3 Bladed Prop Counter Rotating. Design help wanted

[TomHH]

Guys,

I am building an Northrop XP-56 in 1:10 scale. This plane has 2 counter rotating 3 bladed pusher props. I need some help with selecting suitable props.

Here the theory how I understand it. The first prop rotates clock wise. The second prop rotates counter clock and needs a smaller diameter and larger pitch than the first. I can adjust performance by varying the RPM difference between the props.

Now my questions. What props would be suitable for this project, 9"x6" plus 8"x8". Can I do some calculations for this problem and where do I get the equations? The biggest problem, where do I get such props?

Best Regards, Tom

[omega blood]

Good luck trying to find somthing like that. I would be very difficult trying to find counter rotating props much less a gearbox for them.
Here is a discussion I started on CRP's
http://www.rcgroups.com/forums/showthread.php?postid=1494464#post1494464

[Hal Ray]

You can do it with a gear for one prop and a small timing belt for the other, that way one will turn right and the other left. Size your gears or belting so the back prop turns faster (don't know how much faster it has to go).
Master Airscrew has right and left three blade props around the size you want.
I did not think of this, there are plans from Model Builder for the XP-56 with that type gear/belt box.

HRH

[TomHH]

Thanks, looks like that the only available props are the Master Airscrew 3-blade 9x7 left/right. So adjusting has to be done with a higher RPM of the second prop. Anybody with a hint how to calculate he required RPM delta?
Regards, Tom

[max z]

www.varioprop.de
2-3-4 blade props, replaceable blades with adjustable pitch. Largest lefthander is 8.2 inch. Look under smallest hub size.

Max.

[Viper2000]

as I understand it most contraprops are setup so that both props turn the same speed.

In a real WWII type installation you'd find two constant speed props driven from the same engine - the back set of blades might adopt a slightly higher pitch.

However "on design" the difference would be rather small, as the wash from the first prop would only be a couple of knots at speed. And the swirl that the first prop imparts to the flow will act to reduce the forward speed the back prop sees (in terms of alpha at the blade, not dynamic pressure).

This is quite easy to visualise by drawing a velocity triangle.

If the prop is reasonably efficient then the swirl will not be sufficient that the back prop would adopt the same pitch as the front prop. But the error would be only a couple of degrees.

So I'd just use identical props.

Of course if you're being devious you could set up the gear box like a car differential. That way the power will be shared as it's needed between the props.

Remember that you'll need to have quite a complex arrangement when it comes to prop addaptors - you're going to have concentric shafts and thus you'll need two different prop adaptors.

Contra props in service were often found to be a downright pain on piston engines, because they never reall had all the bugs ironed out before the jets came along (with the exception of the installation on the Shakleton). On a turboprop you can cheat by having two powerturbines. And anyway even if you do go for one turbine and a clever gearbox there's no vibrational load to contend with (at least compared with a piston engine). So if you do this, I'd suggest going electric to increase reliability.

Matt

[max z]

you're going to have concentric shafts and thus you'll need two different prop adaptors

Yes, that occurred to me too after I posted my suggestion for a Varioprop. That makes it not a suitable candidate for the rear prop. I the first place, the maximum shaft dia. is only 4 mm, but more importantly the pitch adjustment mechanism is at the front, in the way of a throughgoing shaft. Still it's a nice arrangement....:)
Max

[Three-Wire]

Search in the open discussion forum for “Precious Metal” - Mark Rittinger tried co-axial counter rotating props some time ago and ran into the problems inherent with R/C use of this technique. I wanted to try it as well but have not been able to get around to setting up the tests necessary to make it work.

The issue as I understand it is this - On a full-size aircraft, they pitch of the blades is adjustable and can be set depending on the flight regime. On models this isn't the case. It's easy to see the amount of work each prop is doing by looking at the amps they are drawing (assuming you use an independent geared motors to drive each prop). This is easy to do on the bench (static case) but not really possible to do in the air. Optimized for static, you quickly run out of thrust at a low forward velocity (again, see the Precious Metal threads).

What we need is a wind tunnel test to match the load on each prop at, say, 30 mph. I'd say, go with a left-hand rotation 3-blade in front and get a vario prop with the closest matching blade style, but about 1/4 to 1/2 inch less diameter in the rear (smaller diameter gets away from the tip losses of the front prop). Then adjust the vario prop to match the amp draw of the front prop at speed (hanging them out a car window or the bed of a pickup maybe). Then it may work... Just a thought. Sorry for the long post.

[TomHH]

thanks for your replies. I found some advice on DJaerotech (I did some editing):

Contra Rotating Propellers
The swirl energy from a spinning prop dissipates through friction with the surrounding air. To recover the maximum of whatever swirl energy is available, the two props should be as close to each other as possible. However, that also worsens the vibrational effects of the blades passing each other. The real key to getting the most out of any pusher installation, including one with a tractor up front as well, is to get the airflow into the rear prop as clean as possible. Any fat fuselages, bracing, struts, and especially any flying surfaces or any large bodies that are to one side of the prop's axis can spell serious trouble.
So keep the inflow as undisturbed as possible, and also have it as symmetrical around the axis of the prop as possible. Anything that creates turbulence is bad, and anything that deflects the airflow to a different angle (so that the angle of attack seen by the blades varies as they sweep around the disk) is even worse. Wings, tail surfaces and deflected control surfaces can be serious problems. A thin, shoulder-mounted wing mounted well ahead of the prop on a slender, smooth fuselage is better than a high wing and a lop-sided fuselage right in front of the prop.
What's that bit someone else mentioned about different diameters due to "slipstream contraction", and what about the need for different pitches and/or rpm's for the two props? A prop makes thrust by grabbing chunks of air from in front of it, and accelerating them out behind. About half the acceleration occurs in front of the prop, and the other half behind. The reaction to the force required to accelerate the air's mass shows up as thrust. Because the air has to be accelerated to make thrust, the velocity of the air behind the prop is faster than the velocity in front of the prop.
As the velocity changes, the roughly cylindrical stream of air flowing through the prop has to obey Bernoulli's principle. If its airspeed increases, then the cross-sectional area (and therefore the diameter) of the stream has to decrease in proportion to that in order for the volume of the flow to remain constant. If this were not so, the flow through the prop would violate the law of conservation of mass and energy, which happens to be one of the most inflexible laws in all of Newtonian physics. Thus, the diameter of the inflow to the prop is actually larger than the prop at some point upstream of it, then contracts during that first half of its acceleration until it is equal in diameter to the prop when it reaches the prop disk. It continues to contract after it passes through the prop, during the second half of its acceleration. This is that "slipstream contraction" that some other posters to this thread have mentioned. This means that a second prop, aft of the first one, that is supposed to be working with the slipstream of the first prop, needs to be a little smaller in diameter in order to match the boundaries of the now-contracted slipstream.
Just how much faster (and therefore how much smaller in diameter) depends on a number of factors. For the ratio of slipstream dynamic pressure to free-stream dynamic pressure, Daniel E. Dommasch's "Airplane Aerodynamics" suggests an equation, which with a little algebraic juggling gives us:
Qt = Q + [(4 * T * g) / (D^2 * Pi)]
where: Qt = dynamic pressure ("ram air pressure" minus the static pressure) in the fully developed slipstream well aft of a prop Q is the dynamic pressure in the freestream well ahead of the prop, and outside of the propwash T = thrust D = prop diameter and of course "Pi" is 3.141592...
Dynamic pressure ("Q") is equal to one-half the air density, times the velocity squared. If we plug that back into the formula and do some more algebra, we get:
Vt = SQRT [V^2 + (8T * g / rho / D^2 / Pi)]
where: Vt = the velocity in the fully developed freestream in meter per second "SQRT" means you take the square root of the result of the formula inside the [ ] V^2 = the freestream velocity squared, T = thrust in kg, rho = air density in kg/m^3, D = prop diameter in meter, g = gravitational constant, 9.81 m/s2.
Suppose we have a twin-engined model that weighs 1 pound, and we're planning to modify it into a twin contra-rotating arrangement. Let's also assume that the L/D (essentially the same as the glide ratio) at our expected cruise speed of about 25 mph (11.2 m/s) is about 4:1 (I know that sounds low, but remember, typical cruise speeds are higher than best gliding speed, and besides, this airplane has a bunch of extra stuff hanging out in the breeze). This means our drag is equal to the weight divided by the L/D, or 0.25 pounds. In level flight, that is also equal to the total thrust.
Let's also assume the front prop is doing about 55% of the work (0.063 kg of thrust) to allow for the lower efficiency of the aft prop. We'll define the prop as having a 6" diameter (0.152 m).
Plugging all of that data into our formula:
Vt = SQRT [11.2^2 + (8 * 0.063*9.81/1.025/0.152^2/3.1416)] = 13.9 m/s
which is equal to 50 km/h, That's a velocity ratio of 1.2, or 20% more than the freestream velocity. This means that if the aft prop is far back enough to sit in the fully developed slipstream from the forward prop, it will need either 20% more pitch (the preferred solution) or 20% more rpm (which opens several other cans of worms). In addition, the slipstream contraction will be SQRT (1/1.2), or 0.913 . That means the aft prop should be 91.3% of the diameter of the forward prop, or just a little less than 5.5" diameter.

I helped advise a guy recently who scratch-built a VERY giant-scale electric model of the Voyager. As I recall, his original setup used the same size props on both ends. It flew much better when we put a prop with more pitch on the aft motor. So, that's all there is to it! Just correct for slipstream effects on the rear prop, and keep the inflow into it as clean and undisturbed as possible. You will probably not have as much prop efficiency as a pair of tractor props with nice clean inflow, but it shouldn't be too bad.

[mrittinger]

I found most of this to be true. If you read the thread you'll see what I ran into. I DID succeed in getting the PM in the air with two gearedCR props on her. It needed about 3"+ more pitch in the rear prop. I would say MINIMUM.I had them very close together also, about 3/8" or so. I think if I had been able to find a reverse pitch prop with the right pitch it would have flown better. As it was it hung in there in the air, but it wasnt a speed demon. It DID have an odd sound.
I contacted the Precious Metal guys and found out they do run more pitch on the rear prop, and it has slightly smaller diameter due to what was refered to as a conical airflow rolling off the front prop tips.
Heres my non scientific advice from what I learned: Run the extra pitch on the rear prop. Run the front lower pitch prop at the pitch you would NORMALLY USE on the airframe, and run it reverse. It's easier to find lower pitch reverse props compared to higher pitch props. Run lower gear ratio or higher wind motor on rear prop to accelerate it above front prop. Make the airplane light, it gets heavy quick with added cells/motors/gears/esc's.Trim a tiny bit of diameter from the rear prop. Good Luck!

Mark

Mark

[TomHH]

Mark, thanks for your reply. I will do exactly as you advise.

Regards, Tom

[mrittinger]

Be ready to experiment too!

[TomHH]

Yep, will do so and use 2 bladed props.

actually I will start with speed 280 motors and use the Titanic Airlines gearbox. Then I have the choice between 1:2 and 1:3 gear ratio, Graupner 5X2 and Günter 5X4 props, and speed 280/280 Race motors. That gives me 27 combinations.

Regards, Thomas

[mrittinger]

Those props are too small for the 280 on a gear. They would be more appropriate on a tiny motor, but not the 280. I would think you'd need closer to a 7" prop. I was using 9" diameter on 6 volt 400's with 2.3 ratio. Remember, you need the pitch on the FRONT prop that you'd normally use with ONE prop, then even more pitch on the rear.
I would try a 7/4 front, and a 7/7 rear. You might find yourself carving some props like the rubber power guys to get that. With the 400's, I ran out of props with the available pitch I needed, like a 9/9 reverse (it doesnt exist.)
Mark