P-factor

[Birgir]

I think I learned about the P-factor the hard way.

What do you think?

 

 

Edited February 14 by Birgir

[Alan Gorham_]

You took off with a lot of up elevator straight into a stall.

[James Green2]

Those trölls are at it again..

[Birgir]

Guilty as charged :-(

[Cuban8]

Rudder input might have saved it along with not so much  grunt until airspeed over the tail surfaces had more effect. Very clear yaw to the left......right wing advances then  rises......roll to the left.........up elevator adds to the problem giving the flick.  Perfect example of ailerons not being effective in this situation. Not much room on your flying 'patch' so I understand your haste to get it away. Guess you were fortunate not to have hit the street lamp or building. Just one of those things TBH - did subsequent flights go OK?

Cubs can bite!

 

A friend of mine had a Decathlon of a similar size, and the model was a shocker to get away without something similar happening. Never really pinned down a fault with the model, after just about everyone in the club had a go with it. Once away it flew nicely and landed well - it simply needed very careful handling during takeoff.

Edited February 14 by Cuban8

[Birgir]

Thanks Cuban8
I think you are spot on.
The outrunner axel was bent so subsequent flights haven't happened yet.
Next time I will practice some taxiing before takeoff
B.

[Martin Harris - Moderator]

The lack of rudder compensation was compounded by rapid application of full throttle.  I (almost) always advise gradual and progressive opening - with typical model power to weight ratios vastly exceeding full size, it's easy to get into the situation avoided by pilots of high powered piston engined fighters where giving full throttle too early would lead to loss of control.  Not only do you have better control authority as airspeed increases, but it's much easier to feed in progressive correction - there's a tendency to overcontrol otherwise.

 

If the wing is stalled, application of aileron makes the situation worse by increasing the effective angle of attack of the stalled wing over the span of the aileron.

 

P.S. P Factor is only one of several influences that pull a model to the left during take off.  For anyone unfamiliar with it, this is the best graphical explanation that I've come across

 

...as you can imagine, the side of the propeller disk with the upgoing blade produces less thrust due to its lower angle of attack and this is coupled with the change in airspeed as the propeller blade moves backwards and forwards relative to the airstream as it rotates, producing asymetric thrust.  These effects are more pronounced on taildraggers on the ground run - a typical tricycle model won't experience them until it rotates but gyroscopic, torque and helical airflow induced effects will still try to turn it left.

 

Edited February 14 by Martin Harris - Moderator

[Cuban8]

1 hour ago, Birgir said:

Thanks Cuban8
I think you are spot on.
The outrunner axel was bent so subsequent flights haven't happened yet.
Next time I will practice some taxiing before takeoff
B.

Try to not be in so much of a hurry to lift the model off - give yourself at least twice the distance so you have a chance to use the rudder to keep the model into the wind, then lift the tail and accelerate to flying speed on the main wheels. Check your elevator movement - too much is a bad mistake, as is elevator expo which you really don't need on this type of model. You just need enough elevator response to gently rotate the model into the air, and once there, no more movement other than that which is needed to keep the the nose up in turns. Rudder about 3/4" to 1" either way.

My guess is that 1/4" movement either way of neutral elevator will be more than enough for first flights - presumably C of G is no more than 30% of chord back from the leading edge.

Once the  model is settled and you start to get to know it, by all means experiment with control movements and CG to make thinks more lively if you want to.

 

[Birgir]

Thanks, I value your comments very much.
 

I‘ve also been thinking about the CoG and perhaps I should add a bit more lead before next try.  Now the CoG is just shy of 75 mm from the leading edge of a 170 mm wing cord (44%). This might also be a contributing factor.

I need to put in a new Outrunner or put a new shaft in this one before next flight.  It will take some surgery to get this one out.  There is a build video on my YouTube channel that explains this. I would appreciate your thoughts about that video also.

My channel is at: https://www.youtube.com/@birgiredwald

B.

[Martin Harris - Moderator]

I wouldn't go behind 30% for a first flight - quite likely to have contributed to your misfortune.

[John Lee]

2 hours ago, Martin Harris - Moderator said:

P.S. P Factor is only one of several influences that pull a model to the left during take off.  

 

Yes, for tailwheel aircraft with propellors rotating in the conventional direction there are 4 forces all of which are trying to make your model turn left.

 

1) Torque

2) P-factor

3) Gyroscopic precession as the tail lifts

4) Spiralling slipstream affecting the fin & rudder.

 

You need to be on the ball from the moment the model starts moving until it is, proverbially, back 'locked in the hangar'.

[Simon Chaddock]

Agree about the 4 factors but it is amazing how folk seriously under estimate the effect of motor torque.

With electric full power torque is there as soon as you wack open the throttle and remains until you close it again!

Little or no aerodynamic control to resist motor torque until full flying speed is achieved. Only the rudder in the prop slipstream will be effective instantly to keep the plane straight as the throttle is opened.

As has been pointed out many planes have considerably more power than is needed to fly so consideration has to be given to how much and when it should be applied.

We live and learn. 

[Martin Harris - Moderator]

I've seen a few hand launch incidents with high powered models.  I certainly launch my EFExtra at part throttle after watching some others' attempts!

[Cuban8]

Sorry, but no "perhaps" as you say about it - having the balance point at 44% of chord on that model is quite incorrect and will make it a handful.

Where did that figure come from?

No further back than 30% and chances are you'll be fine. 

Edited February 14 by Cuban8

[Birgir]

Dear Cuban8,
 

I have a confession to make.
 

When deciding the CoG, I simply added some lead to the front, assuming that 50g was quite a lot. Then I placed my fingers under the wing and thought, "This doesn’t seem too bad." I didn’t even check the plan.
 

But this is the essence of our hobby. If you don’t think things through, if you don’t give the task at hand the respect it deserves, you will be punished. And during takeoff, I failed to respect the process and thought I could just go straight into the air.
 

This is what I love most about our hobby. If you are careless and inattentive, you will be punished within seconds. But once the disappointment fades, you realize that this is just a game—one that teaches you valuable lessons to apply in the real world.
 

I will now check the plan and make amends.

[John Lee]

[Birgir]

Thanks to everyone above for their comments.  
I have now made some amendments to the model and a short video about my experience.  
I’ve included some of the comments from above in the video and hope that is ok.
Here is the video:

 

[J D 8 - Moderator]

It may just be the camera picture but at the end of video it looks like the wing is set with anhedral, this will not help with lateral stability where as some dihedral would.

[Birgir]

Thanks - it's straight - I promise 🙂

[Peter Jenkins]

Just one more point Birgir, if you are taking off in a cross wind, as soon as the aircraft starts to move, and sometimes before if its a strong wind, the model will act like a weather cock and the fin/rudder will yaw the aircraft into wind.  If you had a wind coming from the left when you take off then that will also have added to the yaw to the left.

 

I always think to myself before I start the take off, which way will the wind swing the aircraft so which rudder to apply, left or right?  On a strong wind day, you might start with rudder deflected, in the correct direction, before you apply power and then adjust with the rudder to keep the aircraft straight.  Remember also that applying, say, right rudder will also cause the aircraft to roll to the right as airspeed rises so be prepared for a touch of opposite aileron to the rudder to keep the wings level - it won't be much aileron but it might stop the wing being tipped over.  Also at no/low speed, especially for a high wing model, the wind can "get under" the wing and blow the model over as well.  So, thinking about what the aircraft can do before you apply power will help to prevent situations like this occurring.  Also, if you need to land on a runway as opposed to into wind, the cross wind take off will help you to remember that on approach the aircraft's track should be along the runway heading but the aircraft's heading will be to the windward side of the runway.  As you round out to land, you will need to squeeze on enough rudder to align the aircraft's axis with the runway and the direction to move the rudder will be the same as you used for take off.  Again, be ready for some opposite aileron to prevent a roll in the direction you have applied rudder.

 

There is a lot going on when taking off and landing and indeed flying a good circuit to give a good stabilised approach path is the precursor to a good landing.

[Birgir]

Hi Peter,

Thank you so much for your detailed and insightful advice regarding crosswind take-offs and landings. Your explanation has certainly helped me understand the nuances and considerations that need to be taken into account when dealing with strong winds.

I will definitely keep your tips in mind, especially the points about anticipating the wind's effect on yaw and the necessity of using rudder and opposite aileron to maintain control of the aircraft. Your suggestions about thinking ahead and preparing for wind conditions before applying power are invaluable and will undoubtedly guide me in future scenarios.

For now, I plan to wait for calmer weather before making my next attempt. Your post has given me a lot to reflect on and I feel more confident about managing crosswinds in the future.

Thanks again for your helpful post.

Best regards,

Birgir