Building a old Midwest mach 1 and the plans show no right or down thrust.Hate to have to put it in after its finished.I know i put some right thrust in the tiporare i just finished.

I built my Mach I per plans and after flying it for 2 1/2 years, it had no indications that it needed to be modified in regards to any right or down thrust. It flew great as designed. I hope this helps.

This debate has been going on for decades. The popular thinking back then was to set everything up 0-0-0. Seemed to work at the time. Earlier, when planes had positive incidences and semi-symettrical airfoils etc, a bit of right and down thrust was an absolute necessity.

I believe in Murphy's Law (and that he was an optimist). No matter how hard you try to achieve a "zero" engine offset, 99.99% of the time it will be "up", "down", "left" or "right". With current equipment and AC designs, "up" thrust and "left" thrust are usually worse than "down" thrust and "right" thrust. I'd suggest going with "positive error" and incorporating ~1 degree of both down-thrust and right-thrust.

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When inverted, right thrust becomes left thrust, thus pulling the climbing model (assume an extended vertical ascent) to the left, which torque and P-factor are already trying to do. I use zero-zero and learned how to fly the model through these shifting modes.

Buy the proper tools for setting up the model, if you do not already possess them, and setting incidence and angles will be a snap.


Ed Cregger

True, but note that when inverted, the vertical stabilizer/fin "moves" to the "bottom" of the fuselage. Where the original right-thrust was counteracting the left-yaw induced by the interaction between the spiral prop-stream and the upward-extending vertical fin; the inverted "left"-thrust is now properly counteracting the "right"-yaw induced by the interaction between the spiral prop-stream and "downward" vertical fin.

Free-flighters have long known that moving the vertical fin from "above" to "below" the engine thrust-line will change a power-on left-turn into a power-on right-turn.

Back in the "old days" when I could win a few pattern contests, the best flying airplane I
ever owned was set up 0-0-0.

tommy s

Too bad it doesn't work that way on most pattern models. I know the theory, but I've yet to see it work as theorized.


Ed Cregger

However you set it up initially, flight testing will likely reveal some need for change.

Terry in LP

I agree that flight testing does reveal what is needed. Changing just the weight of a pattern ship will have effects upon trim.


Ed Cregger

Does up elevator become down elevator ? When you set right thrust do you set it canopy or belly facing you ?


Albert

As you can probably tell from the various wanderings within this thread, you're going to get a lot of different opinions. I suggest that you build the plane according to the "plans", and go have fun flying it. With the design of the Mach 1 and the high-speed at which it flies, you will probably not notice any effects from incidental, small deviations in the thrust line.

I'm gonna have to disagree with you on this Ed. That's just not true.

gyrocptr is right on this one and I have seen it practice. I had a gas powered Extra once with one of those screw on gas caps. The cap came loose and fuel came dribbling out. When I got it down I could see the swirl patterns around the fuse that ended on the left hand side of the vertial stab. I believe that's the main reason we need right thrust.

Not my quote but I think it explains it well:
"A propeller pushes air not just horizontally to the back, but more in a twisting helix around the fuselage (clockwise as seen from the cockpit). As the air whirls around the fuselage it pushes against the left side of the vertical tail, causing the plane to yaw to the left. The prop wash effect is at its greatest when the airflow is flowing more around the fuselage than along it, i.e., at high power and low airspeed, which is the situation when starting the takeoff run. "

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There is tons and tons of stuff that I don't know factually, so I always appreciate it if someone tells me when my thinking is wrong by conventional wisdom.

I am aware of the propeller slipstream vortex and I'm not trying to weasel out of the consequences of such a force acting upon the model. It's just that I've never seen it demonstrated in reality with the old pre ballistic and ballistic models. That does not mean that it is a false assumption. It simply means that I haven't experienced it on high wingloading, high speed models - yet.

Let's keep the dialogue going.


Ed Cregger

My opinion would be to build in right thrust per the designers specifications...If they say none..Then that is what I'd do...They may have done things aerodynamically which take care of the need for adding right thrust... (nice)

To add to the discussion:
There are several forces acting on the aircraft which tend to yaw the aircraft to the left while at high power setting and high angle of attack...P-Factor, Torque, Propeller Slipstream Effect and Gyroscopic Procession are all kinda acting in unison...(These forces act on the longitudinal axis (roll) under the same conditions)

P-Factor remains the same weather the aircraft is upright or inverted and climbing...Now when the aircraft is upright: The pilot will add Right Rudder to counter P-Factor...However when the aircraft is inverted: Left rudder is applied to counter P-Factor...As a buddy once told me (a Pitts Pilot)..."If your pullin' on the stick it's right rudder..If your pushin' on the stick it's left"

Have Fun...

Chuck

Just to keep the dialogue going and as food for thought, here's a picture from an old textbook. It shows the propwash effect on a twin engine airplane. Though I'm far from being able to criticize the statements on propwash effects on the vertical tail of models or even free-flight models, I know some propwash effect on real airplanes.

The picture draws attention to the fact that this effect or at least an important part of it is not caused by the vertical tail. The propeller's slipstream may be seen as a gyro that is deflected downward by the lift-producing wing. This makes for a precession and the sideward force on the airplane.

On the other hand, pattern models don't use flaps and are set up with zero-zero incidence and zero downthrust. The described effect has to be quite small on a ship from the 'golden era' because it flies fast at a small lift coefficient. (The wing downwash angle and thus the slipstream precession depends on lift coefficient.) But it is still there because some rudder is needed to avoid deviations in upgoing (high-lift) parts of maneuvers. The older models from the early 1960s clearly show the effect because they have a semi-symmetrical airfoil and some decalage and are flying slower.

Still I wouldn't rule out the other part of the propwash effect, that acting on the vertical tail. I could fancy a connection to the propeller's diameter/pitch, rpm, and efficiency. Ballistic models had a small high-rpm prop with bad efficiency, meaning a fast-turning but small-diameter slipstream. Maybe it's gyro effect is small but it's aerodynamic effect on the tail is big, at least compared to later models with a big low-rpm prop. Maybe even the gyro effect is actually small due to the speed of the ballistic models while the aerodynamic effect is still there. After all many models have some right thrust even for the vertical part of patterns.

Anyway, when researching the Brushfire (here on RCU) I found out that 0-0 setting is recommended. That's probably because the vertical tail of this model is nearly symmetric (above and below the centerline). On the other hand, also Phil Kraft had 0-0 on his models with a quite high located vertical tail. I would agree with him and Ed Cregger, use zero-zero setup and get used to the natural behavior of the airplane. At least that's what I'm trying to do.

I tend to agree w/ Ed - although the effect is there, the BPA-style planes fly at such a relatively miniscule angle of attack that it is not noticeable. Also, the older patterns did not have high alpha maneuvers, the verticals were done at high speed or idle (top hat, stall turn, etc). Planes like the Banshee, my own Peppermint Pattie, etc all were set up at zero right thrust.

Thanks Guy's for the input,I have a 35% and 40% Edge and Extra and they have some serious right thrust.Was just wonder on the older pattern ships never really flew them until now.

Most of the older, "ballistic" Pattern ships had 0-0-0 setups. With the way they were flown, this worked out very nicely. A very few had other force arrangements. What you did find was that the designer/builders added/reduced the amount of incidence, up/right/down/left thrust as was required to get the airplane to fly the way they wanted it to.

Even dihedral angles can be changed after building...I once read where Don Lowe would saw a slot fore and aft at the wing centerline from the topside to about 3/4 through the wing. Squeeze in some epoxy, weight the wing to a slightly-higher dihedral angle, and allow it to cure. Then add a wrap of 6-ounce 'glass cloth around the center of the wing. Presto! You've altered the dihedral angle. If you cut up, from the bottom of the wing, you could reduce the angle by using the same method. The small kerf would allow a change of a degree or slightly less.

It's all part of "tuning" the model to suit how you wish for it to fly.