Official Freewing Twin 70mm SR-71 Blackbird with Gyro EDF Jet Thread

[Evan D]

People have reported that they have tried and failed.

[GliderGuy]

As my dad used to say, “If that’s not a strong repair, then I am a monkey’s uncle.” I went for strength and not beauty - LOL.

I lost the “C” clip, so ended up using a bolt/nut and added a dollop of Foam Tac on the nut to secure it.

If you don’t have a 3DP, and you decide to strengthen the anti-torsion arm on the nose gear before it breaks, carbon sheeting and Foam Tac seems to do the trick. Time will tell.

Carbon was also added to the strut side of the anti-torsion arm. 2 days of curing and the glue is solid.

Regardless of your approach, strengthen the anti-torsion arm BEFORE it breaks.

I also generously coated the bottom portion of the arm in JB Weld epoxy which you see if you zoom in. You could use carbon and FT glue on that part of the arm as well. Epoxy wouldn’t hold the broken top part of the arm.

The MRC 70mm Panther jet main that Evan suggested arrives tomorrow. It has an aluminum anti-torsion arm. I’ll report back as to how it fits into the SR-71 nose gear assembly.

-GG

[Evan D]

From an old show... Verrry interesting.

[Kitchnpassmastr]

Ref the post about avoiding paint peeling when masking your model. I’ve had great luck with applying two coats of water-based polyurethane with a brush first before doing anything with the model like decals or painting. Not only do the decals go on better and blend well, but low tack tape is not prone to peel off the paint.

[GliderGuy]

The repair lasted 4 flights and all were good main-first landings. Back to the drawing board to repair the anti-torsion arm.

25 total flights….getting the hang of it and loving it!

-GG

Update: The outer end of the part that the bolt goes through is metal. Hmmm…may be an all metal part after all, but I didn’t investigate by scraping paint off.
This time, I wrapped the whole thing in steel aircraft safety wire…minus the hinge areas, of course.

More landings will test it. The 70 mm Panther Jet main with the aluminum anti-torsion arm did not arrive today. So, that experiment is delayed another day.

[GliderGuy]

Hi Aros and all…

Ahhh the discoveries to be made when shaking down newly released models.

Disregard anything I have posted before about the anti-torsion arm being plastic. They are most definitely BRITTLE metal.

After 30 total flights (4 more made at sunrise today), one of them broke on a main gear. She stops pretty short when a main rotates 90 degrees….and it was a smooth landing!?!?!?

I did come up with another solution for a “keep it flying” fix, but a well designed 3DP plastic piece that is less brittle is needed.

Evan’s suggestion to try the ones off the FW 70 mm Panther experiment is delayed in shipping. Hopefully, that anti-torsion arm design will fit and is stronger.

Some of y’all with 3DP expertise might take a stab at a totally new and stronger and less brittle design for this part.

Now…off to fix arm #2 so I can fly it tomorrow.

The asphalt surface I fly off is not the silky smooth, fine aggregate type. I would classify the surface as small pebbles. No doubt the small pebble bumps are stressing the arms, but a ton of flights with other models are flown without gear issues being caused.

Until a more robust fix is available, operating the SR-71 off rough surfaces (grass if possible) will likely cause these arms to snap within a few flights.

-GG

[GliderGuy]

Sure….but no need for any lightening voids. The SR-71 obviously puts a lot of stress on this arm, so the stronger the better. It’s on bottom, so scale for me is a don’t care. Not flying a $700 bird for lack of a $5.00 part is a downer.

Of course, they’ll need to be designed with the triple tang design of the SR’s design.

Thanks! This is good feedback.

-GG

[3D-Seth]

Thanks for sharing the issue GG. Did you see any cavities or anything in your parts when they broke? Its not something we've seen yet, so we will be keeping a close eye on these.

[GliderGuy]

Thanks for sharing the issue GG. Did you see any cavities or anything in your parts when they broke? It’s not something we've seen yet, so we will be keeping a close eye on these.

No cavities or voids in the faces of the broken metal, but the broken metal faces are grainy. Appears like what we used to call pot metal.

If a change is made, I’d be happy to put them to the test.

It is highly suggested to reinforce them (your engineering as to how) BEFORE they break. Why? It’s a challenge to attempt to fix them once they break. I’m still not sure I have successfully accomplished this.

Thanks.

-GG

[Shaun Evans]

Hi

Has anyone seen the inside of the aft fuselage? Strange question, know, lol. I need to build some wood structure in the aft center stinger area, but I have no idea where it's hollow and where it's solid foam. This is for a conversion.

[DCORSAIR]

No cavities or voids in the faces of the broken metal, but the broken metal faces are grainy. Appears like what we used to call pot metal.

If a change is made, I’d be happy to put them to the test.

It is highly suggested to reinforce them (your engineering as to how) BEFORE they break. Why? It’s a challenge to attempt to fix them once they break. I’m still not sure I have successfully accomplished this.

Thanks.

-GG

I think maybe slamming the nose down like it does on landing could be the reason for it breaking that part, but everybody flies and lands different so some may last longer than others, if all of a sudden we start seeing this part fail then we know its a bad design. I put 18 flights on mine so far and I worked on my landings this morning, having a long runway helps because I figured out how to hold the flare while adding in power at the right moment will make for a nice landing and not slam the nose down.

[GliderGuy]

I think maybe slamming the nose down like it does on landing could be the reason for it breaking that part, but everybody flies and lands different so some may last longer than others, if all of a sudden we start seeing this part fail then we know its a bad design. I put 18 flights on mine so far and I worked on my landings this morning, having a long runway helps because I figured out how to hold the flare while adding in power at the right moment will make for a nice landing and not slam the nose down.

The real proof will be how the anti-torsion piece “might” stand up to a large number of landings like my AL-37, B-2, and PJ-50 do.

These all have a huge number of landings, and don’t require gear parts (other than tires) to be replaced. Case in point…count their total landings in the thousands.

I’d expect the SR-71 to be able to reach large landing counts (without breaking), too. I was a surprised to see early landing count failure. My expectations have been set by my excellent FW experience related to quality.

The piece’s failure is probably related to the small pebble asphalt surface trying to torque the nose and main gear. And…after the first 3 landings of the SR-71..,,most have been good. As you have done…got her landings dialed in.

Failure count = 1 nose and 1 main. Same part failed.

-GG

[DCORSAIR]

The real proof will be how the anti-torsion piece “might” stand up to a large number of landings like my AL-37, B-2, and PJ-50 do.

These all have a huge number of landings, and don’t require gear parts (other than tires) to be replaced. Case in point…count their total landings in the thousands.

I’d expect the SR-71 to be able to reach large landing counts (without breaking), too. I was a surprised to see early landing count failure. My expectations have been set by my excellent FW experience related to quality.

The piece’s failure is probably related to the small pebble asphalt surface trying to torque the nose and main gear. And…after the first 3 landings of the SR-71..,,most have been good. As you have done…got her landings dialed in.

Failure count = 1 nose and 1 main. Same part failed.

-GG

We will know soon enough when more guys get more and more flights on this bird, it is a hoot to fly and a very nice looking bird but flying at sunrise this morning did have me nervous at times because at certain angles it got very hard to see.
I just went out and checked my gear for any sign of trouble but all looks good, it sure looks like a plastic piece but your saying its actually metal??

[GliderGuy]

Yes…definitely metal. I agree…definitely a hoot to fly.

-GG

[GliderGuy]

Unfortunately not…no photo possible. I’ve already encased the parts in my 2nd attempted repair so I could fly more. So far, 4 flights on repair type #2 have passed muster. Hope the repairs hold until the spares become orderable.

Also, as noted in prior posts, The FW 70 mm Panther Jet mains are enroute. Evan suggested I try the anti-torsion arms from them. A Thursday last week delivery is pushed into Monday.

The metal fractured without any signs of a bend. The two broken pieces fit back together cleanly without any distortion. Embrittlement sign = yes.

Two have snapped the same way…no bending and all broken faces were grainy.

-GG

[DCORSAIR]

I can never apply decals properly it seems, I got most of the top side ones on with no issues but messed up the stars and bars, I was doing good at first but don't ask, I hate applying decals so I could use a set if anyone has the stock decal sheet you didn't use.???

[EDF-Jetpilot]

Any chance of a photo of the fracture surface. Preferably magnified. "Grainy" appearance is a classic sign of embrittlement.

https://gearsolutions.com/department...re-mechanisms/

A rough surface on metal parts usually occurs for one reason only: it is a cast part, i.e. cast and not milled. Cast parts are much more susceptible to breakage than milled parts.

[GliderGuy]

A rough surface on metal parts usually occurs for one reason only: it is a cast part, i.e. cast and not milled. Cast parts are much more susceptible to breakage than milled parts.

Which supports my prior “pot metal” comment. Broken cast iron pot surfaces look just like the broken surfaces of the anti-torsion arm. A 3DP version of this arm in a strong plastic and not brittle material may be a better solution than the cast metal part.

One thing I neglected to mention. When these broke, it was about 40F. These parts may be less suceptible to being brittle when the air is warmer. If you have only flown your bird in 60F temps, that may be why yours haven’t snapped.

-GG

PS Flying cold-soaked LiPos below 50F has been stated to be harmful to the battery. Mindful of this precaution, I was keeping my batteries in a very warm truck.

[GliderGuy]

Update on the paint match….
The foam block I inverted her on compressed the area just ahead of the parachute doors. Note to self….use more foam blocks and support in more areas.

This area required filling and sanding to restore. The Tamiya satin black proved to be much too shiny. A better match is the following:

RUST-OLEUM FLAT BLACK
2X ULTRA COVER PAINT+ PRIMER

Once well dry, buff the newly painted area with a dry Mr. Clean Magic Eraser sponge. This sponge has a very fine abrasive which is perfect for getting the newly painted flat black area to a matching look.

It takes several thin coats to “hide” the filler texture.


-GG

[EDF-Jetpilot]

Which supports my prior “pot metal” comment. Broken cast iron pot surfaces look just like the broken surfaces of the anti-torsion arm. A 3DP version of this arm in a strong plastic and not brittle material may be a better solution than the cast metal part.

One thing I neglected to mention. When these broke, it was about 40F. These parts may be less suceptible to being brittle when the air is warmer. If you have only flown your bird in 60F temps, that may be why yours haven’t snapped.

-GG

PS Flying cold-soaked LiPos below 50F has been stated to be harmful to the battery. Mindful of this precaution, I was keeping my batteries in a very warm truck.

Exactly... I think the SR-71 is really made for smooth runways, not grass and gravel. Your gravel runway probably puts too much strain on the pivot points on the wheels, which jolts and tears the undercarriage too much over time. I don't think it's a material defect, because no one else has reported this problem. Things like this would pile up quickly if it were a real problem, like the elevator on the first MIG-29 delivered back then. However, if you can find a suitable milled part made of solid aluminum, it will hold, a printed plastic part is far too weak in my opinion, unless there is at least a printed CFK. Although realistically I think that the problem cannot be solved on your runway.