# Electric and Counter Rotation

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#### henryk

##### Well-Known Member
=100 HP differential gearbox have 2.5 kg mass (circa 0.5 l volume)

and work at temperatur 40 deg.Celsie ...
BTW=classic gear (reductor 3:1) have similar weight and volume,
big reaction momentum.

CR differential gear have 1/3 of this reaction momentum.

#### blane.c

##### Well-Known Member
HBA Supporter
Logic dictates than when using two propellers counter rotating, that the loss of one propeller is greater than a 50% loss in efficiency. So if the loss of a propeller keeps you flying "ok".

#### henryk

##### Well-Known Member
Logic dictates than when using two propellers counter rotating, that the loss of one propeller is greater than a 50% loss in efficiency. So if the loss of a propeller keeps you flying "ok".
F=m*a...=m*(a1+a2)

(second propeller accelerate air mass too...)

in case of differential gear the RPM of ich is in inverted proportion
to the drag momentums.

#### daveklingler

##### Well-Known Member
Maybe. It sounds great - "I don't have all that energy in spiraling wash" - and that has its appeal to the point where a bunch of Roll Royce Griffons were built with contra rotating props and a Royal Navy airplane was produced with two turbines each running a prop on one shaft.

We start somewhere around 85% efficiency with just a prop, then we run a second prop with the attendant losses in the gear train and in a second prop thrashing the air in order to get back some of 15% we were losing. Generally you do not make enough extra efficiency to carry the extra weight. Some folks have improved the net efficiency but only at specific points in the operating range, while the rest of the operating range breaks even or loses.

If you want to do it, knock yourself out. A gear geek's party. But unless you scheme out something special or really new, it will likely be no more efficient as turning one good prop.

Billski
Like I said, contra-rotating props are kind of the killer app for electric motors. You can get redundant motors for not much extra weight, because motors have gotten so amazingly light anyway. There is no gear reduction.

The title of the thread is "electric and gear reduction counter rotation".

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#### RonL

##### Well-Known Member
HBA Supporter
Like I said, contra-rotating props are kind of the killer app for electric motors. You can get redundant motors for not much extra weight, because motors have gotten so amazingly light anyway. There is no gear reduction.

The title of the thread is "electric and gear reduction".
I agree with what you said, but I think the latest bit of conversation caused your eyes to play a trick on your mind, the thread title "Electric and Counter Rotation"

#### wsimpso1

##### Super Moderator
Staff member
Log Member
Like I said, contra-rotating props are kind of the killer app for electric motors. You can get redundant motors for not much extra weight, because motors have gotten so amazingly light anyway. There is no gear reduction.

The title of the thread is "electric and gear reduction".
If you read me just a little less narrowly, you get the same conclusion. The second prop, whether gear driven in the opposite direction or by splitting power into two motors, still requires extra "stuff" to run that a single prop does not. In electrics that is unavoidably some extra weight from duplication of many elements, bearings between the shafts, etc. And running more blades, even with areas reduced, is unavoidably some extra blade drag from more root and tip interfaces, blade wake interactions, etc. The added weight must be carried (induced drag on the airframe increases) and the efficiency gains are less than we would hope. The efficiency has to result in more power expressed in moving air than the complexity and weight costs you or there is no point in the exercise.

The reason we already know about this, is these same things have been gone through on a number of other similar manner. Single vs multiple piston engines driving props, single vs multiple turbine engines driving props, single vs multiple jets, counter rotating props on piston and then on turbine powered airplanes, single vs multiple turbine engines on helocopters, etc. The arguments that motors are small and light so multiplicity has less impact have been made with jets and turboprops (fixed wing) and turboshafts (helos).

In engine driven systems, it has only rarely been thought to be worth the trouble and weight. In electrics it may be a little more broadly applicable, but the same issues of duplicated systems do come to the fore and eat into the benefits...

The one real beauty in coaxial props with multiple motors for electrics is in redundancy. Carefully designed, we can prevent many single point failures from causing complete power loss at any one prop (or rotor), and that has considerable value in Failure Mode Management. Even then, you may have to build in quite a bit of excess power - the good engine is nominally making one-half the nominal power of the system, and it is trying to express that power while moving air through a prop and motor that are likely windmilling and can be sucking up significant power. In addition, the powered prop is likely being dragged down (max rpm-wise) some by the interactions with the unpowered prop, which can further reduce the expressed power. Say you are now down to 30% real power expressed as moving air when one is dead - are you able to fly level or are you just carrying the airplane to a different location for the forced landing? Adding in enough reserve power on one eats into your glide range and overall efficiency too.

The problems of a gas engine running two props instead of one show up when you go with one electric motor or two to do the same thing. The magnitudes may be a little different, but the problems end up being very similar...

Billski

#### blane.c

##### Well-Known Member
HBA Supporter
Just find a convenient place to locate, no extra gearing needed.

#### henryk

##### Well-Known Member
View attachment 85351 Just find a convenient place to locate, no extra gearing needed.
=one motor,NO gear, two propellers=reactionless drive,
very energy efficiant ... "CR FLIGHT" !

#### proppastie

##### Well-Known Member
Log Member
=one motor,NO gear, two propellers=reactionless drive,
very energy efficiant ... "CR FLIGHT" !

#### BBerson

##### Well-Known Member
HBA Supporter
Coaxial props are less efficient than a well designed single prop. The only reason to use a coaxial is limited ground clearance or something.
Coaxials were used for some helicopters for torque cancelation, but the nagatives are more than the positives so they are rare.
Briefly, a prop accelerates a tube of air. Trying to add more momentum with a second prop to that high speed air is less efficient. Better to put the second prop in unaccelerated air.

#### RonL

##### Well-Known Member
HBA Supporter
View attachment 85351 Just find a convenient place to locate, no extra gearing needed.
Let's look at that on the workbench

They don't show the readings on those meter boxes, but I think they might indicate some interesting results.

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#### pictsidhe

##### Well-Known Member
Contra props can run efficiently at higher advance ratios than singles. Though it isn't a common need at our speeds...

#### daveklingler

##### Well-Known Member
I agree with what you said, but I think the latest bit of conversation caused your eyes to play a trick on your mind, the thread title "Electric and Counter Rotation"
Oh, heck, I meant to type "electric and counter rotation". Sigh.

There. I fixed it.

#### henryk

##### Well-Known Member
Let's look at that on the workbench

View attachment 85364

They don't show the readings on those meter boxes, but I think they might indicate some interesting results.
-are they say,whot distance is optimal ?

(my english is not anoth to understand speech!)

#### henryk

##### Well-Known Member
Coaxial props are less efficient than a well designed single prop. The only reason to use a coaxial is limited ground clearance or something.
Coaxials were used for some helicopters for torque cancelation, but the nagatives are more than the positives so they are rare.
Briefly, a prop accelerates a tube of air. Trying to add more momentum with a second prop to that high speed air is less efficient. Better to put the second prop in unaccelerated air.
=auer model (1.3 m diametr) give us 14 kG thrust force from 1 kW
battery power (24 V *40 A )...not bad ?

K=14/1,36 *sqr(14/1.33)=33 (no motor & controller efficiancy !)

#### RonL

##### Well-Known Member
HBA Supporter
-are they say,whot distance is optimal ?

(my english is not anoth to understand speech!)
They did not say much, but I think they were talking about finding the most perfect spacing between the props. But what came to my mind is the possibility of having in-flight, computer sensed adjustments, that include spacing between the props and speed of each prop in relation to each other.
A way to maintain the most ideal airflow through the props, even as weather conditions change, air density and wind gust, being two.

#### henryk

##### Well-Known Member
They did not say much, but I think they were talking about finding the most perfect spacing between the props. But what came to my mind is the possibility of having in-flight, computer sensed adjustments, that include spacing between the props and speed of each prop in relation to each other.
A way to maintain the most ideal airflow through the props, even as weather conditions change, air density and wind gust, being two.
-CR system with differential gear is ADAPTIVE one=

RPM of propellers is in invert proportion to the actual drag momentum ...

BTW=most coaxial gears have constant (+1) / (- 1) proportion and neads second reductor... (n/1).

="auer" is differential inverter and integrally circa 3:1 reductor,summary 3kg mass!

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#### henryk

##### Well-Known Member
View attachment 85426

-CR system with differential gear is ADAPTIVE one=

RPM of propellers is in invert proportion to the actual drag momentum ...