Quick question on motor thrust angle

[hankg]

When talking about motor thrust angles, are the direction of angles measured from the point of a person sitting in the cockpit looking toward the front of the aircraft or looking from the front of the aircraft toward the propeller.

I've always taken for granted the position of angle direction was taken from a cockpit view forward so my planes have the motor angled down and toward the right (starboard side) but I had a conversation with another flyer today and wondered if I had it backwards because I was viewing the aircraft improperly.

Regards,

Hankg

[gramps2161]

You have the right idea on the down and right.

[hankg]

Gramps,

Thanks...I thought I had it right but at 65 years old, I'm starting to second guess myself on a few things.

Regards,

Hankg

[gramps2161]

Hankg I know that feeling seems the info goes into the grey matter then gets lost some where along the way.

[FlyWheel]

Thrust angle is actually a rather arbitrary term. The way I understand it, a line "drawn" along the propellor shaft (the direction the prop is pulling/pushing) and extended back, or forward for a pusher, should intercept the center of lift. Too far back and the plane will nose up as power (thrust) increases, too for forward and it will nose down.

This angle is in relation to the line of level flight. Side angle is to counteract torque.

Posted via Mobile Device

[solentlife]

Side angle is to counteract torque

Not strictly correct but is the common answer. Torque is where the law of force and reaction to that force come in ... the motor turns a propellor that has to move air. That means a force applied. The motor is also being held by the mount which ideally should not turn ...

OK .. lets move on. The propellor actually creates a fast spiral of moving air that flows round and back along the fuselage. When it gets to the back of the plane - it hits the vertical rudder and stab ... causing it to deflect to the side ... usually to right, nose to left. Effectively yawing the plane to the left. Right thrust is then applied to the motor mount to counter this yaw.

It can now be seen that it is not torque but a combo of a little bit of torque and a lot of yaw.

So sayeth the Book of Aerodynamics I have on my shelf.

[Leadchucker]

Originally Posted by hankg

....... toward the right (starboard side) but I had a conversation with another flyer today and wondered if I had it backwards because I was viewing the aircraft improperly........

You were on to something by mentioning starboard. The sides remain the same no matter how you view the aircraft if you use starb'rd and port as referances.

[Toby B]

So sayeth the Book of Aerodynamics I have on my shelf

Pray tell, whath' the title of that book?

Thanks!

t

[JetPlaneFlyer]

I've no idea what the book was but the effect if spiral prop wash is explained on a number of on-line texts, such at this one: http://www.av8n.com/how/htm/yaw.html (section 8.4)

[Larry3215]

Originally Posted by solentlife

Not strictly correct but is the common answer. Torque is where the law of force and reaction to that force come in ... the motor turns a propellor that has to move air. That means a force applied. The motor is also being held by the mount which ideally should not turn ...

OK .. lets move on. The propellor actually creates a fast spiral of moving air that flows round and back along the fuselage. When it gets to the back of the plane - it hits the vertical rudder and stab ... causing it to deflect to the side ... usually to right, nose to left. Effectively yawing the plane to the left. Right thrust is then applied to the motor mount to counter this yaw.

It can now be seen that it is not torque but a combo of a little bit of torque and a lot of yaw.

So sayeth the Book of Aerodynamics I have on my shelf.

I think your book is also a bit out of date

P-Factor is an aerodynamic effect that causes propellor-driven planes to yaw when they are flown at high power and low speed (takeoff and climbout, for example.) At low speeds, the plane flies at a substantial angle of attack, and so the airflow is not parallel to the plane's axis. Relative to the plane, the airflow is directed several degrees upwards. Now the prop axis is normally parallel to the plane's axis. As the prop rotates, on one side the blades are travelling upwards and on the other side they are travelling downwards. (On most planes, the prop turns clockwise, as seen from behind, so the left side goes up and the right side goes down.)
The upwards angle of the airflow causes the downward (right) side of the prop to have a greater airspeed and angle of attack than the upward (left) side. So the downward (right) side of the prop generates more thrust. Pull harder on the right side of the plane than on the left and the plane will yaw to the left.
This is one of the reasons why most real prop planes need a certain amount of right rudder to keep them straight during takeoff and climbout.
The other factor that requires right rudder on takeoff (in planes with clockwise props) is spiral propwash. The sideways component of the spiral propwash strikes the vertical stabilizer from the left (in conventional single engine configurations), also causing a yaw to the left. In general, the spiral propwash effect is a lot stronger than P-factor.
You also need right aileron to keep the plane straight to counteract the rotational torque from the engine(s).

[Larry3215]

Steve beet me to it - and with more detail

[JetPlaneFlyer]

Larry,

both spiral prop wash and p-factor play a pert but on models p-factor is a relativly small contributor because on models prop speed is high, forward speed low and angle of attack generally less than full size planes.

P-factor is a bigger issue when angle of attack is very high and where the forward speed of the plane is significant in comparison to the angular velocity of the prop... such as in fast flying full scale warbirds.

[kenchiroalpha]

Originally Posted by JetPlaneFlyer

I've no idea what the book was but the effect if spiral prop wash is explained on a number of on-line texts, such at this one: http://www.av8n.com/how/htm/yaw.html (section 8.4)

Hi Steve
That there is an excellent text
I highly recommend any one interested in our hobby and in aviation in general read it
http://www.av8n.com/how/htm/#contents
Have read it before and lost the link until now
Thanks ever so much
Take care dear friend
Yours Hank

[Larry3215]

Just finished reading the link you posted Steve.

I could swear I had read recently that the the helical prop wash effects were less significant than the P=factor effects as far as yaw - but that reference says just the opposite.

I seem to remember reading that the helical effects were actually more of a rolling than yawing force. partly because they also impact the left and right elevator halves.

Was I day dreaming?

[Larry3215]

We were posting at the same time

I was thinking primarily of take off situations and tail dragers in particular.

[JetPlaneFlyer]

Originally Posted by Larry3215

I seem to remember reading that the helical effects were actually more of a rolling than yawing force. partly because they also impact the left and right elevator halves.

Was I day dreaming?

Possibly dreaming on the rolling issue.. It's true that the impact of the helical wash on the tail and wings causes a rolling torque but this acts opposite to the engine reaction torque and actually helps to partly offset it... a bit like the stators in a ducted fan...

Yes, in the fixed wing world P-factor is greatest on a taildragger when the model has a very nose high sit.. But P-factor is actually a much bigger issue on helicopters than on fixed wing planes.

Steve

[Larry3215]

Thanks Steve!

[solentlife]

Originally Posted by Toby B

Pray tell, whath' the title of that book?

Thanks!

t

A 1970's classic reprint of a UK Modellers bible ! No I will not sell my copy and as far as I know it is out of print and no longer available.
They did a run of books ... Scale, Helicopters, Aerodynamics, Slope, Thermal etc.

It's a treasure trove of wing sections ... NACA, Clark etc.

[solentlife]

Originally Posted by Larry3215

Just finished reading the link you posted Steve.

I could swear I had read recently that the the helical prop wash effects were less significant than the P=factor effects as far as yaw - but that reference says just the opposite.

I seem to remember reading that the helical effects were actually more of a rolling than yawing force. partly because they also impact the left and right elevator halves.

Was I day dreaming?

Most likely not day-dreaming ... but as another says - model props are generally fast rpm and low P-effect. It's same situation with large slow props vs high speed props on boats ... large slow ones impart a part roll to the vessel ... in fact dipping one side deeper in water than other until speed comes up and vessel then NEARLY evens out.

But all I can say is be careful what source quoted - and I say to myself as well ... there are online and written texts that are discrediting the low vs high pressure factors that cause a wing to lift ... all backed up with seemingly gooid scientific research ...

In my humble opinion models being fast, light, high rpm motor / prop combos exhibit pretty standard characteristics when pushed ..

Pull of ground too early and steep and she'll roll .. then cartwheel to LEFT. Based on prop / motor direction and fuselage wanting to turn opposite. Wings don't have enough airflow to stabilise it.
It's the one killer that you see again and again and again ... honest answer : How many cartwheels in at take off have you seen to the RIGHT ? I can't remember any ! I can remember plenty of planes 'yawing to right and then left roll in' due to pilot frantically trying to fight it into the air !!

In fact I can say that lack of Downthrust effects are rarely seen on ground as they usually manifest themselves when throttling up / down in flight. Sidethrust is usually trimmed out and again shows itself when throttling up / down again ... that's my observation ... maybe I'm wrong.

[RC Fan Chris]

Stumbled on this thread trying to figure out the reason why thrust angle was needed, and the torque explanation wasn't cutting it for me.
Very in depth, and scientific, just the way I like it!