I have a micro canard pusher if the motor has upthrust in a pusher
configuration does this push the nose down?
It seems to fly better at 1/3 to 1/2 throttle than at full throttle. If you are flying and you chop the
throttle the nose comes up.
From : Don Stackhouse
This whole business of "up" or "down" thrust can make more sense if you relate it back to where the thrust line goes in relation to the C/G. In particular, you need to be aware of the vertical location of the C/G.
The vertical location of the C/G is something most modelers generally ignore. However, the C/G is a single point, and it has a location in the fore-and-aft sense (the "longitudinal C/G", the one most of us are familiar with), the side-to-side sense (the "lateral C/G", something that modelers occasionally obsess about, but in actual practice is usually not that important), and in the up-and-down sense (the "vertical C/G").
The vertical C/G (together with the longitudinal C/G) is especially important with regard to thrust lines, and also in the location of landing gear, particularly for "conventional" (i.e.: tailwheel type) landing gear.
If you suspend the model from a thread, perhaps from somewhere around the cockpit, and then plot the line defined by the thread on the side view of the plane, the C/G will lie somewhere on that extended line. If you suspend the model from a thread at a different point, perhaps somewhere around the tail, and plot that line on the same side view, the point where the two lines cross will show the vertical and longitudinal location of the C/G. Since the wing is one of the heaviest structures in most planes, the vertical location of the C/G tends to be near the wing, typically a little above it in low wing airplanes and a little below it in high wing airplanes.
Now, the key to why we went to all this trouble:
Once you've determined the vertical and longitudinal position of the C/G and plotted it on your side view, draw the thrust line on the side view as well.
If the thrust line passes below the C/G, it will tend to push the nose up when you add power. The bigger the perpendicular distance between the thrust line and the C/G (the "moment arm" that the thrust is acting through), the stronger this effect.
If the thrust line passes above the C/G, more thrust will tend to push the nose down. Tom, this is the situation you describe.
If the thrust line passes through the C/G, thrust will neither push the nose up nor down.
Simple, eh?
Now, there can be some tricky situations. In particular, it should be obvious that for an engine mounted directly above or below the C/G, such as the Lake Amphibian's pylon-mounted engine, or a wing-mounted engine like on a Wingo, Multiplex "Teddy" or our Roadkill Series Curtiss-Wright "Junior", there is no practical thrust line that will pass through the C/G. In that case you have to rely on secondary effects. Typically the thrust line is arranged so that the propwash blows at a small downward angle against the top of the horizontal stabilizer, creating a nose -up effect to counteract the nose-down effect of the high thrust line/low C/G relationship.
When you increase power with this arrangement, the prop moves more air, making more nose-up effect to counter the increased nose-down effect from the increased thrust. Properly done, this can work very well in flight, but tends to be less effective at the beginning of takeoff run. Airplanes like this often need to start takeoff with a gradual increase in power, allowing the stabilizer to get some decent airflow across it before we start demanding a lot of downward lift from it to counteract the nose-down effects of thrust. Holding full up elevator will also obviously help, although for that to work you still usually need at least a little airspeed. The velocity from the slipstream alone at the very beginning of takeoff roll usually isn't enough to make that much lift all by itself.
Obviously this won't help if the stabilizer is not where the propwash can hit it, such as a canard layout. If you have a canard with an extremely high-mounted engine, you're pretty much out of luck. Back to the drawing board!
Regarding my earlier comments about landing gear location, if you put the mains of a taildragger way ahead of the C/G, it tends to push the nose up at touchdown, making the plane prone to bouncing. The Piper Cub is one example of this. If you put them too far back, then the plane is prone to nose-overs.
The general rule of thumb is to take a side view of the airplane in a level attitude (such as it would be in a "wheel" landing, as opposed to a "3-point" or "full-stall" landing), plot the C/G location, then draw a line angled downwards and forwards from the C/G, angled forwards from vertical somewhere between about 5 degrees and 15 degrees, depending on how resistant to bouncing you want the plane to be, vs. how resistant to nose-overs. The wheels should be on that line, and far down enough to provide adequate prop clearance when it is in a slightly nose-down attitude.
Landing gear location can be an issue for tricycle arrangements as well. If the mains are too far aft, there may not be enough elevator authority to lift the nose for rotation at takeoff. Also, once it does start to lift, the change in angle shifts the C/G aft, closer to the main wheels, making the airplane suddenly easier to rotate nose-up, making it prone to "leaping" into the air, in some cases almost uncontrollably. If the elevator is not sitting in the propwash, this tends to make the problem even worse. Pushers in general, and canard pushers in particular, are especially prone to this problem. The Prescott Pusher required humongous amounts of runway for takeoff, and the Rutan canards such as the VariEze and even the Voyager are known to have problems with long takeoff runs plus a tendency to surprise their pilots with sudden rotations once the nose finally does un-stick. In fact, part of the reason why the Voyager dragged its wingtips on the takeoff for the round-the-world flight was due to some attempts they made in the landing gear setup and the takeoff technique to restrain the overenthusiastic liftoff tendencies, which could have triggered some catastrophic flutter problems. Unusual airplanes often force their designers and flyers into some unusual compromises.
Moving it towards the wing means moving the C/G aft, which reduces pitch stability. It also reduces yaw stability, and yaw stability can be a major problem on canard aircraft, because the effective moment arm for the vertical tail tends to be very short. Be careful, and keep an eye on both parameters.
As you can see from my comments above, what you observe agrees with what we would expect from the analysis. However, it's not necessarily "normal" any more than it would be normal for any other type. Canards as a group do tend to have short vertical tail moment arms, making them, as a group, somewhat more prone to yaw stability issues. They also tend to have long distances from the prop to the C/G, making them, as a group, somewhat more sensitive to small changes in the angle of the thrust line, since those angle changes multiplied by the long horizontal distance results in a greater perpendicular distance from the thrust line to the C/G. However, tractor airplanes with very long noses, such as the TBM 700, P-51 Mustang and the Pilatus PC-6 Turbo Porter also have this tendency.
As with most issues in airplane design, a certain type may be more sensitive to a particular design issue than other types, but that does not automatically mean that every aircraft of that type will have that problem. In the end, as usual, it all depends on the details, and on how well the designer does their homework.
Don Stackhouse
DJ Aerotech
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February 9, 2010
