Power application ?

[Vfrpilotpb]

I wonder if any of you Fwgers would satisfy my probing mind, I fly helicopters and when applying power, dependant on the model you counter this power application by feeding in either right or left pedal, what happens in a FW craft when you increase or decrease power? :)

[Evo7]

Same thing. More power -> increased yaw tendency (left for clockwise propellor) -> apply more (right) rudder to correct.

(For a Single engine, anyway - not sure about twins)

[distaff_beancounter]

In F/Ws the yaw is similar, but is only really noticable just after applying full power for takeoff.

If you don't use the rudder to keep the aircraft straight, some of the more powerful aircraft, will run off the side of the runway :eek:

Radial engines tend to produce a more noticable yaw.

All taildraggers need active use of rudders on takeoff, to keep straight.

Most of the smaller light twins have engines rotating in the same direction, therefore have more yaw on takeoff than singles.

The more expensive light twins have counter rotating props, to eliminate this yaw (but I don't usually get my hands on any of those :( )

[FNG]

I would say that yaw with power changes is noticeable in flight as well as on takeoff: if you enter a steep climb or pull to the vertical for a loop or similar manoeuvre you will experience yaw unless you counter with rudder.

[distaff_beancounter]

FNG ..... You are perfectly correct

..... But then I usually only indulge in nice gentle ladylike flying ;)

[A and C]

Distaff_beancounter could you please tell me why a radial engine will produce more yaw than an inline or opposed engine of equal power output.......Thanks.

[Noggin]

distaff_beancounter


"Most of the smaller light twins have engines rotating in the same direction, therefore have more yaw on takeoff than singles."

How do you work that one out?

[distaff_beancounter]

I knew that it would be dangerous answering this thread. I have been flying so long, that my feet on the rudder pedels, work to correct yaw with no obvious connection to my brain :confused:

A and C .... You are probably correct. I am confusing the centrifugal forces of the engine, with the effects of the prop, arn't I? I have only got the odd hour on radials, so no doubt you can correct me on this one.

Noggin ..... I do fly twins most weeks. On props rotating in the same direction, the thrust line of each engine is on the side of the down-going prop. On most modern USA built twins the props run clockwise, as seen from the pilots seats. So the thrust line is to the starboard of the centre line. On counter rotating props this effect is balanced out. You are now going to tell me that this does not effect yaw on the takeoff run, & you may well be right. :rolleyes:

Now tell me what is wrong with my above load of cobblers :D

[FlyingForFun]

Not much to add to the posts so far, except details:

First of all, the original poster said he is a helicopter pilot. I've never flown a helicopter, so I may be wrong, but I believe that the direction of the yaw in most western aircraft is opposite to that experienced in helicopters. (Although eastern block aircraft usually have the propeller rotating the other way, so the yaw in these aircraft is the same direction as a helicopter.)

There is more yaw at low speed. (I don't think this is true in helicopters - but I'd be interested to know if I'm wrong...) This is because the yaw is caused by the rotating slip-stream off the propeller hitting the side of the vertical stabiliser - i.e. hitting the vertical stabiliser with an angle of attack. When stationary, this angle of attack is quite high. When moving at 100kts, though, you have to do some vector arithmetic to add the prop slipstream to the free air, and the total angle of attack is quite low. This is the reason why many pilots only notice the yaw on take-off, or when entering a steep climb - both of which are low speed, high power manoeuvres. Also, the rudder is less effective at low speed (less airflow over it) so a large amount of rudder input can be required. It is noticeable when cruising, but not as much.

I don't think the thrust line being to the side of the engine, which distaff-beancounter is talking about, is relevant in most cases (but see taildraggers, below!) This effect is called p-factor, and only applies when the aircraft has a positive angle of attack. Once the nose of the aircraft is pointed up, the angle of attack of the down-going blade is slightly higher than the angle of attack of the up-going blade, and so very slightly more thrust is produced on this side. (Very hard to visualise, but if you think about it long enough, you'll see it's true.) As with the slipstream effect, this is more noticeable at lower speeds - once the aircraft has a bit of speed, the result of the vector sums will show that the difference angle of attack between the blades is minimal. This is true of singles as well as twins, and is rarely noticeable in a tricycle-geared aircraft (which has a zero angle of attack until it rotates at a fairly high speed). I think the reason you get more yaw in a twin is because you've got more (double) the horsepower. The actual effect would be slightly less than double, though, because the engines aren't directly in line with the vertical stabiliser, as the engine on most singles is.

Taildraggers... The slipstream effect in a taildragger is exactly the same as in a tricycle. There are several reasons why taildragger pilots are more active on the rudders - but propeller slipstream is not one of them. First, p-factor is relevant, because the aircraft is at a very high angle of attack at the start of the take-off roll. When the tail is raised, the p-factor stops being relevant, but you will get gyroscopic procession during the period when the tail is being raised (but not once it has been raised). Finally, having the centre of gravity behind the main wheels means that the aircraft is unstable, and every minor excursion must be actively corrected - whereas in a tricycle, the aircraft will pretty much keep itself going straight (the same way a car will straighten itself if you take your hands off the wheel).

Of course, the pilot isn't actually aware of any of this as he flies - he just applies whatever control inputs are required to keep the aircraft straight.

And finally - radial engines. I'm not sure about extra yaw with radial engines, but there is definitely extra yaw if you're lucky(???) enough to fly an aircraft equipped with a rotary engine - one of those antique eccentricities where the whole engine rotates about the crankshaft. The engine has a huge amount of angular momentum, which results in the engine trying to twist the aircraft in the opposite direction. (All engines do this, but the extra momentum of the rotary engine means it's (apparently) actually noticeable.) In flight, this will be noticed as a roll to the left. On the ground, the rolling effect will force the left wheel onto the runway, causing more friction on the left of the aircraft, and therefore a yaw to the left.

And the disclaimer - I don't really know what I'm talking about, and the information in this post is my own version of things I've read or picked up along the way. I could well have got some of it (or all of it!) wrong, so I'm waiting for corrections...

Hope that helps!

FFF
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[distaff_beancounter]

FFF ... I am glad that you came up with your detailed post.

At present, I regularly fly tricycle low wing singles & twins. In the past I did a full taildragger course, on a high wing Citabria, & I have flown a Harvard.

As I have been flying for a few years, I think that my feet have acquired a conditioned reflex, so that they automatically react to a yaw, whatever the cause. (At least it usually works on the simulated engine failure after takeoff, on the annual MEP proficiency test)

At the same time, the little grey brain cells are depleting, so I have forgotten the techie stuff, about how the yaw is created in the first place :confused:

[DFC]

A helicopter with it's main roter blade turning will try to turn the body of the aircraft in the opposite direction to the blades. A tail rotor is used to balance this force.

If power to the blades is increased then this force is increased and the tail rotor must produce more force to counteract the increased tendency of the body to turn in the opposite direction to the blades.

This effect is most noticeable at low speed.

In a tricycle aeroplane, the same thisg occurs with the propeller. Thus when the power to the prop is increased, the reaction is for the fuselage to rotate the opposite way. This again is most noticeable when sitting on the runway.

If the prop rotates clockwise as seen from the cockpit then the fuselage will try to twist anti-clockwise. This paces extra downforce on the left wheel. This extra downforce will tend to make the aircraft turn to the left unless this movement is opposed by the use of rudder (steering).

The prop generates a corkscrew airflow which hits the fin at an angle of attack. This in the case of a clockwise prop will make the fin want to "fly to the right". Thus the nose will move to the left. When designing the aircraft, the designer ensures that this tendency is removed at cruise speed and power.

At slow speed and high power, extra right rudder is required to counteract this movement. Thus when in the climb or during the take-off roll, extra rudder input is required to keep the ball in the middle (aircraft running striaght).

In a twin, the engines will not normally be placed in front of the rudder. Thus the corkscrew airflow will have no effect.

Hope that covers it in non technical jargon.

DFC

[englishal]

Aircraft like the seneca have counter rotating props and the combined centre of thrust is more or less along the aircraft centreline, and so to answer your question, in the Seneca there is no noticable yaw effect for change in power settings.

Cheers
EA

[Ivchenko]

Interesting thing about yaw effect with power on at low speed in fixed wing a/c is how extreme it is in high power aeroplanes. Remember reading (think it was in Feet Wet) about a low hour pilot on something hot like a Bearcat, who put on full power for a go around, failed to anticipate the yaw and as a consequence the a/c rolled inverted (fatally for the poor pilot).

Also, non aero pilots are sometimes surprised to hear that the most critical control surface (or perhaps just the one that takes the most thought) when flying a loop is the rudder.

[Fujiflyer]

A and C

I seem to remember that one of the several forces producing yaw during take off is that due to gyroscopic precession. This force will produce yaw when the nose is raised (or lowered). Its magnitude is proportional to the amount of angular momentum the rotating mass has hence a radial engine (which has a greater rotating mass) makes the effect more noticeable than with a "normal" a/c.

I've got a feeling that the effect is seen as a right hand swing when the tail of a taildragger is raised.

Fujiflyer :)

[FlyingForFun]

Fuji,

Gyroscopic precession from a normal clockwise-rotating (when viewed from behind) prop will produce a left hand swing (requiring right rudder) when the tail of a taildragger is raised.

The way to figure this out is as follows: raising the tail could be achieved by pushing on the top of the prop. Imagine placing your finger on the top of the prop, then follow it round, in the direction of the prop (clockwise), for 90 degrees. Your finger would now be on the right hand side of the prop, at 3 o'clock. So the force from the gyroscopic precession will be as if it is being applied at this point. If you were to push the right hand side of the prop of an aircraft, the nose would swing to the left.

Incidentally, when the aircraft is rotated, gyroscopic precession should, in theory, produce a yaw to the right. However, I've never heard anyone mention this before. Anyone know why this is? I'd guess that the effect is negligable compared to the slipstream effect, but I'd be intereste to hear if anyone knows for certain.

FFF
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[A and C]

Fuji
I cant see how the gyroscopic mass of a radial can be much more than eny other engine after all it has a very short crankshaft that would be lighter than an inline engine but is likely to have an extra cylinder...........however if you are thinking of a rotory engine as was fitted to aircraft in WW 1 I would agree with you as the whole engine assembly turns on this type of engine and the crankshaft is fixed to the airframe.

As for taildragers swinging when the tail is lifted I think that this is due to the reduction of A of A on the down going prop blade and thus the thrust line moving towards the center of the prop , this is known as assymetric blade effect.

[FlyingForFun]

A and C,

The asymmetric blade effect (p-factor) is noticed during take-off in a taildragger, but not in the way you imply.

P-factor is present before the tail is lifted, and will cause a left-yawing tendency. Once the tail is lifted, the blades are symmetrical - as in a tricycle - and the p-factor disappears. The rudder required through take-off roll will be reducing throughout the roll due to increased rudder authority as speed builds - but there will be a marked reduction after raising the tail, due to the removal of p-factor.

Gyroscopic precession, which fujiflyer is talking about, also causes a yaw to the left. This is only present during the period when the tail is actually being raised - it is not present before the tail is raised (unlike p-factor), nor after it has been raised. This is dependant on the rotating mass, and I agree with you that fujiflyer may be confusing radial engines with rotary engines. Side note: from the pilots point of view, gyroscopic precession can be controlled by adjusting the rate at which the tail is raised. If the tail is raised quickly, it can be quite severe - a good reason to apply forward stick gradually.

This topic has now gone way off of Vfrpilotpb's original question about yaw due to power changes! :D Both p-factor and gyroscopic precession are dependant power, but it's probably worth pointing out that, during the cruise, the primary (I'm tempted to say only, but I'd need to think about it a bit before committing to that) reason that yaw is encountered when changing power settings is neither p-factor nor gyroscopic precession - it's slipstream effect.

FFF
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[Evo7]

Going offtopic is half the fun... :)

[A and C]

FFF .....exactly the point I was trying to make that the required rudder decreases as the tail is lifted.

I have found that with all the "flying club bar" myiths most people new to taildraggers use lots of rudder and it is the reduction in required rudder that catches them by surprise.