Official Freewing MiG-29 Fulcrum Twin 80mm Thread

[radfordc]

“Stock everything”...,does that include the wires driving the 425BBs? Or did you install a larger gauge wire when you upgrade to the 425BB servos?

Bypass the BB with elevator? Stock push rods?

Thanks for sharing so we can better understand.

-GG

Same servo wires that came with the upgraded Freewing servos supplied by MRC. Bypassed the BB. Heavier elevator pushrods.

Not sure why it's hard to understand that this plane will pitch down hard if you use full flaps at high speed? I don't think it would matter what servos were installed.

[nuts-n-volts]

The stock wiring on this jet is very thin gauge. It reduces power via the extra long runs, especially to the elevators as they are always under a load. There are also a lot of servos drawing power as well. Add in a much bigger twin jet with larger surfaces including FFS. It all adds up. If you are running the stock BEC voltage, i.e. 5.1V, it doesn't take much for the system, while drawing extra power, to drop down in voltage as a whole. So, if the flaps are down, the retracts are operating, and the elevators are now working with more than their usual load (flaps down), you could go below 4v. This could create a situation where the servos don't have enough power to hold their position. The elevators are especially susceptible as they are always working against a load (keeping the nose up). You will notice this first, even if the rudder and ailerons are also being affected by the low voltage.

[T-CAT]

nuts-n-volts brings up a valid point. Unfortunately, a system is often only as good as its weakest link.

Although, I’ve had good luck with the stock Freewing external BEC on all the PNP jets that include it, I like to test the voltage for any drop that could cause issues, especially when I have upgraded components that will increase the load. Although the Freewing F-14 is a different jet, the external BEC couldn’t handle the load during the wingsweep activation and the load dropped below 4.5V (which is where I have my telemetry set to trigger an alarm). This dropped occurred without moving any other surface, only the wingsweep. This was rare occasion considering I hadn’t had any issues with other Freewing external BEC’s on other jets, but it reiterated why it’s important for me to check the system, especially when I upgrade anything that could alter operation.

[GliderGuy]

Radfordc....The below is NOT intended to rub salt in your wounds BUT as a PRECAUTION to those still flying. I feel your loss and frustration. We’ve all been there.

=========

Agreed...It has been shown by actual measurements and posted in earlier posts that the stock elevator wire may create an IxR voltage drop sufficient enough so the elevator servos get less than 4 volts in a worst case condition of loading the full flying stabs.

It isn’t that the plane pitches down...it is that the servos are being forced to work at low voltage...and they can’t provide the force necessary to hold the nose up. The servos have been shown to freeze/stall. So the end result is loss of control as the nose drops....the pilot “feels” no response to back stick inputs. It “seems” to be an “interesting” aerodynamic aspect of the MiG, but the aerodynamic loads on the servos exacerbate an ELECTRICAL issue with the small stock servo wiring etc. as has been shown with actual measurements in early posts.

The early posts/measurements show that it is advisable to 1) replace the long wires to the elevator servos with a larger diameter wire....20 gauge at least and 2) find a servo with minimum operating voltage specs of less than 4 volts ( there are some higher torque servos with less than 4 volt min op specs) to have the smallest chance of a loss of control event. But at least use bigger wire to the FFS servos.

Many mods have been put forth in this forum that address the increased aerodynamic loads imposed on the full flying stabs and structure while flying fast with flaps down. The more mods implemented, the less likely a loss of control event will occur in a worst case electrical and aerodynamic load condition. Stated as a precaution....not implementing mods increases the likelihood of a loss of control event in a worst case condition. Some mods are more critical than others....especially when the worst case condition of flying fast with flaps down and multiple servos working at the same time is encountered on a flight. T-CAT’s weakest link mentioned above.

The MiG is an awesome EDF, but she must be modified.

-GG

[4qr1hu9n7]

They add up.

I mentioned the "low voltage" issue weeks ago. Unfortunately it fell on deaf ears. I sent an email to "Motion" regarding this issue. It too fell on deaf ears. Oh well. They know what's best for the customer.

[Evan D]

Really? From the first deliveries we talked about upgrading servo extensions, using shorter ones, bypassing the MCBe and other things...

[Griffon Wings]

And she is no more... One moment without proper thought and disaster. I flew a normal flight and went to land. On downwind I flipped the gear switch. Without thinking, I then flipped the flap switch. The plane immediately pitched down out of control with no effect from the elevators. My normal procedure was to lower the gear and wait until the plane had slowed to approach speed before lowering the flaps. This time I just didn't do the right thing.

This is the second major crash due to the planes "interesting" aerodynamics. I'm done.

Oh brother i’m so sorry , my smashed so hard i have very few parts left over. I saved some electronics and lights .
Sorry for the lost of your great bird !

[radfordc]

If the stabs are being bent down severely I don't think any servo will save the plane.

[radfordc]

Radfordc....The below is NOT intended to rub salt in your wounds BUT as a PRECAUTION to those still flying. I feel your loss and frustration. We’ve all been there.

=========

Agreed...It has been shown by actual measurements and posted in earlier posts that the stock elevator wire may create an IxR voltage drop sufficient enough so the elevator servos get less than 4 volts in a worst case condition of loading the full flying stabs.

It isn’t that the plane pitches down...it is that the servos are being forced to work at low voltage...and they can’t provide the force necessary to hold the nose up. The servos have been shown to freeze/stall. So the end result is loss of control as the nose drops....the pilot “feels” no response to back stick inputs. It “seems” to be an “interesting” aerodynamic aspect of the MiG, but the aerodynamic loads on the servos exacerbate an ELECTRICAL issue with the small stock servo wiring etc. as has been shown with actual measurements in early posts.

The early posts/measurements show that it is advisable to 1) replace the long wires to the elevator servos with a larger diameter wire....20 gauge at least and 2) find a servo with minimum operating voltage specs of less than 4 volts ( there are some higher torque servos with less than 4 volt min op specs) to have the smallest chance of a loss of control event. But at least use bigger wire to the FFS servos.

Many mods have been put forth in this forum that address the increased aerodynamic loads imposed on the full flying stabs and structure while flying fast with flaps down. The more mods implemented, the less likely a loss of control event will occur in a worst case electrical and aerodynamic load condition. Stated as a precaution....not implementing mods increases the likelihood of a loss of control event in a worst case condition. Some mods are more critical than others....especially when the worst case condition of flying fast with flaps down and multiple servos working at the same time is encountered on a flight. T-CAT’s weakest link mentioned above.

The MiG is an awesome EDF, but she must be modified.

-GG

Perhaps you are right. Maybe the photo/video evidence of the stabs being highly deformed when the flaps are deployed at high speed isn't the real problem, or if it is maybe it can be overcome with brute force by the servos. Easy enough to test....I assume you have made all the necessary "mods" to your plane. Take her up high, go fast, and drop the flaps to 45 degrees and see what happens. I think I remember one guy reporting that he did this and ended up crashing, so if you try it be cautious.

I'm now convinced that the one thing I should have done to keep the plane safe is to just not use flaps at all (or very small deflections if desired for takeoff). The plane really doesn't need them.

[radfordc]

Sorry to disagree, but this sounds much more like an electrical issue as I've been saying all along. If he was running everything thru the BB and it is limited to 5.1V, he probably overloaded the system with both gear and flaps running... and elevator! Lose voltage with all these operating simultaneously and you will lose elevator authority. I'd like to know if his system was stock or what.

And, I disagree with you. The elevator servos were plugged into the Rx and a redundant 4.8v NiCd battery was also plugged into the Rx in parallel with the BEC. I doubt loss of voltage was the problem. Lots of "ifs and buts" being thrown out about why this Mig dives into the ground under certain circumstances. The only physical evidence I've seen are the photo/videos of the horizontal stabs being deformed down under aerodynamic loads.

[GliderGuy]

Perhaps you are right. Maybe the photo/video evidence of the stabs being highly deformed when the flaps are deployed at high speed isn't the real problem, or if it is maybe it can be overcome with brute force by the servos. Easy enough to test....I assume you have made all the necessary "mods" to your plane. Take her up high, go fast, and drop the flaps to 45 degrees and see what happens. I think I remember one guy reporting that he did this and ended up crashing, so if you try it be cautious.

I'm now convinced that the one thing I should have done to keep the plane safe is to just not use flaps at all (or very small deflections if desired for takeoff). The plane really doesn't need them.

Actually the deformation of the stabs DOES indicate the servos are under heavy load...especially since the center of pressure is aft of the pivot point.

Source voltage being good and important does not mean good voltage at the servos as has been demonstrated.

Heavy load = more current = greater IxR voltage drop = less voltage at the servos = less able to move = need for LARGER wires and high torque servos....even better, ones with lower min op voltage specs. It is an additive aerodynamics AND electrical issue.

I totally trust the observations and many/most of the suggested mods and video evidence to date. I will pass on the invitation of performing further experiments.

I follow your “usual” process of slowing, dropping gear, then drop flaps. So far....all good and tons of flights completed. In this hobby, things work until they don’t. So I am under no delusions of being bullet proof.

The need for larger wire is a precaution worth stressing again.
-GG

Edit: Source for 20 AWG wire

https://www.hacker-motor-shop.com/Servos-and-servo-accessories/Servo-cables/DITEX-Servowire-Extension-0-50qmm-80cm.htm?shop=hacker_e&SessionId=&a=article&ProdNr=50025080&p=8312

[Airguardian]

Stock everything except the elevator servos. I used Hitec HS-425BB servos there.

We have seen a few crashes already with upgraded servos and linkages.
Although those 425BB don't seem much of an upgrade over the stock ones anyway but still, I think this should start to convince some of the skeptical that kept insisting that this is a servo-only issue.

The stock wiring on this jet is very thin gauge. It reduces power via the extra long runs, especially to the elevators as they are always under a load. There are also a lot of servos drawing power as well. Add in a much bigger twin jet with larger surfaces including FFS. It all adds up. If you are running the stock BEC voltage, i.e. 5.1V, it doesn't take much for the system, while drawing extra power, to drop down in voltage as a whole. So, if the flaps are down, the retracts are operating, and the elevators are now working with more than their usual load (flaps down), you could go below 4v. This could create a situation where the servos don't have enough power to hold their position. The elevators are especially susceptible as they are always working against a load (keeping the nose up). You will notice this first, even if the rudder and ailerons are also being affected by the low voltage.

Also, the less nose-heavier the better. And the reflex... and shortening wires / increasing gauge. ( :

Easy enough to test....I assume you have made all the necessary "mods" to your plane. Take her up high, go fast, and drop the flaps to 45 degrees and see what happens.

I may do that in due time. Not on a rush though.

I'm now convinced that the one thing I should have done to keep the plane safe is to just not use flaps at all (or very small deflections if desired for takeoff). The plane really doesn't need them.

FWIW, I use flaps quite a lot (alas, not at great speed) and haven't had any issue with mine (then again I made quite a few mods).
I find they are really useful to land the plane at a slower pace, otherwise I overshoot easily. Having flaps fully deployed helps me keep speed in check during approach when I dive to clear the treeline and this is massive. Since I'm not yet using vectored thrust, doing the same by abusing high alpha and knowing the thrustline issues and locked high-alpha conditions of this bird would be terribly dangerous. It all depends on your field conditions and space to operate I guess.

If the stabs are being bent down severely I don't think any servo will save the plane.

+1
At the very best, you'd recover at a slow-rate which could be not enough to pull out. So in addition to the electrical problems, structural still applies too.
Then again, glassing the stabs is pretty easy as I did on mine (Glassing their assemblies not so much though!).
Here are some pics I made that hadn't shared yet I think... I also used twisted servo wire for the stabs with slightly larger gauge:

[GliderGuy]

FYI - One source for the twisted and heavier gauge extensions....from prior posts. They offer a choice of lengths. This equates to 20 AWG wire.


https://www.hacker-motor-shop.com/Se...0025080&p=8312

Drop in, higher torque servos WITH lower min operating voltage spec is the MKS HV69.

Unfortunately, no reverse option is available. Thus, I resorted to not re-wire one servo but instead am using a servo reverser....so far, so good.
Futaba SR-10 reverser. Very stable with temperature changes, too.

The key for me was having the lower min op voltage spec along with the higher torque spec....even at the lower voltage. This overrode my concern about adding another potential failure point....the reverser.

-GG

[kallend]

I mentioned the "low voltage" issue weeks ago. Unfortunately it fell on deaf ears. I sent an email to "Motion" regarding this issue. It too fell on deaf ears. Oh well. They know what's best for the customer.

It was "mentioned" long before that, and even quantified by making measurements.

[xviper]

Take her up high, go fast, and drop the flaps to 45 degrees and see what happens. I think I remember one guy reporting that he did this and ended up crashing, so if you try it be cautious.

I did just this last November when all these crashes started to get more numerous. I did 2 videos - one with a camera on the wing pylon pointing at the elevator and one from my hat cam (which wasn't really pointed very well but it gave enough information that I posted it anyway). I did several passes at max throttle on the return leg of my circuit (left to right). At max throttle and airspeed reached max, I dropped the flaps to 1/2 on one pass and then pulled on the stick to climb. Next pass, max throttle again and then full flaps and climbed again. You can see on the video the flap deployment speed and that not very much elevator was needed to actually climb quite steeply. You can also see the elevator arm didn't really bend or warp in any way. This was with the upgraded elevator servos and extension as provided by Motion. BB by-passed for elevator, chokes not used at this time (installed later). I now have flap speed set at 2.0 seconds. I can't remember what it was set at for this video but you can see on the video how it was. I drew no conclusions from these videos but neither did anyone else. I just did the test and posted them just for kicks.

Here's the pylon cam:

Here's the hat cam:

[GliderGuy]

Thanks for the videos....the pylon cam does (once again) confirm that an aerodynamic down-load is placed upon the FFSs upon flap deployment at fast speeds. You can see the tip of the FFS move down upon flap deployment at the faster speed. Since the center of pressure is aft of the FFS's pivot point, this means an increased load on the servos is happening.

Increased load = higher servo current = increased IxR voltage drop in the servo wiring = less voltage available at the servos = need for larger diameter wire to the FFS servos AND upgrading to higher torque servos for good measure (servos with lower minimum operating voltage specs...even better).

I don't see any appreciable deformation of the FFS during flight w/o flaps being deflected. Further evidence (for me) in support of why I decided to not glass the FFSs. But, this is my choice and not a suggestion for anyone else nor a negative comment about those who have.

Consideration for thought...even with the FSSs being glassed, the deformation goes away, but the increased aerodynamic loads will still be present with flaps deployed at faster speeds. AND...the amount of deformation will decrease as the speed decreases.

-GG

[The43rdHammer]

Airguardian could you not just put CF rods along the stabs (underneath). Although you’ve made a really neat job of it, I don’t want to glass mine (yes I’m vain, I admit it it 😂).

BTW nothing (reasonably priced) beats a Savox servo 👍👍👍

[Airguardian]

At the time of the flap tests you are not flying fast, so it's not conclusive. One can tell from the sound.

[Airguardian]

Consideration for thought...even with the FSSs being glassed, the deformation goes away, but the increased aerodynamic loads will still be present with flaps deployed at faster speeds.

Yes, I'm not suggesting to only glass the stabs and forget everything else. But I think it's kinda necessary for best performance and peace of mind, in addition to addressing the electrical power-feed and distribution issues, installing tougher servos, etc.

Airguardian could you not just put CF rods along the stabs (underneath). Although you’ve made a really neat job of it, I don’t want to glass mine (yes I’m vain, I admit it it 😂).

It will help but it's not quite the same.
From a structural design point of view, making the skin work is more efficient and probably lighter if done carefully.
Having the skin not only adresses bending moment but also torsional loads, which are a big thing here too since the stab has a lot of aft sweep.

BTW nothing (reasonably priced) beats a Savox servo 👍👍👍

I use those on helis but they get hot!

[xviper]

At the time of the flap tests you are not flying fast, so it's not conclusive. One can tell from the sound.

Well, I guess you can't tell from the sound. I was at max throttle before I finished the turn. Besides, I never made any conclusions.