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HomeScale✈️AircraftScale HeliScale Helicopter Main Discussion Docs  Gazelle Build
07-29-2013 04:34 AM  6 years ago
Joel Rosenzweig

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Marlborough, MA - USA

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Oldie1,

Well, stay tuned. I haven't given up yet, and maybe we'll arrive at a solution that helps all of us out. That's what keeps me going on this. I'd like it to work for me, and I'd like it to work for others. It's a nice model, and I'd really like to fly it.

I don't think Willis has any bad intent. He must have some configuration that works for him, but it doesn't match what either of us are doing. Of course, TF Model does not spec exactly what rotor head, blade length, profile, to use, or head speed, or an all up flying weight, etc etc. That's part of the problem. There are too many unknowns. I hope to get to the bottom of it. Ideally, before the flying season is over!

I'll share, as I figure all this out.

Joel
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07-30-2013 04:08 AM  6 years ago
heliadictaholic

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Hobart, Tasmania - Australia

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G'day guys,

I was thinking about the travel of the control linkage where the servo wheel approaches the end of travel. The linear movement of the push/pull rod reduces as the ball link moves from a "straightish" line to an arc.

Is it possible that this could be the reason why it is difficult to get the last bit of pitch on the blades?

One more thing, it is no wonder we are having problems with fan control under load - I needed a small sleeve for something so I deliberately distroyed a spare 2mm ID bearing to get the central ring of the bearing. You won't believe how small the balls in the bearing are.... Put one on the end of your finger and it is so tiny you would not want to blink because you would loose it. I measured one of the balls, it was only about 0.7mm - might not sound too tiny but it is "tiny"....
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08-05-2013 02:38 AM  6 years ago
heliadictaholic

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Hobart, Tasmania - Australia

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G'day Guys,

Anything to report Joel?

I have 3 of these TFModel fenestrons and with my limited flying experience, I am a bit afraid to lift off just in case things go wrong and I have difficulty recovering from a fan holding problem. Having said that, I would only be hovering and gently moving about pretty much tail in and some side on. If the heli ended up nose in, I could easily get confused - although getting much better with the pod and boom arrangement.

Anyway, I want to 3D model the fenestron and try to make some changes to the design. You might remember that I 3D modelled a Gazelle scale rotor head and had the parts CNC machined in China, so no reason I could not model the fenestron.

There are a couple of things that I would like to address:
1. Redesign the aluminium hub to incorporate thrust races on the blades - there is a problem with this in that the bearing recess does not have enough space for standard thrust races so it might be necessary to machine/precision grind the thrust race washers down to a smaller outside diameter.
2. The fenestron housing/duct appears to he fairly straight - but should open out at about 4degrees (8degrees total cone/evase). This would be a fairly big modification - hopefully item 1 is all that is needed.
3. Redesign the spindle to include a larger diameter actuator rod and increase the tiny bearing diameter from 2mm I.D. to maybe 2.5 or possibly 3mm.
4. The aluminium actuator arm mount, is screwed to the "plastic" inner cover over the sprocket, by 4 tiny screws. Talking about using huge power servos is bound to rip the tiny screws out of the plastic. I would replace the plastic inner cover with aluminium and tapped holes for the actuator mount.

My aim with any modifications would be to allow us to spin the fan a bit faster and maintain better pitch control....

If you guys have any suggestions that I could incorporate, please let me know. I will do the 3D model and send you the 3D models or PDF's so that you can comment.

I do not know the cost of CNC machining the update fan parts but my custom scale rotor head cost me $1000.00 - some of the head parts are quite complicated compared to the fenestron parts. If you guys have any contacts with CNC machinists maybe you can talk to them on "our" behalf. My CNC machinist is "Wanfon Prototype"....

As I said, I have 3 of these fans so I am going ahead with some redesign and I am happy to provide the 3D models to you guys if you want them - for your/club own use.

Anyway, if you have any improvements you would like to see in the 3D model, please let me know. I am going to start on the hub tonight......

Jim
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08-05-2013 01:30 PM  6 years ago
heliadictaholic

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Hobart, Tasmania - Australia

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G'day guys,
I have taken some measurements and it looks like fitting thrust races to the fan blades will be possible - just!! - it will need the aluminium hub to be re-made. I will post the 3D model when complete...

Some pics - Thought you might be interested to see how the fenestron is built. The fan that I dis-assembled is one of the first versions - hopefully TFModel have sorted out the assembly errors. This fan had the blade bearings installed incorrectly - the same as my BT fan. I will now need to pull my Gazelle completely apart to check things out.

There were 4 belt alignment bearings missed from my Gazelle fan.....

Side cover removed

Outer actuator disc removed

Blade bearings were installed incorrectly. The spacer should be between the flanged bearing and the standard bearing.

Looking into blade mount you can see the standard bearing where the spacer should be.

This bearing can drop out. Needs to be locked in. I used a spring loaded centre punch set on light impact. Probably only need 3 pop marks.

This pic shows the left hand thread - do not rotate the fan blades by hand, you could easily un-screw this joint. This is what happened to my BT fan - on the bench luckily.

In my last post, I mentioned the tiny screws into plastic. These screws do concern me especially when we need to use a powerful servo to actuate the blades.
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08-06-2013 01:04 PM  6 years ago
heliadictaholic

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Hobart, Tasmania - Australia

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G'day Joel,

I have drawn the hub in 3D. The hub is designed to take 2, standard bearings 2x5x1.5 and 1, 2x6x3 thrust race in each blade mount.
There is a small amount of clearance around the blade mount on the old hub that I was able to use in order to fit the 6 OD thrust race.

If you PM your email address, I can send the AutoCAD file - or I can save in other formats if you do not have access to AutoCAD.

Next I am going to draw the sprocket cover - which is the piece that has the actuator fixed with tiny screws into plastic. This is the piece I do not trust.....

Tomorrow (Wednesday) I will contact my CNC guy and get a price to CNC the new hub.

By the way, If you would prefer this subject to be moved away from your thread, let me know.

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08-08-2013 04:29 AM  6 years ago
heliadictaholic

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Hobart, Tasmania - Australia

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G'day guys,

I have sent the 3D model of the "thrust race" version of the fenestron blade mount hub to TFModel - asking if they can machine the part....

I hope the addition of thrust races to each blade will make a difference and reduce the amount of stress on the servo drive system.

I was going to have the piece machined by the guy who machined my scale Gazelle rotor head, but as the fenestron is a TFModel item, I think it would be better for them to do any updates - it is their design...

Waiting eagerly for their reply.....

Jim

--------------------------------------------------------------------------

Just heard back from TFModel
I will get the parts machined by my guy in China.....

Hmmmm, maybe I am wasting my time on this.... We will soon see.....
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08-08-2013 01:12 PM  6 years ago
heliadictaholic

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Hobart, Tasmania - Australia

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I have drawn up the sprocket cover and I will have re-made from aluminium. I do not like the way the blade pitch actuator mounting is screwed to plastic by 4 TINY screws. With the talk of using a high powered servo to actuate the fan blades, I just do not trust those screws into plastic.....

I have 3 fenestrons and the 3 new aluminium covers will cost $120 ($40 ea)... Money well spent in my opinion.

With this mod and the new blade mount hub with thrust races on each blade, hopefully spinning the fan a bit harder will not impact on the blade actuation system. If it does, I will go further..... The duct is not optimal - should open out at about 4degrees each side (8deg total)

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08-08-2013 02:53 PM  6 years ago
Joel Rosenzweig

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Marlborough, MA - USA

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Jim,

I don't have any updates yet. I have not found a configuration or set of changes that will make the fenestron work properly in my model. I have not root caused the reason for the lack of tail response. Therefore I cannot claim that any of the changes you are making are going to solve the problem because the nature of the problem has not yet been identified.

I simply don't know if the parts you have concerns about and have redesigned will address the fenestron yaw performance or not.

I do think that we are still dealing in the realm of speculation until the true root cause has been found. Once that is accomplished, it can possibly be addressed. Then other weaknesses can be looked at and resolved as necessary.

Willis told me that he will be measuring fenestron power on their flying model so I'll have more data to work with. I don't have that data yet. But it is critical to know before going much further.

The design itself is a copy of the Hirobo fenestron. The primary difference is the drive system uses a belt instead of Hirobo's torque tube system. My belt buzzes under load, and maybe that means that it does not reach full RPM. Still to be proven as a problem. The rest of the design worked OK for Hirobo, so whatever faults it has are shared between them and was apparently good "enough" to fly. No doubt that some items could withstand design changes though for better performance or efficiency or longevity.

Joel
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08-09-2013 12:17 AM  6 years ago
heliadictaholic

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Hobart, Tasmania - Australia

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G'day Joel,

Yes I understand exactly what you are saying. Having said that, I believe thrust races on the blades will be a good engineering upgrade - radial bearings can not take too much axial load - the original bearings are "radial" bearings and the size of the balls in a 2mm ID radial bearing are so tiny - does not look good.

The tiny screws that hold the blade actuator to the plastic sprocket cover have to go.....

I am not an engineer but I work in a mechanical engineering department.... I don't know if my mods are overkill but if I need to spin the fan faster, loads will only increase. I would hate to lose a heli because I over-spin the fan and have something fail.

Maybe with your belt buzzing issue, you could try to modify the motor mount similar to what I did with my BT when I had the problem of the belt jumping teeth while riding up the side of the motor sprocket. I also added a bearing to the end of the motor pinion shaft. My mount design relieves the stress on the motor output bearing and provides correct belt alignment. I can tighten the belt and not stress the motor mount and ease the load on the motor output bearing.
Just a guess, but maybe you should check the motor bearing to ensure it is still good.

One thing to check with Willis is the main blade length he is running in his fenestron test.
I would have liked to run 800 blades on my Gazelle but difficult to find 800 blades with the correct dimensions at the hub end - so 700 it is.

Have a good weekend

Jim
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08-09-2013 03:03 AM  6 years ago
Mojave

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Palos Verdes, Ca. USA

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FWIW, I agree with both of you on this issue... Joel has a valid point, the design has not proven to be the culprit, yet...

Heliadddict: from a design standpoint, you have a very valid point. This design is lacking the thrust bearings. But then again, it did/does work for hirobo... I like your upgrades, but they will most likely never be employed at the mfr's end, due to cost constraints. In order to produce this unit at an affordable price for us (the dirty masses), some corners get cut.

I'm very interested in seeing if either of you get this issue figured out. This is one of the reasons that I didn't buy this kit... I just don't have time for science projects
Barry
All helis and planes have an expiration date stamped on them...you only find it after you crash!!
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08-09-2013 10:47 AM  6 years ago
heliadictaholic

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Hobart, Tasmania - Australia

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G'day Barry,

A big part of this hobby for me is the building part. I have 3 TFModel fenestrons - none of which I have had the confidence to fly yet I have an Aussie made BT, TFModel BT and now a TFModel Gazelle... I am still in pod and boom mode until I get some experience.

You may have seen my "almost" true scale Gazelle/Blue Thunder rotor head.

If I see a need, I will go after it - hence the head and some fenestron work. I like to redevelop or try to improve on stock items when I can, and hopefully if I re-work something, it might just help someone else out. This is why I have offered my 3D Cad files to anyone who wants them.

In the past, I have posted pics of my BT/Gazelle rotor head, but I don't think I have posted any on the heli.... Here are some pics FYI

By the way, The TFModel fenestron is a very nice piece of work - (that's why I have 3 of them), even though I think things could be a bit better....

Its Friday night here - sitting back with a beer listening to some "Fleetwood Mac"

"Over to you Joel..."
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08-09-2013 11:46 AM  6 years ago
heliadictaholic

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Hobart, Tasmania - Australia

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Me again,

Some interesting reading..... Fenestron thrust is not just from the thrust from the blades, but also from the lip at entry to the duct - suction force.... The TFModel blades are extremely close to the intake lip and when fenestron pitch is given, the blades actually encroach into the lip area which could upset the suction force at the lip.... Intake lip suction on the full size version can be 50% of the fenestron thrust....

Anyway, have a read... It is about the full size Gazelle and fenestron stall in certain conditions...

http://www.aaib.gov.uk/cms_resource...URP%2001-92.pdf

When I get the upgrade CNC parts for my fenestron, I might work on an add-on lip at the fan intake to try to improve air entry conditions.... Something else to play with.....

Regards,

Jim
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08-09-2013 01:19 PM  6 years ago
Mojave

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Palos Verdes, Ca. USA

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Hi Jim, I understand completely I'm definitely not downplaying your desire to fix an issue with the fenny, as this may help others that have this issue. I followed your custom rotor head thread from day 1 and really enjoyed your build. The only pitfall that I can see, is most folks don't have the extra cash to fix a manufacturing issue. If in fact the problem is a design flaw, it should be fixed at the cost of the kit supplier. The fact that you have offered your design services at no cost is very kind on your part and quite a selfless offer to those that could not have done this on their own dime.
Barry

P.S. Enjoy the beer Your music selection is great I just saw Fleetwood Mac in concert a couple months ago...
All helis and planes have an expiration date stamped on them...you only find it after you crash!!
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08-09-2013 02:59 PM  6 years ago
Joel Rosenzweig

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Marlborough, MA - USA

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Over to you Joel
Reminds me of the handoff between NASA mission control in Houston and their counterparts in Australia when they were communicating with the folks flying the Apollo missions.

In any event, I'm 3000 miles from home right now so more fenestron experimentation will have to wait for another week until I return. Maybe I will have heard from Willis by then, too.

Safe trip y'all.

Joel
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08-10-2013 09:45 PM  6 years ago
oldie1

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New Zealand

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Hi to all,
Keep up the good work I watch with interest.

But as I have said before and Barry has just reiterated TF should be putting their hands up and doing something to fix the problem. I do not expect to buy an expensive piece of machinery that does not work and then to get no backup from the makers.

The fact that TF are still offering these items for sale with a known design fault is bordering on the criminal.

Cheers Tony
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08-14-2013 03:30 AM  6 years ago
heliadictaholic

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Hobart, Tasmania - Australia

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I drew the fenestron shroud/duct to include 4 degree expansion on the leaving air side of the fan as well as a square outlet lip (better square than radiused on the exit face).

The radiused lip on the intake should be 15mm based on calcs I downloaded, but 15 is not easy to achieve without protruding too far out from the side of the tail..... 10mm is achievable.

Having said all that, I have stopped work on the duct. I got a price for a high quality/strength 3D print - way too much for me.... $668.00...

I am now going to draw just an add-on radiused lip for the entering air side and the square lip on the discharge. I will give those a try... It would have been nice to get the 4degrees expansion, but, not this time....

Anyway - here is a pic of what I drew.

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08-14-2013 06:25 AM  6 years ago
Joel Rosenzweig

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Marlborough, MA - USA

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Jim,

It's interesting to note that the Hirobo fenestron performs properly, and has the same duct (as far as I can tell) as the TF Model fenestron. I understand the theory behind the larger duct, but I'm wondering if that's really the problem, or if we're dealing with some other issue entirely.

It would be great if you test flew one of your three models to get some data to add to this discussion. I'm not sure what else to try with mine. I have run out of ideas for now. And I'm still waiting for Willis to get back to me with some power data that he was going to measure on his own model.

I admire what you are doing with the parts re-design, as it's really a great piece of work you're doing. I just wish we knew what the real problem was so that we were not shooting in the dark and throwing money at the wrong problem. I've put as much money into this as I'm willing to for now. It's not yielding the results I want. If I knew that there was a definitive answer to the problem, I'd consider spending more money to resolve it, but, we don't know what that is yet.

In any event, I'm happy to experiment more if there are ideas that are zero to low cost to implement. I'd still like to find a solution to the problem. I just don't know what else to try that fits within a reasonable budget. 3D printing a new duct to try it out does not fit that profile.

However, perhaps there's a cheaper way to try a larger duct. For example, if I used plastic to lengthen the duct tube, would that have the same impact? Or does it have to terminate in a particular shape in order for the air to flow properly?

If I created a ring that I glued to the left hand side of the fenestron, that simply extended the length of the duct, would that work? That would seem to be a cheap experiment. But, I don't want to both to craft such a duct if the idea is all wrong anyway.

Joel
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08-14-2013 07:15 AM  6 years ago
heliadictaholic

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Hobart, Tasmania - Australia

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G'day Joel,

I based my duct design on the full size version of a fenestron where they state that the inlet duct should be radiused at the intake edge to "0.2xR" - which gave me about 15mm. The duct should open out from the back of the blades (at full pitch) at about 4 degrees and the discharge should be square to get good air separation. That is 3 things I was trying to incorporate into a new duct. I had a look on the net at the full size versions and yes, they all have a good radius in the intake and square edge at the outlet.

I was extremely interested where a British investigation into an incident with a full size Gazelle which suffered fenestron stall at hover close to the ground - they stated that the radiused lip at the air entry of the fenestron can produce 50% of the fenestron thrust. They referred to this as "suction force" so testing a radiused intake is the least that I think I could try.

I certainly agree, there is a limit on what we can spend on trying to find a solution for the stall. I have a 3D printer, I might print one of my ducts and set it up on a test rig - "my Aussie made BT" - still not built, but I can use the boom/tail and measure the thrust comparison between the stock unit and the test duct. Unfortunately, my 3D printer is a very simple version (RapMan) and I would not trust the strength of any structural "printed" parts in a permanent installation. Printing a radiused lip and a square lip should work. You may have seen somewhere on RR where I 3D printed my scale rotor head before going to CNC.

I asked TFModel if they could provide a small section or their 3D model of the tail that I could load into my PC so that I could get a good fit on any printed part. Unfortunately they can not help, I have to take a lot of measurements and photos and try to develop a 3D model of the tail around the fan before I can finalise the test duct shape or radiused lip where it meets the tail. The fit of any lip must sit neatly and blend perfectly against the tail. The tail has a constantly changing profile at the edge of the fan casing so reproducing the 3D model is not going to be a simple process.

In relation to the parts that I am having CNC'd at the moment (blade mount hub with thrust races and the sprocket cover) are purely upgrades to a (I think) safer option. I want to be able to spin the fan a bit faster - if that is the solution - and not be too fearful that a radial bearing is going to fail or the tiny screws into plastic may pull out under high servo load.

Unfortunately, I am busy at the moment turning the void space under my house into a decent hobby room for my wife (I might get a small corner) - she has just purchased a $6000.00 kiln for her glass slumping and glass fusing hobby....

I downloaded a program "123D Catch" which is a program where you take some photos of an object, and it then sends the photos "to the clouds" (what ever that means) and they send back a 3D model.... The impressions I have of 123D Catch is pretty much crap!!. The 3D model did come back, but the 3D model is a perfect representation of what the tail of my Gazelle would look like if it were made of plastic and left sitting in front of the heater for a few hours...

Anyway, I will do what I can on the 3D print as soon as I get the tail drawn in 3D

Jim
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08-18-2013 07:39 AM  6 years ago
heliadictaholic

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Hobart, Tasmania - Australia

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G'day Guys,

Joel, have you had any feedback from TFModel yet?

I have just completed adding the extra pinion support bearing to my fan motor. One of the pics below shows the comparison between the old style BlueThunder mount and the new Gazelle stronger design.

You can see in the pics that I have added belt guide rollers to each side of the mount. The reason for this is that the belt runs extremely close to the inside of the boom tube and may touch the belt while running.

---Just wondering if anyone might know the KV rating of the stock motor supplied with the Gazelle? I asked TFM and I got this reply:
4s 500kv,6s 800-900kv.....

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08-18-2013 09:23 PM  6 years ago
Joel Rosenzweig

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Marlborough, MA - USA

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Hi Jim,

No word from Willis yet.

Your motor mount looks good. I like the bearing arrangement you used. I'm wondering how you tension this assembly properly once the boom is already installed onto the machine though?

Do you have some sort of screw adjustment that lets you tension the belt?

Joel
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