Scramble

In real size, quite a number of planes (Gannet, Shackleton, Antonov, etc) were equipped with coaxial contrarotating props. I'd like to know whether the front and the aft prop had a different pitch (for example, the aft prop with a coarser or a finer pitch) and why. I've never been able to find out for sure. Anyone who can tell exactly (please no 'I think' 'They should' 'Probably') and refer to websites containing evidence ? Thanks in advance.

Chris300s

My Feedback: (1)

Scramble,

I know you were looking for a more clinical definition but...
I can tell you that the rear propellor losses a lot of efficiency. In full scale the second set of props is smaller in diameter and coarser in pitch. I can only assume that it is a trade off; lossing efficiency due to shorter span (lower aspect ratio), gaining efficiency due to cleaner airstream (tip vortices) and coarser pitch.
I guess that the system is prone to vibration that could destroy the prop. Harmonics between the blades that would fatigue them prematurely. That is why the 2nd blades are smaller in diameter? At our Reynolds numbers, the second blades would have to be much more aggressively pitched compared to the first set. Of course you could use what's availible and overcome the efficiency loss with more power..
Keith Shaw has a good working system on his Bugatti racer. It's beautiful! For full scale do a google search for "Precious Metal" or chech out www.aafo.com and www.warbirdaeropress.com. Both feature the Griffon powered P-51 air racer "Precious Metal". "Red Barron" and "Miss Ashley II" also raced with counter rotating props. I point towards the air racers because they went to the 6-blade system trying to gain better efficiency over the WWII bombers/transports.
You may find Ron Buccarelli's email address (Precious Metals owner) on one of the sites, he could tell you why the Whittington brothers set it up that way.

Update - OK, I was on the right path. According to Adrian Popa, Directors Office, Hughes Research Laboratories, "Aircraft propeller tips also move at high radial velocity generating large trailing vortices. Counter-rotating propeller tips create vortices that rotate in opposite directions, clockwise and counter clockwise, which significantly reduces the net trailing vortices by cancellation. This increases propeller efficiency and reduces the workload on the engine, just as the trailing ducks have a reduced workload. "
http://www.madsci.org/posts/archives...4680.Eg.r.html

Scramble

Thanks a lot Chris, for your time and searching work providing precious information sources.

Scramble

Hello Chris
Last findings :


The question you submitted to MadSci Network on
Wed Jan 21 06:23:23 2004 -- Coaxial counter-rotating airplane propellers pitch has been answered. We will include a copy of the answer below, or you may visit our site by going to:
http://www.madsci.org/posts/1076952505.Eg.q.html
We thank you for your support.

ï‚· Kieran Kelly, [email protected], MadSci Network

Your question was answered by:
Martin Smith Engineering, B.E., M.EngSc., Uni of Qld / airline pilot
This is a fairly difficult question to answer completely, but easily answered generally. Unfortunately I have not been able to find details of the gannet propeller pitch. Below is what I have been able to gleen from the net or deduce for myself.
In general if you have a counter rotating prop system the two props will need to have different pitches.
A propeller is a wing. For maximum efficieny you need to control the angle of attack (the angle between the airflow and the wing chord). This becomes complicated for a propeller because as you move out from the hub of the prop the speed of the blade (thus angle of attack) changes, and as the propeller cuts through the air at different forward velocities the angle of attack changes.
Now for a counter rotating system the second disc is meeting already disturbed air. In general the first disc will cause a "swirl" of air behind it. The air behind the fist disc is travelling backwards faster now, but it also has a rotation applied to it. The increased backwards speed of the airflow tends to require a higher pitch on the second disc, the rotation tends to cause a requirement for a lower pitch. The second disc has different pitch requirements to the first, however just what those requirements are changes depending on the type, speed, use etc of the system.
Finally it should be noted that a pilot does not directly control propeller pitch (in aircraft where there is any involvement of pilot with propeller pitch). Variable pitch propellers almost always use a constant speed device. This device alters propeller pitch to maintain a constant rotational speed. The pilot adjusts for engine speed and the CSU adjusts propeller pitch to maintain that speed.
This article talks about pitch requirements for contra rotating helicopter
blades: <a
href="http://www.helosim.com/coaxialarticle.htm)http://www.helosim.com/coaxiala
rticle.htm
A close up of a contra roating system for a torpedo - the obvious
difference is blade design for each disc: <a
href="http://www.larwe.com/sub/img/mk3_tail_hires.png)http://www.larwe.com/sub/
img/mk3_tail_hires.png
from here <a
href="http://www.larwe.com/sub/groton.html)http://www.larwe.com/sub/groton.html


The AN-70 is a modern aircraft with contra rotating propellers:
<a
href="http://www.aeronautics.ru/nws002/ai016.htm)http://www.aeronautics.ru/nws0
02/ai016.htm
(http://www.aeronautics.ru/img002/an70-progres-d-27-
drawing.jpg)http://www.aeronautics.ru/img002/an70-progres-d-27-
drawing.jpg
Martin Smith




MadSci Network
http://www.madsci.org/
[email protected]

Very complex business I should say
Regards

Chevelle

My Feedback: (2)

Just a thought but I would imagine that having true working counter-rotating props isn't necessary for power. Have you thought about just having the second prop just spin from the prop wash from the first? The reverse pitch will make it spin in the opposite direction.

stregaracer007

How about having the second prop bolted on to simulate counter-rotating props? In other words, two 3-blade props stacked on top of each other. You'd have to reduce the pitch.

MalcolmL

Hi Scramble (Claude) - this doesn't answer any of your questions - I have a question (or two) for you - is your query academic or are you planning to build a model with Contra-Rotating props ? And if so, which one do you have in mind ?
Hi Chris300s - I'm obviously not looking in the right places - where can I found out about Keith Shaw's Bugatti ?
MalcolmL

Scramble

My question is purely academic, but then, suppose I wake up one day with the urge to build a Westland Whirlwind, how am I going to tackle that prop problem without knowing how it was done on real-life aircraft ?
Here is some info for you, also there was more on E-zone if my memory serves me well ...
http://www.geocities.com/CapeCanaver.../momfeb00.html
http://members.aol.com/kmyersefo/ampsep03/ampsep03.htm
Thanks for the interest.

Chad Veich

My Feedback: (60)

I just happen to be building a model of an aircraft with counter rotating propellors and was wondering what others thought of Chevelle's idea above. My intention was to simply have the counter rotaters for static and fly with a single prop but the idea of running a dummy prop behind has me intrigued. What are the drawbacks if any?

MalcolmL

Hi Claude (Scrambler) - that would be a great choice - I've had in mind building a model of it too for a very long time ......... if you would like to email me direct, we could exchange information.
True C-R props, driven by an i/c glow engine (as well as electrics), have been done successfully. I have a preliminary design and a working mock-up of a C-R gearbox for .60 to 1.20 size engines. I've been looking for someone who is interested in co-operating with me to machine the gearbox components.
Incidentally, moved by your academic interest, I looked up some reputable 3-views, articles and close up photos. There is no mention of the two props being of different diameters and/or pitch. They are frequently referred to as "pairs" which suggests (but not conclusively, I agree) they are identical. Apparently however, on the full size system, " .... control of pitch change is critical and must be sensitive to ± 2 - 5 rpm with a pitch change of around 5° per second .... " - quoting Don Middleton writing in Aeroplane Monthly. If the diameters then are equal, does it follow that pitch would/should be the same ? The model already flown successfully had the same diameter and pitch props front and back. My gut feeling (and I don't have a very large gut !) is that the same diameter of fixed pitch props (as the case may be) on a model would work fine.
Best Malcolm

Flypaper 2

The VTOL Pogo has even length blades also. Been thinking of building an electric version using differential gears from the electric cars.

MalcolmL

Hi Gord (Flypaper2) - the Convair XFY-1 POGO - a great C-R Props prototype - possibly the ultimate - and VTOL to boot ! That and the Lockheed XFV-1 Salmon. What fantastic RC scale models these would be to see flying ! The POGO has been on my projects list for a long, long time - hah ! And yes - apparently the same diameter blades from the photos and 3-views of the original types. MalcolmL

saramos

The XF 11 is a counter rotating prop plane that I would consider a dream project to model.
This photo is of the second prototype using standard four blade props instead of the counter rotating blades used on the first prototype.
If I recall correctly, the first prototype crashed as a result of the pitch reversing on one of the counter rotating props while in flight, nearly killing Howard Hughes.

Wing span: 101'5"
Lenght 65.5'
Height 23'3"
Wing area 983 sqft
Weight 37,100 empty, 58,800 max takeoff

Power: 2 Pratt & Whitney R4360 radial engines, 3000 hp each
Speed: 450 mph
Celing: 44,000'
Range: 5000 miles

Chevelle

My Feedback: (2)

What a great plane to model. Here are some clearer pictures...

Trapnell

My Feedback: (1)

I may be sticking my you-know-what out a little with this but, I believe the term to be used is contra-rotating. Counter-rotation applies to planes like lightnings and Piper Seminoles - planes where the propellers of the opposing engines turn in opposite directions. To put it simply, the second propeller straightens the airstream in an attempt to recover the energy in the swirling stream of air behind a normal propeller. Airplanes with incredible amounts of excess horsepower (Bear Bomber, etc) use them so prop diameter doesn't get too large (tip speeds too high). The Pogo and the Salmon used such props to counter the torque effects of swinging a large prop at near-zero airspeeds.

Flypaper 2

Hi Malcolm:
The advent of the Li- Poly batteries and brushless motors make it a lot more feasable. When are you starting yours

MalcolmL

Gord - just can't get away from noise, smell, sludge, vibration of engines ........ MalcolmL

Chad Veich

My Feedback: (60)

Trapnell, I'm sure you are correct and if you look at Scramble's original post that is what he identifies them as. I said "counter-rotating" in my post above but I'm actually building a model of an airplane with contra-rotating propellors. No, it's not the Spitfire in my Avatar! I am aware of the difference but old habits are hard to break!

MalcolmL

Hi Guys - counter-schmounter ! I believe we're all on ther same wave length anyway - it's 2 props on coaxial shafts turning in opposite directions. MalcolmL

ampbomber

My Feedback: (7)

Hi all you contra-rotating prop lovers,

Check out the info at this site.

http://search.atomz.com/search/?sp-q...&sp-m=1&sp-s=0

MalcolmL

Hi George (ampbomber) - interesting post - thanks for the lead to DJ Aeroteech. BTW they got it wrong in #3 about Contra-Rotating props (if I read it right) and Contra-Dict themselves in #1 anyway - about C-R props on the P-38 - but got it right in #6 ! MalcolmL

Scramble

Well, I am happy to see my question started a whole train of posts. Thank you everybody. To make matters absolutely clear, we are talking coaxial props. I've come across counter-rotating and contrarotating indifferently, but I think counter-rotating more specifically applies for example to the Lightning, and are also distinguished as supra divergent or supra convergent (speaks for itself). We deal with coaxial (or tandem) props. As far gearboxes (at least for electric power), two chaps at least have international repute in the field, Keith Shaw and Dave Chinery. I am told Dave Chinery published a series on such props and gears in RCME '99 articles. I mailed them but haven't received any answer yet. I easily agree the aft prop doesn't have to be smaller in diameter. But what about the pitch ? The question stands ...

Scramble

Of course coaxial props have variable pitch, but that doesn't change the question. Let us say we change from cruise pitch (large) to take-off pitch (small), well the pitch difference (if any) between the props might stay, e.g. cruise pitch 5 for front prop and 3 for aft prop, take-off pitch 3 for front prop and 1 for aft prop. Two pilots at least should know, Ron Bucarrelli who flies Precious Metal, and John Roxburry, a helicopter pilot who worked for Wallis Jet restoring Shackletons, but I've been unable to contact them so far. Incidently, Precious Metal engine and props system are ex-Shackleton. On the Shackleton preservation site I found a brief mention allowing to think that the aft prop pitch was smaller. My question is pending at Aeroplane Monthly but five issues have gone by without any mention of it. Now, modelwise, what prompted me to inquire deeper into the matter was the collision between Keith Shaw's Bugatti with a larger pitch aft prop and a twin rotor in tandem (contrarotating) EDF design by Ralph Dvorak, in which the aft rotor has a smaller pitch, because, to quite him, the aft rotor has a smaller physical pitch, but a larger effective pitch when it meets the swirl coming from the front rotor. [sm=confused.gif]

ampbomber

My Feedback: (7)

See if this helps clear up some of the questions. Here's a dissertation from Don Stackhouse:


OK, so we've learned that pushers are usually a detriment unless you really
do your homework, contra rotation is not generally worth the trouble on
models, but if we're going to do it anyway, we should try to keep the
airflow into both props as clean, smooth and uniform as possible. 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) / (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 / rho * D^2 * Pi)]

where:
Vt = the velocity in the fully developed freestream in feet per second
"SQRT" means you take the square root of the result of the formula inside
the [ ]
V^2 = the freestream velocity squared (velocity in feet per second)
T = thrust in pounds
rho = air density in slugs/ft^3 (.00238 at sea level standard day
conditions)
D^2 = prop diameter in feet

Other units will work as well, just make sure that you use the same system
of units throughout (no fair mixing feet in one variable with inches in
another, or metric units with English, etc.!).

Ok, now that half of you are getting glassy-eyed and most of the rest are
running for cover in a mad panic, let's clarify that terrifying blast of
algebra with a practical example:

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 ( multiply by 22 and divide by 15 to
get 36.67 fps) 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 (0.138
pounds of thrust) to allow for the lower efficiency of the aft prop. We'll
define the prop as having a 6" diameter (0.5 feet).

Plugging all of that data into our formula:

Vt = SQRT [36.67^2 + (8 * 0.138 / .00238 * 0.5^2 * 3.1416)]

which is equal to 43.99 feet per second, or 30 mph. 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. See, that
wasn't so hard, was it?

If you plan to do this a lot, I suggest coding these formulas into your
favorite spreadsheet program, such as Excel.

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.

Scramble

Brilliant and conclusive. Thanks to all. [sm=thumbup.gif][sm=thumbup.gif][sm=thumbup.gif]