Official Freewing MiG-29 Fulcrum Twin 80mm Thread

[Raydar]

Interesting data on the servos slowly overheating and losing power, people comment o the plane elevator just going mushy and the results you produced reflect that.

I want to know if the servos recovered functionality when cooled. But even if they did not the partial slowing down and not centering certainly would be enough to end in disaster.

Question is it unreasonable to stall these servos so long and not expect failure, I for one have burnt out plenty servos by accidentally letting them bind.

I imagine those starting in a higher ambient temp would reach servo issues sooner.

[GliderGuy]

Also....More work for Firebird, but how do the stock 17gm servos behave after 6 min? It’s possible the new upgrade (no history) servos behave differently than the stock (lots of history) servos do.

-GG

[Firebird]

Firebird - Thank you!!!

The missing piece is actual flight load telemetry data you plan to gather later when you fly.

Then the work begins related to extrapolation of flight loads to the time servos might fail to respond.

Eventually I see guidance from this being something like “Push it hard, generate high loads, land before this amount of time”. Or simply - upgrade beyond the MRC upgrades.

Speaking of which....what is a servo that can stand the stress and will fit...which ones might be considered?

-GG

Thank you Gliderguy,

me personally - i wouldn’t recommend any servo below 8Kg. Even this is giving you barely 727gram force at the tip of the elevator.

The blackhorse mig29 is 20cm longer, has 20cm more wingspann and is about 1,8kg heavier. This aircraft has approximately same recommended CG location and approximately same pivot point position then the freewing mig. The pushrod geometry is close to the freewing version (regarding lenght of lever).
Blackhorse recommends the Futaba BLS 352 which has 18Kg. Even if the plane is a bit bigger, the numbers tell a lot.

[Firebird]

Interesting data on the servos slowly overheating and losing power, people comment o the plane elevator just going mushy and the results you produced reflect that.

I want to know if the servos recovered functionality when cooled. But even if they did not the partial slowing down and not centering certainly would be enough to end in disaster.

Question is it unreasonable to stall these servos so long and not expect failure, I for one have burnt out plenty servos by accidentally letting them bind.

I imagine those starting in a higher ambient temp would reach servo issues sooner.

Raidar,

the servos work fine again after just a moment of releasing force on them - as long as you don’t force them to deliver high torque again too soon.

[kallend]

... good stuff

The missing part is the unknown trim load needed.

Elsewhere I calculated the position of the stab's center of pressure with respect to the pivot axis. That would be a better place to locate the trim force than the tip of the elevator.

[GliderGuy]

I imagine those starting in a higher ambient temp would reach servo issues sooner.

Please see post #2643 on Nov 1st regarding high ambient temp experience. I didn’t see any issues operating at high ambient over several hundred flights...many in rapid succession.

But....my typical flight pattern has a 1/2 throttle slow down period of 1 min at the end to conserve battery capacity and reduce the charge time needed. Not done for hot servo considerations at all. I may have adopted a flight plan that kept me from seeing the servos going non-responsive. Luck?

-GG

[nuts-n-volts]

Firebird, Thanks so much for doing a very well coordinated and documented test. I think it really speaks for itself as to why these failures are happening. It would also show that if you really loaded the FFS up by having a much more forward CG, the servos would fail earlier. I'm thinking of the very first crashes over in Asia where the planes only did a circuit or two and then crashed. Those first videos impacted me early on as a system slowly failing and that this problem had something to do with the voltage in the system as well as a few other things all adding up. I appreciate all your hard work as it was something I didn't have time for. That is why I come to these threads... for the help of others. The bottom line is that I feel much more comfortable flying this thing now that we all have stats showing the problem and knowing at least a little on how to go about fixing it. Well done!

[Raydar]

The fact the servos cool and work perfectly again also fits the failure remit perfectly.

Larger more powerful servos to fix the issue and disipate the heat better, stock servos and other micro servos perhaps not up to the job, who would have guessed😂.

thanks for the effort you put in.

[Firebird]

Firebird, Thanks so much for doing a very well coordinated and documented test. I think it really speaks for itself as to why these failures are happening. It would also show that if you really loaded the FFS up by having a much more forward CG, the servos would fail earlier. I'm thinking of the very first crashes over in Asia where the planes only did a circuit or two and then crashed. Those first videos impacted me early on as a system slowly failing and that this problem had something to do with the voltage in the system as well as a few other things all adding up. I appreciate all your hard work as it was something I didn't have time for. That is why I come to these threads... for the help of others. The bottom line is that I feel much more comfortable flying this thing now that we all have stats showing the problem and knowing at least a little on how to go about fixing it. Well done!

Thank you Nuts-n-volts, appreciate!

All of us together - we as team will be able to fix the issues and enjoy this beautiful aircraft without fearing the loss of it.

[kallend]

You also sound like D. Trump

Is there a reason you find it necessary to be insulting towards engineers in general and me in particular?

[Firebird]

Please see post #2643 on Nov 1st regarding high ambient temp experience. I didn’t see any issues operating at high ambient over several hundred flights...many in rapid succession.

But....my typical flight pattern has a 1/2 throttle slow down period of 1 min at the end to conserve battery capacity and reduce the charge time needed. Not done for hot servo considerations at all. I may have adopted a flight plan that kept me from seeing the servos going non-responsive. Luck?

-GG

There might be nothing wrong with the servo itself. I could imagine that the internal protection circuit of the servo board lowers the output to the motor to protect the servo under continuous high load from overheating. That would result in (at least momentarily) less available torque.

[kallend]

Thank you Nuts-n-volts, appreciate!

All of us together - we as team will be able to fix the issues and enjoy this beautiful aircraft without fearing the loss of it.

These are the numbers for the MiG stab's center of pressure (CP) position:

The pivot axis is located almost exactly at the leading edge of the Mean Aerodynamic Chord (MAC).

The CP of a symmetric airfoil is located at 25% MAC and doesn't move with AoA over reasonable (small) values.

This puts the CP at a full 25% of MAC behind the pivot axis.

The MAC is 17.8cm (or 7") so 25% of that is 4.45cm (1.75")

In other words, whatever aerodynamic force F has to be provided by the stab. for trim or maneuvering purposes, it exerts a moment about the pivot axis of 4.45*F kg.cm or 1.75*F oz.in (using appropriate units in each case for force).

(In contrast, the FW SU35 FFS has its pivot axis almost exactly at the CP)

[PieterO]

Anyone had issues with toe-in or in my case toe-out on the mains? One main had quite a bit of toe-out after a few flights. I ended up forming the axle forward to fix the problem after disassembling the retract assembly and not finding a problem or an easy way to adjust the toe-in. Formed both axles but I had to form one so much that the wheel had to be pressed on the axle with some persuasion. Once on the axle the wheel worked fine and handling is great now. In the end the adjustment was quick and dirty but worked.

Yes, same issue here. Also disassembled the retracts but didn't find a good solution. So you just bent the axles?

[Jacckall]

Anyone had issues with toe-in or in my case toe-out on the mains? One main had quite a bit of toe-out after a few flights. I ended up forming the axle forward to fix the problem after disassembling the retract assembly and not finding a problem or an easy way to adjust the toe-in. Formed both axles but I had to form one so much that the wheel had to be pressed on the axle with some persuasion. Once on the axle the wheel worked fine and handling is great now. In the end the adjustment was quick and dirty but worked.

Hello!

Same problem here! Can you post photos, how you fix this problem? :) Thank you :)

[PieterO]

Partial list of fixes that might work against the crash tendency

Here my personal list of partial fixes that might work:

• Exchange elevator servos, at least with the Freewing upgrades, possibly for even bigger ones. [x] done
• Use much bigger, sturdier pushrods for the elevators. [x] done
• Use longer screws to screw in the control horn on the elevators, possibly completely through the elevator, with a counter plate on the upper side. [ ] not done
• Change pivot location in the elevator. [ ] not done
• Really screw in the rudders, so that they stabilize the a-bit-too-flexible tails that carry the elevator, possibly glue them in. [x] done
• Exchange the leads to the tail servos, use much shorter leads (50cm) with a thicker wire gauge. [x] done
• Reroute the leads to the tail servos away from ESCs and fans. [ ] not done
• Bypass the blue box whereever easily possible, for rudder and elevator. [x] done
• Bypass the blue box whereever possible, also if that means rewiring of the wing servo connector. [ ] not done
• Move the receiver back into the rear/wing root compartment, but position the two antennae wisely to avoid shielding by batteries. [x] done
• Connect each elevator servo independently to the receiver and operate them as tailerons. [x] done
• Move CG 10mm back from factory-recommended position and dial in some reflex in ailerons and elevators in order to reduce the "always up"-stress on the elevators. [x] done
• Remove factory BEC. [x] done
• Change the BEC output to a slightly higher voltage than 5.0V. [x] done
• Possibly exchange *all* servos for HV models and go to 7.2-8.4V. [ ] not done
• Add a separate receiver battery or at least receiver capacitor. [ ] not done
• Initially fly like grandpa. [ ] not done

Anything wrong or missing?

Cheers,
Henrik

One more option:
Use the inner hole of the elevator servo horn to reduce the applied torque on the servo. (didn't do this myself (yet) by the way)

My solution for now is your last option :-)
I fly like grandpa for now, but still enjoy it vey much!

[fredmdbud]

Telemetry - test 8 Bluebox / Wings connected / continuous load



I had the opportunity today to run another test on the mig. The test is an attempt to simulate a 4minutes straight and level flight interrupted by full elevator up command. The full up command is applied to show the maximum available moment of the stock upgrade servo related to the flight time, servo temperature, voltage at the servo and

I am not so sure about your premise - it seems you are looking at a purely static and zero airspeed case. And it also seems you are balancing the elevator torque with the mass of the rest of the plane forward of the CG - what about the rest of the plane? If you put the plane on a balance stand, you don't need that much force to bring the plane to level.

And zero airspeed ignore the induced lift (downward) forces of the elevators (in Newtonian terns, the reactive force coming from deflected airflow).Likewise it also ignores the torquing effect of lift pressure.from the lifting body (additional nose-lowering force behind the CG).

Any device drawing current will heat up (from resistive elements). That the servo heated up slowly is not a surprise - slow indicating a relatively low current draw, until the temperature reaches an equilibrium - was this test left running until the temperature stabilized?

[HaroldAnderson]

Hello!

Same problem here! Can you post photos, how you fix this problem? :) Thank you :)

Here is a picture of the 'formed' axle. You remove the axle from the gear by loosening the 2mm set screw at the bottom of the gear. Place the axle in a vice and carefully tap it with a hammer in the direction you need it to go. You can see the vice mark on the axle at about 45mm reference point on the ruler. The short portion of the axle was in the vice and you lightly tap the outer part. It may take several tries to get it right but it only takes 5 minutes to do. It's redneck but no more toe-out. Happy landings again!

[crxmanpat]

I just conducted a very non-scientific experiment.

First I removed the carbon tubes from the control rod (I have a small CF tube that fits snugly over the control rod and then inside the stock CF tube). Plugged in a battery and held the rear of the elevator. Moved the elevator stick, and the control rod flexed VERY EASILY! I was able to go to full throw on the elevator servo by holding the elevator in place, and the control rod flexed to match the servo arm movement.

Next I put the stock CF tube back on and tried again. This time the control rod flexed inside the tube until it had no more room to flex. The servo arm only moved enough to give the full amount of flex available inside the CF tube, then stalled.

So I would say one of the following two options are a MUST DO on this plane:

1. Do what I did and put a CF tube over the control rod where the ID of the tube will just allow the control rod to slide in, then put the stock CF tube over that so that it slides over the control rod clip at the servo arm.

2. Replace the entire control rod setup with 4-40 or similar.

[fredmdbud]

I just conducted a very non-scientific experiment.

First I removed the carbon tubes from the control rod (I have a small CF tube that fits snugly over the control rod and then inside the stock CF tube). Plugged in a battery and held the rear of the elevator. Moved the elevator stick, and the control rod flexed VERY EASILY! I was able to go to full throw on the elevator servo by holding the elevator in place, and the control rod flexed to match the servo arm movement.

Next I put the stock CF tube back on and tried again. This time the control rod flexed inside the tube until it had no more room to flex. The servo arm only moved enough to give the full amount of flex available inside the CF tube, then stalled.

So I would say one of the following two options are a MUST DO on this plane:

1. Do what I did and put a CF tube over the control rod where the ID of the tube will just allow the control rod to slide in, then put the stock CF tube over that so that it slides over the control rod clip at the servo arm.

2. Replace the entire control rod setup with 4-40 or similar.

Yup, if the gap between the rod and the CF tube is eliminated, the stiffness of the tube will mitigate any bending. Even more so if you use concentric/nested CF tubes, or even shove 2 more control rod pieces to prevent the flex inside the tube.

[leithalweapon]

Is there a reason you find it necessary to be insulting towards engineers in general and me in particular?

One thing I learned when I got my engineering degree was to never assume your the smartest person in the room. Odds are your probably not.
Also, it's not always what you know, but who you know, so let's keep as many people on here as we can.