Ivan Pettigrew Planes - Build Threads and Videos - Page 51 - RC Groups
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This thread is privately moderated by LukeZ, who may elect to delete unwanted replies.
Apr 04, 2009, 04:56 PM
Slip the surly bonds...
Sopwith Mike's Avatar
Hi Slider,

I have deliberately kept the model simple, so no flaps or gear. The drawbacks are loss of realism (especially the flaps, which look great in the various photos of the real thing) and lack of amphibious ability, but the complication of a working and waterproof UC were just too great. The benefits are the very low wing loading and the structural integrity of the wing and fuselage.

It would be relatively simple to make a working set of flaps with those huge "outdoor" hinges, and it would be fun setting them up and flying with them, but it would make the whole thing much more accident-prone for someone like me who is not a terribly skilled pilot. The UC is a much more difficult project (there are some posts on this thread a few pages back about the various options) but obviously not impossible, just not worthwhile on this type of model, in my opinion.

Sopwith "keep it simple" Mike
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Apr 04, 2009, 06:07 PM
What could possibly go wrong?
nickchud's Avatar
I'm with you Mike ... "Keep it simple".

Here's a diversion, sorry..

Whilst travelling I’ve been reading Max Arthur’s book on the Dambusters – A compilation of diary entries and comments by the aircrew and groundstaff involved. It’s frightening and inspiring. They trained to fly the Lancasters at 240mph at 60ft. On the raid they lost some planes that were flying at 100 ft – too high and too easy to shoot. Others hit powerlines. One crashed when the pilot was blinded by a searchlight. One turned back before crossing the Dutch coast – he lost his bomb when it hit a wave. The bombs were so big that the bomb doors were removed and they hung out underneath. On the way back there is a breathtaking account of one crew that flew over the Zuider Zee at 50ft. As they approached the sea wall defences they dropped another 10ft. The front gunner, completely exposed in his Perspex turret was desperately duelling with the gunners on the ground, firing up at them. But as the huge plane roared up to the wall, they saw the A-A gunners diving for their lives to get out of the way. The pilot later wrote that they must have thought this was a Kamikaze crew.

I'll have to have another go at building another Lancaster one day. I was looking forward to seeing the 70% model from Ivan's plans that Rick Bell has started. My world just isn't big enough for another large plane, not 100" anyway.

Well we can all dream!

Nick
Apr 04, 2009, 11:23 PM
Registered User
Slider388's Avatar
yeah mike, I know what you mean. Rather have a model that flies great but be a little less realistic than have one that you can crash easily or mess up. I think I started that discussion about the UC on the catalina, it does looks like a really complex project to get everything to work perfectly and lock.
Nick, thats awesome. The dam buster was an insane invention, ingenious really. Having the electric motor start spinning the bomb before it was dropped so that it would literally skip over the anti torpedo fences and sink down when it hit the dam wall to blow up at the weakest point was just purely ingenious. Also, being in a huge bomber flying at 40ish feet at top speed would scare the s*** out of pretty much anyone, not to mention the pilot. Nerves of titanium and insane reflexes.
Apr 05, 2009, 06:32 AM
Registered User
pentaxman's Avatar
Shed loads of practice I would guess Slider.

The idea of flying anywhere with people shooting or firing missiles at me would scare me enough to persuade me to stay on the ground.
Apr 05, 2009, 09:18 AM
What could possibly go wrong?
nickchud's Avatar
They were all just boys! Even Guy Gibson who led the raid was only 23.

I was going to say at that age you think you're indestructible, but those guys knew the risks they were taking.

phew!
Apr 05, 2009, 10:25 AM
Registered User
Slider388's Avatar
Its just crazy to think about, must have taken ages of training.
Apr 14, 2009, 02:12 AM
Registered User
Trevorh's Avatar

Pettigrew Retracts


Having now done five landings and take-offs with the Welkin, this is probably a good time to summarise my experience of adapting Ivan's retract design.

1. Geometry
The secret of the design is the 90deg angle formed, in both the 'up' and 'down' positions, between the operating crank and the hairpin bend in the top of the retract leg.

The price paid for this inbuilt locking function is that, at mid travel when the crank pin passes close to the pivot point of the leg, there is very little mechanical advantage. This can be eased somewhat if the retract angle can be reduced below 90deg. Bear in mind though that the angle swept by the crank must increase accordingly, such that the two always add up to 180deg. A retract angle of 80deg and a crank sweep of 100deg is a good compromise.

Moving the pivot point forward, so that the leg is effectively raked back, enables the torque rod portion of the leg to absorb some vertical shock as well as horizontal shock loads. Use the full width of the nacelle to make the torque rod portion of the leg as long as possible.

2. Fabrication
As someone who never got to do metalwork at school, anything mechanical is a major challenge. The beauty of Ivan's design is that the only metalworking involved is bending up the u/c leg and operating crank. If, like me, you find even bending the hairpin slot in the u/c leg a daunting prospect, then I recommend making the slot a few millimetres over-long. The end of the hairpin can be filled with epoxy later and then filed and ground to optimise the effective slot length and the contour of the bend.

Nylon saddle clamps against ply or hardwood make effective bearings for both the leg and the operating crank. Make sure though that your wire turns smoothly in them. I fitted the leg bearing clamps with hex screws into captive nuts to make the later removal and re-fitting of the legs more practicable.

3. Servo selection
I used Hitec HS85MG servos. These are normal (i.e. non-retract) micro servos. Whilst they are plenty powerful enough, I'm not sure I would go this route again. Some considerations are:

i) They are much faster than a retract servo, so you need to have a servo slow function on your Tx (and be careful to check the switch position before switching on!) otherwise things can get a bit violent.

ii) Retract servos drive at full power right to their end point, then stop. A normal servo 'homes in' on its target point and reduces power as it gets nearer. This means that the end point is less well defined when under load - for example when drawing u/c doors closed. This can also result in a higher residual current drain than with a true retract servo.

4. Performance and durability
I used 3mm wire for the legs and the crank. The legs are quite long but the wheels are light and the system operates well without the need for any counterbalance springs. I have only five landings in the log book so far and the attached picture gives some idea of the ground conditions. The model does tend to nose over when taxiing but lands and takes off okay, so in practice, one wouldn't want to operate off rougher ground, even with a fixed u/c, without changing to bigger wheels.

I hope this is of some help to anyone contemplating home made retracts. You can read more about the development of the Welkin units in my project diary .

Trevor
Last edited by Trevorh; Nov 21, 2010 at 03:07 AM. Reason: Updated weblink
Apr 14, 2009, 09:25 AM
Registered User
RiBell's Avatar
Quote:
Originally Posted by nickchud
I'll have to have another go at building another Lancaster one day. I was looking forward to seeing the 70% model from Ivan's plans that Rick Bell has started. My world just isn't big enough for another large plane, not 100" anyway.

Well we can all dream!

Nick
Nick:
Stay tuned I'm hoping to get back to it soon. I've just about got the Solant repairs finished. Then it's most likely back to the Lancaster.
Rick
Apr 14, 2009, 09:46 AM
Two left thumbs
I'll be watching for the resumption of your Lancaster build too, since I'll build Ivan's Lancaster wing and tail - plus an additional fin - for my York project.

Geoff
Apr 14, 2009, 11:32 AM
Registered User
pentaxman's Avatar
That looks to be a very nice approach in that shot you have posted.
Does it need 2 pilots though or is that just for back up
Apr 15, 2009, 05:20 AM
Slip the surly bonds...
Sopwith Mike's Avatar
Yes, PM, you guessed, that's the back-up pilot we all have here... Actually, the man in the large house in the background, the one with the big tower, he looks after our interests. Perhaps.

The Minicat I crashed and very nearly abandoned is completely repaired and waiting for a calm evening and high tide for a test flight. It has turned out pretty well considering the damage, though as with all these things, another few hours on the details would have made her look more like a new plane.

One of the motor shafts was bent to 45 degrees in the crash, and was replaced, but I thought the other was undamaged. On fitting the props and running up the motors last night I found that there is a very small bend in the other shaft. It does not seem to have much effect, but it's something else to worry about!

More details at http://sopwithmike.mudefordsc.org/ht...a.html#Repairs

Mike
Apr 15, 2009, 06:32 AM
Registered User
paulbw's Avatar
Hi Mike,
That is a very nice job on the repair - one of the hardest tasks in modelling too - at least for me! Sounds like you'll be needing a second new motor shaft though?!
Paul W.
Apr 15, 2009, 10:18 AM
What could possibly go wrong?
nickchud's Avatar
Trevor!

I really like the work you've done on those retracts, I wish that resource had been available when I started mine. However, I succeeded in the end and I can bear out what you say. I do recommend these: Hitec HS-75BB Ball Bearing Retract Servo which cost me £18.99 each from BRC Hobbies. If I had not elected to build my Mossie with separating wings, I could have managed with a single servo.

Mike!

That's a beautiful Minicat, I'm delighted you fixed it.

I'm looking forward to meeting you two guys and seeing your models next month.

cheers

Nick
Apr 15, 2009, 11:05 AM
Registered User
pentaxman's Avatar
Mike

Those repairs are not noticable in the slightest.
I also love the colour scheme and look forwards to seeing this machine in person.
Very well done sir.
Apr 15, 2009, 02:55 PM
I´m the "Before Club"
carabin's Avatar
Congratulations Mike
Excellent repair work and (like mine) a nice decoration.
I wonder , How much weight is the model after the repair?
I have built more heavier than Ivan. Aprox.2, 5Kg (higher wing load)
I can not wait for the premiere
keep us informed of their next Flight
Carlos
Last edited by carabin; Apr 15, 2009 at 04:54 PM.


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