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One engine or two? Tests say two.

Old 05-29-2011, 07:48 PM
  #1  
z-8
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Default One engine or two? Tests say two.

I've been experimenting with twin engine, single shaft brushless systems. The basic idea is to extend a common shaft through two brusless motors. Since brusless systems have so little friction, the efficiency of running one motor or both motors in the twin config is very similar to the numbers attained when physically separated. This requires two ESCs on a Y harness, each powered by independent batteries.

Cost savings occur on many levels. For example, one larger ESC is generally more than twice as expensive and often more than twice as heavy as two half-size ESCs. But the largest cost savings occurs with batteries. Since there are two batteries operating in parallel, you can buy both half-size, and half-C rating, batteries. This results in a much less expensive option for the same performance, and/or doubles the available battery technology with regard to C rating . Since batteries are the only recurring cost in the power system, savings can be substantial.

Here are general advantages that I've found of a twin engine approach:

- Less system weight per unit thrust ~ or more power per weight
- More torque at mid throttle
- Lower power system cost (considering motors, ESCs, and batteries)
- Doubles the available battery C-rating
- No thrust asymmetry like other twin engine designs
- Less frontal area ~ less drag
- Better crash survivability
- More servo power
- Reliability advantages of two motors
- Reliability advantages of two ESCs
- Reliability advantages of two BECs
- Reliability advantages of two batteries
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Old 05-29-2011, 08:51 PM
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Z8,

Your experiments are interesting but I'd like to see some back up to some of your claims, because from where I'm standing they just don't ring true.

For instance cost and weight... I've checked and if you pick items from the same manufacturer and same product line it's always MORE expensive to have a two x 50% twin, compared to one x 100% single. Two x 50% is heavier too.



Taking an example of Hyperion G3:
  • Two x 3 cell 1100mAh: cost = $55.90, weight = 200g
  • One x 3 cell 2200mAh: cost = 47.95, weight = 196g
The same is even more true for ESC's.. lets look at castle creations:
  • Two x CC Pheonix Ice 50: cost = $199.90, weight = 119g
  • One x CC Pheonix Ice 100: cost = $139.95, weight = 96.5g
It's also true for motos, using Hacker as an example:
  • Two x Hacker A30-12L (500W each): cost = $123.98, weight = 10.2oz
  • One x Hacker A40-10S (1000W ): cost = $99.99, weight = 7.43oz
These were just random examples but clearly illustrate that watt for watt EVERY component in a twin engine set up is both heavier and more expensive. I'm confident you would have to search long and hard before you found a case where using twin motors, ESC's and batteries was lighter and/or cheaper..please give examples if you think otherwise.


It's also incorrect to say that you could run lower 'c' rating batteries.. If each battery in the twin set up was 50% capacity then the 'c' discharge rating would have to be the same as one single battery in order to deliver the same total power.

As for reliability.. It's basic common sense that two of anything stand twice as much chance of experiencing a unit failure of as one does... ok you are very unlikely to experience 'total' failure, but a partial failure (failure of one of the motor/ESC/battery systems) is twice as likely as in a single motor system. Reliability of twins is therefore, at best, a mixed blessing. In reality it would mean that things went wrong twice as often, which in my book is a bad thing.

I don't have the data to disprove your claims for more torques at part throttle but based on the fact that it sounds intuitively incorrect and all your other claims don't hold water i suspect it's 'made up on the spot' too. Generally it's a fact that lower kv motor have more torque, also larger motors tend to have lower kv than small motors, so if anything the single large motor will have more total torque than the twin, at any throttle setting.

Frontal area is a total red herring.. the diameter of the motor is never an issue in frontal area of a plane, other design constraints invariably govern frontal area. We are not talking radial piston engines here! About the only time when this may make some sense is for an EDF.

By all means prove me wrong with some hard facts.

Steve
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Old 05-29-2011, 09:53 PM
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oh.. and as for crash resistance..

Not sure how you could claim that twins are any more resistant to crash damage. I'd have expected that on average having twice as many components 'in harms way' would result in more frequent damage?

Also the one thing from personal experience that is most prone to damage is the prop shaft. Thin 'spindly' prop shafts with large props attached are especially prone to bending or snapping, even due the mildest of prop strikes... Invariably with a twin like yours you will always end up with a relatively large prop on a relatively small diameter shaft, making the shaft MUCH more prone to bending and/or breakage.

I do appreciate you sharing your experiments, and they are quite interesting, but the fantastic claims you make just seem IMHO to be totally without foundation.
Sorry but other than 'novelty value' I'm just not seeing any advantage whatsoever in your twin set up

Steve
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Old 05-29-2011, 10:37 PM
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Taking an example of Hyperion G3:
  • Two x 3 cell 1100mAh: cost = $55.90, weight = 200g
  • One x 3 cell 2200mAh: cost = 47.95, weight = 196g
I'm sure you can find all sorts of bad deals where lipos of half the C rating and half the capacity cost about the same. Here's an example where buying both half the capacity and half C-rating generates the usual discount:

Turnigy 1300mAh 3S 20C Lipo Pack = $9
Turnigy 2650mAh 3S 40C Lipo Pack = $25

Recurring battery savings = 28%


The same is even more true for ESC's.. lets look at castle creations:
  • Two x CC Pheonix Ice 50: cost = $199.90, weight = 119g
  • One x CC Pheonix Ice 100: cost = $139.95, weight = 96.5g
Again, it's easy to find a bad deal and extend that to any scenario. Here is what happens when shopping for a good deal:

Turnigy Plush 30A = $12
Turnigy Plush 60A = $36

One time cost savings = 35%


Motors are more difficult to compare, due to wide differences in design and value. As one example, the combo I showed above costs $40 (2 x $20) and out-RPMs a $70 Power 25 with the same prop. It weighs 4.8 oz vs the Power 25's 6.7 oz.

Cost savings: 45%
Weight savings: 30%

(With more power)
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Old 05-29-2011, 10:50 PM
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Originally Posted by z-8 View Post

z8rc.blogspot.com
The most advanced RC site on the planet; probably all of the planets.

Why does your blog link go to the DISNEY channel? Nevermind I found it....

have a good one
cr

http://www.wattflyer.com/forums/showthread.php?t=58665
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Old 05-29-2011, 11:23 PM
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You can get wit dis

Or you can get wit Dat

I'm thinking one engine/motor is more efficient then conjoined engine/motors.
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Old 05-29-2011, 11:41 PM
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z8,

As i said in my original post.. If you want the same total amp rating you cant compare batteries with half the capacity AND half the 'c' rating. For the total peak Amps to remain the same the c rating would have to be the same. Your two half C rated and 50% mAh capacity batteries would only deliver a total of half the Amps of the larger higher 'C' rated battery.

taking your example:
1300mAh x 20c = 26A; two of these together = 52 Amps max discharge
2650mAh x 40c = 106 Amps max discharge

If you want to make a fair comparison compare the two x 1300mAh batteries to one of the same C rating: http://www.hobbyking.com/hobbyking/s...idProduct=9271
what do you know.. The single battery is much cheaper and lighter yet again!


As for ESC's.... Your 'good deal' is an ESC that you cant buy because it's out of stock Anyone who has ever ordered from Hobbyking knows NEVER EVER order items that are shown as stock level 'BK'.
How about comparing the same type Turnigy ESC's that are in stock:
2 x plush 40A = $46.20
1 x plush 80A = $39.99

Also consider that chances are you would not need a double Amp rated ESC because larger motors tend to run more cells and so higher voltage and relatively little or even no increase in amps. Larger higher voltage motors can work with a ESC little or no larger than a smaller lower voltage motor.. So cost and weight savings of the single motor are actually often much larger.

And the weight still increased for the twin however you look at it....

Last edited by JetPlaneFlyer; 05-29-2011 at 11:59 PM.
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Old 05-30-2011, 12:16 AM
  #8  
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Can't you buy them by the dozen? I am thinking why stop at 2 or 3? Just use this motor for everything! You can just add another till your are right with the world. Since you say they are 400w motors you power small stuff to 50% monsters. Just slap another motor and esc on till your happy.

I'm in!

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Old 05-30-2011, 12:20 AM
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As for motors being hard to compare.. i dont agree.. you just take motors from the same manufacturer one that has roughly half the power rating of the other.

taking your much favoured Super tiger motors as as an example:

Super Tiger 400 (two off) = 290W total = $43.98, 100g
Super Tigre 10 (one off) = 320W total = $23.99, 69g

So in this case, same manufacturer, 'watt for watt' the larger motor is close to half the cost and not miles away from half the weight...
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Old 05-30-2011, 12:58 AM
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Originally Posted by CarpenterDave View Post
You can get wit dis
I'm thinking one engine/motor is more efficient then conjoined engine/motors.
Actually it is really common to join cylinder banks to create a larger aviation engines....



But in the case of electric there is more of a benefit vs a single bank since you also double the battery discharge technology and bandwidth.
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Old 05-30-2011, 01:03 AM
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Originally Posted by z-8 View Post
Actually it is really common to join cylinder banks to create a larger aviation engines....


Not since the jet age, Larger, 'Aviation' engines are now jet engines.

Although I do appreciate the art of the multi cyl engine... been a classic VW enthusiast for 30 years.
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Old 05-30-2011, 01:04 AM
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Originally Posted by JetPlaneFlyer View Post
z8,

As i said in my original post.. If you want the same total amp rating you cant compare batteries with half the capacity AND half the 'c' rating. For the total peak Amps to remain the same the c rating would have to be the same. Your two half C rated and 50% mAh capacity batteries would only deliver a total of half the Amps of the larger higher 'C' rated battery.

taking your example:
1300mAh x 20c = 26A; two of these together = 52 Amps max discharge
2650mAh x 40c = 106 Amps max discharge

If you want to make a fair comparison compare the two x 1300mAh batteries to one of the same C rating: http://www.hobbyking.com/hobbyking/s...idProduct=9271
what do you know.. The single battery is much cheaper and lighter yet again!


As for ESC's.... Your 'good deal' is an ESC that you cant buy because it's out of stock Anyone who has ever ordered from Hobbyking knows NEVER EVER order items that are shown as stock level 'BK'.
How about comparing the same type Turnigy ESC's that are in stock:
2 x plush 40A = $46.20
1 x plush 80A = $39.99

Also consider that chances are you would not need a double Amp rated ESC because larger motors tend to run more cells and so higher voltage and relatively little or even no increase in amps. Larger higher voltage motors can work with a ESC little or no larger than a smaller lower voltage motor.. So cost and weight savings of the single motor are actually often much larger.

And the weight still increased for the twin however you look at it....
I think you missed the point - which is that the double motor naturally doubles the C rating since you have two batteries discharging into the system instead of only one. ESC bandwidth is doubled as well, as is the servo current available.

So if you buy 50C Lipos, this motor configuration results in 100C performance by design.

There are always price anomalies, but I've found shopping for the best possible deal results in a lot less money (and usually less total weight) buying two half size ESCs, likely because the tech engineering challenge is halved. Same goes for motors, due to basc materials limits, it is easier and more efficient to run two motors at a fraction of the technological limit than to run a bigger one right on the edge.
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Old 05-30-2011, 01:08 AM
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Originally Posted by Nitro Blast View Post
Not since the jet age, Larger, 'Aviation' engines are now jet engines.

Although I do appreciate the art of the multi cyl engine... been a classic VW enthusiast for 30 years.
Not really, jets are efficient for high speed applications where you must move a small amount of air a lot (including many-mach exit velocity with a converge-divergent nozzle). But props are massively more efficient for low speed applications where you may move a lot of air a little.

Thrust of the two engines can be the same, or it can different, depending on the momentum relationship created by mass flow and exit velocity.
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Old 05-30-2011, 01:18 AM
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Originally Posted by z-8 View Post
I think you missed the point - which is that the double motor naturally doubles the C rating since you have two batteries discharging into the system instead of only one. ESC bandwidth is doubled as well, as is the servo current available.

So if you pay for 50C Lipos, you get 100C performance by design.
Uhm, no.. You don't. I see what you mean and it makes total sense, but the C-rating actually stays the same. Why? Because with parallel coupling, you double not only the maximum amp draw, but also the Ah-capacity. And since the C-rating is the ratio between amp draw and capacity, the C-rating stays the same with you double both, right?

1300 mAh 20C allows 26A maximum discharge. So if you double up, you get 2600 mAh and 52A maximum discharge. But 52000/2600 = 20, so you're still left with 20C.

But I see what your point is. You double the maximum possible amp draw.
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Old 05-30-2011, 01:47 AM
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Originally Posted by NJSwede View Post
Uhm, no.. You don't. I see what you mean and it makes total sense, but the C-rating actually stays the same. Why? Because with parallel coupling, you double not only the maximum amp draw, but also the Ah-capacity. And since the C-rating is the ratio between amp draw and capacity, the C-rating stays the same with you double both, right?

1300 mAh 20C allows 26A maximum discharge. So if you double up, you get 2600 mAh and 52A maximum discharge. But 52000/2600 = 20, so you're still left with 20C.

But I see what your point is. You double the maximum possible amp draw.
Why doesn't capacity stay the same? You have two, half size batteries discharging in parallel = same system capacity.

You can definitely measure the effect using the double-motor itself in single-motor mode. If you attach a power meter to one side, the min voltage recorded under max throttle is a lot higher when both motors are operating than with only that one motor operating.
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Old 05-30-2011, 02:05 AM
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Originally Posted by z-8 View Post
Actually it is really common to join cylinder banks to create a larger aviation engines....



But in the case of electric there is more of a benefit vs a single bank since you also double the battery discharge technology and bandwidth.
So if I fly 7:00 minutes now, will I fly 3:30 or 14:00 minutes according to your science?


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Old 05-30-2011, 02:52 AM
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Originally Posted by CarpenterDave View Post
So if I fly 7:00 minutes now, will I fly 3:30 or 14:00 minutes according to your science?
How about a question to answer the question:

If I put a less restrictive air intake and exhaust system on a car engine, resulting in more top end horsepower, does maximum range increase or decrease?
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Old 05-30-2011, 05:56 AM
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Originally Posted by z-8 View Post
Actually it is really common to join cylinder banks to create a larger aviation engines....
I've wondered about the double rows of cylinder banks. Is it because a single row of cylinders with the same horsepower would wind up being a much larger diameter engine with a LOT of drag caused by wind resistance? The power output of these model radial engines is often a lot less than a similar displacement twin cylinder engine.

And, you don't see any radial engines being used nowdays, nowdays you see only the 4 and 6 cylinder aircraft engines for the smaller private airplanes. And perhaps a few custom modified engines for the home built airplane crowds.
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Old 05-30-2011, 06:42 AM
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Originally Posted by z-8 View Post
I think you missed the point - which is that the double motor naturally doubles the C rating since you have two batteries discharging into the system instead of only one. ESC bandwidth is doubled as well, as is the servo current available.

So if you buy 50C Lipos, this motor configuration results in 100C performance by design.

There are always price anomalies, but I've found shopping for the best possible deal results in a lot less money (and usually less total weight) buying two half size ESCs, likely because the tech engineering challenge is halved. Same goes for motors, due to basc materials limits, it is easier and more efficient to run two motors at a fraction of the technological limit than to run a bigger one right on the edge.

z8,

Either you dont know how 'c' rating works or you are deliberately feigning ignorance to try to prove a point.

I'll tell you how c rating works to avoid any more of this none-sense:

To calculate discharge rate of a battery you multiply battery capacity in mAh by 'c' number to give maximum allowable discharge rate in milliamps, (divide by 1000 to get Amps.)

So if you buy 50C Lipos, this motor configuration results in 100C performance by design.
If the two 50c batteries are each 50% capacity (as you state they are) then this statemant is totally incorrect. ... Two 50c batteries only = one 100c if the capacity of each individual battery was the same meaning the mAh capacity of the twin was twice that of the single, which would of course mean that the twin was MUCH heavier and more expensive (though would run longer).

If you have a half size battery and you also half the 'c' rating you end up with quarter of the discharge rate in Amps... not half. It matters not a jot if you have two x 50% battery or one x 100%.. it's the total capacity multiplied by 'c' rating that determines maximum Amp discharge.

In any case, if two small batteries were really lighter and cheaper (which they almost invariably are not) you could easily exploit it with a single ESC and motor by coupling the batteries in parallel with a 'Y' lead.. so it's all a mute point anyway.

As for me picking unusual examples to try to illustrate my point... I've just picked examples totally at random, in fact I've tried to deliberately used well know 'benchmark' product lines or even the same product lines as you referred to. You on the other hand have been unable to come up with one example to back up your claims (excluding apples and oranges comparisons or 'out of stock' pricing)
It is simply not true that it's normal for two 50% batteries/ESCs/motors to be cheaper and lighter than one 100%... I challenge you to come up with genuine mainstream examples where that is the case, comparing products that are in other respects similar (same manufacturer, same quality, actually available to purchase).

The only time that the twin might make some sense is at the very largest end of the electric motor range where a single motor of the desired power was either unavailable or was very limited in availability and so priced very highly. For mainstream product lines it just doesn't stack up.

Steve
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Old 05-30-2011, 06:54 AM
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Guys, you are wasting your time debating with mr z. He's just making this stuff up as he goes. Any argument or point of fact you put forth will just be ignored or rebutted with more silliness

As Steve has pointed out, all of his claims are nonsense and this thread is no different than any of his others.
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Old 05-30-2011, 07:01 AM
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Originally Posted by z-8 View Post
Actually it is really common to join cylinder banks to create a larger aviation engines....


But in the case of electric there is more of a benefit vs a single bank since you also double the battery discharge technology and bandwidth.

Double row radials were produced for two reasons:
  1. More power was required for new faster/larger aircraft. Double row engine was a way to doubled power while without re-designing the entire engine (simply bolt two existing units together).. This made the new engine much quicker to develop which was critical at the time (war)
  2. Radial engines have large diameter making the cowling aerodynamically inefficient. Two rows prevented diameter increasing.
The first point only applies to electrics if you are talking about very large power outputs (several KW) which may be unavailable in a single unit.

The second point is totally inapplicable to electric motors because electric motors are very small in diameter compared to a radial piston engine and rarely if ever pose any issues in regard to cowling.. In fact the spinner on the prop is normally of greater diameter than the motor!


But in the case of electric there is more of a benefit vs a single bank since you also double the battery discharge technology and bandwidth
this comment just shows your continues lack of understanding of how battery 'c' discharge rating works!.. And as for 'bandwidth', how does that apply to a battery
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Old 05-30-2011, 07:08 AM
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Originally Posted by Larry3215 View Post
Guys, you are wasting your time debating with mr z. He's just making this stuff up as he goes. Any argument or point of fact you put forth will just be ignored or rebutted with more silliness

As Steve has pointed out, all of his claims are nonsense and this thread is no different than any of his others.
Larry,
You are quite correct..

I will not feed the troll any further!

Steve
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Old 05-30-2011, 07:08 AM
  #23  
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Hey now,
A few minor bits from my perspective;
I've run multi motor banks before. Both outrunners on a comon shaft and inrunners sharing a gear box. In both cases I did it because back then huge motors weren't available to me and in the case of the geared set up I later used both drive sets in split up on two different models. This ability to split them up later makes up for some of the short comings.

On the dyno both systems sort of fought each other a bit and so you couldn't just double the nbers and get an accurate idea of out put.

Also one of you complained about using "bad deals" to make the others idea look bad. The one quoted prices of Castle products and the other came back with Turnigy. Yes Castle may cost more than Turnigy but that doesn't make it a bad deal by any means. I've used both and I'd never choose a Turnigy over a Castle again they're cheaper alright. Cheaper in every way. Yuck.

Same with the Hacker vs. Turnigy motor bit. I'm not sure about the Hacker outrunner but their inrunners, I'd much rather pay for a motor running at ninety percent effeincy than one that runs at only so ty five percent. If I'm puttin a thousand in I'd be much happier getting nine hundred out than six fifty. Your online twin would be fun to play with but switching from your twins at sixty five to a single running ninety makes far more sense.

On the other hand if you ran one motor on a hollow shaft and ran the second with a solid behind it you could do contra rotating props. Now that would be nifty fun.
RobII
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Old 05-30-2011, 07:19 AM
  #24  
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Good points Rob. The only reasons to do something like this are as Steve said earlier - for the fun of it or because there isnt a single motor large enough.

These dual motors have been done before a few times - Axi, MGM, Hacker and one other I cant recall off the top of my head.

They have all fallen by the wayside for three reasons - they invariably cost more than a single larger motor and they are also heavier and less efficient - not more efficient.

So, all of z-8's claims are just pure hog wash. Id advise paying no attention to any of his claims
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Old 05-30-2011, 07:26 AM
  #25  
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Originally Posted by kyleservicetech View Post
I've wondered about the double rows of cylinder banks. Is it because a single row of cylinders with the same horsepower would wind up being a much larger diameter engine with a LOT of drag caused by wind resistance? The power output of these model radial engines is often a lot less than a similar displacement twin cylinder engine.

And, you don't see any radial engines being used nowdays, nowdays you see only the 4 and 6 cylinder aircraft engines for the smaller private airplanes. And perhaps a few custom modified engines for the home built airplane crowds.
A lot of classic airplanes, like the F4F Wildcat, F6F Hellcat, F4U Corsair, F8F Bearcat, PBY, C-47, and B-24 used twin radial engine designs within each nacelle. While some post-war planes like the B-36 used four stacked radials inside each of its 6 piston engine nacelles.

Radials connect each cylinder to a common point on the shaft, so every other cylinder fires sequentially around the circle, so you need an odd number to naturally cycle into the power and exhaust strokes. Twin radial engine aircraft like the 14-cylinder (even number) Wildcat fired in this sequence only within each radial bank, effectively making it a twin 7-cylinder engine on a single shaft. I'm sure that reduced frontal area had a lot to do with it, as well as simplicity of manufacturing. Being air-cooled, it would be hard to stack too many radial engines behind one another.

There are radials in use today, some of the more famous users being the Pitts Model 12 & 12S Python, Yak-54/55, Su-26, and the Waco YMF.

The point is that stacking two or more motors on a single shaft is nothing new in aviation. It's tried and true.
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