Official Freewing MiG-29 Fulcrum Twin 80mm Thread

[fredmdbud]

Anyone ever heard of VFE? I see on MRC website they have some info on VNE which everyone on here should read.
If you exceed VFE expect parts of the plane to start coming off and a crash.
We don't know the VFE or have an airspeed imdicator. Doing it on accident is one thing, but doimg it on purpose you've become a test pilot and the aircraft doen't have an emergency parachute.
If it were a full scale plane this would be pilot error for exceeding VFE.
The video makes me feel better about the plane. Now I know when it happened to me, i screwed up and most likely didn't put the flaps up after takeoff. If it happens again the first thing I'm gonna do is check the flap switch.
Sorry about your plane, but I think we all owe you a few bucks to get a new one! This video is like the Patterson video. Definitive proof!

"VFE, the maximum velocity at which the airplane can be flown with its flaps fully extended, is the high-speed limit of the white arc. Flying at speeds greater than VFE with full flaps can result in damage, perhaps to the point of losing one or both flaps." - AOPA

Losing a flap is somewhat a little more recoverable than lack of elevator response.

There also is V_A - Design maneuvering speed. This is the speed above which it is unwise to make full application of any single flight control (or "pull to the stops") as it may generate a force greater than the aircraft's structural limitations.

[Airguardian]

Indeed, a good demonstration of the forces at play - a 17g servo never really stood a chance did it...

It's not about the servos.
The torsion of the assembly sending the stabs into anhedral changes the geometry of the linkage and prevents servos from actually producing any sort of pitch up regardless of torque. It is structural.

You can test this in your garage, go full pitch up with your jet and force anhedral on the stab assembly with your hands, you'll see the stabs automatically reduce pitch deflection from the geometry that ensues.

Why not cutting off 2cms of the trailing edge of the elevators, or even its tip

That is a horrendous idea IMO.
Not only you will totally mess scale looks but doubt you will reduce the problem.

Look, for the aircraft to balance with flaps down, the tail needs to make a certain down-force regardless of its shape.
If you make stabs shorter, you will need more deflection to achieve said force, but ultimately, you will end up applying the same force if you want your jet to balance.
And the most important factor that everyone seems to miss... this is no longer about servo torque, but how the linkage geometry gets distorted by the structural torsion of the stab assembly, making it impossible for the servo to pitch the stabs up, regardless of torque.

If it were a full scale plane this would be pilot error for exceeding VFE.

but I think we all owe you a few bucks to get a new one!

'VFE' for this jet is extremely low, obviously. That's a design/manufacturing concern, not pilot error.

I think Freewing/Motion owes him those bucks, instead. ;)

[fishface5]

VFE usually denotes when damage can occur to the flap surfaces.

Again, flaps in this example are being used to demonstrate a structural weakness in the airframe. There's nothing to say this can't happen in a clean configuration - particularly if there are variances in manufacturing or assembly.

My crash was with gear up, flaps up

[fishface5]

I hope Motion will help all of us out that have crashed there Migs in some way. I lost the jet and both batts I hate swallowing 800.00 for a bad design

[leithalweapon]

It's not about the servos.
The torsion of the assembly sending the stabs into anhedral changes the geometry of the linkage and prevents servos from actually producing any sort of pitch up regardless of torque. It is structural.

You can test this in your garage, go full pitch up with your jet and force anhedral on the stab assembly with your hands, you'll see the stabs automatically reduce pitch deflection from the geometry that ensues.



That is a horrendous idea IMO.
Not only you will totally mess scale looks but doubt you will reduce the problem.

Look, for the aircraft to balance with flaps down, the tail needs to make a certain down-force regardless of its shape.
If you make stabs shorter, you will need more deflection to achieve said force, but ultimately, you will end up applying the same force if you want your jet to balance.
And the most important factor that everyone seems to miss... this is no longer about servo torque, but how the linkage geometry gets distorted by the structural torsion of the stab assembly, making it impossible for the servo to pitch the stabs up, regardless of torque.



'VFE' for this jet is extremely low, obviously. That's a design/manufacturing concern, not pilot error.

I think Freewing/Motion owes him those bucks, instead. ;)

I started to reply, but realized you have no idea what your talking about and will never learn.

[mshagg]

It's not about the servos.

As far as root cause is concerned, that's correct for sure.

However, the fact that the entire fuselage extension is twisting and bending gives us an indication of the load on the stabs.

Even if no twisting was present (e.g. I posted a video which showed high loads on the stab, but no significant anhedral developed) it is a clear demonstration that any micro servo is going to struggle up against these forces.

Even if you reinforce the tail section of the fuselage, I would watch Fred's video and ask yourself - is a tiny micro class servo (even if it is a whizz bang uber expensive coreless pro-modeller one) - really up to the task of overcoming those forces? I mean the whole rear end of the jet is disappearing out of frame like it's made of rubber...

[fredmdbud]

Anhedral, anyone?

[GliderGuy]

POSSIBLE REVELATION.....

Take a REAL close look at the video. Watch the area along the bottom of the vertical stab attachment. There is no downward movement...no shifting downward...no CW or CCW twisting is seen.

What appears to be, upon first look, the rear of the plane (FFS support area) bending and disappearing out of site is actually the FSS bending down out of sight.

Nothing wrong with making the rear area stronger, but from what I am seeing...air loads are bending the the FFSs down, and the support area is stable more or less.

This downward bending of the FFSs (more evident at the tips), combined with the knowledge that the CP is so located rearward of the pivot point, would be an indication of the loading being imposed on the servos.

I submit for your study and comment that the geometry of the support area is minimally changing. If it were changing, there would be deformations/changes of the top area at the bottom of the vertical stab attachment line. There's none to be seen.


Looks like a case of perception not being reality. Initially, I was fooled, too.

-GG

[radfordc]

Also telling is the amount of movement of the vertical fins due to turbulence. Given the amount of bending of the horizontal tail I'm not sure that any servo could deal with it. Full scale aircraft can be turned upside down from vortexes created by other aircraft.

[RC30Flyer]

Still assembling mine, and after watching video, I applied down force to the stab pivot rod and there is torsional flex following the red line I drew on my photo. I'm thinking that a nice piece of CF plate epoxied to the side should solve this issue. I remember back in the day with the Phase3 F-16 that this mod was a "must do". Was the guys flaps deployed in that shallow dive or was that just air pressure twisting the stabs out of sight?

[GliderGuy]

RC30Flyer....watch the video closely...he flashes "FLAPS DOWN" very briefly just before starting the dive. For the rest of the video prior to this, there is very little downward "twisting" of the stabs. Take note...don't fly yours with flaps down very fast if you wish to avoid the crash.
Once you attach the vertical stabs, the movement you see drastically decreases.

Strengthening this area won't hurt...but that's very likely not what is causing the problem.

-GG

[fredmdbud]

For your consideration - 2 frames from my video overlaid: reference shot on ground during flight surface check, and the other during flaps-down dive.

While the right horizonal tail is being pressed down, the right vertical tail is being pressed inward:

[GliderGuy]

I agree...This shows only a small movement....not matching the large movement of the FFS captured at that time in the video. And there is turbulence to consider, too (flap generated?). Let's see what the others see after they re-study the video.

I'm not saying the rear doesn't show some deformation...the amount doesn't match the amount of movement of the FSS.

The perception is that the rear of the fuselage is disappearing out of site. It's not. Pretty much all the movement is in the FSS flexing downward under air loads...seemingly caused by the flaps.

-GG

[RC30Flyer]

RC30Flyer....watch the video closely...he flashes "FLAPS DOWN" very briefly just before starting the dive. For the rest of the video prior to this, there is very little downward "twisting" of the stabs. Take note...don't fly yours with flaps down very fast if you wish to avoid the crash.
Once you attach the vertical stabs, the movement you see drastically decreases.

Strengthening this area won't hurt...but that's very likely not what is causing the problem.

-GG

Holy crap you're right... never saw that!!! Only saw that flaps were retuned to up... was this done for testing purposes? If so, get this man a new jet, and a job testing jets for motionrc so we can get a "finalized" "tested" jet. I'm so freaked out about maidening mine once I'm finished assembling, I have considered selling it and going back to the F-22.

[Airguardian]

Anhedral, anyone?

There's a difference between built in anhedral, and anhedral caused due to structural failure, messing up the linkage geometry.

As far as root cause is concerned, that's correct for sure.

However, the fact that the entire fuselage extension is twisting and bending gives us an indication of the load on the stabs.

Even if no twisting was present (e.g. I posted a video which showed high loads on the stab, but no significant anhedral developed) it is a clear demonstration that any micro servo is going to struggle up against these forces.

Even if you reinforce the tail section of the fuselage, I would watch Fred's video and ask yourself - is a tiny micro class servo (even if it is a whizz bang uber expensive coreless pro-modeller one) - really up to the task of overcoming those forces? I mean the whole rear end of the jet is disappearing out of frame like it's made of rubber...

Totally agree there with you. My point is that upgrading servos without reinforcing the stab assembly will do nothing to prevent the crashes.
Once the assembly is beefed up, the next weak link are the servos, sure, but let's not lose the focus as to the more immediate and proven point of failure. ;)

I started to reply, but realized you have no idea what your talking about and will never learn.

Maybe it is you that will never learn.

Take a REAL close look at the video. Watch the area along the bottom of the vertical stab attachment. There is no downward movement...no shifting downward...no CW or CCW twisting is seen.

That argument is pretty flawed. It takes very minimal deformation from that section (that wouldn't show up on such a camera recording) that will produce the huge anhedral seen by the stabs. Again, you can test this on your own MiG. Take vertical stabs away, apply force to the shaft and see how the piece deforms. It is pretty clear once you do this exercise.

I'm not saying the rear doesn't show some deformation...the amount doesn't match the amount of movement of the FSS.

It does not have to. It's on the center of the torsional axis.

[fredmdbud]

Holy crap you're right... never saw that!!! Only saw that flaps were retuned to up... was this done for testing purposes? If so, get this man a new jet, and a job testing jets for motionrc so we can get a "finalized" "tested" jet. I'm so freaked out about maidening mine once I'm finished assembling, I have considered selling it and going back to the F-22.

Naw, you can just do what I do - have flaps mixed with your gear channel. No gear, no flaps, no thinking.
Overall it's a kittycat until you back it into a high-speed flap/divebomber corner lol

[fredmdbud]

Wow - they censored "fussycat" (replace with f with a p) lol

[GliderGuy]

Hi Airguardian...But what if the FSS is flexing under load and there is "the" very little deformation that you reference? Ref the prior post photo showing overlays with/without FFS deflection before/after the dive w/flaps. Please explain why a small deformation causes problems. I'm not seeing it / getting it from your diagrams. Appreciate your patience.

I do plan to stiffen the area by laying in some carbon fiber sheeting. .

-GG

[fredmdbud]

Here is a video of the Eflite Su-30. Notice how the vertical tails deflect with full up elevator/FFS - looks predominately as a result of wing flex (correlation of events); the Su-30's tail axles are at the end of the tail booms.

[janmb]

I started to reply, but realized you have no idea what your talking about and will never learn.

Someone needs a mirror