Official Freewing MiG-29 Fulcrum Twin 80mm Thread

[themudduck]

No, and I'm not sure that one crash in 2 dozen flights is a record I aspire to.

You misunderstood me, and I didn't crash. Where did I say that I crashed? What I said was that if the servo is being constantly overloaded by aerodynamic forces as YOU suggested, then the servos would be overloading all the time. And they are NOT. My point is that they are not being overloaded to the point of failure on a regular basis. Its an unusual and rare occurence. Both servos simply stop working seemingly at random.

[themudduck]

Hi Guys I just measured the wing incidence on the Mig . At 0 degrees on the wing the stab is at 6 degrees negative. To my thinking the CG is way off or this plane does not have enough wing area to support it's weight. I can see why the servos are possibly overheating.

Eric, the position of the stab is dependent on the CG. If the model is nose-heavy then the incidence must be adjusted. Its simple, if you want to have less incidence on the stab then move the CG back. The CG has nothing to do with the wing area.

[kallend]

By the way I was working on full sized A/C before you were a gleam in your daddys eye, .

Richard Driftmyer CMSgt USAF Retired .

In that case you must be AT LEAST 91 years old or were doing it illegally

As for my experience which you seem to denigrate, I've been flying R/C since 1964. Made my own servos back when I couldn't afford to buy them. First transmitter I made had vacuum tubes. I've had two of my original designs published and one kitted. I created the world's first R/C flight simulator. I'm an instrument rated pilot and own my own plane. And I've taught airplane design to engineering students.

[kallend]

My point is that they are not being overloaded to the point of failure on a regular basis. Its an unusual and rare occurence. Both servos simply stop working seemingly at random.

Not rare enough, apparently, nor particularly random - the failures seem to occur in one particular phase of flight and generally after a few minutes flying time.

Not everything fails the first time it's overstressed (mechanically, thermally or electrically).

[Eric D]

Eric, the position of the stab is dependent on the CG. If the model is nose-heavy then the incidence must be adjusted. Its simple, if you want to have less incidence on the stab then move the CG back. The CG has nothing to do with the wing area.

[Eric D]

I know CG has nothing to do with wing area. I said it may not have enough area to support it's flying weight. A thrust line issue may also cause an excess of up elevator needed.

[Bajora]

A few more shots of clubmate Michael Troys Mig 29, now with afterburners

[kallend]

My point is that they are not being overloaded to the point of failure on a regular basis. Its an unusual and rare occurence. Both servos simply stop working seemingly at random.

How do you know the servos stop working? Where is the evidence of that?

What we know is that elevator authority is lost which may be a servo issue, or a pushrod issue, a control horn issue, an aeroelasticity issue, an aerodynamic issue, a power supply issue, or a combination of these items.

[HaroldAnderson]

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.

[Gilatrout]

A buddy of mine lost his Mig on the maiden. Straight and then nose down and done.

He felt he may have lost power too.

Anyway, the only explanation we could come up with that explains both elevators being unresponsive is the BEC failing.

Throttling down would reduce load on Bec and allow the caps to recharge restoring power and control to the plane.

The trick is finding a way to reproduce the problem on demand.

[janmb]

Yet another 😔

Maidening mine tomorrow - got upgraded bec though, but clearly not tons of confidence at this point

[janmb]

A few more shots of clubmate Michael Troys Mig 29, now with afterburners

Beautiful!

[mshagg]

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.

Yep, noticed both mains toeing out last time i was flying the mig. Ground handling was sensitive, as you might expect, although no problems taking off or on rollout. It's on the list of things I need to take a look at in the hangar.

[Firebird]

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

at the receiver.




Task:




Assuming the mig requires a negativ lift of 100gram at each elevator tip to fly straight and level at cruise for 4minutes - how much force is required by the elevator servo and how does the continuous load on the servo (fighting the nose down moment) influence the temperature and voltage at the servo / receiver and how does the available servo torque change accordingly?

How much force is available at the tip of the elevator at the end of this scenario?




Calculation:




Due to the way the servo force is transmitted to the elevator, we need to calculate the related force on the servo related to 100gram force at the elevator.




F1 x S1 = F2 x S2




F1= 100g = 0,981N (newton)

S1= 22cm

F2= X

S2= 2cm




0,981N x 22cm = X x 2cm




0,981N x 22cm

———————- = 10,791N

2




10,791N = 1100gram




According to the physics leverage law formular the servo need to provide 1100 gram continues elevator servo force assuming you have 100gram aerodynamic force on each elevator tip.




1100gram on each elevator servo = 2200 gram total shared by both elevator servos.



Setup:


Jeti Mui150 telemetry sensor soldered into the servo power line directly in front of one upgraded stock servo. One carson telemetry temperature sensor taped to one servo, transmitting temperature data to a second remote display. Both upgraded stock servos mounted into a benche vise - forcing them to deliver their force equally to a kitchen scale. Both elevator servos connected via the stock elevator servo cables - connected to the related elevator outputs of the bluebox. Both wings connected via the stock wing connection cables into the bluebox. Bluebox connected via the bluebox elevator and flap patch cables into the related channels of my jeti rex12 receiver. Stock BEC directly powering the receiver. Flaps idle in up position- no force. No other equipment connected.

Find below the supplied force, voltage and temperature at the servo with the related voltage at the receiver.

1. All servos idle / no force. Voltage at the receiver 4,97volt, voltage at the servo 4,8volt, temperature at the servo 21.7 degrees celsius (f) delivered force 0gram.
2. Full elevator up command applied to test initial available force of both elevator servos. Delivered force 3961gram (both servos together) Voltage at the receiver 4,87volt, voltage at the servo 3,9volt, temperature at the servo 21.8 degrees celsius (f).
3. Timer started and elevator trim applied to settle a force of around 2200gram (cruise flight) from both elevator servos on the kitchen scale. Trim and force settled at time 0min24sec. Voltage at receiver 4,96volt, voltage at servo 4.7volt, temperature at servo 23,2 degrees celsius (f), delivered force 2227gram.
4. Timer at 0min59sec. Servos lowered output force to 2177gram. Voltage at receiver 4,96, Voltage at servo 4,7volt, temperature at servo 24,9 degrees celsius (f)
5. Timer at 1min10sec. Full elevator up command applied. Maximum available force of both servos together 3146gram. Voltage at the receiver 4,89volt, voltage at the servo 4.0volt. Temperature at the servo 25.5 degrees celsius.
6. Timer at 1min29sec. Servos back in cruise flight force at equal trim setting. Delivered force 2150gram. Voltage at receiver 4,96, voltage at servo 4,7volts. Temperature at servo 26.3 C.
7. Timer at 2min10sec. Full elevator up command applied. Maximum available force 3098gram. Voltage at receiver 4,89volt, voltage at servo 4,0volt, temperature at servo 28.1 C. (F)
8. Timer at 2min40sec, servos back in cruise flight, delivered force 2188gram. Voltage at receiver 4,96V, at servo 4,7volt, temperature at servo 29.1 celsius. (F)
9. Timer at 3min10sec. Full elevator up command applied. Maximum available force 2476gram. Voltage at receiver 4,88volt, voltage at servo 4,0volt, temperature at servo 30,3 celsius (f)
10. Timer at 3min28sec, servos back in cruise flight, delivered force 1693gram. Voltage at receiver 4,96V, voltage at servo 4,7volt, temperature at servo 30,8 celsius (f)
11. Timer at 4min5sec. Full elevator up command applied. Maximum available force 2512gram. Voltage at receiver 4,88volt, voltage at servo 4,0volts, temperature at the servo 32.0c (f). The test was supposed to here but i continued monitoring the reaction of the servos.
12. Timer at 4:40, cruise trim, supplied force 2217gram. 4.96V at receiver, 4.7volt at servo, servo temperature 33.7 C (f)
13. Timer at 4:59, cruise trim, supplied force 2205gram, voltage at receiver 4,96volts, voltage at servo 4,7volts, temperature at servo 33,4 c (f)
14. Timer at 5:04, full elevator up command applied, maximum available force 2454gram, voltage at receiver 4,89volt, voltage at servo 4,0volt, temperature at servo 33,6 c (f)
15. Timer at 5:29, cruise trim, supplied force 2065gram, voltage at receiver 4,97volt, voltage at servo 4,7volt, temperature at servo 34,3 c (f)
16. Timer at 5:59 cruise trim, supplied force 2059gram, voltage at receiver 4,96volt, voltage at the servo 4,7volt, temperature at the servo 34,9 c (f)
17. Timer at 6:05, full elevator up command applied, maximum available force 2438gram, voltage at receiver 4,89volt, voltage at servo 4,0volt, temperature at servo 35 c (f)
18. Timer at 6:29, cruise trim, servo didn’t return to cruise trim position after last full elevator command, supplied force 2425gram, voltage at receiver 4,96volt, voltage at servo 4,7volt, temperature at servo 35,1 c (f)
19. Timer at 6:38, full elevator up command applied, maximum available force 2419gram, voltage at receiver 4,89volt, voltage at servo 4,0volt, temperature at servo 35,2 c (f)
20. Timer at 6:40, cruise trim, both servos not following command, supplied force 2418gram, voltage at receiver 4,97volt, voltage at servo 4,7volt, temperature at servo 35,2 c (f)
21. Timer at 7:36, full elevator up command applied, servos reacting very little on command, maximum available force 2477gram, voltage at receiver 4,89volt, voltage at servo 4,0volt, temperature at servo 35,9 c (f)
22. Timer at 8:09, cruise trim, no reaction of servos, supplied force 2454gram, voltage at receiver 4,96volt, voltage at servo 4,7volt, temperature at the servo 35,7 c (f)
23. Timer at 8:10, full elevator up command applied, no reaction of servos, supplied force 2454gram, voltage at receiver 4,89volt, voltage at servo 4,0volt, temperature at servo 36,2 c (f) End of test.

Summary:


The simulated continuous load of 100gram on the outer tip of the elevator (recalculated with physics leverage law in actual servo load 1100gram per servo = total elevator load 2200gram shared by both servos during simulated cruise) interrupted by full elevator up command, caused the servos to lower there maximum available torque over the duration of the test. Both servos delivered together 3961gram at 3,9volt during first full elevator up command, at assumed landing time after 4:05min they delivered a maximum of 2512gram at 4,0volt (equals maximum capable force at the outer elevator tip of approximately 114gram). Assuming the mig needs about 100gram downforce at each elevator tip for straight and level cruise, the servo might be close to stall after continuous stress. At around 5:50minutes the servos didn’t correctly follow their commands anymore until they fully stopped to follow at around 8minutes. The measured temperature at the servo rised from 21,7 c at the beginning to 36,2 c (f) at the end of the test. The voltage at the receiver remained stable for the related condition. The voltage at the servo dropped under load in the beginning to 3,9volt, later remained stable at 4,0volt under load and 4,7volt delivering assumed force for cruise trim.

[Firebird]

Some more pics of the test:

[Drifty]

In that case you must be AT LEAST 91 years old or were doing it illegally

As for my experience which you seem to denigrate, I've been flying R/C since 1964. Made my own servos back when I couldn't afford to buy them. First transmitter I made had vacuum tubes. I've had two of my original designs published and one kitted. I created the world's first R/C flight simulator. I'm an instrument rated pilot and own my own plane. And I've taught airplane design to engineering students.

You also sound like D. Trump

[HK111]

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

[GliderGuy]

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

[HK111]

Assuming the mig needs about 100gram downforce at each elevator tip for straight and level cruise, the servo might be close to stall after continuous stress. At around 5:50minutes the servos didn’t correctly follow their commands anymore until they fully stopped to follow at around 8minutes. The measured temperature at the servo rised from 21,7 c at the beginning to 36,2 c (f) at the end of the test.

THANKS!

So the servos are getting warm, are stressed, are close to their limits and maybe just failing...

Cheers,
Henrik

[GliderGuy]

Henrik- You may want to search this forum on the word “ferrite” and “RF choke” and read those posts.

-GG