Author Topic: Contra Drive  (Read 4378 times)

Offline Chris Currie

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Re: Contra Drive
« Reply #15 on: 19, March 2016, 07:18:13 AM »
Hey Angus,

I'll let you test out my new budget alternative to a contra system. Simply bolt this on behind a conventional prop set-up and bingo, perfect axial thrust!  ;)

Offline Stuart Mellor

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Re: Contra Drive
« Reply #16 on: 20, March 2016, 09:32:10 PM »
Hi Brian,
Yep - get the difference. The Brenner types act like a car differential - stop 1 wheel (or prop) & the other prop receives all the power.
 
Adverrun types will turn their props at the same rpm regardless of pitch of either, although there still has to be some load sharing I think, in that if you reduce pitch on 1 prop, rpm will increase for them both.

One thing we all have difficulty with - If, say you were to run traditionally on, say, the tractor prop only- you would achieve an 8 min flight as normal, with normal current draw. Now (for me) the difficult bit! - using 2 props obviously does not double the load, or amps would go through the roof - so it must mean a massive increase in prop efficiency using 2 props to achieve similar current draws.

How can this be explained? Can it be that the original designers of contras knew of this phenomenon -or was it pure luck?


This from Wikepedia:

''When airspeed is low, the mass of the air flowing through the propeller disk (thrust) causes a significant amount of tangential or rotational air flow to be created by the spinning blades. The energy of this tangential air flow is wasted in a single-propeller design. To use this wasted effort the placement of a second propeller behind the first takes advantage of the disturbed airflow. The tangential air flow also causes handling problems at low speed as the air strikes the vertical stabilizer, causing the aircraft to yaw left or right, depending on the direction of propeller rotation.

If it is well designed, a contra-rotating propeller will have no rotational air flow, pushing a maximum amount of air uniformly through the propeller disk, resulting in high performance and low induced energy loss. It also serves to counter the asymmetrical torque effect of a conventional propeller (see P-factor). Some contra-rotating systems were designed to be used at take off for maximum power and efficiency under such conditions, and allowing one of the propellers to be disabled during cruise to extend flight time.

Advantages and disadvantages[edit]

The torque on the aircraft from a pair of contra-rotating propellers effectively cancels out.

Contra-rotating propellers have been found to be between 6% and 16% more efficient than normal propellers.[2]

However they can be very noisy, with increases in noise in the axial (forward and aft) direction of up to 30 dB, and tangentially 10 dB.[2] Most of this extra noise can be found in the higher frequencies. These substantial noise problems limit commercial applications. One possibility is to enclose the contra-rotating propellers in a shroud. It is also helpful if the two propellers have a different number of blades (e.g. four blades on the forward propeller and five on the aft).[citation needed]

The efficiency of a contra-rotating prop is somewhat offset by its mechanical complexity and the added weight of this gearing that makes the aircraft heavier, thus some performance is sacrificed to carry it. Nonetheless, coaxial contra-rotating propellers and rotors have been used in several military aircraft, such as the Russian's Tupolev "Bear" Tu-95.''

Stuart
« Last Edit: 20, March 2016, 09:51:39 PM by Stuart Mellor »

Offline Serious Power

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Re: Contra Drive
« Reply #17 on: 21, March 2016, 09:07:31 AM »
Hi Stuart,
As you know, I'm not an expert on things aerodynamic. However even the experts have to resort to 'trial and error' type testing to get the applied aero correct.
But I'll try for you.


Amp draw ; Assume using a pre-existing F3A motor (which Brenner did). The load is designed in and is a combination of chosen prop load and gearing ratio. His V3 uses 10 : 1 give or take.


Efficiency; Using the power to produce a 'straightened' airflow seems to be part of story. I think the rest is about 'drag'.  The drag from the much larger prop area seems to be more than off-set by the huge reduction in prop speed - ; 3500rpm  v  7000rpm.
I think (haven't time to look it up) that the 'square' of the speed is used in drag calculations.


Brian

Offline Stuart Mellor

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Re: Contra Drive
« Reply #18 on: 21, March 2016, 04:38:29 PM »
Hi Brian,

Aha -didn't realise rpms are now in the 3500 area. You are right , of course, about the square law re drag - double the speed & there's 4x drag. Not sure how that is mathematically related to rpm -probably a more complicated relationship.

Perhaps Peter can tell us?

It does look though - that efficiency of our props increases by more than the 6 - 16% area mentioned in the article. BUT - people say down line breaking is improved with contras - that means, er...   more drag - thus defeating the previous 'less drag' argument.

All in all, fascinating stuff - just when we thought we knew it all!

Stuart

Offline Sam Wragg

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Re: Contra Drive
« Reply #19 on: 21, March 2016, 06:29:07 PM »
Yes fascinating stuff.

Blade cavitation.........Like I said, cavitation was the only way I could describe the effects of blade cross-over and Angus's description of cavitation  just about sumarises the meaning of the word. Many hours of computer modeling go into the design of a ships screw so to reduce the effect of blade cavitation.simply because cavitation reduces the lifespan of the propellor. You also get cavitation in hydraulic pumps when there is air in the system and once again this causes premature wear.

Moving onto a finer pitch prop to propel forward does not necessarily mean an increase in revs. For example when the comp season ended I used to (for a break) diversify from f3a and mess about with different aspects of our glorious hobby. My fun fly model I once had was fitted with a piped super tigre G34.

The recommended prop for this set up was a 9x7. Unhappy with the  noise  I tried a 11x3.5? I was sum what taken by suprise of the effects. The revs had dropped, throttle response was instant and the power was up with unlimited verticals + better braking effect on the down lines.

If it's correct what Brian says about the blades rotate indipendant of each other with power bieng delivered to the prop that's the easiest to drive changes my thoughts.

Effectivly the contra gearbox must be like a differential on a car whereas all the power is delivered to the wheel with the least resistance. If this is true  I can't see the benefits of having 2 props of different dia or pitch.

This bring me on to my train of thought of "does blade cross-over generate resistance making the contra less efficient than it is/could be?"

Sam
« Last Edit: 21, March 2016, 06:34:07 PM by Sam Wragg »

Offline Serious Power

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Re: Contra Drive
« Reply #20 on: 21, March 2016, 07:31:33 PM »
Sam,
I outlined, briefly, the 3 contra types currently in use.
One of the 3 types employs a differential gear box ! - the other two do not !!


Are you suggesting a contra without 'blade cross over' ?


Brian

Offline Sam Wragg

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Re: Contra Drive
« Reply #21 on: 24, March 2016, 09:04:07 AM »
Hi Brian,

I wish I could give you a definataive answer one way or the other, without playing with a Contra drive system my ideas are mearly based on what I know and cannot be put into practice.

I would say a Contra rotating drive system would probably be more efficient with some form of propellor syncronisaition?.....efficiency, as in less power to drive the system......we all know  in battery powered propulsion systems power is capacity.

I think at this level we probably all have  some  form of understanding  how a propellor  works with the corkscrew effect concerning  flow of air the propellor creates....... the helix angle of the corkscrew effect is governed by the pitch of the prop

In a Contra system you have  a left and right handed rotation of air so when you get blade cross-over it must have some effect.

Example........ The front prop has a left hand (Negative) pitch prop with an helix angle of 40 deg. The rear prop has a positive pitch of 45 deg . So for a split second as both blades pass each other you have  a shear effect of 85 deg of flow of air. This shear effect must consume more power.

Maybe I'm wrong .......

T'Owdlad

Offline Serious Power

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Re: Contra Drive
« Reply #22 on: 26, March 2016, 07:54:25 AM »
Hi Brian,

Aha -didn't realise rpms are now in the 3500 area. You are right , of course, about the square law re drag - double the speed & there's 4x drag. Not sure how that is mathematically related to rpm -probably a more complicated relationship.

Perhaps Peter can tell us?

It does look though - that efficiency of our props increases by more than the 6 - 16% area mentioned in the article. BUT - people say down line breaking is improved with contras - that means, er...   more drag - thus defeating the previous 'less drag' argument.

All in all, fascinating stuff - just when we thought we knew it all!

Stuart


Hi Stuart,
Sorry, I meant to respond and got distracted.
Responding to you always requires some thought  :)


The difference in braking mode has to be about the 'relative' size of the various forces.
In braking the extra area of the two very large props is indeed the source of the braking power.
While braking the prop speed is low and the work load is low.
Now form drag becomes a 'relatively' large figure in the equation while induced drag is 'relatively' low.


Another way of looking at it;
The two large props are better (more effective/efficient) than one smaller one whether they are pushing or pulling.


Brian