Moderators: richierich, ua900, PanAm_DC10, hOMSaR

ojjunior
Topic Author
Posts: 994
Joined: Thu Jul 13, 2006 12:31 am

### What is the dynamics or reason behind the turboprops rotating blades in opposite directions?

Does this generate additional power?
How is it advantageous for the aircraft?

For example, the A400M rotates blades in opposite directions on each wing

The AN-22 rotates blades in opposite directions on the same axis.

Curious about the engineering behind it...
The real reason it does exist...

FluidFlow
Posts: 786
Joined: Wed Apr 10, 2019 6:39 am

### Re: What is the dynamics or reason behind the turboprops rotating blades in opposite directions?

It is to balance out the forces. You have less vibrations that way. you can observe this on helicopters, if the rotor in the back fails the cabin start to turn counter to the rotation of the blade. The same force applies on the nacelle of the propeller engine. To balance that our you have counter rotation on the second set of fan blades.

Karlsands
Posts: 235
Joined: Sat Sep 16, 2017 2:53 pm

### Re: What is the dynamics or reason behind the turboprops rotating blades in opposite directions?

P factor

LightChop2Chop
Posts: 159
Joined: Mon Jul 30, 2018 3:33 pm

### Re: What is the dynamics or reason behind the turboprops rotating blades in opposite directions?

FluidFlow wrote:
It is to balance out the forces. You have less vibrations that way. you can observe this on helicopters, if the rotor in the back fails the cabin start to turn counter to the rotation of the blade. The same force applies on the nacelle of the propeller engine. To balance that our you have counter rotation on the second set of fan blades.

boats / ships with twin (or more) screws almost always counterrotate for the very same reasons.

kczombie
Posts: 13
Joined: Wed May 23, 2018 2:58 pm

### Re: What is the dynamics or reason behind the turboprops rotating blades in opposite directions?

BTW, I just love the A400. Just a sweet thing to look at.

ojjunior
Topic Author
Posts: 994
Joined: Thu Jul 13, 2006 12:31 am

### Re: What is the dynamics or reason behind the turboprops rotating blades in opposite directions?

FluidFlow wrote:
It is to balance out the forces. You have less vibrations that way. you can observe this on helicopters, if the rotor in the back fails the cabin start to turn counter to the rotation of the blade. The same force applies on the nacelle of the propeller engine. To balance that our you have counter rotation on the second set of fan blades.

Makes all sense, thanks for that.

kczombie wrote:
BTW, I just love the A400. Just a sweet thing to look at.

Yes it is.
I'm assuming you enjoyed seeing Mission: Impossible-Fallout Effect, didn't you?

masi1157
Posts: 440
Joined: Sun Feb 06, 2011 11:56 am

### Re: What is the dynamics or reason behind the turboprops rotating blades in opposite directions?

FluidFlow wrote:
It is to balance out the forces. You have less vibrations that way. you can observe this on helicopters, if the rotor in the back fails the cabin start to turn counter to the rotation of the blade. The same force applies on the nacelle of the propeller engine. To balance that our you have counter rotation on the second set of fan blades.

That may be true in the case of 2 fans rotating on the same axis, but on the A400M it isn't so, it has 1 fan per engine (axis).

Gruß, masi1157
517 different segments on 101 airlines to 212 airports in 55 countries

WayexTDI
Posts: 1953
Joined: Fri Sep 21, 2018 4:38 pm

### Re: What is the dynamics or reason behind the turboprops rotating blades in opposite directions?

masi1157 wrote:
FluidFlow wrote:
It is to balance out the forces. You have less vibrations that way. you can observe this on helicopters, if the rotor in the back fails the cabin start to turn counter to the rotation of the blade. The same force applies on the nacelle of the propeller engine. To balance that our you have counter rotation on the second set of fan blades.

That may be true in the case of 2 fans rotating on the same axis, but on the A400M it isn't so, it has 1 fan per engine (axis).

Gruß, masi1157

It still balances the torque effect when the engines are accelerated, and prevents the plane to veer left or right during the take-off roll.

jimatkins
Posts: 58
Joined: Mon Apr 24, 2006 1:57 pm

### Re: What is the dynamics or reason behind the turboprops rotating blades in opposite directions?

Heard somewhere the A400 has counterrotating props to sweep air up over the wing for more lift. Counterrotating blades like on a Tu-95 or An-22 are to keep the blades shorter to avoid compressibility effects on the tips of a long blade approaching speed of sound as they rotate. Lose efficiency if that happens. Also bad effects from the sound waves produced. Douglas C-133 had skin fatigue issues from the noise of big 3-bladed props close to the fuselage.

Max Q
Posts: 8631
Joined: Wed May 09, 2001 12:40 pm

### Re: What is the dynamics or reason behind the turboprops rotating blades in opposite directions?

LightChop2Chop wrote:
FluidFlow wrote:
It is to balance out the forces. You have less vibrations that way. you can observe this on helicopters, if the rotor in the back fails the cabin start to turn counter to the rotation of the blade. The same force applies on the nacelle of the propeller engine. To balance that our you have counter rotation on the second set of fan blades.

boats / ships with twin (or more) screws almost always counterrotate for the very same reasons.

Do they ?

Sounds like a good idea but never heard of that
The best contribution to safety is a competent Pilot.

GGg

dr1980
Posts: 196
Joined: Tue Jun 14, 2016 9:55 pm

### Re: What is the dynamics or reason behind the turboprops rotating blades in opposite directions?

As a counter example, on the Q400 both props spin in the same direction so crews need to make trim adjustments when power changes are made to keep the aircraft level.
Dave/CYHZ

strfyr51
Posts: 5106
Joined: Tue Apr 10, 2012 5:04 pm

### Re: What is the dynamics or reason behind the turboprops rotating blades in opposite directions?

the practice also allows for a smaller Vertical stabilizer with less rudder authority.

Florianopolis
Posts: 343
Joined: Sun Mar 29, 2015 2:54 pm

### Re: What is the dynamics or reason behind the turboprops rotating blades in opposite directions?

Have we considered the possibility that they know it works, but don't know how ?

When developing another quad-engine turboprop with very powerful engines, Lockheed had such a hard time getting their arms wrapped around stall on the C-130J that they just gave up and installed a stick-pusher.
http://www.flightglobal.com/news/articl ... oblem-804/
http://www.flightglobal.com/news/articl ... her-24837/

(Remember the AC-130J written-off after departing controlled flight during a max-sideslip test flight that stalled the vertical stabilizer, and the recovery exceeded the G-limit?)

So perhaps Airbus decided they wanted it to be symmetrical, played around with it in the wind tunnel and CFD, and ultimately determined that "down between the engines" counter-rotating props was the best solution. http://www.flightglobal.com/news/articl ... rs-181755/

Did it solve everything? It probably solved asymmetry problems with propwash, but the airplane still has weird aero problems. Apparently refueling helicopters is still too frightening, and be careful jumping paratroopers out both sides of the airplane at the same time: they might bang into each other behind the airplane. http://aviationweek.com/defense/a400m-h ... ch-project

PS: You know the P-38? They built it with counter-rotating engines, decided the prop rotation was creating unfavorable interactions with the boomed stabilizers, and switched the engine sides. Aerodynamics is weird like that.

LightChop2Chop
Posts: 159
Joined: Mon Jul 30, 2018 3:33 pm

### Re: What is the dynamics or reason behind the turboprops rotating blades in opposite directions?

Max Q wrote:

Do they ? Sounds like a good idea but never heard of that

Even the twin Mercury 200s on my boat are counter rotating. If both propellers turn is same direction the boat noticeably tilts to starboard when up on plane and you either have to trim one engine slightly higher, shift some weight around or install trim tabs to counteract. it not dangerous just not as efficient.

kurtverbose
Posts: 581
Joined: Thu Jun 26, 2014 9:33 pm

### Re: What is the dynamics or reason behind the turboprops rotating blades in opposite directions?

From FlghtGlobal in 2004: -

Airbus Military has chosen a unique "handed" propeller concept for the A400M airlifter in a bid to mitigate some of the technical risk associated with integrating what will be the West's most powerful turboshaft engines on to the airframe.

Also known within Airbus as the "down between the engines", or DBE, configuration, describing the downward sweep of the blades between each pair of wing-mounted, 10,700shp (8,000kW) engines - the design will ensure a symmetrical airflow over each wing and allow a simpler flap system and smaller horizontal stabiliser and fin, according to the manufacturer. It will also reduce vibration and noise levels in the cargo bay and allow air-drops to be performed more effectively.

The downside is that while the turbomachinery in all four engines will be identical, two engines will require additional gearbox components, a larger oil system and a mirror-image propeller design, increasing support costs.

Airbus Military commercial director Richard Thompson says "concerns had always existed"about rotating all four 5.33m (17.5ft) - diameter propellers in the same direction, after Lockheed Martin hit propwash problems in the C-130J development programme. He adds that adopting the handed approach meant "we had to do a lot of trade-off studies, which we had the resources to do only after launch.

"This is the most effective solution. It is a risk-reduction measure which has been confirmed by windtunnel tests to be the right direction. The impact on cost is marginal in terms of 30-year life-cycle costs."

Production of parts for the first EuroProp International (EPI) TP400-D6 turboprop has begun and first run is scheduled for August next year at MTU Aero Engines' Ludwigsfelde final assembly site. Series production is due to get under way in late-2007 or early 2008.

The engine programme will effectively require two certifications - one for each gearbox/propeller configuration - separated by around three months, says R-R Deutschland TP400 programme director Dr Christian Poensgen. It is expected that the first flight of the A400M, due in October 2007, will be performed with only one of the two engine/gearbox-propeller configurations certificated.

StereoTechque
Posts: 186
Joined: Sat Nov 28, 2015 7:24 am

### Re: What is the dynamics or reason behind the turboprops rotating blades in opposite directions?

kurtverbose wrote:

The downside is that while the turbomachinery in all four engines will be identical, two engines will require additional gearbox components, a larger oil system and a mirror-image propeller design, increasing support costs.
]

Counter rotating props on the same axis are for reducing the counter torque effects produced. You can find the same mechanism on the shafts of PW1000G Engines. Counter rotating props on adjacent engines of the same wing results in changing the gearbox, dont know how will it require a larger oil system..
Looking California.. Feeling Minnesota.... R. I.P. Chris Cornell...

longhauler
Posts: 6488
Joined: Sat Mar 06, 2004 12:00 am

### Re: What is the dynamics or reason behind the turboprops rotating blades in opposite directions?

Karlsands wrote:
P factor

I guess no one read this.

It is the main reason for counter rotating props on twin engine propeller aircraft. Basically, what it means is that the down going blade is producing more thrust than the up going blade at any angle of attach higher than zero.

Counter rotating props, clockwise on the left engine, counter-clockwise on the right engine (when viewed from behind) will have the highest thrust closer to the centerline of the aircraft. In a single engine situation, the amount of rudder is the same regardless of the engine lost. Aircraft without counter rotating props (both clockwise when viewed from behind) will require more rudder with the loss of the left engine than the right. (Termed the "critical engine"). And performance will always be predicated on the loss of the ciritical engine.

As noted above, some new and advanced aircraft, like the Q400, (for example) do not do this. There are many reasons ... things like commonality of engines/props for left/right or that there is so much excess power, counter rotating props are not necessary.
Just because I stopped arguing, doesn't mean I think you are right. It just means I gave up!

CowAnon
Posts: 140
Joined: Fri Nov 03, 2017 12:03 am

### Re: What is the dynamics or reason behind the turboprops rotating blades in opposite directions?

For the Kuznetsov NK-12 engine that runs on the Tu-95 and An-22, I'm guessing that the contra-rotating fans have an equal split of power between the front and back propellers. The Progress D-27 engine has the front propeller absorbing more of the power than the back propeller, though -- a 59/41 percent split, IIRC. Air swirling won't be a problem (capturing that wasted energy is the purpose for contra-rotating fans), but does the unequal power distribution cause balance issues?

vikkyvik
Posts: 12598
Joined: Thu Jul 31, 2003 1:58 pm

### Re: What is the dynamics or reason behind the turboprops rotating blades in opposite directions?

masi1157 wrote:
FluidFlow wrote:
It is to balance out the forces. You have less vibrations that way. you can observe this on helicopters, if the rotor in the back fails the cabin start to turn counter to the rotation of the blade. The same force applies on the nacelle of the propeller engine. To balance that our you have counter rotation on the second set of fan blades.

That may be true in the case of 2 fans rotating on the same axis, but on the A400M it isn't so, it has 1 fan per engine (axis).

Gruß, masi1157

Why do they have to be on the same axis? Props on different axes are still imparting torque to the aircraft.

Chinooks, for example, don't need a tail rotor because they have counter-rotating main rotors.....on different axes.
I'm watching Jeopardy. The category is worst Madonna songs. "This one from 1987 is terrible".

stephanwintner
Posts: 79
Joined: Thu Jan 03, 2019 5:04 pm

### Re: What is the dynamics or reason behind the turboprops rotating blades in opposite directions?

longhauler wrote:
Karlsands wrote:
P factor

I guess no one read this.

It is the main reason for counter rotating props on twin engine propeller aircraft. Basically, what it means is that the down going blade is producing more thrust than the up going blade at any angle of attach higher than zero.

Counter rotating props, clockwise on the left engine, counter-clockwise on the right engine (when viewed from behind) will have the highest thrust closer to the centerline of the aircraft. In a single engine situation, the amount of rudder is the same regardless of the engine lost. Aircraft without counter rotating props (both clockwise when viewed from behind) will require more rudder with the loss of the left engine than the right. (Termed the "critical engine"). And performance will always be predicated on the loss of the ciritical engine.

As noted above, some new and advanced aircraft, like the Q400, (for example) do not do this. There are many reasons ... things like commonality of engines/props for left/right or that there is so much excess power, counter rotating props are not necessary.

Thanks for that explanation. I did actually read Mr. Karlsands' comment.But even as an aircraft engines engineer, I didn't know what P factor meant....I imagine many others didn't either....

masi1157
Posts: 440
Joined: Sun Feb 06, 2011 11:56 am

### Re: What is the dynamics or reason behind the turboprops rotating blades in opposite directions?

vikkyvik wrote:
Why do they have to be on the same axis? Props on different axes are still imparting torque to the aircraft.

You are right, for the "static" torque the need not. I was taken away by "dynamic" in the thread title and was thinking about imbalance and vibration, which could in fact partly be compensated with both propellers rotating about the same axis.

Gruß, masi1157
517 different segments on 101 airlines to 212 airports in 55 countries

DFW17L
Posts: 275
Joined: Fri Nov 20, 2015 2:53 am

### Re: What is the dynamics or reason behind the turboprops rotating blades in opposite directions?

kczombie wrote:
BTW, I just love the A400. Just a sweet thing to look at.

Yes, the A400 is a sweet bird. I bet the Italians had something to do with it. They have such a great sense of style.

Starlionblue
Posts: 20350
Joined: Fri Feb 27, 2004 9:54 pm

### Re: What is the dynamics or reason behind the turboprops rotating blades in opposite directions?

DFW17L wrote:
kczombie wrote:
BTW, I just love the A400. Just a sweet thing to look at.

Yes, the A400 is a sweet bird. I bet the Italians had something to do with it. They have such a great sense of style.

The cockpit is rad as all hell. Most of the pushbuttons, levers and such are the same as on the A350 so it feels familiar, but the panels are all black for that extra covert ops look.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

### Who is online

Users browsing this forum: leoben, RetiredWeasel and 35 guests

### Popular Searches On Airliners.net

Top Photos of Last:   24 Hours  •  48 Hours  •  7 Days  •  30 Days  •  180 Days  •  365 Days  •  All Time

Military Aircraft Every type from fighters to helicopters from air forces around the globe

Classic Airliners Props and jets from the good old days

Flight Decks Views from inside the cockpit

Aircraft Cabins Passenger cabin shots showing seat arrangements as well as cargo aircraft interior

Cargo Aircraft Pictures of great freighter aircraft

Government Aircraft Aircraft flying government officials

Helicopters Our large helicopter section. Both military and civil versions

Blimps / Airships Everything from the Goodyear blimp to the Zeppelin

Night Photos Beautiful shots taken while the sun is below the horizon

Accidents Accident, incident and crash related photos

Air to Air Photos taken by airborne photographers of airborne aircraft

Special Paint Schemes Aircraft painted in beautiful and original liveries

Airport Overviews Airport overviews from the air or ground

Tails and Winglets Tail and Winglet closeups with beautiful airline logos