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Author Topic: Martin-Baker M-B V 21" span rubber.  (Read 2630 times)
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p40qmilj
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« Reply #25 on: August 05, 2017, 12:42:24 PM »

 Grin  i know what that tree is thinking

" oh yumm, yum.  a MB 5..... come to papa you tasty little treat!" Roll Eyes Roll Eyes Roll Eyes Roll Eyes Roll Eyes Roll Eyes Roll Eyes Roll Eyes

jim  Grin
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Prosper
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« Reply #26 on: August 11, 2017, 10:00:45 AM »

Hi Jim! That tree has indeed collected a few models in its time. Luckily the wind was blowing the MB 5 away from it, and I guess the zoomed lens makes it look a bit closer than it is.

Problems Sad. For this scale build I replaced the original ball bearing with a plain bearing - a drilled-out nylon 'nose button'. This was sloppy so I decided to re-do it and dismantled the unit to add a new brass bearing and a new bearing cone. This allowed me to photograph each step and I've posted this dreary stuff on my other contra thread here starting at reply 21.

But there's still friction in the system! Every single spinning element works fine individually - only when put in the model does it bind. I think the original ball bearing had enough play in it that it could accommodate whatever axis the whole spinning rig wanted to adopt. This new plain bearing has effectively zero play, and some tiny eccentricity is enough to cause binding. Rats. A system like this has enough inherent friction even if well built; any undue friction will really sap the power available to the props.

I've managed to reduce the friction largely by shaving a bit off this edge or sanding a new angle on that edge - but it's still there. Looks like a re-refit will be needed.

Anyway I video'd a static run. Please note that the airframe is not finished and finally painted yet! The camera does bad things to both the props and the sound. The mic. greatly exaggerates the clattering of the motor in the tube and the props flicker round one way then the other like a wagon wheel in a movie film. The deep hum or drone is the blade noise. The rear prop spins more slowly than the front, partly because of its greater blade pitch and partly the rogue friction. Maybe an interaction with the front prop too. The rear prop stops before the front one because of the unwanted binding. Average rpm through the run is about 1330 so say >665 for the front prop and <665 for the rear. It doesn't seem enough to move the model, but it flies even with just the last few hundred turns.

Stephen.

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strat-o
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« Reply #27 on: August 11, 2017, 12:36:01 PM »

This is really impressive.  I like the close tolerance of all the moving parts:
1. You got everything situated really closely together
2. It works and doesn't appear to wobble!
3. Propellers tracking nicely.
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Prosper
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« Reply #28 on: August 11, 2017, 02:57:20 PM »

Thanks Marlin. I'm pleased with how close the spinning parts are to each other, the only concern is that the contra assembly is what bears the brunt in a hard arrival. So far it's only landed in long grass or weeds. There's a ring of quite firm foam set in the nose so that despite the small clearance between the rear prop hub and the fuselage, it should absorb a lot of shock. That's the idea anyway Shocked . The mockup model, which used these same components mostly, would hit the turf and end up with the whole powertrain, tube and all, ahead of the airframe. In fact I think the model would hit on its receding chin; the heavy powertrain would be ejected, and the airframe would recoil back from the powertrain. It did this any number of times without damage so that's fine with me.

Stephen.
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tom arnold
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« Reply #29 on: August 11, 2017, 03:47:25 PM »

How do you adjust the thrust line for trimming?
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Prosper
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« Reply #30 on: August 12, 2017, 03:49:31 AM »

Hullo Tom,

the downthrust question is important. First of all, I make a rear bearing which can be moved up and down. This means that the shorter the torque tube, the more the thrust angle can be varied. This model is saddled with a pendulum and that reduces the angle available. Also, a shorter tube means that in this 'classic monoplane fighter' layout, the rear bearing can be manipulated from the cockpit opening. A longer tube requires some form of access to the rear fuselage.

Tom I know you know what follows, but for what it might be worth to someone:

The need for downthrust (in any model) can be altered by varying power output, since downthrust is arguably just a symptom of too much power. Setting plenty of downthrust is pretty much automatic in S&T 'duration scale' style building - light low-drag airframes, big motors and shallow-pitch commercial props - but if you have a heavier, draggier airframe with less rubber, and a coarse-pitched prop, then things look different. It's down to whether the model can ride out the initial power burst and still have enough revs to make use of the final few hundred turns of the motor. If it lands with turns on board then it's asking for a finer prop pitch to match the installed motor, and this finer-pitch, high RPM prop might cause power stalls after launch.

Then there's the importance of the thrustline in trimming, but in this area downthrust or upthrust can be indicated. I've found that with low-wing, shallow dihedral models the thrustline will be near neutral. These are always quite heavy models with a smaller % of overall weight given to rubber compared to S&T models.

For a torque-tube type contra-rotating model, the thrustline may need to be adjustable but ought not to be a show-stopper IMO, at least if you can vary the blade pitch of your propellers and spend time matching props and motor size as a part of the trimming process.

Stephen.
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tom arnold
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« Reply #31 on: August 12, 2017, 02:05:59 PM »

Good explanation and thanks. It shows the difference between our colonial thinking and FAC fetish on time aloft and the British approach of flight realism and shorter times. In FAC flying you need to make use of all the power you can squeeze out of a motor and translate that into brisk climb which needs downthrust as a given. If you can slow down the expenditure of power through a coarse pitched prop, that initial power burst is held in check, the launch speed is lower and you don't need (much) downthrust, as you say. Good thinking and I am anxious to see how the flights go as I can see use of the coarse pitch prop in both flying arenas.
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« Reply #32 on: August 12, 2017, 03:35:38 PM »

Tom wasn't that you with an MB 5 in model builder 10-15 years ago?
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« Reply #33 on: August 12, 2017, 04:03:34 PM »

Yes, and the only thing left of it is the canopy. I have often picked up that canopy and considered building another underneath it. It flew good while it was alive though.
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« Reply #34 on: August 12, 2017, 05:35:29 PM »

Ha ha ha I still have the article and plan. I was so gonna build that. I forget why I didn't... maybe the front end? I like to pick my battles, but the text you wrote almost hooked me.
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« Reply #35 on: August 13, 2017, 05:10:14 AM »

Last evening I did another static run but with 1470 turns. That's 0.75 of the supposed breaking turns, and that's what I consider full turns - not just the number, but the motor's getting hard by then. It ran a minute again despite the 'mystery friction' issue. I wonder if there's a question of 'running-in' in a rig like this? I forgot to say that there's a small hole in the rear prop hub through which I can inject oil onto the main bearing, which should quickly find its way where it's needed. Anyway that's more like the static RPM I was looking for, based on the mockup. I was pleased as well that there were no turns left jammed at the back of the torque-tube. Then just now. in a [very unusual, unfortunately] light breeze, I got a sweet flight on 300 turns - it wandered off rocking slightly either way, straight for Jim's tree (reply 25) but managed to veer slightly and flew beyond it. The fact is it's well outgrown my little patch; needs a bigger field so it's down to the cattle and the weather now. It flew 'til the turns ran out (seemed like 10-12 seconds maybe).

Quote from: Tom Arnold
as I can see use of the coarse pitch prop in both flying arenas.
Definitely! A few yrs back I got a bit evangelical on this site re. encouraging people to make their own props, but I done give up, 'cos no-one was heeding the word. I always seem to end up with a coarse or very coarse pitched prop for best performance - I'm not talking about a scale look but duration. That includes draggy biplanes too. The only exception I recall is the mockup Westland Whirlwind I made, that loved 28° pitch at 75% prop diameter; performance fell right off at 32° and magically returned when back at 28.

Quote
It flew good while it was alive though.
I'm not surpised - the layout has a lot going for it.

Stephen.
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Prosper
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« Reply #36 on: August 14, 2017, 02:29:17 PM »

I got in a couple of 500-turns flights yesterday when the cattle were in a far corner. The model stalled at the end of each flight in quite a big way - rearing up and diving for the dirt in a very determined manner. That's not good for something with two props spinning in opposite directions. To my mind it clearly needed down elevator and upthrust. There was no point adopting the 'increasing power output in lieu of upthrust' line, since the thrustline was above where I guess the centre of drag to be. More power would just pitch the bird nose-down even more, requiring more up elevator and more stall in the glide. The existing torque tube couldn't be moved lower because of the pendulum, which I didn't want to shorten, so I made a new torque tube thismorning, with a narrower centre.

I photo'd it next to the original tube that started this project a year and more ago. Original tube 4.25g; current tube 2.0g and less rotational momentum. This has allowed me to move the rear bearing down about 5mm. I then wound the motor up to 1540 turns without it breaking, and got a static run with no turns left jammed in the back of the tube, so the new tube seems sound. It has a choke in the centre - a disc with a 12mm hole cut in it.

I also added some noseweight to get the balance point forward to about 25% of root chord. The model glides very well for something with a big food blender at the front, and will take a hard throw without stalling, so when the rain/wind/cows go away I'll see if it can get some longer flights in.

Stephen.
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Re: Martin-Baker M-B V 21" span rubber.
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malc
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« Reply #37 on: August 14, 2017, 04:04:27 PM »

Still totally fascinated by this,
One question - why does the smaller tube have to flare out at each end?

Malc.
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« Reply #38 on: August 14, 2017, 06:32:46 PM »

Hi Malc, your question has two answers, from my lookout. One is simple: I didn't want to have to make two new endcaps for the torque tube (a couple of hours work, or more) so the diameter of the tube front and back had to remain the same.

The second is more woolly: I knew yrs ago, from trying to get very long motors crammed into short hook-to-peg distances, that the unwinding motor would form a 'skipping rope' which would use all the available diameter and absorb a great amount of its own stored energy just to flail its own weight around, and waste more energy in wiping itself against internal structure. I realised that a very delicate touch in the middle of the motor would cancel this tendency. Then when I started putting pendulums in models they interfered with the rubber motor. I had to place a shield or guard of some kind to protect the pendulum from being swiped by the motor. This shield provided the delicate touch necessary to damp the skipping-rope, and ever since I've put a choke or damper or restrictor or call it what you will halfway along my motor run, whether there's a pendulum or not. In this torque tube there's a central baffle (another possible term!) about 12mm in diameter, and this tames the motor centrally, but it still needs some space fore and aft to unravel its knots. The fully-wound motor gets knotty enough to pretty much fill the whole torque tube. If the whole tube had the diameter of the central portion (18.5mm O.D.) I think the motor would just lock up, or at least waste most of its energy rubbing against the inside of the tube.

Stephen.
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