gothkrieger View Post
    Your plane always has a "trim" state. The trim that allows you to fly hands free offers the "least" drag. If you have to add any sort of "extra" control force to maintain your flight state (eg: level flight, slight climb etc.) you are adding "more" drag to your aircraft making it less efficient. You are working against your trim, that is not efficient.
    Negative, unless you're in a multi-engine (counter-rotating props) or jet aircraft. In a single engine piston driven plane, there will always be torque from the propeller, which makes the aircraft want to roll in the opposite direction of the spin of the propeller. The LEAST drag would be with all control surfaces and trim tabs at zero, which would make zero extra drag but would put your plane into a roll (and dive, thanks to gravity). Anything done to the aircraft to keep it from rolling, whether it be trim tabs or control surfaces, will create drag, as it has to displace enough airflow to generate force equivalent to the torque, but in the opposite direction.



    -Irish

  7. #7

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    I think we are talking past each other. I am talking about "extra" drag induced by control input that is in addition to your trim setting. This is valid regardless if its a jet or not. Even a jet will not remain in level flight at all flight speeds without trim. Zero trim will eventually end in zero flight and that is not a practical discussion about drag.

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    Quote Originally Posted by Gothkrieger View Post
    I think we are talking past each other. I am talking about "extra" drag induced by control input that is in addition to your trim setting. This is valid regardless if its a jet or not. Even a jet will not remain in level flight at all flight speeds without trim. Zero trim will eventually end in zero flight and that is not a practical discussion about drag.
    Ahhh... Referring to "extra drag," "all flight speeds," and "level flight" make a big difference. Those weren't given as conditions of your earlier statements. You had said, "The trim that allows you to fly hands free offers the "least" drag." That statement, taken by itself without any qualification, is not true. You also said, "If you have to add any sort of "extra" control force to maintain your flight state (eg: level flight, slight climb etc.) you are adding "more" drag to your aircraft making it less efficient. You are working against your trim, that is not efficient." That is also not always true, as you could have to add control input to supplement (work WITH) the trim, rather than to work against it. In that case it doesn't cause more drag, but is just more of a workout for the pilot.



    -Irish

  9. #9

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    My comments are in line with the original posters "intent", and not in regards to "other cases". Best abide by his comments as they are correct.

  10. #10
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    Quote Originally Posted by Gothkrieger View Post
    My comments are in line with the original posters "intent", and not in regards to "other cases". Best abide by his comments as they are correct.
    Actually, they're not. His original intent was not for straight and level flight. It was how to trim for the least drag when entering combat. His comments are not correct. If you go into combat trimmed for straight and level high speed flight (as he suggests), your tabs are set for slight up elevator and (in an American fighter) right aileron (roll), to compensate for gravity and torque, respectively. Any time you have to dive or roll left, you're applying control forces which are "fighting" your trim tabs, thereby creating significantly more drag than you would if all tabs were at zero. When you enter a dive, you'll have the upward force of the elevator trim tab fighting the downward force you're applying to the elevators, thereby producing both upward (tab) and downward (elevator) airflow. Likewise, to roll left, you have both roll right (tab) and roll left (aileron) airflow fighting each other.

    I stand by my original statement. The LEAST drag, going into combat is with all tabs at zero. That way, your control inputs NEVER have to fight opposite trim, which is what adds drag.

    *Edit* BTW, fighter tactics for the PTO taught American pilots to roll left instead of right, whenever possible, when trying to get away from Japanese fighters. The Japanese props spun the opposite way from ours, so they would have to fight their torque to roll left while our pilots would be rolling with their torque. Likewise, most Japanese pilots knew to roll right to get away from our fighters. It didn't make a huge difference, but every little bit would help.



    -Irish
    Last edited by O'Donovan; 10-21-2014 at 01:22 AM.

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