Is Propeller Spiraling Slipstream a myth or provable fact? 1

[Majortomski]

Hello ladies and gentlemen! I’m a total newbie to this forum, and I have used the search engines here on the forum to no avail in finding an answer to my question.

Just so we’re all on the same page I am referring to the phenomenon of spiraling slip stream the theory that the propeller induces a spiral of air around the fuselage that strikes the fin/rudder as some angle of attack that causes a yawing force. Said to be cancelled if there is a sub rudder or if the rudder is placed outside the slipstream as on an Erocoupe. Supposedly present all the time. This is not to be confused with the turbulent spiral that is visible off a propeller tip in humid air, which flows the wrong way to support the theory.

The reason that I question whether or not it is a myth is because I have never seen this phenomenon quantified. The aerodynamics of an airplane are cookbook plug and crank mathematical operations. Take a set of interactive equations, plug in a bunch of numbers, and it cranks out the answers of area and angle of attack for all of the flight controls. The one thing missing in all those equations is the mathematical definition of the slipstream. Such that for a given horsepower, a given number of propeller blades we should get an answer as to how much the fin should be offset to correct for this supposedly ever present spiral. By the way before the publishing of “Stick and Rudder” this theory didn’t exist.

Now to be honest I have seen one brief equation mentioned in a very old NACA which was summarized as the angle of attack of the vertical fin due to this effect, was at MOST 3 degrees off centerline, again an insignificant value when considered against the extreme yaw encountered by most S.E. aircraft in a climb.

So, have any of you ever seen this effect quantified?

 

[rb1957]

ok ...
a propeller producing thrust is rotating. the swirl velocity of the wake of a propeller is easily calculate (if you know how). it is no myth.

in WW2 several twin engined planes had propellers rotating in opposite directions, to balance out the wake.

in modern times, engine power increased so that some applications have counter-rotating propellers (Tu-44, Gannet), the point being that counter-rotating propleers reduce, significantly, the swirl of the slipstream.

 

[Majortomski]

"the swirl velocity of the wake of a propeller is easily calculate (if you know how). it is no myth."

Well I've been looking for roughly 20 years now and I haven't found that methodology.

"in WW2 several twin engined planes had propellers rotating in opposite directions, to balance out the wake."

Please, name ONE. The P-38 had counter rotating props to resolve other aerodynamic problems, but they rotated in the "wrong" direction, they made two critical engines instead of one.

All of the WW-II vintage aicraft with "H" tails have the tails in the upper half of the supposed swirling slipstream which would make the condition worse, not better.

I believe most of the modern counter rotating props are due to length of landing gear vs available power to be absorbed issues.

Sorry rb, but your answer is the canned answer that I always find in the performance and pilotage books, but no one actually knows the math to prove the swirl exists.

Thanks

T

 

[MiketheEngineer]

I don't know if is true or not - thoughs I supect it may.

Cessna always put a little more dihedral in the right wing (less lift) to offset the torque of the engine - plane trying to turn left. This may also have been done becasue of what you say. They always told me it was for torgue.

One day as a student pilot - I added power way too quick - apparently - and almost shot off the left side of the runway. Plane too slow and dihedral efeect had not yet come into play. I am still here - so no big deal.

 

[rb1957]

ok, the propeller blade is an airfoil at incidence.

The propeller blade is an airfoil, creating lift and drag. These forces resolve into thrust and an inplane component acting in the tangential direction. These in-plane forces (from different blades) are in phase, all acting CW or CCW depending on the rotation of the blade. These cause the swirl. It is possible that there is no readily available math for this swirl velocity 'cause it depends on the after-body. Also I'd anticipate that the swirl velocity is significantly less than the axial velocity of the slipstream, so you may not feel it.

"Well I've been looking for roughly 20 years now and I haven't found that methodology." tonight i'll look up my 4th year propulsion engineering text; i told my wife they'd be usefull one day.

I think i had the P38 in mind ... if the props are rotating in opposite directions, why are they "two critical" ones ??

as for counter-rotating props, yes in part, the engine power needed more than a single propeller, as you say, the radius of the prop is limited by many things, and the number of blades is also limited; however, it is a huge complication to arrange for counter-rotation ... why would they do this if there wasn't a performance pay-back ?

 

[Majortomski]

A critical engine in a multi, esp twin engine airplane is the one that if it fails the remaining engine causes the most advers yaw due to the P factor thrustline running down the outside of the nacelle. In the case of the P-38 both engine turned outboard over the top, so that if one fails the other always causes the most adverse yaw condition.

 

[rb1957]

i was thinking about your question on my drive home ...

the drag component on the propeller blade (acting in a tangential direction) is minimised in modern propellers due to pitch control; this optimises the airfoil force vector to be in the thrust direction. older fixed pitch propellers would be set to maximise thrust at a particular airspeed, and would be inefficient at different speeds (wasting energy in creating swirl).

i was also thinking about the classical picture of a prop in flight, with the nice cork-screw motion of the tip. i think this is the case for a prop with pitch control. without pitch control, the drag of the prop is sort of causing the prop to "skid" through the air.

another reason there isn't a "canned" equation to calculate the swirl is that it is dependent on the airfoil of the blade.

i'll give you the P-38, what about my observation on the counter-rotating props ? what about my drag forces ?

 

[GregLocock]

As a net effect on the control volume of air it must be a fact by conservation of momentum. The motor exerts a torque on the aircraft, this must be counterbalanced by a torque applied to the air by the prop.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[thruthefence]

IE critical engine: On an aircraft that has both props rotating clockwise when viewed from the cockpit: The distribution of thrust across the 'disk' is not equal, at higher angles of attack, such as in a climb, the descending blade side develops more thrust. Thus, the r/h engine is further away from the center line of the aircraft, and has a better " mechanical advantage " for yawing the aircraft, thus making the L/H the "critical engine" if it fails. Critical because you have to add more rudder to control the yaw, then you would had the r/h engine failed.Any unnecessary induced drag when you have just lost half your thrust, is indeed "critical". Two light twins aircraft that I am aware of,( Piper Seneca & Beech Duchess ) the r/h engine rotates counter clockwise, viewed from the cockpit. eliminating the 'critical engine'. The P-38 also had counter rotating props, tho they rotated L/H CCW, and R/H CW, giving you TWO critical engines??

Anecdotal unscientific evidence of 'spiraling' propeller slip stream:
Look at any turboprop powered aircraft. The soot deposits from the exhaust travel aft down the fuselage in a distinct spiral pattern.

 

[Surestick]

"as for counter-rotating props, yes in part, the engine power needed more than a single propeller, as you say, the radius of the prop is limited by many things, and the number of blades is also limited; however, it is a huge complication to arrange for counter-rotation ... why would they do this if there wasn't a performance pay-back ?"


The performance payback is the additional power available when it isn't limited by propeller size.

 

[rb1957]

you missed the point surestick ... having decided to use two props to absorb the engine power (mind you there are many other ways to do this other than 2 props), you "could" have both props turning in the same direction (out of phase by 1/2 a blade-spacing), though i don't think it's been done; typical practice i think is to have the props counter rotating which is a significant mechanical complexity and cost, so i reason there must be a reasonable pay-back (which i think is recovering the swirl).

 

[FieldTeam]

Is it too far off base to associate this question to the differences between pusher and tractor type engine systems.
Simplified:
One of the ideas of a pusher (rear mounted) engine is to eliminate the vortices from the propellers from striking the fuselage (which causes noise, among other things).
This would be considered a spiral of air around the fuselage that would go all the way to the tail.

 

[thruthefence]

As a matter of semantics;

The large, powerful engines, Turboprop, or recip, that use two props rotating co-axially, but in opposite directions, are referred to as "contra-rotating".

A twin engine aircraft having propellers that turn in opposite directions, are said to have " counter-rotating " props.

 

[Dan320]

Majortomski,
If the wake isn't spiralling, what then is the cause of the need for counter rudder application at take-off? In a climb, the downgoing blade will perhaps have greater lift and try to turn the a/c. However, in a take-off roll in a nose-wheel a/c with little or no AOA to begin with, the nose still wants to swing immediately after application of power. Spiralling theory explains well what's going on even if it may be incorrect. Let's tuft the nose and windscreen and see for ourselves!

 

[rb1957]

dan, there's some contribution from propeller gyroscopic forces

but the OP has gone quiet ... maybe he's thinking, maybe resting, maybe pining

 

[Majortomski]

Ahh, now you've all started down the path!

The aircraft yaws on take off due to p-factor. The THRUST of the propeller is higher down one side of the fuselage than it is on the other side.

If the spiral is true, then the momemt the aircaft lifts off the ground, then it would ROLL right with 5 times the force of the yaw to the left. We always see this spiral drawn in side profile. Draw it in top profile and the completely ignored effects on the wings and the horizontal stabilizer show the plane rolling the direction the opposite of the yaw. But this is never shown or discussed.

The prop tip spiral mentioned above also goes the wrong way to support the theory.

Furthermore, the prop is nothing but an airfoil. Are we saying that the wing induces air particles to move forward in the same direction as flight aft of the trailing edge?

 

[Dan320]

If we consider what is actually happening (as opposed to the usual wind-tunnel mind picture) in flight we see that the wing is moving through still air.
The wing profile and aoa causes higher pressure on the bottom side and lesser pressure on the upper side to achive lift, and in doing so displaces air downward. It is not entirely unthinkable that the forward motion of the wing will cause a (small) forward movement composant of the displaced air. That would be the equivalent of a spiralling propeller wake.

 

[Dan320]

To add to my previous post, in the wind-tunnel mind picture the airspeed is decreased on the bottom side (and increased on the upper), if we say the free-stream airspeed is some discrete number and the lower side airspeed is less, mathematically that is the same as saying "a wing travelling through still air will impart a forward motion to the air deflected by the wing." Of course the main air movement is in the downward direction.
This reasoning supports the spiralling wake theory.
The magnitude is another matter. Perhaps by relating chordwise pressure charts to resultant airspeeds could give us an indication. CFD?

 

[GregLocock]

What about where you have axially coincident contra rotating props -eg Fairey Gannet? There is no net torque at the engine mounts, (ideally), so no need to accelerate the propstream rotationally.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[rb1957]

what forces are being applied to the airflow by the propeller ? ... lift and drag.

the ideal condition is when the resultant is in the thrust direction. old (fixed pitch) propellers achieved this for one airspeed, modern (variable pitch) propellers can achieve this over a much wider range.

in the off-design condition there is a thrust (fwd) component and a drag (tangential) component. the tangential component is causing swirl.

i had to think a little about the propeller on the plane, flying forward; alot of the apparent rotation of the flow through the propeller is taken up by the forward motion of the plane, remember the pictures of the propeller tip advancing (cork-screw like) through the air.