Design > Stunt design
Thundergazer design: Understanding its decalage
Teodorico Terry:
Hi -
I just bought a Thundergazer kit from RSM and I have been spending some time looking at the drawings trying to understand how to built it. By the way, the RSM kit is very nicely done with good wood and nice hardware.
So looking at the drawing and assuming that the wing is shown at 0 degree incidence this is what I noticed:
1) The design uses down thrust, 1/8" over the length of the engine bearers.
2) The tail plane has a slightly positive incidence, 1/64" up at the stab's leading edge.
3) The CG is rearward of what I have seen in other C/L designs, CG at 3 7/8" with a wing cord of 12" at the root, almost 33%. Considering the taper of the wing it would place the actual CG a little closer to 28-30% mark.
I also fly R/C, mostly the AMA pattern (Advanced) and they way in which the model is set-up would make me thing that it is in a tail heavy condition. The use of down thrust is pretty normal but in most cases the wing is set-up with a slight positive incidence (0.5 -1.0 degree) with the stab neutral. The Thundergazer in a sense is set-up with negative wind incidence if the stab is neutral.
Does anyone understand the reason why it might be set-up that way?
Thank you,
Teo
Mark wood:
Yes, many here do understand the reason behind the trim elements in the airplane. First CG location is generally related to the Mean Aerodynamic Chord, MAC. When a wing is swept rearward the MAC moves aft so what you've estimated is likely off due to not having determined the MAC location. Doing that you'll find the relative CG isn't as far aft as 33% MAC and is likely further forward depending on the sweep and taper.
The thrust line and tail incidence are related to the propeller and trim bias. An RC pattern aircraft virtually never has to deal with propeller precession when compared to a CL model which has to deal with it continuously. The CL model is traveling in circle which means the propeller is pivoting about the Z axis continuously. The precession translates this through 90 degrees to the Y axis which results in a nose up or nose down moment depending on the rotation direction of the propeller. For a counterclockwise propeller this means a nose up moment is created. To compensate for this we can do one or both of two things, change the thrust line or change the tail plane incidence. Adding a little down thrust will help offset the precession as will a little tail incidence. Sometimes the thrust line is moved up or down for the same reason. Note that this trim correction is only good for one flight velocity and resulting rotational rate.
The tail plane incidence has been found to be beneficial in removing some flight trim bias. An airplane may have a difference in the trim from upright to inverted. For instance, the upright trim may be close to "hands off" while the inverted trim may have a "heavy" feel to it. Unlike the RC Pattern airplane placing the CG on or close to the neutral point is not very desirable as this makes the controls "twitchy" which is why we see the more forward CG being used for the CL models. By adding a little ail incidence the upright control feel becomes a little more pronounced and the inverted trim becomes a little less "heavy". So, by adding a little positive incidence the control position bias becomes more symmetric between upright and inverted. Having the balanced control moment upright and invented makes the airplane easier to fly the maneuvers.
Teodorico Terry:
Motorman,
Looking at the kit it would be difficult to build it with the molded leading edge sheeting as designed. Even if you use the existing holes so that you can use a traditional JIG (i.e. rods/tubes) there is not way to support the 1/2 ribs. I am thinking that I might use the alternate process using two L-shaped extrusions to support the LE and trailing edge. To do this I will have to add a leading edge strip, probably 1/8x1/4. However, if I add a leading edge strip then I can probably use the traditional jig. I know that I will have to notch the ribs but since the C/L is marked it should not be a bid deal.
I think that in some respects the design is overly complicated with laminated spars which use CF for reinforcement. I will probably make some changes along the way. My intent is to build it as a one piece model in an effort to save weight as mine will be electric. We will see how it goes.
Teo
Teodorico Terry:
Mark,
Thank you for your explanation.
Brett Buck:
--- Quote from: Teodorico Terry on January 27, 2022, 06:35:42 AM ---Hi -
I just bought a Thundergazer kit from RSM and I have been spending some time looking at the drawings trying to understand how to built it. By the way, the RSM kit is very nicely done with good wood and nice hardware.
So looking at the drawing and assuming that the wing is shown at 0 degree incidence this is what I noticed:
1) The design uses down thrust, 1/8" over the length of the engine bearers.
2) The tail plane has a slightly positive incidence, 1/64" up at the stab's leading edge.
3) The CG is rearward of what I have seen in other C/L designs, CG at 3 7/8" with a wing cord of 12" at the root, almost 33%. Considering the taper of the wing it would place the actual CG a little closer to 28-30% mark.
I also fly R/C, mostly the AMA pattern (Advanced) and they way in which the model is set-up would make me thing that it is in a tail heavy condition. The use of down thrust is pretty normal but in most cases the wing is set-up with a slight positive incidence (0.5 -1.0 degree) with the stab neutral. The Thundergazer in a sense is set-up with negative wind incidence if the stab is neutral.
Does anyone understand the reason why it might be set-up that way?
--- End quote ---
Yes, and the simple answer is that he experimented extensively with both thrust lines and stab alignment, and found that it worked best that way, after having the downthrust influenced by his 4-time National Champion mentor/hero Ted Fancher, and the positive incidence suggested to him by one of his dimwitted flying buddies.
A slightly more complex answer is that as long at the prop is spinning in the conventional direction, there is a fair bit of nose-up pitch torque from gyroscopic precession, and some benighted individuals think this explains why many airplanes end up rigged with lots of down elevator at neutral flap. This just builds it in, or in the case of the original airplane, was shimmed in using the removable tail hardware. The theory is that this compensates for this nose-up torque, making it neutral both for sensitivity and control loading.
Brett
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