MajorTomski

Back when my good professors at Parks were trying their best to beat an understanding of aerodynamics into my thick headed scull in the 1970's, they did succeed in making me understand that aviation science is one of precision and attention to detail.

Rush Limbaugh summarized this best in the 90’s: "Words mean things!"

Why then do we, as a community, continually use the word "torque" when we mean "P-factor"?

Specifically the application of right thrust and down thrust to mitigate "torque effects".

Torque is a rolling force about the Y axis of the airplane. It is the result of the engine trying to twist the propeller against the air loads that are applied to the propeller. Offsetting the thrust line in any direction; up, down right or left cannot and does not do anything to mitigate the magnitude or effects of this torque.

P factor is the yawing force caused by the differential thrust across the face of a propeller blade that is a combination of the difference in relative velocity and the angle of attack of the blade.

Down thrust contributes to minimizing the source of P-factor, right thrust moves the remaining P-factor yawing force closer to the CG and minimizes its effect.

Can we please use the proper terms for the forces in question?

Thank you rant over

pkoury

My Feedback: (7)

How about just learning the correct names of aircraft structure. Here on RCU I have read posts that call the horizontal stabilizer the: tail wing, secondary wing, and even elevator. How about postings describing the rudder when they really mean the vertical stabilizer.
It is a hobby and many model aircraft pilots have no clue as to how and why the model does what it does, I just grin and bear it.

rmh

some full scale "experts' are just as bad -
The info my brother got at flight school was frankly a bit goofy - but what was required for getting a priviate liscence.
The terms misued for airframe are often a bit strange -
"the little back wing" being my favorite.

da Rock

Because the community wouldn't know P-factor if it walked up and whapped 'em in the face with a pie.

The vast majority would tell you the nearest they could figure out what p-factor might be is........ an excuse for flying poorly their last flight.

Lnewqban

When I call some stores, they call me dude.

Some people offers you silence instead of a simple thanks.

One of my beautiful neighbor talks to her children using the F*** word twice in a sentence.

............The worst part is not what they say, it is that they don't care about improving what they say.

BMatthews

Well, actually downthrust counters the effect of too much pitch stability brought on by the CG being too far in front of the neutral point. It's got nothing to do with P factor effects.

In tail draggers a strong P factor results in a bad yaw at takeoff. But there's also going to be an associated torque effect at the same time. You're certainly right though that some side thrust will deal with the P factor effect.

But many of our models will also roll into a yaw as well since many of them use some dihedral. With such models adding a bit of yaw with some right thrust will produce enough roll couple to cancel out much of the torque induced roll. So due to the design of the craft the right thrust CAN do both.

fledermaus

A few years ago here on RCU, I asked why we refer to the "center of gravity" or CG of our planes, when we are really only defining the axis of longitudinal balance for our aircraft.

People got pretty snarky at my comment, but of course I was being pedantic and probably deserved to get slapped (at least a bit).

I guess it takes all sorts to make the hobby what it is, if you don't like the terminology someone is using, just smile and nod...

Jet_Plane

If we are splitting heirs then I'll point out that P-factor is not really an issue in normal flight because you only get P-factor if the propeller disk is significantly misaligned from the direction of travel, and that only occurs in high alpha flight or on the ground with a taildragger. Also P-factor, evenin high alpha attitude,is much less an issue on models than on full size aircraft because of the relative slow forward speed of models compared to full size.

Most of the reason that right thrust is required is because of asymmetric loading on the vertical stabiliser due to spiral slipstream. Also as pointed out by BMatthews on many planes right thrust can help offset rolling momentdue to torque becauase of yaw-roll couple.

http://home.earthlink.net/~x-plane/F...y-PFactor.html

Steve

charlie111

I was taught that the screwing affect of the prop wash on the vertical stabilizer caused that.Right Rudder on take off.I know that

pimmnz

The only problem I have with that (spiral flow) theory is that, when you actually put a cotton thread streamer right behind the prop, and watch the tail end of it down by the fin, there ain't enough 'spiral' to tell which side of the fin it will go...The problem is that great big 'airflow splitter' called a wing, which tends to prevent any spiral flow happening. The other objection I have is that you find the exact same thing happening with a pusher wing, where the only thing behind the prop is...nothing. Torque, of course, is always there, so long as the prop is turning.
Evan, WB #12.

da Rock

With our model airplanes on takeoff, a huge number of them see the same situation, and that situation benefits torque. We see it every day.

The guy who firewalls the throttle as soon as it looks like the model probably is pointed down the runway. He's somehow decided that by getting the takeoff over with as soon as possible is the best way. Same theory as "if you're going to run the red light, do it as fast as possible to reduce the time you're at risk of a collision."

There are a couple of regulars at one field who haven't taken off straight down the runway in years. Most takeoffs are about 10' worth of roll, and a LEFT TURN at right angles to the runway.

What their plane has going on during this.............

It's got a prop that's "flying" 100% worth. An engine that's just gone to full power and is accelerating that prop. And an airplane that doesn't have a single part "flying" yet. Until those important parts, the ones that control pitch, yaw, and roll, have some airspeed, they aren't going to stop the plane from pitching, yawing, and rolling wherever the forces that model is feeling choose to pitch, yaw and roll that sucker.

Thankfully, most of those planes' surfaces get enough grip on the air to stabilize the pitch/yaw/roll in time to give that pilot a plane headed about 80 degrees to port and angled up a bit, often with a bit of roll throw in.

I've often wished to be able to wave a magic wand and turn those guys' engine and prop to a clockwise rotation just to see how they'd react to the plane making a hard right

MajorTomski

Ahh YES! another one sees the 70 year old lie!'

Did you guys know that spiraling slipstream "theory" didn't exist in avaition before the publication of "Stick and Rudder"?

Did you know there isn't a single equation in all of aviation to account for this supposed fact?

Did you know that the spiraling slipstream ignores what would actually be happening to the airplane if it existed.

Did you know that the supporters argument that the slipstream doesn't affect twins with H shaped tails because the H tail is buried in the spiral and so it cancels out the effect, is false for most H shape tailed twins and quads?

More later.

bogbeagle

A way to check this might be to experiment with a water-screw.

It should be possible to observe the behaviour of suspensions in the water behind a prop. I'm thinking of a boat's prop, rotating in a glass of water, with some polystyrene beads floating in it. Or something.

If there exists a helical flow in water, then it is a fair bet that the same effect will be present in air.




Come to think of it, when I use a kitchen mixer, the fluid in the bowl does rotate. Mind you, someone did point out that, for a torque to exist at the prop-shaft, there must be a helical component to the airflow ... action-reaction.

rmh

When in doubt - I test-
I built a pusher pattern plane - and tested the flow in front of and behind the prop
Flow in was pretty straightforward- the little string I was holding showed this.
The flow out was adifferent thing altogether
the high pressure air formed a decreasing diameter which started about 10% in from the tips - a trumpet shaped airflow.
I could not tell if this was spinning -but it did have a direction-as shown by the little string which would move along outer edge of the cone.
Experts with flow testing devices may/may not confirm this.

bogbeagle

Oops! I thought better of this post and deleted it.

AA5BY

On my wall hung the skeleton of a plane started thirty years previous... a Das Box Fly, a .20 size biplane (tail dragger with skid). It had been a gift from a student in appreciation of instructor training and got built but never acquired an engine.

A LHS was going out of business and had a Saito .30 at half price so was acquired to finally provide flight power for the Das Box Fly. As I started covering, I posted some comments about working on the old plane to get it air worthy. Received in reply were several nostalgic comments from guys who had owned the plane back in the seventies who remembered that it was a great flyer, but had horrible ground manners.

During the maiden... the surprise came that it was in fact a great flier but suffered none of the expected ground handling problems. I've no reason to believe the gentlemen were wrong in their claims of what I should expect... so what I'm left wondering, is there a contrast between their experiences with an era two stroke and my use of a four stroke?

The four stroke operates at less rpms and compensates by using longer blades. Would these variations reduce P factor or vortex coupling or whatever theory/s one believes apply?

UStik

The following might explain that (more air accelerated less and thus rotating less).

Evan, the best explanation I found so far (and the attached picture) is from an old (1954) Swiss aerodynamics textbook written in German. It was known for its simple yet correct explanations. It points out that propwash effects exist even on twins where the tail is not hit by propwash, or it is but due to deflection of propwash, caused by gyroscopic precession. The explanation in my words goes as follows:

A good way is considering the rotating slipstream to be a gyro. When it streams across the wing, it is deflected downward like any air flow that is streaming across the wing. But deflecting a gyro has a side effect. If a slipstream turning clockwise, seen from behind, is bent downward, some gyroscopic forces will bend it clockwise also seen from top. And this turning the slipstream clockwise (action) makes for turning the airplane counterclockwise (re-action).

I believe that even applies if the helical propwash is bent down by the wing downwash on a pusher...

Jet_Plane

So explain why the Hawker Hurricane, among many other aircraft, has the vertical stabiliser offset a degree or two to the right in order to counteract spiral slipstream? The Hurricane by the way was designed about 10 years before the publication of Stick and Rudder, when according to you spiral slipstream hadn't been invented

Doh!

fledermaus

FWIW: [link=http://www.nrc-cnrc.gc.ca/eng/multimedia/cavitation-tunnel.html]http://www.nrc-cnrc.gc.ca/eng/multimedia/cavitation-tunnel.html[/link] showing the spiral cavitation pattern caused by a ship propeller in water, in the National Research Council of Canada's experimental water tunnel.

[link=http://www.flightglobal.com/airspace/media/mokkel/propeller-vortices-2-7104.aspx]In this picture of a C-130 taking off[/link] one can see condensation vortices behind the props which are similar to the cavitation in water. The pressure changes around the prop blades cause water vapour to condense along the path of the blade.

One thought I have, however, is that these helical patterns do not necessarily mirror the airflow. The axial velocity of the air in the propwash is much greater than the tangential velocity (if it wasn't, it would be a pretty crappy propeller) and there is not that much momentum transferred tangentially as a result. This suggests that the prop wash effects might not be that substantial, but I'm not sure about that conclusion.

Maybe I'm just blowing smoke...

pimmnz

Of couse, I have done the 'pin and string' test, or I wouldn't have given the results. The airplane was my Atlas, OS.61 and 11x8 prop and it was standing still at the time, I think as soon as it starts to move any displacement of the flow behind the prop will very quickly align with the FSD. You can all try it yourselves...Choose a model with the least fuselage behind the engine...As for offset and cambered fins, heck, even the WW1 Fokkers had them, I bet there are other WW1's...The Me 109 has a cambered fin, even the P51 has fin offset, but all to counter TORQUE. Much easier than the pilot having to hold a deal of rudder at any airspeed other than that that balances the torque. I think you might find that one of the gripes pilots held against the early versions of said '109, was that it really needed a rudder trimmer. Most of the others all had pilot adjustable trimmable rudders. As for 'spiral airflow', you could even inject smoke into the airstream to see if it is wrapped up around the fuselage, and down the other side, in the spiralling airflow. I have tried, with those IMAC models with smoke on, but have failed to detect it yet. If it were a detectable effect, then the exhaust streaks on the noses of most WW2 fighters should be aligned different, one side to 'tother. They don't appear to be, either now, or in photos from 'back in the day'. I have seen 'spiral airflow' but that was from a domestic fan, with really big wind shovels, static, and fairly slow RPM compared with a model airplane prop. And it had nothing behind it. I can confirm RMH's observation that the stream behind the prop appears to contract, but that should be no surprise, the moving air behind the prop will be at a slightly lower pressure (due to its velocity) than the surrounding, and the rest of the air does what it always does, and moves from higher to lower pressure. As for the visible spiral effect from prop blade tip vorticies, thay are simply standing vorticies showing where the blade tip was and in no way indicate the direction of the air behind the prop, else they would point backwards. As for the twin explanation, well, you could use the fan example, then whack a splitter (wing) across the flow, and watch what happens...but then most will already have guessed what does happen. Much more interesting is watching the streamers as you increase the AOA of your 'wing'...But this is all kids stuff, and one would have thought that enquiring minds would have already done much of this...As for the 'Published Explanations', I cannot guess, they may have been written as a simple explanation for pilots or laymen and entered the realms of fact simply by longevity. But then I have been told several times that I am an unprincipaled cynic...
Evan, WB #12.

bogbeagle

OK.

Back to my prop in a glass of water.

If you dropped some steel balls into the glass and activated the prop, wouldn't those balls begin to roll around in the bottom of the glass?





Alternatively, if there were no helical flow of air from a prop, how would the engine's shaft experience any torque? I mean, if the airflow was purely axial, then the propshaft would experience thrust, but no torque ... I think that this would be impossible.

MajorTomski

Please read my statement again. I agree the PRIMARY purpose of down thrust is to help with pitch problems due to power changes. I said the down thrust AIDS in minimizing P factor by placing the propeller more perpendicular to the local airflow thus minimizing the source of the P factor in the first place

In ANY single engine propeller aircraft P factor will cause a yaw at ANY AOA higher than cruise configuration. It will be a bad yaw during the demonstration of slow flight.

For the torque of the propeller to affect our models it has to have a mass that is relatively high when compared to the mass of the airplane. For most “sport” and Pattern models this is not the case. A typical trainer weighs 6-8 pounds the prop weighs around ONE OUNCE.
I offer the mass-induced or air load-induced torque on a TYPICAL Glow powered RC plane is insignificant. There isn’t enough torque to induce the roll coupling you refer to. This can easily be tested by placing a scale under each MLG of a model, restrain it and run it at full throttle. The difference in weight on the scales will give us the actual torque. HOWEVER in small light 3D airplanes the mass of the prop is high enough to affect the plane, resulting in TORQUE ROLLS not YAWING.

And that turn is due to P factor more thrust running down the right side of the fuselage. You can demonstrate this in flight. Take a trainer and transition to slow flight; nose high, just enough airspeed and power to keep it in the air just above a stall. Now, without adding any other control input (more difficult than it sounds) punch full throttle. The airplane will YAW left not roll left, until it accelerates away from the stall speed.

Actually sir, you’re confusing cavitations vortices with the spiraling airflow theory. The tip vortices on the C-130 are supposedly perpendicular to the supposed spiral in the downstream airflow.

And the F-4U Corsair’s fin cranks over 6 degrees when the flaps are lowered. In both cases, the fin is offset to counter a yawing tendency. No one has attributed the cause of the yaw to “spiraling slipstream”. Again, I said the spiral theory did not catch on until after 1944. At my work we have one of the largest aviation libraries in the country. I’ve researched over two dozen stability and control text books. Spiraling theory is not mentioned prior to 1944, and after that it is only stated as accepted factual explanation of why the plane yaws. The books never go into an actual analysis of whether or not you can mathematically predict its existence, t and therefore design out its effects.


Back to the original intent of my original post; It is quite evident from just these few posts that the hobby is full of a lot of theories and preconceptions that have nothing to do with the actual engineering or science involve. I guess I’m just beating my head against the wall hoping for some improvement in the accuracy of the sport.

Sorry if I've caused this to become a tempest in a teapot.

Lnewqban

Whatever happens makes airplanes fly and propellers pull.
That very phenomena is interpreted in as many ways as observers try to, as this thread and many more show.
Our brain needs the concepts and axis and divisions to make sense of these things.

There is only one force and one acceleration; however, we divide then in so many components, that the phenomena ends up being confusing.

By forcing a propeller through a mass of air, a unique reaction happens, which is beneficial to our purpose of moving the airplane forward.
We could say that torque is the reaction caused by the drag of the blades and that the thrust is the reaction caused by the lift of the blade.

Since lift without drag cannot happen, thrust without torque cannot exist either.
More AOA means more lift and drag; hence, a prop moving in a path that is non-perpendicular to its rotation plane will produce an asymmetric reaction, which we like calling P-factor.

Once leaving the rotating blade, what force would cause the stream of air rotate for long, if any rotation requires a centripetal acceleration?

rmh

There ain't one -
You don't need text books to proove this
simple observation will do-
remember - in nature everything tends to static and equal condition.

The higher pressures irrespective of momentum will -unless confined - simply reach the static or ambient conditions
Even the dreaded wing tip vortices -which DO exist - gradually "go away'
Nothing mysterious about it.

cutaway

If the spiral notion is so minimal, what explains the differing power patterns on high/low thrust and top fin vs underslung fin FF designs? One combination can produce a natural left climb pattern, another a right.