nisam nasao racunanje ali evo podesavanje ( testnim letovima)
Setting The Thrust Line
Most models require the engine to be set at an angle relative to the fuselage centreline and is typically 2½° right thrust and 1° down thrust. The engine mounting will also be offset to the right when viewed from the front. For a clearer understanding of thrust lines, see our online Thrust Angles section. Setting the thrust line correctly is probably the single most effective thing you can do to improve the handling of your model. It is also easy to test fly for and easy to correct - a real "no-brainer".
Simply put, right thrust is required to compensate for the spiralling airflow from the propellor hitting the tailfin while down thrust stops the model climbing when the throttle is increased. (Unlike a trainer, where more throttle equals more power equals more height, the aim now is for more throttle to equal more power. With today's high drag or "constant speed" models, a considerable increase in power only gives a modest increase in airspeed.)
To get the thrust line set properly, you need to eliminate pitching and yawing with throttle use.
It is essential that you have decided on the engine/propellor combination that you will use and that the engine is "on song" and well run in. It is also essential that the model is trimmed to neutral. If this isn't the case, you'll be wasting your time.
Establishing The Side Thrust.
Fly your model straight into wind at full throttle and pull up into a vertical climb. The theory here is that as the airspeed bleeds off, a continued vertical climb is now fully dependent on engine power alone. Do not use rudder (or rudder trim) to correct for any deviation from the vertical. Your model will do one of three things.
If the model continues to climb vertically, the side thrust is correct.
If the model gradually, then more noticeably, veers to the right, the model has too much right thrust.
If the model gradually, then more noticeably, veers to the left, the model has too little right thrust.
This test should then be repeated but this time start by heading downwind. If you're happy that you know what you have to do for side thrust, you can go straight on to check the down thrust in the same flight. Repeat this process a few times to ensure consistent results.
Establishing The Down Thrust.
Fly your model straight into wind at full throttle and horizontally. Abruptly close the throttle and keep it closed for 2 or 3 seconds. Open the throttle fully again. Do not use the elevator (or elevator trim) to correct for any variation in height - unless of course it's screaming straight for the ground! The model will do one of three things.
When the throttle is closed, the model slowly arcs nose down (as the airspeed drops) and when the throttle is re-opened, the model should continue towards the ground in a straight line. Recover (Duh!) and congratulate yourself that the down thrust is correct.
If when the throttle is closed, the model arcs nose up and when the throttle is re-opened, the model goes nose down, you have too much down thrust.
If when the throttle is closed, the model arcs nose down and when the throttle is re-opened, the model goes nose up, you have too little down thrust.
Repeat this test a few times to make sure you're getting consistent results.
Changing The Thrustline.
Whoopee! My favourite - an engine out job... This can usually be done at the flying field.
Do not attempt to change the thrustline by sticking things under the engine mounting lugs. The correct way is to alter the angle between the rear face of the engine mount and the front face of the firewall. There are several ways of doing this.
Perhaps the easiest way is to place a washer or washers on the bolts that hold the engine mount to the firewall between the mount and the firewall. Most mounts have four bolts holding them to the firewall so a few minutes thought and you should be able to work out what goes where.
To increase down thrust, add washers to the top of the engine mount.
To decrease down thrust, add washers to the bottom of the engine mount.
To increase right thrust, add washers to the left of the engine mount (when viewed from above).
To decrease right thrust, add washers to the right of the engine mount (when viewed from above).
Down thrust and side thrust can be changed together.
Check that there is still adequate clearance around the engine and exhaust then ensure everything has been re-tightened correctly and try re-testing. There should be a marked improvement. If it's not perfect, it's engine out time again! Keep repeating the whole process until the model successfully and consistently "passes" the thrust line tests.
Once you are happy with the thrust line settings, you can make the installation permanent by machining the rear of the engine mount to the appropriate angle. If you have had to add a considerable amount of side or down thrust, you will probably gave to move the position of the engine mount to bring the propellor/spinner back onto the fuselage centreline
Remember, if you change the propellor to a diffent type or size or change the engine, you will almost certainly have to alter the thrust line again.
i malo o tome
I will assume that you have the most common type of motor/prop configuration (at the front with the prop spinning counter clockwise when looking at the prop from in front.
Ok let's start with side thrust. The spinning prop creates a helical prop wash. This is swirling air mass which looks like a little tornado extending back towards the tail of your plane. The center of the tail is roughly in the eye of this tornado, but you'll notice the vertical stabilizer shoots upward through this tornado and the propwash ends up hitting the fin on the left side (looking forward). So when you hit the gas, the fin feels this air hitting it in the side and yaws the plane to the left. To correct this we use right thrust. This effectively yaws the plane to the right whenever the you throttle up and offsets the yaw produced by the helical prop wash. Note that the horizontal stab also feels this helical propwash, but since it feels a little force up on the left and down on the right, the net pitching moment is zero. There is a small roll torque produced which wants to make the plane roll right, but this is a good thing as we'll see in a moment.
The other reason we have a little right thrust built in is to offset the motor torque. If you have ever held a plane up under full power (or even more noticeable is just the motor spinning the prop), you will have surely felt the torque. Since the motor is torquing the prop clockwise when looking from the back towards the front, there is an equal and opposite torque which the air puts on the prop and motor. This makes the plane want to roll left under power. We don't have any real way of compensating for this roll torque that is proportional to throttle setting. Mixing some aileron in with a computer radio seems like a good idea, but turns out to be a less than perfect solution because the amount of roll torque the ailerons produce is dramatically effected by airspeed. We use the right thrust to compensate a little for this effect because even though it doesn't really fix the roll torque, we recognize that the left roll tendency will want to make the plane turn more left under power than not. Yawing to the right as a function of power is a less than optimal, but a "better than nothing" solution for general purpose flying. Note that the 3D guys have a special maneuver that exploits this motor torque. It's called a torque roll.
Ok - down thrust.
Sometimes the motor thrust line is placed either above or below the center of gravity of the plane. This is sometimes done for scale reasons or practical considerations like ground clearance. Whatever the reason, if the thrust line were placed below the CG (easy to do on a high wing plane), then a nose up pitching moment will be created which will get larger as the power in increased. To offset this one could angle the motor down to compensate for this effect. Note that positioning the motor higher will also correct this problem, but may not be possible. This is only part of the story though. Lets look at a low wing plane. Here the vertical CG is pretty low and it's easy to put the thrustline higher than the CG. Why is downthrust sometimes still required? This is because as the wind over the wing increases (both from the direct prop blast as well as the resulting increase in airspeed) the wing generates more lift and the plane begins moving upwards (even though it's horizontal attitude is still the same). This upward motion forces air down on the horizontal stab and creates a nose up pitching moment. This moment is loosely related to power setting, but also has a part that is dependent more on airspeed. We compensate for the part, which is power dependent with downthrust, but the other is left uncompensated. This is why even a well trimmed plane will generally still climb when going fast and descend when slowed down, it just won't pitch dramatically when power setting are abruptly changed.
As to how much down or side thrust to use? That's a tough question. Try changing them and see the effect. I usually start with 2-3 deg right thrust and I try not to use any downthrust if possible, I try to position the thrustline up or down instead. Do you know why?
- Zoom -
And now for the lone voice in the wilderness answer. I was hoping someone would answer this like Zoom did. He did a great job, unfortunately Zoom has quoted the wrong answers that have been preached as gospel for decades.
First let’s get torque out of the discussion. By the laws of physics down thrust or side thrust cannot have any effect on the torque caused by out engines and motors. Unless we’re talking a 2000 Hp engine turning a half-ton propeller then our torque is extremely minor.
Next the mythical slip stream swirl (helical propwash). It's true that on a high humidity morning you can see this spiral around the airplane from the vortex off of the propeller tips. But if you look at it very closely the swirl goes the opposite direction that is given as the explanation for left yaw. So if it did have an effect it should be the opposite of what we are correcting for. Secondly, If the swirl does exist as Zoom reiterated then the same spiral is causing an ENORMOUS ROLLING force to the right by causing more lift on the left wing and horizontal stabilizer and less lift on the right wing and horizontal stabilizer. Ironically in every diagram of the slipstream swirl these forces are always completely ignored, because the vertical fin in the slipstream is the only point the author is usually trying to make. And by looking at the other forces I mention would really mess up this little old erroneous theory. Next watch any aircraft with a smoke system, and at very low airspeeds, and very high power such as a vertical climb or at the verge of the stall, so long as the airplane is still flying the smoke blows straight back under the airplane it never is seen swirling around the fuselage as the theory predicts. And finally we have been analyzing the physics and mathematics of aviation longer than we have been actually flying. In my 25 years as an aero engineer I have never found (and I’ve searched some of the biggest libraries in the country) an aero design book that quantifies the helical propwash. Surely by now some desperate for a topic doctoral candidate has developed a set of equations that for a given horsepower, propeller pitch, flying speed, fuselage length, and fin size will tell you exactly how much to offset the fin for this swirl. These equations do not exist.
So what is the answer? P-factor. P-factor is a difference in thrust on the left and right side of an airplane caused by differences in relative airspeed and angle of attack of the propeller as it moves up on the left side of the airplane and down on the right side of the airplane. In most conventional aircraft this thrust is greater on the right side of the airplane, pulling the airplane to the left. To stop this left turning tendency you can either add left rudder, or aim this lopsided thrust through the CG to minimize this turning force. That’s what right thrust does it compensates for P factor not torque.
Down thrust is a way of reducing the need to retrim an airplane with power changes. As you increase airspeed by adding power the stabilizer will pull downward with a stronger force, and an airplane will climb. You can either add more down elevator to reduce the climb or you can let the extra power pull the nose down. There is an oft-overlooked benefit of downthrust. When added to an airplane it also lowers the angle of attack that causes P-factor, thus reducing the P-factor, and often eliminating the need for right thrust because the P-factor has been neutralized in the cruise condition.
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