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A pitch too far

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Hogster10/03/2013 08:57:35
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I have always had a problem with my Seagull T6 when it comes to landing. Slowing it down for landing requires a very long final approach and throttling back on the down wind leg. Its fitted with a SC70fs turning a 12X7 prop. It cruises at hafl throttle nicely. I thought I would try reducing the prop pitch to help slow the model down quicker for landing. Well, the choice I had to hand was a 12x6 or 12x4. I thought the 12x6 wouldn't make much of a difference so opted for the 12x4.

The change was quite dramatic. I needed pretty much full throttle for general flight and drop in speed when I reduced the throttle was surprising. My first landing was well short and my on my second landing I was too slow and tip stalled at about 2 foot high.

I got away with minimal damage and gained valuable experience in the effects that prop pitch has on a model. Needless to say I have changed the prop to the 12x6 and depending on how that works may venture to trying a 12x5.

Delta Whiskey10/03/2013 09:01:35
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A good lesson in prop effects - thanks Hogster !

bouncebounce crunch10/03/2013 09:25:54
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There is a recomended prop and tuning range for each engine that you buy, all is well written in that discarded instruction booklet. get it out and read again and again and again, otherwise it gets costly, costly, costly. yes prop pitch and diameter can make a change but not as beneficial as you think. If you ask Wizard of Woo he will get frumpy and want to hit his head on the bench because the 40 npage book that comes with the lump of fuel converting machine never gets read. read, read, read.

i12fly10/03/2013 10:10:13
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In support of Mr Hogster the SC instructions are so basic as to be a waste of time (although the last I bought was 2 years ago). They give a prop range but no advice on selection, you have to learn by experience -your own or others. Also the 70FS model prop size is omitted altogether in the instructions I have, they list the 52FS and then jump to 90FS.

I run 13x6 on both my SC70FS and this is fine for my planes and get about 10K revs, so I'd suggest that if you go smaller pitch it would be best to up the diameter otherwise noise could be an issue or maybe even overrev the engine?

Ruprect Spode10/03/2013 10:45:09
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Basically a propellor with a 6 inch pitch means that 'theoretically', for every revolution the propellor will 'screw' through 6 inches of air, therefore by fitting your 4 inch pitch propellor for every revolution it will screw through 4 inches of air. This obviously equates to a 'theoretical' 33.3 percent reduction in amount of air screwed through per revolution. Thats a big drop.

Ruprectface 23

Mike Rolls10/03/2013 11:25:09
500 forum posts
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Posted by Ruprect Spode on 10/03/2013 10:45:09:

Basically a propellor with a 6 inch pitch means that 'theoretically', for every revolution the propellor will 'screw' through 6 inches of air, therefore by fitting your 4 inch pitch propellor for every revolution it will screw through 4 inches of air. This obviously equates to a 'theoretical' 33.3 percent reduction in amount of air screwed through per revolution. Thats a big drop.

Ruprectface 23

But with IC the revs will increase in partial compensation not sotrue for electric, of course..

Mike

Biggles' Elder Brother - Moderator10/03/2013 11:52:48
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One way to think about props is in terms of the "load factor" they put on the motive source. This load factor involves both the prop pitch and its diameter. There are plenty of proposed equations for prop load factor - but they nearly all have one thing in common, they involve multiplying the pitch by the diameter. So we can say that:

Load factor is linked to: pitch x diameter.

What load factor we use is entirely dependent on the engine/motor. To run at maximum effectiveness without overloading we need to keep the load factor constant at the recommended level for the power unit. So a quick inspection of the relationship above will confirm that if the pitch goes down, then the diameter must come up to keep (pitch x diameter) approximetely constant. Unfortunately it is not a simple "an inch for an inch" type of relationship because the diameter is commonly raised to a power in these equations.

It is for this reason that a large change in one parameter without a compensating change in the other is going to lead to problems. Too low a load factor on an IC engine runs the risk of over-reving. Too large a load factor on an electric motor runs the risk of burn-out.

Generally a change of one inch in either on an average size prop that is running well is "OK" - but anything more than this really does need matching compensation in the other paramemeter.

You might like to check out the Feb and March 2012 editions of the mag where I discuss this sort of stuff in more detail.

BEB

Charles Smitheman10/03/2013 11:55:59
226 forum posts
18 photos

The rule of thumb that seems to work for me:

Start with the engine manufacturers recommended prop.

For a fast model use a smaller diameter and a coarser pitch, and vice versa.

One inch of diameter roughly equates to two inches in pitch.

So my Slower speed Wayfarer Biplane powered by a Saito 82 is happy with a 14x6

The Midget Mustang scale racer with the same engine likes a 13x8 I would struggle to land this without flaps.

Slow first world war models such as my SE5a go really well with funfly type props (In my case a Saito 62 loves an APC 14X4W) Slowing this one down for landing is not an issue!

Provided you do not over rev engines, generally they are happy with slightly smaller props, too big a prop can lead to overheating. Engines using tuned exhausts are particularly sensitive to this and can be easily destroyed. But that is another can of worms.

With electrics it can be more dramatic if you swop to a bigger prop you can have a FIRE! Use a wattmeter. Being a bit dumb with electrics I normally pester someone clever and get them to do this for me.

Tangental10/03/2013 13:37:58
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Posted by Charles Smitheman on 10/03/2013 11:55:59:

With electrics it can be more dramatic if you swop to a bigger prop you can have a FIRE! Use a wattmeter. Being a bit dumb with electrics I normally pester someone clever and get them to do this for me.

Want a laugh? Last week I was testing various motor setups through a watt meter, trying to achieve a constant 160 watts. Fixated on the meter I was celebrating the fact I had done so with one combination when I noticed a burning smell from the motor, with sudden realisation I closed the throttle, the motor was red hot although the esc and battery were just slightly warm. The meter cycled through the results achieved, 22amps averaged, motor rated to 15amps, oops!blush (This electric conversion thing might not be 'my thing'. I think I'll keep some of my fleet IC.)

Tange

Dave Bran10/03/2013 14:01:05
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As the JP website Ad specs table before even looking any further suggests 13x8 as the size for that engine, if you believe as told, even a 12x7 could be considered on the small size being down on pitch AND diameter, a 12x4 therefore most likely resulting in ballistic revs and little effective thrust down the throttle range, and in short order goodbye engine territory if the airframe doesn't give up first .

Charles Smitheman10/03/2013 14:47:08
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Hogster,

Just got back from walking the dog, the wife admired the view, I thought about planes- so what's new!

Walkies thoughts:

Bigger diameter = larger braking disc at idle.

Also with my fourstrokes it is not unusual to set a reliable low idle before takeoff, and find that as the engine warms up, and the mixture leans out slightly due to the fuel level in the tank dropping, the minimum set idle speed can increase quite significantly. A low pass and listen to the engine helps, then you can decide if it needs to have a few clicks of throttle down trim. This can make a big difference, particularly with a slippery model.

Dave Bran10/03/2013 16:59:48
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Posted by Charles Smitheman on 10/03/2013 14:47:08:

Bigger diameter = larger braking disc at idle.

Very true, providing you can get the idle low enough. I have a 91 2 Stroke powered Panic. It might be considered a draggy biplane but the 91's torque and power at idle gave a high enough idle even with a low pitch fun fly prop so that landing it was an issue.

I set up idle down on a three way with normal up, idle down in the middle, and cut on the lower, so during normal flight there was little or no liklihood of deadsticks, then on finals as the threshold is approached I switch in the lower idle. If it deadsticks then (and now its fully run in it doesn't for plenty long enough) I can glide it the rest of the way no trouble.

Another way with dual aileron servos is to set up a very small amount of spoileron (both up) which helps lower both speed through drag AND by increasing the incidence angle during approach.

Hogster10/03/2013 18:06:25
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507 forum posts
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Thanks for all the replies fellas. Plenty of food for thought. I understand the theory but there is nothing like seeing how it works out in a practice. wink

The engine is well used and is not at its best and, even with a 12x4, it never seemed to able to reach maximum revs.

The manual states anything from a 12x7 to 14x6. So given the issues with slowing down for landings what size prop would you guys suggest?

Edited By Hogster on 10/03/2013 18:07:03

Biggles' Elder Brother - Moderator10/03/2013 18:44:48
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If you want slow, and if the engine (which you say is tired) will swing it, and if you can get a steady slow idle; then 14X6 will do the job. Low pitch, big diameter, low revs, lots of drag! But don't expect it to accelerate like a scalded cat!

If you're not happy with that then a 13x7 - I know that is the pitch you started with - but with an inch on the diameter the revs will be lower and you will get a tad more drag effect on approach.

As someone who like scale-ish models I always favour a bigger diameter prop! smile

BEB

Frank Skilbeck10/03/2013 18:56:14
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Posted by Mike Rolls on 10/03/2013 11:25:09:

But with IC the revs will increase in partial compensation not sotrue for electric, of course..

Mike

Actually if you fit a smaller prop on an electric motor it will turn at a higher speed, just not by very much. With an IC engine you should prop it so it's working below it's max power on the ground and it can then unload in the air, if you put too small a prop on the engine you'll go over peak power and hence you won't get much of a speed increase. On a four stroke i the revs go too high you may also start to get valve float, it's when the valves springs can't keep up with the valve train inertia so the valves don't close exactly when the cams tell them too, result is reduced power or worse valves meeting the piston.

One thing you could try is a 13 x 6 prop and a throttle activated glow plug power supply so you can get a lower reliable idle.

Tom Sharp10/03/2013 19:14:18
387 forum posts

Of course to make any sense of this discussion you need to start with an OS engine.

cool

Peter Beeney10/03/2013 19:32:57
1593 forum posts
59 photos

Or a Laser…….

PB

Mike Rolls10/03/2013 22:34:18
500 forum posts
22 photos
Posted by Frank Skilbeck on 10/03/2013 18:56:14:
Posted by Mike Rolls on 10/03/2013 11:25:09:

But with IC the revs will increase in partial compensation not sotrue for electric, of course..

Mike

Actually if you fit a smaller prop on an electric motor it will turn at a higher speed, just not by very much. With an IC engine you should prop it so it's working below it's max power on the ground and it can then unload in the air, if you put too small a prop on the engine you'll go over peak power and hence you won't get much of a speed increase. On a four stroke i the revs go too high you may also start to get valve float, it's when the valves springs can't keep up with the valve train inertia so the valves don't close exactly when the cams tell them too, result is reduced power or worse valves meeting the piston.

Frank

That's why I said 'not so true'. The effect of changing prop sizes on RPM is far less marked with electric than with IC - what changes greatly is the current draw. On the question of peak power on IC my experience is with 2 stroks only - never owned a four stroke - but very, very few IC engines in run of the mill models (excepting the like of C/L speed and team race and R/C pylon) are being run at peak power - if they were the noise complaints would be never-ending!

Mike

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