Solution Discovered... Angles and Dangles and Pushers...Oh My! - RC Groups
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Jul 08, 2017, 01:55 PM
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Discussion

Solution Discovered... Angles and Dangles and Pushers...Oh My!


With tractor gyros under our belt, we wanted to work on a pusher. A basic "stick" model seemed the place to start and we took inspiration from the BEGi , a design by Mr. Nowell.

All the materials for a simple mockup were on hand and would be a quick build and a rugged experimental platform to learn about pushers. So we used the angles and dangles from BEGi , the power system and rotor from a Whippit, set it up and went to the field to fly it.

We were pleased when it flew away at the first hand launch, but then it was time to tame the beast. Although stable enough to fly, it seemed very tail heavy, so we landed and adjusted the pitch to give us more down nic. Then an ROG take off and it seemed better, but still tail heavy. Applying thrust pushed the nose up steeply, but we could compensate with down nic, but it was really too much nose up happening for comfort. Any more down nic, and we would be below horizontal. The model was not as stable as we desired, so we landed, went home and glued on a horizontal stab then went back to the field.

Now stability was improved, but we still had the tail heavy going on. Subsequent flights were done, each time moving the battery (and cg) forward until we ran out of room.

From reading on this forum, we learned the vertical cg on a pusher is important, as well as the motor offset. The motor offset was done before the maiden flight, but we had not checked the vertical cg until now. Could it be the cg is above the thrust line, causing the nose up with thrust?

Attached are pics of the model hanging to indicate cg, and a rear view showing motor offset. It looks to us like the thrust line is above the cg. Two small pieces of tape were placed at the vertical cg and thrust line on the front of the mast, and the distance between them is 25mm.

What do you guys think? Can we rule out vertical cg as an issue? If so, what is the next change to try? Thanks!
Last edited by rus1; Yesterday at 07:43 PM. Reason: Solution Discovered
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Jul 09, 2017, 06:09 AM
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Obviously your motor thrust line is above the CG of your craft. But it seems to me that it has some downthrust with respect to the horizontal stabilizer. Being a pusher this might be responsible for pitching up with increasing thrust.

BTW, in general, it should be safer if the thrust line goes through the CG
http://pra.org/publicdl/Presentation...%20version.pps
Jul 09, 2017, 11:46 AM
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Ralf,

Thank you for your advice. This is the first time we have tried to measure cg vs. thrust line on a pusher. Thanks for confirming we did it correctly. The presentation was interesting and helpful. Your observation about the thrust in relation to the elevator is very astute. Indeed the motor is mounted at a 2 degree angle in relation to the horizontal stab, pointing the front of the prop down toward the nose.

The motor was mounted at this angle only because that is how it mounted on the BEGi.

I think the next step would be to lower the motor as far as possible, and at the same time remove the 2 degree angle, putting the thrust line parallel to the horizontal stab. What do you think? That is changing 2 features in one step. Would it be better to do them one at a time?
Jul 09, 2017, 02:02 PM
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Personally I would change both features in one step and then test the new setup.
Jul 10, 2017, 01:49 PM
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Please take a look here: https://www.rcgroups.com/forums/show...&postcount=115
Is the dynamic response of throttle pitching down your gyro?

Discussion continues from post #122 here https://www.rcgroups.com/forums/show...dynamics/page9
Jul 11, 2017, 01:53 PM
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We understand down thrust on tractor gyros, where the propeller down angle pulls the nose down.

On pushers with the motor behind the mast, is a propeller angled down toward the nose still down thrust? That is , does it push the nose down?
Jul 11, 2017, 04:50 PM
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No. If the thrust line of the propeller is pointing below the CG you end up with a (small) torque lifting the nose of the gyrocopter. That's why I am puzzled with your observations: From your photos the motor is mounted above the CG. The propeller thrust line is acting above the CG and should push the gyro's nose down with increasing thrust and counteract the pitching up produced by the drag of the rotor disk. However, you report about a strong pitching up of the copter with increasing throttle. This would be expected when the thrust of the pushing propeller would point below the CG.
Jul 12, 2017, 10:22 AM
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The motor is being lowered, and the thrust line made parallel to the horiz stab. The goal is to get the thrust line on the cg. If we can't get right on, it will be as close as possible, but not below the cg.

Then we will fly again, observe, take video and reassess the situation. Thanks for your helpful input. It is clear more data from testing is needed.
Jul 21, 2017, 12:58 PM
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The motor was moved down as far as possible and mounted parallel to the horizontal stab and fuse. Previously the thrust line was above the vertical cg by 25mm (about 1 inch) with 2 degrees of upthrust. Now the trust line is 12mm(about 1/2 inch) above the vertical cg with no up or down thrust. We had to wait to test fly because of monsoon thunderstorms, but now have the results of the changes.

The problem got worse. This time the nose up and tail down problem was much worse than before. Full down nic needed to be applied continuously, and it was very difficult to gain altitude by applying thrust, although airspeed increased.

So, the thinking now is that the motor was moved the wrong way because the problem worsened. When using the vertical cg as the pivot point, it seems the rotor drag is pulling aft on the mast lowering the tail and raising the nose. Originally the thrust was partially resisting, and by lowering the thrust line to be closer to the pivot point we have reduced this resisting force.

Comments are welcome!
Yesterday, 08:04 PM
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Solution Discovered


We found the solution. The motor mount was modified so we could move the motor up and down the mast at the flying field as needed for testing. The motor was kept parallel with the fuselage with neither up or down thrust. From the last test flights we know lowering the motor toward the CG was going in the wrong direction because the nosing up got worse. Test flights were done, this time moving the motor up the mast in small steps .

Flight performance kept improving as we moved the motor up the mast. The optimum motor location on the mast puts the thrust line 42mm above the vertical CG as measured back at the hanger. At that high thrust position relative to the CG, the propeller thrust line is acting above the CG and pushes the gyro's nose down with increasing thrust and counteracts the pitching up produced by the drag of the rotor disk, just as Ralf said it should in post #7 above.

So, now that we know the correct angles and dangles, we can make a pretty pusher using the rotor and power system identical to those used on a Whippit and our Cactus Hopper. The pusher gyro flies very similarly to the Whippit and Cactus Hopper, but is more maneuverable, and less susceptible to up and down air movements, since we removed the horizontal stabilizer.....it doesn't need one and actually flies better without one.

Oh, and we guess this is another example where what works on full size gyros doesn't apply to model gyros. Ralf, thanks for your help and comments!
Today, 12:07 AM
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