# Contra rotating propellers

### Help Support HomeBuiltAirplanes.com:

#### Dusan

##### Well-Known Member
From disk actuator theory, a "maximum value" of thrust can be easily calculated, and for a 100HP, 1.7m diameter propeller, a thrust "maximum value" of 319Kg is calculated. For a good propeller, you should get to 0.8-0.9 of that, that's about 270Kg thrust. So, as you are not even close to that value, I suspect something is wrong with your setup, either the engine is not matched to the propeller, or most likely your propeller is stalled, as you mentioned the noise.

For 70HP, same diameter disk, the theory predicts 252Kg "maximum value" thrust, your setup has 0.91 of that. As you're getting such a high value, I suspect you have some fresh inflow on the aft propeller, that effect is noticeable from 0.2d rotor separation(Leishmann). Keep in mind that the effect is lost if there is too little separation, or if the props are ducted.

Number of blades. Less blades are always more efficient. A 0.1m blade moving trough air at 200m/s is having about Re=1,400,00. A 0.05m blade is having Re=700,000. For Clark-Y airfoil the max L/D ratio decreases more than 15% with the decrease of Re. That means more blade drag needs to be overcome by motor torque for the same lift(thrust)

#### dino

##### Well-Known Member
HBA Supporter
Torque roll in gyroplanes with decreasing disc load-decreasing rotor thrust during push over

#### henryk

##### Well-Known Member
For a good propeller, you should get to 0.8-0.9 of that, that's about 270Kg thrust.
-can I get some examples ? (practice...)

-russian heli constructors have practical formula for propellers comparation=
("finesse coefficient")

Kf=("specific thrust") * sqr ("load phactor") =( kG/HP ) * (kG/m^2) ^ 0.5

f.e. ="auer" differential CR=

Kf=( 230/70 ) *( 230/2.3 ) ^0.5= 3.3 * 10= 33

#### Dusan

##### Well-Known Member
-can I get some examples ? (practice...)

-russian heli constructors have practical formula for propellers comparation=
("finesse coefficient")

Kf=("specific thrust") * sqr ("load phactor") =( kG/HP ) * (kG/m^2) ^ 0.5

f.e. ="auer" differential CR=

Kf=( 230/70 ) *( 230/2.3 ) ^0.5= 3.3 * 10= 33
It looks similar to McCormick formula, I'll look into this, Henryk, give me a couple of days.

#### henryk

##### Well-Known Member
, I suspect you have some fresh inflow on the aft propeller,
F=m*a a=dV/dt,

F=m *( a1 + a2 ).

BTW=propeller in sum accelerates air molequles,

but NOT DIRECTLY (as tennis racket or turbomolecular pump do).
propeller ONLY works as vacuum pump =decrease pressure in own disc...

air molequles,thanks its own kinetic energy (E=2/3 kT) accelerates (average speed is gained)
and go away thanks its own inertia...

PS=iff temperature is close to condensation point we cant fly on wings and propellers,
only jets can moove !

#### Dusan

##### Well-Known Member
F=m *( a1 + a2 ).
This is essentially correct, and F=2*rho*A*vi^2; but P=F*(vi+v0). For the first propeller the v0=0(static condition) but the second prop's v0 is "seeing" induced velocity from the first one, so the second prop needs more power for the same thrust. If you want to use two propellers, it's always best to put them side by side.

#### henryk

##### Well-Known Member
so the second prop needs more power for the same thrust. If you want to use two propellers, it's always best to put them side by side.
=side by side=double surface, but we have place for only one !

=in auer case (differential gear !) the RPM of propellers is anti-proportional to the drag momentum of them... (selfoptymisation).

#### henryk

##### Well-Known Member
similar to McCormick formula,

-another example=( AIRWOLF)

R912 100 HP + 5-blade 1.6 m propeller= F thrust=180 kG...

Kf=(180/100)*sqr(180/2)=17 (allmost halve of CRDiff. result =33 !)

Last edited:

#### Sockmonkey

##### Well-Known Member
Honestly, gryocopters are so fiddly with their handling I'd just live with the extra drag of using twin rotors on booms synchronized by an interconnecting shaft so their respective disks could overlap without flexing into each other.

#### Swampyankee

##### Well-Known Member
From disk actuator theory, a "maximum value" of thrust can be easily calculated, and for a 100HP, 1.7m diameter propeller, a thrust "maximum value" of 319Kg is calculated. For a good propeller, you should get to 0.8-0.9 of that, that's about 270Kg thrust. So, as you are not even close to that value, I suspect something is wrong with your setup, either the engine is not matched to the propeller, or most likely your propeller is stalled, as you mentioned the noise.

For 70HP, same diameter disk, the theory predicts 252Kg "maximum value" thrust, your setup has 0.91 of that. As you're getting such a high value, I suspect you have some fresh inflow on the aft propeller, that effect is noticeable from 0.2d rotor separation(Leishmann). Keep in mind that the effect is lost if there is too little separation, or if the props are ducted.

Number of blades. Less blades are always more efficient. A 0.1m blade moving trough air at 200m/s is having about Re=1,400,00. A 0.05m blade is having Re=700,000. For Clark-Y airfoil the max L/D ratio decreases more than 15% with the decrease of Re. That means more blade drag needs to be overcome by motor torque for the same lift(thrust)
Not that again.

No. Fewer blades are not always more efficient. We've discussed this numerous times in the past. Like any other lifting device, a propeller's drag is from a mix of induced and skin friction drag. Increasing the number blades reduces the total induced drag. The optimum number of blades is the greatest number that a) doesn't run into adverse Reynolds' number effects (in your case, if a Clark Y*'s properties aren't good, you switch to a different airfoil) b) doesn't run into structural problems at low speeds (constant speed propellers, at least, are operating with extensive areas of stalled flow during the takeoff regime) and, c) doesn't run into choking in the blade roots (you need at least seven or eight blades and a operating Mach number of about 0.7 for that, and it's correctable by adjusting the spinner's contour).

For constant-speed props, a complicating factor is that more blades reduces the loads on the pitch change bearings and the pitch change actuators.

----

* There are a lot of airfoils that are much better than the Clark Y, but what's more important is that there are airfoils that are designed for operation at low Reynolds' numbers.

#### davidjgall

##### Well-Known Member
Fewer blades are not always more efficient.
Goldstein, Glauert, Theodorsen, Kerwin, and Larrabee concur.

#### davidjgall

##### Well-Known Member
If you want to use two propellers, it's always best to put them side by side.
Theodorsen, Dowty-Rotol, and Tupolev Design Bureau disagree.

#### dog

##### Well-Known Member
Theodorsen, Dowty-Rotol, and Tupolev Design Bureau disagree.
View attachment 99170
The transport version of the Tupolev ,could be encouraged to run the props with the tips running
supersonic, and the fuesalage skins adjacent would crack from the sound/vibration, so they put on thicker skins, read about that in a snipet about a joint soviet/japenese airline

#### mcrae0104

##### Well-Known Member
HBA Supporter
Log Member
For the love of perspicuity and all that is legible, please use the forum's LaTeX editor. Click the Insert drop-down menu and select Math. The editor even includes a handy link on using LaTeX.

For example,

F=2*rho*A*vi^2
becomes: $$F=2 \rho A v_i ^2$$, and

Kf=(180/100)*sqr(180/2)=17
becomes: $$K_f=\frac {180} {100} \sqrt {\frac {180} {2}} = 17$$

#### davidjgall

##### Well-Known Member
Nobody uses “perspicuity” in a sentence....

#### davidjgall

##### Well-Known Member
The transport version of the Tupolev could be encouraged to run the props with the tips runningsupersonic, and the fuesalage skins adjacent would crack from the sound/vibration, so they put on thicker skins, read about that in a snipet about a joint soviet/japenese airline
Wow! That had to be even louder than the already legendary loudness of the standard “Bear.” Shades of the XF84-H “ThunderScreech!“

#### Sockmonkey

##### Well-Known Member
Increasing the number blades reduces the total induced drag.
Why is that? I would have thought it would be the other way around.

#### jedi

##### Well-Known Member
Why is that? I would have thought it would be the other way around.
Same reason buzzards have the multiple wing tip feathers and the elliptical wing has the minimum induced drag. More vortices but each one is a fraction of the total vortex strength. The power of a vortex increases exponentially with the strength. The power in a single vortex is greater than two times the power in a vortex half the size. Induced drag is a function of the square cube relation between speed, force and power of the downwash.

Last edited:

#### Sockmonkey

##### Well-Known Member
Same reason buzzards have the multiple wing tip feathers and the elliptical wing has the minimum induced drag. More vortices but each one is a fraction of the total vortex strength. The power of a vortex increases exponentially with the strength. The power in a single vortex is greater than two times the power in a vortex half the size. Induced drag is a function of the square cube relation between speed, force and power of the downwash.
Ohh, gotcha.