Official Freewing MiG-29 Fulcrum Twin 80mm Thread

[Evan D]

A lot of people have opinions... I'd rather hear them than they not post, you just never know...

I would rather people read the threads before asking a question or making a statement that has been well covered though, especially when covered on the same page or just before... I think we've all been guilty of doing this though. :)

[Captain MoMo]

Servotelemetry - test 1

The new receiver arrived and i‘m able to provide servo telemetry data now.

I just made a quick voltage check at the servos to show the voltage difference at the servos under same conditions - Test 1 connected to the stock elevator servo cables via y-cable compared to my upgraded and rerouted elevator cables (will follow soon in Servotelemetry part 2)

Following setup for test1:

Stock BEC directly connected to jeti rex12 receiver. Both telemetry servos connected via the stock elevator servo cables and plugged into one receiver channel via the y-cable provided with the MRC upgrade servos. No other equipment connected to the receiver. No Bluebox. Voltage at the receiver 4.9Volt.

1. Servos in idle - no servo movement: V at the servo 4,7Volt at 0,2ah (each servo)

2. Servos moving free in servo test mode at half speed: Voltage at the Servo: 4,6Volts at 0,7ah (each servo)

3. Servos forced by hand to stall: Voltage at the servo 3,5Volt at 2,5ah (each servo)

Firebird - what gauge servo wire did you use to upgrade? I think you are definitely pointing out to a potential issue. If the voltage drops to 3.5V at the servo what is it at the RX? Some RXs tend to brown out at around 3.5V!!

[janmb]

Also, modern radios wouldn't be losing signal at the rate of these crashes. It's exceedingly rare.

Well, it would be rare if people made perfect installations every time. But as it stands, radio loss is still fairly common, but almost exclusively caused by bad installations in one way or another.

[Captain MoMo]

Well, it would be rare if people made perfect installations every time. But as it stands, radio loss is still fairly common, but almost exclusively caused by bad installations in one way or another.

Yes, but these crashes have occurred in very close proximity - another reason I also doubt radio being the issue. Mine happened less than a hundred feet away from me.

[Firebird]

Firebird - what gauge servo wire did you use to upgrade? I think you are definitely pointing out to a potential issue. If the voltage drops to 3.5V at the servo what is it at the RX? Some RXs tend to brown out at around 3.5V!!

Hello Momo,

I used 0,5qmm x 50cm cables.

[Airguardian]

Firebird, that servo telemetry test was performed with stock servos on a telemetry receiver or were they special telemetry servos replacing the stock ones?

If it is the former, then it is indeed quite damning but as I understand, that is measuring with very specific servos that are supposedly more powerful than stock servos, so the test shows more the weakness of using long, thin wires, right?

If that's the stock servos being tested, then I think this pretty much marks them as the most likely root cause, along (worsened by) using long thin wires and 5V power supply.

Has anyone crashed this jet with stab servo leads upgraded to a thicker gauge and/or shortened?

One more down in Germany. I translate a posting from today at RCNetwork.

"mine went in on the 4th or 5th flight due to the lack of elevator function in a nice 45 degree angle into the field. I flew with a turbine, Xicoy x45, about 50% gas. During the first flights I noticed that the HR function is deficient in all downward angles, to say the least; however, during the relaxed approach nothing more came (through)... Total write-off."

That kinda rules out the ESCs being noisy, as the culprit, lol
(Well, Turbines can be noisy too but... you get the point!)

[GliderGuy]

Firebird - Looking only at the current draw (the video premise is incorrect....they function correctly / reversing when the reverse drive signal gets to the servo).

it appears the Hacker servos draw more stall current. The video stock servos appear to draw only 0.7 amps when stalled vs 2.5 amps for the Hacker servos.

https://youtu.be/0c5aI7DpqkY

Thanks for all your trouble!

-GG

[bandetx]

Upgrading the servos and using the same wires and connectors would only make the problem WORSE because they would just be an even more power hog! Looks like the wires/connectors need to be replaced.

[kallend]

Servotelemetry - test 1



3. Servos forced by hand to stall: Voltage at the servo 3,5Volt at 2,5ah (each servo)

OK, but what is the stall current of the stock (replacement) servos? The voltage drop along the wires is going to depend on that. You can't assume they also stall at 2.5A just because your Hacker telemetry servos did.

[kallend]

Firebird - Looking only at the current draw (the video premise is incorrect....they function correctly / reversing when the reverse drive signal gets to the servo).

it appears the Hacker servos draw more stall current. The video stock servos appear to draw only 0.7 amps when stalled vs 2.5 amps for the Hacker servos.

https://youtu.be/0c5aI7DpqkY

Thanks for all your trouble!

-GG

Exactly!

[leithalweapon]

Way to nose heavy in a high speed dive is accident waiting to happen . Your elevators are working they just can't overcome the speed and g force acting upon them. I'm inclined to beleave it's pilot error not anything to do with the electronics. Fly the thing don't abuse it . Sure you love the speed and everyhing about it but it's not meant to fly it the way some of you are doing. Just my opinion.

Sounds like a full scale.pilot talking?

[janmb]

Yes, but these crashes have occurred in very close proximity - another reason I also doubt radio being the issue. Mine happened less than a hundred feet away from me.

Me too, not saying for one second that radio loss is a probably culprit in most of these cases - on the contrary.

[Jet jocky]

Not all !!

[GliderGuy]

Firebird - Your results are strongly suggesting that using upgraded servos beyond MRC-supplied product needs a wiring upgrade as well.

Data summary suggests:
(a) MRC stock or MRC upgraded servos - Stock wiring has minimal voltage drop even under stalled condition at the rear servos (info from the YouTube video and your 0.7 amp readings)
(b) Upgraded servos that draw more current than MRC servos may need a wiring upgrade to minimize voltage drop at the rear servos

Have you had a chance to look at signal strength telemetry data?

Thanks again for this effort.

-GG

[Skosh25]

can someone post a link to a recommended wiring upgrade?

[Mizer67]

Way to nose heavy in a high speed dive is accident waiting to happen . Your elevators are working they just can't overcome the speed and g force acting upon them. I'm inclined to beleave it's pilot error not anything to do with the electronics. Fly the thing don't abuse it . Sure you love the speed and everyhing about it but it's not meant to fly it the way some of you are doing. Just my opinion.

Most of the crashes I've seen have started from level flight or at most a shallow dive at relatively moderate airspeeds.

Sure there are some aerobatics beforehand in a few videos, but they're very mild.

[HK111]

Big THANKS to Mr. Firebird for making the measurements and posting the results!

[AirHead]

Since we still don't have a clue about what really caused these crashes, I thought of a somewhat strange solution that would mitigate almost all of the possible causes that I have read in the past few days:

How about putting a small (2-channel) receiver in the back of the plane for the elevators only? This receiver could be powered by a small receiver pack, also located in the back of the plane. In my case i could use this 6 grams Futaba R202GF S-FHSS and put each elevator on it's own channel.

This would solve/mitigate the issue, if it's caused by one of these many options:

- faulty y-cable
- problems with bluebox
- interference/rf noise in the elevator servo cables
- voltage drop because of long elevator servo cables
- BEC overloaded
- receiver/antenna's blanked out by large batteries
- receiver brown out because of flying high speed with flaps deployed
- and I possibly forgot a few

What do you think?

*Edit: I just noticed that the R202GF isn't suitable for planes, but you get the idea :-)

Maybe it's time for companies to make a "twin pack" of receivers that use the same "Globally Unique Identifier". That way, you could bind two receivers mounted in different locations in the plane, powered either locally or with shielded split power lines. You would mount one up front, one in the tail and connect with short cables and have them isolated from the ESC's and everything else. Might seem absurdly ridiculous, but crazier things have been tried. Either that, or have a "séance" to bring back Nikola Tesla and see if he can find the "ghost" that lives in this aircraft....

[kallend]

Big THANKS to Mr. Firebird for making the measurements and posting the results!

Let's be quite clear, Firebird did NOT use the stock (or MRC replacement) servos. The ones he used (Hackers) drew far more stalled current (more than 3 times as much) according to his report. As a result his voltage drop readings are not applicable to the stock servos (as GG has already pointed out). Ohm's law.

[HK111]

Firebird did NOT use the stock (or MRC replacement) servos. The ones he used (Hackers) drew far more stalled current (more than 3 times as much) according to his report. As a result his voltage drop readings are not applicable to the stock servos (as GG has already pointed out).

Sure. But you can infer a lot from his measured voltage drop. If the voltage goes down from 4.9V at the receiver to 3.5V at his servo at a current of 2.5A, then we have a wire resistance of 1.4V/2.5A = 0.56 Ohms. If we assume the wire resistance is about constant at these currents and temperatures, then we can calculate the voltage drop for a normal servo. If it stalls at about 1A, the voltage drop will be 0.56 Ohms * 1A = 0.56V. That means a 1A-stalling servo will see about 4.34V.

Does the servo run well at 4.34V? Do we have the stall current for the replacement servos?

Cheers,
Henrik