rmh

Calculated, is not demonstrated .
Any"spiraling force" of the air compacted or disturbed by the prop , has yet to be seen.
Torque reactions due to the resistance of the air -is easily seen and demonstrated
Any spiraling of the air remains a figment of th imagination
work with a pusher prop -which is free from any airframe aft the prop-
measure any spiral flow
use colored vapos or strings or whatever
I would love to see this spiral flow

pimmnz

Any standard fan (You know, the thing you use to keep cool) will show with streamers a spiral flow, been there, done that. It all changes when you introduce a splitter (wing) across the flow, and it don't matter much where in the flow you put it. The premise was that this flow causes a reaction on the fin...well, it don't get that far, and it does not explain why, when the prop is the rearmost part of the airplane, the airplane still exhibits the same roll/yaw reaction...just sayin' - 'an it's one of those things anyone can try...don't take my word for it...
Evan, WB #12.

rmh

I guess I will have to make a ring with streamers to hold in th e flow aft the pusher prop
I did note that the pusher prop flow, rapidly decreased in diameter
no flow near the tips - NONE-.

BFoote

Equal and opposite forces. If plane has an angular momentum imparted to it via the propeller, then via equal and opposite forces said force MUST come from the propeller acting ON the AIR. There is the centrifugal aspect of this as well, but that is easily calculated and taken into account. There is a larger force than this present. Therefore said angular momentum is equivalent to the spiral slipstream. All one has to do is add up the direction of the flow over time in an iterative orientation to see this. Without ANY imperical evidence at all. Now said interaction between said spiral slipstream and the fuselage, wing and quantifying it per delta x from the propeller itself is another matter.

Note, a propeller speeds up the air creating a higher density and therefore a smaller volume. Therefore max spiral will be found about 2/3 outbound on a propeller blade distance from centerline.

Math can be a scary thing sometimes, but its usually quicker in the long run to just build the model in Excel instead of typing long posts on the internet or searching for hours on the internet. At least this is what I find usually the case AFTER doing the blathering of posting and the searching on the internet.

rmh

Yes - props turn and compress air ,.
there is also the ambient air which is not moving -but is always involved .
This quickly "kills" the disturbances.
The math of action/reaction etc., is accepted but the resultant interaction with the ambient air seems to be ignored.

The logic of what is happening is that any twisting of the downstream quickly becomes a straght flow that fades to match the ambient air.
I guess I will do some hands on tests with my thrust stand
This is the same device I use to check power inputs (electric) vs thrust in ounces . A static thrust measurement
. I have been told by "experts" that it is worthless info- funny -it seem to be very usable info.

BFoote

Yes, its angular moment quickly sends said energy into the free stream, but its still after the plane has passed through it. The only real question is its effects in hover where velocity is minimal and surface deflections have little ability to counter their effects. Therefore trying to build a naturally stable platform, every engineers goal, is a difficult proposition. Any typical aircraft have their wheels on the ground canceling this minimal force.

This spiral slipstream is calculated and USED to determine blade angle in all turbines.

It exists, weather its useful to know...

MajorTomski

Wake up old thread!

Finally got around to reading the links in post #19. The second paper is on developing efficient airfoils at each station along the length of the blade. Guess what was burried in the middle of the paper:


THE JET ROTATION IS NEGLECTED!

So much for the slip stream rotation being important in the design of a propeller!

Happy weekend

DwightMann

Anybody that has used a propeller type mixer to mix paint has noticed that the paint will move in the direction dictated by the pitch of the propeller, but also spirals in the direction of the rotation. The same fluid dynamics applies to the airstream behind the propeller on an airplane.

DwightMann

Anybody that has used a propeller type mixer to mix paint has noticed that the paint will move in the direction dictated by the pitch of the propeller, but also spirals in the direction of the rotation. The same fluid dynamics applies to the airstream behind the propeller on an airplane.

rhall999

Quote:

THE JET ROTATION IS NEGLECTED!

So much for the slip stream rotation being important in the design of a propeller! Happy weekend

Yes, You are right.......BUT, reread the text and what you even quoted, especially the part Imade red. Idon't recall anyone ever being concerned with design of the propeller in relation to the spiral slipstream. It is the AIRFRAME the propeller is bolted to that is designed with the spiralling airflow in mind. EG, leading edge of the fin offset a degree or two, bit of right thrust, whatever. It is the ability to calculate the spiral that allows those adjusments to be made to said airframe.

Look in a ducted fan at the design of the stator vanes. You will notice that they are almost alwaysat an angle to the straight through airflowand curved like a old timer free flight airfoil.. Why is that??? Simply because the airflow does NOT flow straight through the duct. It comes off the fan on a spiral. Since it can be calculated how much "angle" there is to that airflow, the can put those stators at the right angle to most effeciently "capture" and straighten that airflow. If the spiral slipstream was a myth, there would be no need for even bothering to put the stators in there, other than to support the motor.

banktoturn

It's pretty clear that the flow behind a spinning propeller would be swirling, but since this seems to be doubted by some people in this discussion, here is one link to a paper that specifically mentions this swirl:

http://ntrs.nasa.gov/archive/nasa/ca...1979016853.pdf

There are many others. If anyone is genuinely curious about this, a Google search with keywords such as propeller, visualization, swirl, flow, ... will produce plenty of information.

banktoturn

rmh

The air does change direction as it passes thru a prop-
the angle causes that - obvious stuff
The rate at which it straightens out is another thing all together.
It is far too easy to confuse motor torque reaction with the airflow.
On a pusher you can demonstrate the the true action quite easily- dun it
The flow quickly becomes a fading column of higher pressure -
Air which flows from a heavy jet wing tip is a different thing -
A ducted fan -yet another thing .

banktoturn

This shifts the discussion from whether the spiral slipstream exists to whether it persists far enough downstream to have an effect on the vertical stabilizer. The answer to that question is "yes", for many configurations. Another few minutes of Google searching yielded NACA Technical Note 2409, from July 1951. Here is the relevant section:

The effects of the slipstream on the vertical tail are often very
important and should also be determined from experimental data, if
possible. The increase in dynamic pressure at the tail caused by the
slipstream is treated theoretically in reference ll6 and is illustrated
by the experimental data of references 50, 5k, 55> 5&> 71 > 7^> and 76.
The experimental data of reference 76 also show that the propeller slipstream
can cause a destabilizing sidewash at the tail which will tend
to reduce the stabilizing effect of the increased dynamic pressure at
the tail. Since these data indicate that slipstream effects on the
vertical tail vary greatly with airplane configuration and propeller
arrangement (single or dual rotation), use of experimental data appears
to be the only satisfactory estimation procedure at present.

Here is the URL at which this paper can be found:

http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA382075

Major Tomski, if you are still reading, it is really easy to find the answer to the question that you have posed on several forums. Just do the Google search, and you will have lots of productive hits. The text above even explains why the simple formula that you seem to require doesn't exist.

banktoturn

MajorTomski

Well, when I began my search for a documented process for mathematically even ballparking the value of the AOA change on the fin due to propeller spiriling slipstream, there wasn't a google to use in the first place.

And once again the 60-year old report you've presented as evidence is steeped in the accptance that the effect exists without mathematical proof. In the paragraphs prior to your quoted document, the author in essence once again simply dismisses the effect on the horizontal surfaces. Sorry but, again, the same spiral that causes a yaw to the left (for most model planes) MUST, it can't just disapear, MUST cause a rolling force to the right that is FIVE TIMES as great as the left yawing force. Either the yawing force from the spiraling slipstream is insignificant and we waste far too much time giving it more attention than it is due. Or something wonderful is happening that stops the plane from rolling to the right.

That is my only point; ALL of the published solutions ignore the true force diagram on the airplane.

banktoturn

You make it sound so complicated; maybe we just trim for it. Do you want to find the answer to this one using a Google search, or wait for me to do it? It's no longer 60 years ago.

banktoturn

buzzard bait

The most persuasive piece I've ever seen on this subject was an article in M.A.N. way back about 1963. The reason I remember it so well is because the author modified a little Goldberg 1/2A Blazer FF plane by substituting for the stock fin a tall rectangular fin that slid up and down through the rear of the fuselage. By moving it up and down he could change the climb pattern left to right. The turn was adjusted solely by moving the fin up or down. What else but a slipstream effect could possibly explain that? It made a believer out of me! I still have the article somewhere.

Jim

MajorTomski

Go for it, find one set of equations that give you a fin offset for a given power and or prop size.

banktoturn

That might not exist. You seem to think that if the "mathematical proof" doesn't exist, then the phenomenon doesn't occur. This isn't true; many engineering phenomena are well confirmed, yet don't benefit from such a tidy mathematical description. Bummer.

banktoturn

David Bathe

Seems strange that there's even a conversation going here. Even more surprising that some people even think the spiraling slipstream is non existent or a myth. Errr yeap... where does one start. We've been fiddling with it for years, folks have wrongly referred to it as "engine Torque" or P- Factor" we know it's something to do with the prop slip stream. How do we know? Well, we've experimented with different sized fins, positioned then in different positions, including underneath. We know it's engine sensitive, the yaw left occures more the higher the engine revs. We know it's prop sensitive... anyone who's spent weeks trimming the right thrust on a pattern plane will tell you that you'll need to start again if you change prop. And we know it's assymetrical because the F3A boys using counter rotating props report the left yaw disappears, the motors needing no right thrust to compensate.

AA5BY

I've no doubt that a force exist... as evident by the obvious yaw experienced by many models. I do doubt that the force is slipstream applied to the aircraft tail or rather that 100% of the force or even the majority of effect is at the tail.

My base of thought is experiential, that planes I've had that had a round cowl and fore section, suffered far less yaw force than those with an asymmetrical cowl or fore section. My Yak 54 with a symmetrical forward section yaws very little going vertical and has no thrust offset whereas I've an Akrobat II with a grossly asymmetrical forward section that suffers gross left yaw even with three degrees right thrust. Another comparison is my Sport 35 design that features a nearly symmetrical fore section with round cowl that suffers very little left yaw going vertical whereas a similar sized Spacewalker having a very high prop center compared to the fore section suffers gross yaw. My Ultra Stick Lite has a nearly symmetrical fore section and a minimal one at that and suffers no left yaw with no thrust offset.

When reasoning why this might be true, the likely explanation is that a lower section of forward symmetry relative to the prop, produces unequal pressure zones either side of the fore section. A higher pressure on the starboard side produces yaw to port.

Simply put, I tend to think that it is not the tail section that is yawing starboard but rather the fore section that is yawing port.

I'm not saying that some slip stream doesn't exist... but I'm concerned we focus on it at the expense of missing the greater cause of left yaw.

A test for this theory would be to do the unlikely thing to a model and invert the prop center. That of course would create likely ground clearance issues on most models but it would be interesting to note if the left yaw is gone and that in fact right yaw might be experienced.

buzzard bait

The great thing about the approach in the old MAN article is that the sliding fin proved the point so simply and effectively. It didn't change any other relationships on the airplane and it demonstrated a proportional effect because the sliding fin could be moved up and down to any position.

I think that changing the thrust line would just start a new set of arguments. Why not start with a symmetrical cowl, show that it doesn't cause yaw, and then add side area to the cowl different amounts in different locations should give a clear picture of what, if anything, it's doing.

Jim

AA5BY

Of course it would... and it just might just put the focus on the primary cause of prop induced yaw on model planes, rather than chasing a possible red herring.

David Bathe

My Stick definately suffered, needed a good 3deg off set.

AA5BY

To clarify, I meant to say that I'm thinking the pressure difference is on the fore section of the plane rather than a tail fin. Though, having said that, I'd be errant to claim that there is a single cause. Just guessing, I'd suggest that left yaw might be 20% caused by port forces on the tail plane and 80% from the starboard forces on the fore section.

I'm completely in agreement that prop forces are the cause of left yaw.

David Bathe

Sometimes a picture's worth a thousand words.