Hi All,

While attending the Button Willows Flying last weekend, I have witnessed some great flying done by many jet pilots with very old jet frames. Some are over 12 years old with hundreds of flights under their belts. My old beater JL F-15 was also 200+ flights strong and had just got a new pipe after the old pipe finally fallen apart. I asked myself why some airframes can last so long and some are lucky to make it back on the maiden. After thinking about all the issues, I had contacted some of our factories, and also guys at the event, including Dantley for some advices. We all agree that it might be beneficial to all of us if we can collaboratively develop proper protocols or guidelines for jet factories to follow in construction of reliable airframes.

We can start from the most critical components of many scale jets - the stabs. I have contacted FB and they are willing to collaborate and works with us on establishing a proper guideline for stab construction.

Even though my background is in Aerospace and mechanical engineering, I am a bit rusty in offering expert advise in the establishment of these guidelines. I would like to seek a group participating in jointly developing the requirements for the project.

Let's start with the stabs.

My goals are

1. to help design well engineered stabs that are so strong that they won't fail unless they are damaged, or have bad servos, wrong linkages, or other flutter issues from other sections of airframe.

2. To establish a QA (quality assurance) for the critical components with documented, serialized construction records from factories as a proof of proper construction in compliance with our guideline or minimum requirements.

3. To simply make future airframes safe to fly by applying the guidelines on the entire airframes.

In the past, I had helped some factories build really strong wings and they work great. I can see the drastic improvements. For this time, I would like to try this open book approach in which we can have all the global talents participating in this project so that all things can be considered through out the phases of this project.

Let me know what you think and advise.

Here is the start.

I am working on the new tail for my MiG 21. Here is a picture of the molds.

The skin material is Airex foam.
The shaft is made of 8 mm Titanium.
The anti rotating pins (2 each side) are 2.5mm steel. - I told factory to double the length and use stainless steel 405 series?
Carbon fiber will be added to the skin - need to know the minimum area needed and location?
Spars - now the prototype is made of balsa, but I prefer using hardwood or carbon fiber plates.

What do you recommend to put in there to make it strong and rigid?

We plan to make some test samples, then they would be sent to a small lab set up in my company there to do some stress, torsion, and vibration test, then determine which configurations are the best choices.

Please no bashing or destructive comments.

Mike

Mike, I commend you on taking this approach and opening up a dialog with the jet modeling community. As a software developer, so I appreciate the "open source" aspect of what you're trying to do here. If excuted well, all manufacturers will benefit. Having a certification process for the manufacturing of these parts is long over due.

Mike, it's been awhile since I did any work with tools like polo-finite or nastran but there may be FEM sw available these days to help with the analysis. I would imagine some of the advanced cad programs have this as a feature or add on, does anybody know?

Hi Mike,

very good .
I have a little Exel Program to find the right position for the shaft.
I will send it to you via mail.
I would place one of the anti rotating pin in a playwood spar an not in a litle wood block, as your pic shows.
If you do that, the torsional force of the shaft will go over the entire width of the elevator.

Christian

Hi Mike, as usual you are one step forward and provide a great service to the jet community.
What you propose sounds wonderful, but I think the problems are more basic than that.
I mean, how can we be discussing the materials and building directions of an airplane when the factory does not guarantee consistency of their products? You can have all the steel, titanium and carbon you want, but if the glue is not well laid and the bulkheads are not actually glued to the skin, what good will the extra reinforcements do?

These factories can go as far as to install the reinforcements reversed and not even see the problem when presented to them through a picture...<br type="_moz" />

Make the rotation (cross) pins much longer and change the structure so the larger pins transfer loads across the full chord.

What percentage of MAC is the shaft placed?

Mike,

KISS concept.

You can leave as is, then have the shop fill the area completely with Two Part Adhesive, reinforced with milled fibers. This would greatly improve the torsional rigidity of the design. Longitudinal forces are pretty much neglegible.

If you pursue this method, shop has to commit to it!! no partial filling with adhesive!! It needs to be evenly distributed all around the torque shaft and the retaining pins. The torque shaft should also be prepped for bonding by removing all oily residue.....I would even etch it or sand blast, only on the area to be exposed to adhesive (mask the rest).

Wow, I made it sound like a real aircraft repair! LOL!

Regards,
David

Wit a composite structure you cant just look at the internals and say "its good".

You have to look at the entire structure including the skin.

What is the layup schedule of the skin?
What thickness airex?
What version airex, c70.75, c70.55, etc?
Where is the placement if the reinforcements and what weight, style, etc?

From what i see in the photos, it looks pretty good.

Some personal things i dont like:

1. No full length root rib to support the shaft along the full chord line (this could be a piece of airex, i just cant tell)
2. No full length rib at the outboard end of the pivot shaft, again to give full support.
3. Two short bolts for anti-rotation. I much prefer one long pin sandwhiched between a three part rib for antirotation.
4. A full span spar that interlocks with the outer pivot shaft rib, or even better yet, one shaped so the pivot shaft goes through the spar and the spar continues to the root rib.

BTW, the method above distributes the torsional load evenly across the WHOLE wood former sections enclosing the torque rod, not just the pin hole pentrations.

Yup, if you mean your glue filling, yes. Like i said, his current setup looks fine as is. There are just things i personally do not prefer.

Yeap, that is what I was referring. Some far east companies do this but only partially and without proper surface preparation.....which is the same as adding dead weight!!

agree with Matt, you should have hardwood ribs from L/E to T/E for the anti rotating pin bond into, use uni carbon on the top and bottom sides during lay-up. This will help to share the load across the span on the stab.

Also id stay away from using balsa skin core as this will reduce peel strength.

Lets not forget about the fuselage structure, where the stabs slid in as well.

My 2 cents :-)

Great thread, looking forward to learning. These threads remind me of the older threads that i read when i first join rcuniverse.

I often save good learning threads to help me learn to build or assemble jets.

Invertmast's f-14 thread is an awesome thread to learn.

Make your torque rod as shown and eliminate some work plus add a lot of strength to your design. Just add a 90deg bend. No fragile pins needed.

I still think this is the cleanest, simplets method. It involves a little bit of custom machining, but well worth it. Evenly distributed torque.

I am curious about the airex material. Are the spars and formers bonded to the skin and then airex is filled from there, because from the pic, it looks like the spar and formers would be glued to the airex? I don't own a CARF product (friends do) and don't know how their fuses and wings, etc, are made using this material. I never had a good look. I would think gluing anything to a thin layer of foam isn't the best way to do it? Like I said, just looking for info as I am not familiar with the layup methods using airex.

Dan

As a curiosity, the full size Mig 21 rotating pin is angled, not perpendicular as your stab. Without any further empirical data or factual aero information I think it is a good idea.

The good news is that your pivot seems to be at the right place, regarding the MAC.

The airex should be sandwhiched by layers of fiberglass cloth. The airex is extremely dense, strong and light! The green stuff is atleast 3 times stronger than a similar thickness balsa

Dude, no problem!!

Nice! LOL.

I am looking for airex in 1mm-1/16" thickness. I know of the local folks that can supply 1/8" thickness but that's too thick for my lay-ups. Where do you guys get thinner airex from? I suppose I can always get herex from Germany but shipping is a bit outrageous

BTW- extreme density of airex sort of defeats the purpose, no? If is isn't lighter than 6-8 lb density balsa, why use it?

Did you miss the "light" statement? Its dense stuff, but i believe it is a a hollow sphere, not solid.

Anything less than 3mm your going to have to get from germany. Im planning on placing a fairly large order of about 50-75 sheets of the stuff mid summer. If your not in a hurry i can put some extra in for you.

Last i asked, flat rate shipping for 24"x40" sheets was about $80-100

Enrique, did you do a calculation of the shaft to MAC relationship from the picture?

In the full scale MiG my guess would be that the pivot axis would align with that little jag in the root chord.

I don't know about the MiG but I have been told in other supersonic aircraft that the pivot location is a trade off between supersonic and subsonic regimes.

No calculations Matt, just eyeballed that the model pivot location (behind the jag) intersects the full size pivot line in a place in wich the stab will rotate in a very similar way as the full size. I presume that the full size has the pivot at the right mac %.

And absolutely, the aerodynamic center of pressure shifts back and forward depending on speed, and seems logical to consider this to avoid situations in wich the flying stab may become unstable.

Yes, Enrique my MiG has that same jag in it. It helps to know you have the pivot near the full size I agree.

I helped with an F-20 model years ago and the pivot was wildly off, and of course there were few clues from the full size as to where to put it (straight root chord, no jags).