Contra rotating propellers

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dog

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=is it possible to made quastion more understand-able for me ?

Not possible,and not important, it is a mis comunication,please disregard.

And thank you for the posts you make of your work with contra rotating propellers and other
efforts.
 

Dusan

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Your "experience "with contra rotating propellers and "straitening vanes" is what exactly?
Ducting? or actual factual aircraft?
Bieng snarky and hoping you have built a plane with "straitening vanes" all at once.
I'm in the process of building a ducted rotor, straitening vanes aircraft, along the lines of this: Aliptera

For the "experience" I can nominate countless papers I read on the subject and experiments I've done. Some are distilled into this short video:

As seen, vanes can counteract torque and recover swirl energy, and this is why I'm of the opinion that vanes in the slipstream are much better than contra rotating props: they recover the swirl rotational energy and also provide control in all 3 axes, even in hovering.

Especially when using a ducted prop there is no reason to use coaxial counter-rotating props, as there is no fresh influx of air. As I've mentioned, the only worth wile application of contra rotating props is high disk loading - like thousands of hp and the prop diameter is limited to 1-2 m diameter.
 

Dusan

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In hover mode it will need about 25kW for 1.8m diameter rotor and 206kg total weight. The lip wing I'm expecting to have about 450N of lift, about 20% of total. Full scale test will show if I'll get that.
 

Dusan

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Actually I'm planning to install a 50KW motor, the power required is a bit higher in transition, calculated at about 30KW.
 
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dog

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I'm in the process of building a ducted rotor, straitening vanes aircraft, along the lines of this: Aliptera

For the "experience" I can nominate countless papers I read on the subject and experiments I've done. Some are distilled into this short video:

As seen, vanes can counteract torque and recover swirl energy, and this is why I'm of the opinion that vanes in the slipstream are much better than contra rotating props: they recover the swirl rotational energy and also provide control in all 3 axes, even in hovering.

Especially when using a ducted prop there is no reason to use coaxial counter-rotating props, as there is no fresh influx of air. As I've mentioned, the only worth wile application of contra rotating props is high disk loading - like thousands of hp and the prop diameter is limited to 1-2 m diameter.
Good then.
A contra rotating prop as you have noted reduces the disk size,and historicaly this has
been applied to very high hp aplications,with complexity and added weight making it non competitive for homebuilt and other smaller planes.
Henryk's developement is light and mechanicaly
simple, which makes it atractive for an aplication
where propeler clearance is an issue,like in
a flying boat.
Ducted fans are also atractive in flying boats as
they need as much static thrust as they can get,
and your points regarding straitening vanes counteracting torque and recovering "swirl energy" while at the same time limiting the possibility of prop strikes on equipment and personel, which is a constant hazard in flying boats and float planes,is also interesting.
So a conra rotating ducted prop, reduced disk size, greater static thrust ,neutral thrust axis,and lowered risk of a prop strike.
 

Dusan

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Disk loading on the Henrik contraption I think is about 40kg/m2, on par with some helicopters. From Leishman - "Principles of Helicopter Aerodynamics" pg.69 swirl loss on a typical helicopter rotor is about 1%. Taking into consideration just that contra-rotating propellers blades are "seeing" only half of the Reynolds numbers of the equivalent single propeller (for same disk solidity, the chord is double for a single prop), the aerodynamic loses will be more than swirl loses. Not to mention loses in the gears and transmission. If you want to get rid of "reaction momentum" as put by Hendryk, use vanes.
 

Dusan

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a conra rotating ducted prop
For a ducted propeller, as I mentioned before, using contra-rotating prop doesn't make sense at all. The duct forces all the air to pass trough both propellers, there is no slipstream contraction, so no influx of fresh air into the downstream propeller, all the thrust advantage of coaxial rotors is lost. Picture from Leishman - "Principles of Helicopter Aerodynamics" pg.101 to show the slipstreams into both rotors. The coaxial helicopter have an advantage only if there is enough vertical rotor separation to enable the contraction of upper rotor slipstream before entering the lower rotor. Otherwise, as per Leshman: "Because the lower rotor of a coaxial operates in the slipstream of the upper rotor its net induced velocity is always higher for a given value of thrust ... and so the net system efficiency is lower." and "Other physical factors that seem to affect coaxial rotor performance include a thrust recovery effect through the removal of swirl losses in the downstream wake, although this effect seems only important at very high values of disk loading ..." so my opinion is unless you need to drive your 1m diameter prop with thousands of hp, just forget about coaxial contra rotating props, the swirl loss is too low to matter.
Screenshot from 2020-07-03 11-09-37.png
 
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Sockmonkey

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One application where I can see contra-rotating props being most useful is in a tractor delta type as most of the thing is going to be in the propwash so you want the airflow straight-ish. Plus your lever arm for roll control is short, so you want to minimize the torque effect of the prop.
 

dog

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For a ducted propeller, as I mentioned before, using contra-rotating prop doesn't make sense at all. The duct forces all the air to pass trough both propellers, there is no slipstream contraction, so no influx of fresh air into the downstream propeller, all the thrust advantage of coaxial rotors is lost. Picture from Leishman - "Principles of Helicopter Aerodynamics" pg.101 to show the slipstreams into both rotors. The coaxial helicopter have an advantage only if there is enough vertical rotor separation to enable the contraction of upper rotor slipstream before entering the lower rotor. Otherwise, as per Leshman: "Because the lower rotor of a coaxial operates in the slipstream of the upper rotor its net induced velocity is always higher for a given value of thrust ... and so the net system efficiency is lower." and "Other physical factors that seem to affect coaxial rotor performance include a thrust recovery effect through the removal of swirl losses in the downstream wake, although this effect seems only important at very high values of disk loading ..." so my opinion is unless you need to drive your 1m diameter prop with thousands of hp, just forget about coaxial contra rotating props, the swirl loss is too low to matter.
View attachment 98878
Without having your reference material I have make a number of assumptions about the example cited, which are that the disks will be
the same size and operating at the same speed,
which are limitations that are not universal.
Henryk has demonstrated that the two disks
can operate at different speeds in a contra rotating propeller.
So that opens up the possibility of two
disks of different size(and pitch) operating at
different speeds in a duct.
To produce the most thrust in the smallest physical dimensions, with a neutral thrust axis,while also providing a
guard against prop strikes to equipment and personel.
The danger of prop strikes in a float plane/flying boat come from the water environment where a
float plane will weather vane with the wind and or
drift with the water current,leading to bad things.
Also interactions with the propeller and water
drive the whole design of flying boats.
Another significant factor in flying boat operations is having enough thrust to get "on step",without carrying more engine power (and weight )than can be used in cruise flight, so they
tend to be slightly overpowered fuel pigs that are slow ,complicated and somewhat risky to operate.
If the the quest for speed is droped, then perhaps
the others can be improved, with perhaps an improvement in empty to gross weight.
 

Dusan

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=theory...

in practice=
Fi=1.7 m , 70 HP =230 kG thrust

Fi=1.7 m, single propeller=
180 kG...
In order to have meaningful results you should compare the rotors having the same solidity, otherwise you're not comparing apples to apples.
 

dino

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I think another application for contra rotating props would be gyroplanes. When rotor thrust is gone as in 0g manoeuvres, rotor control is lost. The torque reaction of the prop cannot be countered and the gyroplane rolls over.
 

henryk

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I think another application for contra rotating props would be gyroplanes. When rotor thrust is gone as in 0g manoeuvres, rotor control is lost. The torque reaction of the prop cannot be countered and the gyroplane rolls over.
=more , some picks ?
 

Dusan

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D=D1=D2...shure !
No, same solidity means same ratio of total blade area to the disk area. I assume you used same propellers for both tests, and that means for single prop you have only half solidity. To have same solidity, you should use a blade that is two times as wide (double cord) for the single rotor, same diameter vs the contra-rotating props
 

henryk

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same solidity means same ratio of total blade area to the disk area.
we was made comparation=

2place big PPG trike with 100 HP R912 +5 blade propeller= 180 kG thrust...

2place trike with G13 70 HP (SUZUKI) + 2 + 3-blade propellers =230 kG. +lower noise !
 
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