Official Freewing MiG-29 Fulcrum Twin 80mm Thread

[mshagg]

The stock nozzles seem to have some pitch up built into the thrust angle, which makes you wonder just how much up trim it would need if they didnt.

My strategy was to match the TV pitch trim with the elevator trim that was established with the stock nozzles (i.e. no notable change in pitch between throttle on/off). This required two trim clicks of up trim from the factory setting (using its own trim clicker and later zerod out mechanically).

You can see here the centre trim marker molded into the nozzle, versus where I trimmed it to fly straight and level with my existing CG/Ele trim. The plane is trimmed for ~1mm reflex and 10mm behind book CG.

[xviper]

The stock nozzles seem to have some pitch up built into the thrust angle, which makes you wonder just how much up trim it would need if they didnt.

My strategy was to match the TV pitch trim with the elevator trim that was established with the stock nozzles (i.e. no notable change in pitch between throttle on/off). This required two trim clicks of up trim from the factory setting (using its own trim clicker and later zerod out mechanically).

You can see here the centre trim marker molded into the nozzle, versus where I trimmed it to fly straight and level with my existing CG/Ele trim. The plane is trimmed for ~1mm reflex and 10mm behind book CG.

May not have any direct relevance, but that’s about how much UP trim there is on both my SebArt MiGs. When left on the factory marks, both planes would do a shallow dive on full throttle. If anything, I prefer to have them climb slightly on power up.

[mshagg]

Yes I learned my lesson on the SU35. The compensating up trim on the elevator meant the plane would balloon up as you throttled down on final approach. Tough plane to land in that configuration lol.

[janmb]

I have a cc bec pro installed on mine and i was curious if running it at 5.5 volts will be an issue. The stock bec is 5v (at best). The servos show that they can go up to 6v. Im im using hitec d85s for the tailerons and know they will be fine, but will the blue box or the landing gear be fine. I couldnt find anything on the specs of those two components. Im only running the wing servos and the landing gear through the blue box

6V is very, very borderline for the blue box, and neither servos nor retracts are confirmed rated for that.

That being said, I would assume the actual risk fairly low, but if going 6V I would at the very least eliminate the BB.

Be very careful listening to "never had a problem" type of responses to such issues. Running electronics over rated volts can appear completely unproblematic - until it suddenly isn't. It is not a matter of either going boom the second you plug in or never being a problem at all. It is a sliding scale, and running higher volts than intended can easily reduce the MTBF significantly.

Also, I've not had a single FW model running at 5.5V stock. All of mine have been in the 4.9-5.1 range.

[RC30Flyer]

Pic of my finalized elevator pushrods. Local hobby store had the 4.5 inch Hangar 9 Pro-Links, but I opted to spend $5 over $17.99 and make my own.

[Gringotuerto]

....

Be very careful listening to "never had a problem" type of responses to such issues. Running electronics over rated volts can appear completely unproblematic - until it suddenly isn't. It is not a matter of either going boom the second you plug in or never being a problem at all. It is a sliding scale, and running higher volts than intended can easily reduce the MTBF significantly.

You are 100% correct. I have been in semiconductor industry for a long time, and we normally use a raised voltage and temperature to "accelerate" failure, to be able to estimate long term failure rate from a short term test. The typical failure mechanism is TDDB (time-dependent dielectric breakdown), where the insulating dielectric that forms a transistor gate "wears out" from having strong electric field across it over time. You normally only have to raise voltage a slight amount (e.g. +20% voltage, and higher temperature also) to accelerate the failure rate around 100x, and therefore predict long-term behavior based on short-term data. Don't take those numbers as gospel, because that's just an example and the exact figures depend on the process technology and design; but the general point is that the acceleration factor is proportional to an exponential of the overvoltage, and exponential means you are playing with fire -- literally and figuratively -- when you go overvoltage. May or may not be a problem, but janmb's point is that absence of immediate smoke doesn't mean anything. You could still be accelerating TDDB by some huge factor, and you will most likely discover that in the air.

[Airguardian]

Thanks for the detailed insight!

[mshagg]

Running a good strong BEC already puts you ahead of the competition, I wouldnt tempt fate by going for more. With a strong BEC and beefier servo extensions cut to the right length you'll minimise any vdroop at the servo and get the most out of the components without resorting to overvolting.

[steven gubala]

Thanks sfcfury, will check into that for the runcam

[janmb]

The stock nozzles seem to have some pitch up built into the thrust angle, which makes you wonder just how much up trim it would need if they didnt.

Sorry, but thrust line is a terribly poor way to solve trim issues. It is (obviously) highly throttle dependent, and you don't want your model's trim to be dependent on neither speed nor throttle setting.

The model needing lots of elevator trim needs to be solved as much as possible with CG (and can be somewhat helped with a little reflex).

For my part, VT nozzle neutral position will be somewhat lower than the stock nozzles. Fixing that thrust line issue is half of the point with installing VT at all for me.

[janmb]

You are 100% correct. I have been in semiconductor industry for a long time, and we normally use a raised voltage and temperature to "accelerate" failure, to be able to estimate long term failure rate from a short term test. The typical failure mechanism is TDDB (time-dependent dielectric breakdown), where the insulating dielectric that forms a transistor gate "wears out" from having strong electric field across it over time. You normally only have to raise voltage a slight amount (e.g. +20% voltage, and higher temperature also) to accelerate the failure rate around 100x, and therefore predict long-term behavior based on short-term data. Don't take those numbers as gospel, because that's just an example and the exact figures depend on the process technology and design; but the general point is that the acceleration factor is proportional to an exponential of the overvoltage, and exponential means you are playing with fire -- literally and figuratively -- when you go overvoltage. May or may not be a problem, but janmb's point is that absence of immediate smoke doesn't mean anything. You could still be accelerating TDDB by some huge factor, and you will most likely discover that in the air.

Precisely! Thanks for the details!

[paulrkytek]

When I asked mrc about setting the voltage on my separate rx bec I was told to stick to 5v as the blue box does not like going above this. I only go to 5.5v when I do not have a blue box.

[Evan D]

Some FW EDF and FL war birds that have a ESC with the built in BEC is at 5.5V and have the MCBe and have no problem. T-33, L-39, F4U...

[GliderGuy]

Merry Christmas everyone!

Light winds and mild temps here will allow me to check out the battery bay cooling modification (post #3575) later today.

I ended up putting a thin, 2 cm long shim under the canopy/lid on both sides to ensure the existence of a small air intake gap after performing more smoke flow tests with the fans turning. The thin gap caused by the shims can’t be seen unless you look really close.

I will report the subjective results later...I don’t have a temperature reading/recording set-up.

In the prior post about semiconductors, both voltage and temperature were stated to impact MTBF, and LiPos live longer if kept cooler, too. This EASY mod should keep the battery bay temps down.

-GG

[Colorc]

I bought the cc bec pro used and had this feeling i should check the voltage output which was 5.5 on the multimeter. I dont have the castle usb adapter to change it, and was hoping that 5.5 would be fine cause the waether is looking nice for today and tomorrow around here. I knew i shouldnt have cut the stock one loose and soldered and mounted the cc bec without checking the settings on it first😅. Just my luck. Looks like im ordering that cc programmer.
I know it wouldnt be too hard to put the other one back in, but for some reason back tracking like that just feels about as much fun as slapping my sunburn.

[Colorc]

Some FW EDF and FL war birds that have a ESC with the built in BEC is at 5.5V and have the MCBe and have no problem. T-33, L-39, F4U...

Good point.

[paulrkytek]

Maybe the bluebox has been improved since I asked or modified for later planes, perhaps a call to MRC is in order !!!

[Elbee]

Merry Christmas everyone! Light winds and mild temps here will allow me to check out the battery bay cooling modification (post #3575) later today.

Merry Christmas, GG. Best on your Check Flight. Ho, Ho, Ho, LB

[skybolt]

I checked with Motion RC about the voltage limits they told me no higher than 5 volts for the blue box or the standard servo.

[GliderGuy]

Merry Christmas, GG. Best on your Check Flight. Ho, Ho, Ho, LB

Thank you! After several check-out flights, I am happy to report that the BEC is barely warm, and the batteries seem cooler too.

I have flown on cooler days, and the electronics and batteries got warmer (pre ventilation mod). What I immediately noticed today after doing the cooling mod is the absence of the warm air “bubble” I used to feel upon opening the lid after a flight.

Pretty sure the vent mod detailed in post #3575 is gonna help come summer.

-GG