Official Freewing MiG-29 Fulcrum Twin 80mm Thread

[kallend]

It would be good to get one of these in a wind tunnel and really find out what's going on.

I am not convinced that the flexing per se is causing the loss of control, but rather it may just be a symptom of the excessive loads developed causing a fatal problem elsewhere.

[GliderGuy]

From what others have posted previously, the analysis done on the FFS center of pressure vs location of the pivot indicates the observed (video and Borat photo) down force on the FFS which causes the downward deflection MUST translate to increasing loads on the servos.

The servos simply are unable to cope with the loads they are exposed to with flaps down in the videoed and photoed flight realm. Borat posted a good photo of the FFS deflection before he crashed. Two documented cases of evidence. No guessing...hard evidence of the flight loads on the FFS from two crashes.

Carefully watching the video, very little deformation is seen happening to the rear support structure...as noted in my prior post.

Airguardian has noted that deformation of the support structure impacts the geometry of the drive. If this is the case, added stiffing to the support structure will help mitigate. My carbon fiber sheets arrive tomorrow. I don’t see video (even the overlayed photo from the video posted) evidence of much support deformation....but the addition of CF won’t hurt.

Don’t fly fast with flaps down!

-GG

[AirHead]

One topic that has been on the forums is rigidity, or lack thereof, regarding EPO. I have a cousin in the plastics industry and when I showed him one of my planes, he was surprised how soft and flexible the foam is. He claims there are methods to make it harder. Apparently one is to leave it in the oven longer which bakes the outside. I told him I'm sure they would have tried it by now. Recently, I was at a sculpture supply shop and they had foam forms that were hard as rock, but lightweight. Different chemistry, but there is a big amount of flex in foam planes....

[GliderGuy]

Thanks AH....I hasten to focus the discussion back to the fact that the bending/flexing of the foam FFS is not the issue or the problem. The flexing is evidence of applied loading in the video flight realm.

It is the loading on the FFS as evidenced by the downward deflection which will be translated to increasing loads on the servos that is the issue.

Again...Evidence says avoid flying fast with flaps down.

-GG

[flyAA]

I just got one of these. I’m by no means an engineer so I will just do what you smart guys say. I fly big ones for work and just press the buttons they tell me to when they tell me to. Haven’t bent one yet lol

Here’s what I’m going to do:

1: use the freewing upgraded servos
2: use inner hole
3: not fly fast with the flaps down
-I’ve been flying full scale for a long time and this has always been something i do without even thinking so shouldn’t be an issue
4: possibly upgrade to 4-40 rods
5: yolo

thanks for all of the good discussion!

[Airguardian]

GG, you are wrong.

Regardless of the load on the servos, the deformation on itself will cause a crash.
You are missing the most important thing: when deformed, linkage geometry changes and full-up servo output DOES NOT produce an up-pitching stab.

Hi Airguardian... [...] Please explain why a small deformation causes problems. I'm not seeing it / getting it from your diagrams. Appreciate your patience

Picture it as a circle. The tip of the stab is on the outer circumference, the part deforming is in the center. There is no visible displacement of the deforming part vertically or horizontally.
You would see it though by removing the vertical stab and looking from the top or from the rear while manually applying force to the shaft.
As a certain angle of deformation multiplies the deflection (arc) over the span of the stab (S = r * x ...Arc =radius*distance to center), a small deformation will produce the anhedral seen in the videos and photos.

Here’s what I’m going to do:

Should reinforce the stab assembly too.

[bandetx]

The deformation could cause the elevators to lock into place so they cannot move. Larger servos wouldn't help.

[GliderGuy]

Thanks...Gotta think more on this to get my head wrapped around it.

-GG

[bandetx]

One topic that has been on the forums is rigidity, or lack thereof, regarding EPO. I have a cousin in the plastics industry and when I showed him one of my planes, he was surprised how soft and flexible the foam is. He claims there are methods to make it harder. Apparently one is to leave it in the oven longer which bakes the outside. I told him I'm sure they would have tried it by now. Recently, I was at a sculpture supply shop and they had foam forms that were hard as rock, but lightweight. Different chemistry, but there is a big amount of flex in foam planes....

Real planes flex. It's stronger than being completely rigid and brittle. https://www.travelandleisure.com/air...ings-bend-test

Though, to be fair, this is just on the wings not on the fuselage holding in the vertical stabs.

[kallend]

Real planes flex. It's stronger than being completely rigid and brittle. https://www.travelandleisure.com/air...ings-bend-test

Though, to be fair, this is just on the wings not on the fuselage holding in the vertical stabs.

Yes they do, and the flexing is thoroughly analyzed by the design engineers (these days using very accurate FEM methods) to ensure that it is not going to be a problem.

[kallend]

The deformation could cause the elevators to lock into place so they cannot move. Larger servos wouldn't help.

That is speculation.

What we know: 1. The recommended CG is inappropriate leading to large forces on the stab even in level flight. 2. The structural stiffness in the tail is inadequate to cope with those forces without producing large deformations which will affect the stab incidence. 3. The stab pivot axis is poorly located so item 1 above leads to large loads on the servos and linkages. 4. The servos, even the replacement ones, are inadequate given items 2 and 3 above. 5. There is a detrimental aerodynamic interaction between the flaps and the stab which aggravates all of the above. ALL of these issues should have been identified and resolved before the design was finalized and put on the market, and before the fanbois put out their "rah rah" videos. 6. Unrelated to the above, it is easy to enter an unrecoverable deep stall at high alpha.

[GliderGuy]

Hi Kallend...Regarding #2....

Study the video closely paying attention to the bottom of the vertical stab. There is very little deformation of the tail structure. Ref my prior post. The deformation is the FFS moving downward.

If the tail structure was deforming down by that much...the bottom of the vertical stab would show it. It’s a case of perception not being reality. The video initially fooled me too.

But I am going to add carbon fiber sheets, regardless.

-GG

[Airguardian]

I did more testing on the workbench and despite all that I previously said still applies I found out that the more pressing concern seems to be deformation of the stab itself.

Fiberglassing the stab with thin GF cloth and WBPU is comparatively easier to doing the proper thing with epoxy and thicker cloth with the tail assembly, so there's that.

[Doug Fales]

[QUOTE=Lcacing;n284648]

I’m flying my MiG at 160mm with RT 5500 45c (770gr) with ordinance. Batteries as far aft as they can go.
I did statically balanced the stabs, about 2oz in total. I think balancing them helps, so if you need to add weight, might as well use that weight for something good.

Sorry not up on the terms. Do you mean with the stab free of linkage you added weight to balance it as it sits in the pivot rod?

[Lcacing]

[QUOTE=Doug Fales;n285132]

I’m flying my MiG at 160mm with RT 5500 45c (770gr) with ordinance. Batteries as far aft as they can go.
I did statically balanced the stabs, about 2oz in total. I think balancing them helps, so if you need to add weight, might as well use that weight for something good.

Sorry not up on the terms. Do you mean with the stab free of linkage you added weight to balance it as it sits in the pivot rod?

Correct, with the linkage off and about 1oz of weight the stab will balance at the pivot point. I didn’t have any lead so I used 4 quarters and 1 penny

https://youtu.be/_yNWCfap6Ag

[janmb]

Picture it as a circle. The tip of the stab is on the outer circumference, the part deforming is in the center. There is no visible displacement of the deforming part vertically or horizontally.
You would see it though by removing the vertical stab and looking from the top or from the rear while manually applying force to the shaft.
As a certain angle of deformation multiplies the deflection (arc) over the span of the stab (S = r * x ...Arc =radius*distance to center), a small deformation will produce the anhedral seen in the videos and photos.

All of that could be explained as a single word: Arm.

[janmb]

ALL of these issues should have been identified and resolved before the design was finalized and put on the market, and before the fanbois put out their "rah rah" videos.

Love the rah rah part lol.

The most funny aspect of this is that the same fanbois keep being in total denied still after the problem(s) being so blatantly demonstrated.

I will agree that the mode of flight required to make this problem become catastrophic probably should be considered at least somewhat outside supported spec, but again, not by much. Even if diving fast with flaps is not something pilots would normally do or be recommended, it is still something a lot of pilots WILL do now and then - most of all because they forget to pull up their flaps from earlier maneuvers or takeoffs.

The design is, at best, marginal. I would rather say flawed.

At the very least we are starting to get a clearer picture of what needs reinforcement.

[HaroldAnderson]

I just got one of these. I’m by no means an engineer so I will just do what you smart guys say. I fly big ones for work and just press the buttons they tell me to when they tell me to. Haven’t bent one yet lol

Here’s what I’m going to do:

1: use the freewing upgraded servos
2: use inner hole
3: not fly fast with the flaps down
-I’ve been flying full scale for a long time and this has always been something i do without even thinking so shouldn’t be an issue
4: possibly upgrade to 4-40 rods
5: yolo

thanks for all of the good discussion!

I have 39 flights on mine and am using steps 1-3 and 5. :-) The only issue I have had was during testing of the flap down during a high speed pass scenario. It only happened once but is very spooky. Luckily I was testing and was up high enough and ready to pull throttle. It happened so fast I wasn't sure if it just wasn't my imagination.
I have a voice notification on my radio to tell me if my throttle is above mid power with flaps deployed.
So....
6. Some sort of alert or mix to disable flaps with high throttle setting or gear up is important with this model too. It is easy to forget flaps. I used to do it all the time until I configured my radio to 'remind me' that I was doing something stupid.

Make sure you know to about the falling leaf situation and how to recover. I have seen that a lot but I am stalling it a lot doing flips and 'almost' cobra maneuvers. Kill throttle and dive...

[Eric D]

It seems like all the troubles started because the batteries could not be back enough to balance at 160 mm plus. A YT video of a turbine powered Mig shows the stab at 0 incidence. The seam line is very close to 0. To achieve this would take the CG to approx. 170mm.

[janmb]

It seems like all the troubles started because the batteries could not be back enough to balance at 160 mm plus. A YT video of a turbine powered Mig shows the stab at 0 incidence. The seam line is very close to 0. To achieve this would take the CG to approx. 170mm.

Which might very well be perfectly fine :D

The mount of nose heaviness these models are designed around is no longer just silly and sad, but outright dangerous.