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Old 01-02-2011, 02:51 PM   #1
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Default B-25 Mitchell Props

I am trying to go with a 3-blade propeller for scale looks and I like the sound of these props. I also want to have counter-rotating props.

I found that the Turnigy 11 X 8.5E 3-blade prop produces the desired results I want with my motor/battery combination. Unfortunately, Turnigy does not make a "pusher" prop in that size. Neither does anyone else. Master Air Screw has a 13 x 8 3-blade pusher but the next smaller size is a 12 x 6 3-blade pusher. I think the 13 x 8 will be over-propped and the 12 x 6 will be under-propped.

I have looked at GWS and APC props and found nothing in my desired size. I would even settle for a 12 x 8 3-blade. Does anyone know of a source for a 11 x 8, 12 x 8 or 12 x 7 3-blade pusher propeller (preferably electric)?

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Old 01-02-2011, 08:44 PM   #2
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I originally flew my B-25 using counter rotating props, (2 blade wooden IC props to get a reverse pitch). Later I changed to APCe electric props, both tractor props, and apart from the model flying better there was absolutely no hint of any torque affect or swing.

So unless the counter rotating is an absolute must have, personally I doesn't make any difference, and that's with three twins and a couple of four motor planes experience.

To me, the prop is the most important part to make a model fly well, without a good choice of diameter and pitch it could put the model at risk. One reason why I don't bother with scale props. I prefer to see my planes fly well, rather than look good while stationary.

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Old 01-02-2011, 09:17 PM   #3
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Ray, thanks for the info on the counter-rotating props. You have confirmed what other modelers have said - does not make a significant difference. As I said, I will stay with the 11 x 8.5 3-blade props from Turnigy and see how she flies on the maiden. Got about 125 watts/pound with that setup and a reasonable amp draw.

BTW - What model B-25 do you have? What is the power system (motors, ESC and battery rating)?

Thanks again for the info.

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Old 01-03-2011, 04:10 PM   #4
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My B-25 was scratch built some years ago, (first flew in '93, and still flown occasionally), mostly balsa construction.

Span = 93"
Flying weight = 10lb - 8oz
Motors = Graupner Speed 600 12v brushed
Gearing = 3.6:1 belt drive
Props = APCe 14" x 12" (4000 rpm)
Battery = 2 x 7 cell Nimh 3300mAh

Also has retracts and a bomb bay, (two bombs).

I can't remember the current draw, (never wrote it down), but it will be less than 30A/motor, so approx 500 to 600 watts total. (50 to 60watts/Lb).


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Old 01-03-2011, 05:46 PM   #5
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Ray, your scratch built B-25 looks great. It also came in a lot lighter than mine (71 inch) and has retracts. You are a great builder. Great job!

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Old 01-12-2011, 01:37 AM   #6
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I agree with Vic there Ray

Very nice looking plane.

Cheesy poofs are what Yankees get when they eat Southen Food!! bub, steve
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Old 01-13-2011, 01:40 PM   #7
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If you are going to have scale props (nice!) why counter rotating which of course is not scale on the B25?
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Old 01-14-2011, 11:43 AM   #8
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Default Counter rotating 3-blade props

Originally Posted by quorneng View Post
If you are going to have scale props (nice!) why counter rotating which of course is not scale on the B25?
I was only going to use them to offset the effects of an engine failure. Since the my desired 3-blade counter rotating prop size is not available, the effect of one engine failure is minimal on planes this size AND the reliability of electric motors is high, I am not pursuing that anymore.

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Old 03-31-2012, 03:23 PM   #9
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Just looking and geting inspired
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Old 05-04-2012, 09:53 PM   #10
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Vic, I have every sympathy with the struggle to get the right ready-made props. Even if you do find one that also comes in 'pusher' multi-bladers can be pretty expensive. That was at least partly why I now make up all my own from 2-blade wood props - either Zinger of Turnigy. I have just made up four 3-blade props for my latest build and compared to buying them ready-made it was equivalent to getting four for the price of ONE ready-made. I actually worked out that it was also equivalent to being paid a bit over 40 per hour for my time spent doing it. If you haven't thought about making up your own before, then now might be a good time to do so - I'll post a couple of photos for you if you're prepared to think about it. My philosophy about props is that I assume I will use scale diameter props at the start of designing the power train so the model looks right either static or flying - and in photos which often stop the prop and give away a non-scale configuration.

Nice try - but no cigar!
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Old 05-04-2012, 11:08 PM   #11
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Actually I should be able to do better than a couple of pics - I had forgotten I posted this on YouTube!...

Nice try - but no cigar!
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Old 05-05-2012, 09:22 AM   #12
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Howdy Vic,
Here's a set up you may want to consider.
http://www.hobby-lobby.com/3_blade_h...32890_prd1.htm
I myself will be buying a 4-Bladed set to check out for a P-51 build that's coming up. As far as the reviews go I'm thinking there may be some whining going on. Just my thinking though. IMHO to get what you want, sometimes you have to put forth the effort.

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Old 05-05-2012, 09:58 AM   #13
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Originally Posted by eye4wings View Post
Actually I should be able to do better than a couple of pics - I had forgotten I posted this on YouTube!...
Very interesting "How To" but you didn't state what type of glue you used. Also I'm curious as to how strong the props are? Have you had any failures in the glued hubs & what size motors are you using them on? I suggested this to a "wood working" shop owner flying buddy and in his opinion he didn't think it'd be a safe thing to do but I still wonder from time to time about doing what you've done.

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Old 05-05-2012, 11:31 AM   #14
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I've not had a failure yet John.

There may be several reasons - or different ways of looking at it.... chief among them being that the wooden props I buy as main ingredients are capable of taking the hammering given to them by IC engines firing off once per revolution, so using them on an electric powered model probably means that they are at least twice as strong as they need be. Thus working on the hubs I could afford to 'lose' a lot of strength - not that I think I lose much, if anything.

A professionally produced prop relies on butt joints (where I scarf joint) metal plates top and bottom (I use glued on 1/16 inch ply - sometimes) and a steel pin (where I use wood screws, which pull the parts together) so using ordinary white wood glue I reckon I probably have pretty comparable strength.

Casting blades has been a problem with some off-the-shelf makes - probably because the metal pins act as spacers and don't allow the propeller blades to be clamped firmly by the prop nut. So if the blade hub dries out and shrinks there is no safety net. No such problems with DIY. We can give each joint our personal attention.

I have used these props on up to 53mm dia motors, but see no reason why they should be size limited. After all as the motor sizes go up so does the size of the prop and the amount of wood in it. Perhaps more telling is that I normally over-prop my motors so that I can reduce the cell count used to power the setup. This means in turn lower revs and higher prop efficiency using pitches much closer to full-size practice. I cannot think of any disadvantage in making up your own multi-blade props - and 4-blades are easier than 3 and need no screws - and the cost saving clinches it for me.

I never did see the point of spending more than absolutely necessary on what goes into the hobby. Some seem to delight in spending $300 on a motor which probably only offers an improvement in efficiency of 10-15% over one that could be bought for $40, but I take more pride in frugality... and making props is a profitable activity from that point of view.

A scale 4-blade prop on your P-51 would make all the difference in my opinion. Up the blade size and lose a cell or two. Go for it!

Nice try - but no cigar!
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Old 05-05-2012, 01:06 PM   #15
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I would preach caution about going up in size.
Yes the amount of wood goes up but the forces involved go up much faster, at least to the square of the prop diameter.
Full size wood props had multiple laminations (you in effect only have one) and even then they eventually had to go to 'inserted' blade with metal hubs above a certain horse power.
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Old 05-05-2012, 05:54 PM   #16
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Granted, but they were on IC engines. We are a whole lot kinder to our props. I have run 20"x10"x3 on a 60" span warbird. I don't know the span of John's P-51 of course but I'm guessing it won't be even 1/4 scale.

Nice try - but no cigar!
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Old 05-05-2012, 06:17 PM   #17
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Originally Posted by eye4wings View Post
Actually I should be able to do better than a couple of pics - I had forgotten I posted this on YouTube!...
Thanks, I find this interesting. I will look into this more.

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Old 05-05-2012, 08:06 PM   #18
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Originally Posted by eye4wings View Post
Granted, but they were on IC engines. We are a whole lot kinder to our props. I have run 20"x10"x3 on a 60" span warbird. I don't know the span of John's P-51 of course but I'm guessing it won't be even 1/4 scale.

Thanks for expanding on your process. I understand it much better now
No I don't have a 1/4 scale Mustang
I do have several different size kits though from the Ace Simple Series kit that I plan on building into a speed demon (for me anyway) using this motor from HURC, http://www.headsuprc.com/servlet/the...450-Jet/Detail
I have 2 HOB P-51 kits, the .20 & .40 size and a unknown brand .46~.60 sized Miss America Mustang that I'm planning on installing this 1400W outrunner in it, http://www.headsuprc.com/servlet/the...h-04%29/Detail
I also used the Ace Simple Series kit for patterns to build a P-82 several years ago but never have put any radio gear in it yet. Looking at motor choices for it that will fit into the cowls that I scratch built to resemble the way it should. Other than the Simple Series speed demon Mustang I plan on, want to use 4-bladed props.
Before I start on the Mustangs though I'm trying hard to finish up my Special Twin scratch build that's taken almost 3 years to build & I'm also trying to finish up my Wing Mfg. A-26B Invader as well.
If I can ever figure out how to work the photo download from my camera to my computer I'll start posting photos again. All it wants to do is download every photo on my camera at one time
Hopefully I can figure it out soon.

Vic,
Sorry to get so far off topic on your thread.

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Old 05-05-2012, 08:19 PM   #19
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Glad to hear it Vic!

Another good thing about moving to wooden props is that the plastic or glass-filled types are pretty heavy, so you will also save some weight. Only a little bit compared to the total weight of airframe, but 'every little helps' after all.

The two props I showed at the end of the video were 13" x 8" x 3 - which is right on target for what you want.

This isn't exactly the prop you probably need (I just happened to have it bookmarked) but it has a lot of blade area and you won't find a cheaper prop to start with - it is a bit chubby though for a proper scale look...
http://www.hobbyking.com/hobbyking/s...ler_13x10.html

Nice try - but no cigar!
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Old 05-05-2012, 09:07 PM   #20
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Perhaps a few thoughts on scale modelling wouldn't take us off subject too badly since I notice that there seems to be a divergence of opinion on what constitutes 'scale' in just this one short thread.

One of the main aims for me is to get the speed something like right for the size of model. This means building light and flying slow - which really helps give the impression of size and gives the model a real presence in the air. But it limits the number of days the model can be flown to those with under 10 knot winds... or it does for me anyway - I don't like to have to fight hard to keep my model on a straight course. I'm not of an age to want too much adrenaline in the system!
One of the things I detest is seeing a beautifully detailed scale WW2 model being flown around as if it was an F-15 in a hurry - destroys the whole effect for me.
I'm not one who has ever tried to simulate rivets (I'm in it for a quick effect -strictly 'sport-scale') but the camera can effectively stop a propeller and even if it isn't visibly off scale to the observer on the ground a photograph showing a tiny two-blade prop on a P-51 (or any other warbird) is just as much a give-away that it's a model in the air as on the ground.
Inapropriate aerobatic manoeuvres are also a no-no in my book, which might put me at variance with Vic if the purpose of his departing from scale by having opposite rotation is to be able to do aerobatics with his B-25. I don't want to see it!

Likewise your 'speed demon' P-51 John - that motor is meant for ducted fan with a small diameter fan that wastes at least half the possible flight time of your batteries because of inefficiency. Put four blades on that and you'll have such a small prop on the model that it'll look ridiculous - or you'll risk a fry-up!
I think you would be much better to look for a motor with under 1000kv then you'll be able to use a larger prop that will run far more efficiently and increase your flight time. Of course if the model doesn't have wheels there's no point bothering changing from a little plastic 2-blade prop - you'd break it every landing - and somersault to a broken tail end in all probability.

Anyway 'yer pays yer money an' takes yer choice'... Or 'whatever floats your boat'... That's just my opinion... and I wouldn't dare charge tuppence for it!

Nice try - but no cigar!
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Old 05-05-2012, 10:03 PM   #21
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Twice so far my reply hasn't displayed. It did this to me the other day too. So I'm trying the Quick Reply just to see if it does the same thing.

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Old 05-06-2012, 07:43 AM   #22
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I've had a glitch or two as well John and wonder if some site maintenance has been going on.

In fact I meant to PM you about downloading from camera but didn't do it because my mind was taken off it by one of them - which I found was due to software I am running on my PC. I am running NoScript as a way of protecting against undesirable intrusion but on some sites I have to remember to tell it to 'temporarily allow all this page'. This is one of them.

If you are using the Windows down-loader the third window that opens will display all the photos it finds on your camera. At the top of each is a tick box which has them all ticked by default, but under them there is a box that will deselect them all and then allows you to select only the ones you want.
Other down-loaders I can't speak for as I don't use the camera's own systems although they are installed for their drivers. (I have two cameras of different makes and don't like my life to get too complicated!) They ought to have some way of doing the same I would have thought.

Nice try - but no cigar!
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