GoldwingRc 91" Slick540 60cc Observations

[Peter Jenkins]

I think that you will always have to have some elevator trim if there is no incident adjustment for a tail plane.  At the end of the day, a minor bit of elevator trim is not going to cause you any other problems like asymmetric elevator movement.  If the aircraft doesn't react in pitch to sudden power changes then measuring the down/up thrust just tells you what the setting is that works best for your installation.

[Adrian Smith 1]

Sounds good to me, Peter. In this case it might be best to leave well alone!

[Adrian Smith 1]

Now I have bit of weather related work shop time, I decided to go over the Slick after about 3 hours flying time. First I regularised the servo arms to allow for the trim and neutralised the TX trims. I still have a fair bit of up elevator dialled in so I thought I would measure a few incidences. Firstly, the motor is exactly 0 degs to the datum line. In absence of guidance that seems fine. 

First the main wings.

 

 

Both the port and starboard main wings are at 0 degs as good as damn it to the datum line. 

Now the odd thing is that the tail planes are both reading at +1.0 degs. They are fixed as per the ARTF build so I am puzzled why that should be. Indeed I am not sure what effect that would have in flight. I am sure there are a lot cleverer chaps here that can tell me.

The last thing I am going to recheck is the CoG to satisfy myself she balances mid-range. I don't have too many worries there, as she is unstable enough for aerobatic flight but stable enough to control as normal.

 

 

[Peter Jenkins]

Hi Adrian

 

Thanks for setting out your motor, wing and tail plane settings plus the fact that you need a lot of up elevator trim for level flight.

 

Clearly, for a symmetrical wing section to produce lift it must be at a positive angle of attack.  In such a case, the wing produces both a lift force and a moment that tends to pitch the wing up and so we use the tail plane to provide a counter force to this - a down force.

 

With your setup, the wing is at 0 deg so will need to be raised to a positive AoA to generate lift.  The tail plane is set at +1 deg so, in relation to the wing, with air flowing over both, and controls at neutral, the TP will produce an upward force tending to push the wing incidence to a negative figure.  To counter this, you have had to use "a fair bit of up elevator trim" in order to generate a down force from the whole tail plane.  That down force is set for your selected level flight speed to pitch the wing to the incidence that at that airspeed generates sufficient lift to balance the entire weight of the aircraft, and the additional lift needed to counter the down force generated by the TP, in order to maintain level flight.  The motor being at 0 Deg will now provide a small element to add to the lift force arrived at by multiplying the thrust force at 0 deg by the sine of the angle of incidence of the wing at level steady flight multiplied by the motor's thrust force.  

 

So, at steady level flight at your datum speed, lift force is generated by the wing, a small amount from the motor and a decrement from the tail plane with the resultant net lift force matching the aircraft's weight force.

 

Ordinarily, I would expect to see the wing at a small positive AoA, somewhere between 0.7 and 1.0 deg with the TP at 0 deg.  Your starting position is why you have to have so much up elevator trim.  

 

If you could increase the wing incidence so that it was in that range rather than at 0 deg, and reduce the TP incidence to 0 deg then you might find that amount of up elevator trim will be different according to: 1) is greatly reduced; 2) requires some down trim or 3) is zero.  You can then adjust the TP incidence so that you no longer need any elevator trim to adjust the TP force to be the required value at that speed and weight of the aircraft.

 

On my current F3A machine, I only have wing incidence adjusters so I have to have some elevator trim for my chosen level flight speed.  In my case, my motor thrust line is -1 deg (I have a contra fitted), wing initially at 0.7 Deg to eventually 1.25 deg and TP at 0 deg but with some up elevator trim (can't remember how much this is physically since I "zeroed" the servo position by adjusting the threaded clevis.  The reason for the increase in wing incidence from the starting value of 0.7 deg was to cater for the requirement to fly a vertical upline hands off to address a pitch to the canopy with the original setup.

 

The pitch to the canopy in vertical flight i.e. no wing lift, meant that there was too much down force from the TP in that state.  Counter intuitively, increasing the wing incidence steadily from 0.7 deg to 1.25 deg meant that I needed the fuselage tail to be raised a little bit to reduce the wing incidence back to that required for S&L flight at my datum speed and therefore the TP down force to be reduced a little bit.  I continued to increase wing incidence until the required down force needed from the TP was sufficiently reduced to achieve S&L flight on the vertical upline such that the remaining TP down force did not cause a pitch to the canopy on a vertical upline.  The slight down thrust from the motor also contributes to this summation of forces and moments.

 

Conversely, a pitch to the undercarriage on the vertical upline would have required an increase in the TP trim force to counter that and that would have required a reduction in the main wing incidence.

 

It all sounds a bit counter intuitive but if you sit down with a piece of paper and draw some force arrows it will become a bit easier to follow.

 

Hope that helps.

 

Lastly, I would not describe the aircraft as being unstable enough for aerobatic flight but rather its stability margin (which is what it's called) is reduced thus enabling aerobatics more easily while the aircraft is still in the stable regime i.e. the CG is still in front of the neutral point.  In the full size world, this would equate to stick force per "g" being pulled.  In an airliner you might want something like a 50 lb pull force per "g" whereas in an aerobatic aircraft like an Edge that might be around 10 lb per "g" or less - e.g for a 6 g pull the pilot would need to exert a pull force of 60 lb so you might actually want less stick force per "g", say 5 lb in such cases.  You could certainly fly the same manoeuvres with a higher stick force per "g" but it would become tiring and require a good deal of strength.  For models, this equates to the servo torque that we have available and why we specify such higher torque servos for aerobatics as we want the surfaces to move fast on some manoeuvres e.g. snap rolls.  That is also why we move the CG to a more aft position while still retaining positive stability and possibly neutral stability.  I have to say, I don't like flying aircraft with neutral stability!  Conversely, a CG too far forward makes the aircraft "heavier" to fly as I recounted with stick force per "g" above.

 

Of course, in today's full size world, you can make an unstable aircraft feel stable  by using flight control computers that alter the feel of the aircraft as its speed varies.   Gyros do the same for us but watch out if the gyro fails with an unstable aircraft!

[Adrian Smith 1]

A lot to take in there, Peter and I am most grateful for your help and explanation. You have put it all so succinctly that even I have followed the gist of it and it's clear.  I did wonder if it was the positive TP incidence with the main wing at 0 deg that was causing my use of excessive elevator trim upwards, without of course knowing for sure why!  (I know now! 😀) Poor choice of words to use "unstable" in relation to aerobatic flight and I acknowledge that and like your description. I am pretty sure the Slick CG is more rearward from the mid-point, but it doesn't bother me, but I will check for sure at the next workshop visit. Incidentally now I have an extended bar for my Robert meter I am going to go  fully over the Capiche  and measure all angles just to satisfy myself and report back on that particular thread. Thanks again 👍