ORIGINAL: alasdair
Rob2160
You make a good and very valid point, that a particular elevator angle places the aircraft in trim at a particular angle of attack and hence a particular speed.
Two follow-on points though
- when flaps are deployed the principles remain exactly the same, but all the numbers change, and how much (i.e. the new trimmed AoA or airspeed) depends on pitch coupling which varies from aircraft to aircraft.
- All the full size flying instructors I've ever met said "re-trim, re-trim, re-trim" whenever you change anything, power, speed, flap angle,socks, whatever
I hope you teach the same. I assume that you advocate that the aircraft should be in trim at 1.3*Vs on the approach??? In which case please explain further the following quote
"When landing a real aircraft you aim to approach at 1.3 x the 1 g stall speed in the landing configuration.
Which means, flying a stable final approach at the correct speed, you should be holding steady back elevator of about 55 to 60% deflection.."
In full size flying we are constantly taught that the secret to a good landing is a good approach, in trim at the correct speed (1.3*Vs as you said).
That is what my posts above were trying to get at. You set the elevator trim to the angle which, with full landing flap, will AUTOMATICALLY lock the angle of attack and trimmed airspeed at the optimum approach speed, 30% above stalling speed (in landing config as you said).
Carrying more airspeed than this (due to not enough up elevator) is what causes excessive float on landing.
Yes please excuse that I didn't differentiate between real and RC control systems...
I agree completely with your follow on points..
1. Stated perfectly and I agree 100%
2. Absolutely, yes the aircraft should be in trim at 1.3Vs during the landing approach..
You quoted two of my lines but left out the one in between that said "disregarding flaps" Yes, flaps do change the equation.. but for my explanation of that specific comment, it was disregarding flaps..
We practice flapless landings in the Hawker 900XP simulator each year for recurrent and yes.. when on final at 1.3 Vs (clean) the control yoke is WAY back during the final approach.. right into your stomach.. IE much further back than it is in a normal landing with flaps 45.
The Hawker is a great pilots plane because it has good old fashioned pulley and cable flight controls and no hydraulic assistance.. You fly it and you feel it flying.. Pure aerodynamics at work.
As you described there are numerous variables and I was over simplifying the explanation.. Most RC aircraft trims do not operate in the same way aerodynamically as a full size trim system.
IE.. in a full size aircraft.. if you change speed / power etc... your elevator position will change to achieve straight and level for example. Trimming in a full size aircraft does not change the elevator positon, it simply releives the pressure required to hold the elevator in that position.
In an RC aircraft the trim works by simply changing the neutral position of the servos..and you can never feel how much control force is being applied back to the servo, so the use of trim in RC aircraft is not to releive the force on the servo but to give the desired flight path with stick neutral. This may actually result in continual force required by the servo. Very different to a trim in a full size aircraft where you are always trying to remove this control force your hands are experiencing. (the organic servo if you like)
In RC flying you are trimming an RC aircraft so that it will fly essentially straight with all the RC gimbals centred.
I have never seen anyone who constantly re trims elevator on an RC plane every time he climbs, descends or lands.
It is actually a slightly different technique....
My comment was confusing and for that I apologise, it was in the context of an RC aircraft that is trimmed for normal straight and level flight at normal power settings.
When reducing power and speed in the RC aircraft you will need to hold back elevator on final approach as the aircraft is "trimmed" for higher speed and power.. and will therefore naturally want to drop its nose unless you compensate with more elevator..
In a real aircraft the trimming and re trimming is constant during all phases of flight and on final, yes your elevator will be generally further back than it would be if flying faster in the same configuration, but the trim is aerodynamically assisting this elevator deflection and therefore there is no pilot force required to hold the elevator in this position. the trim is doing so..
The difference really is because an aircraft gives you feedback.. you can feel control forces change with speed, power etc etc.. the RC transmitter simply has springs that try to centre the joystick and the tension on the spring never changes regardless of the speed or power setting in your aircraft.. So you don't have any real feedback as you do in a real aircraft.
The only real feedback in an RC aircraft is Visual and perhaps aural.
On a side note.. I found it very interesting learning to fly RC collective pitch helicopters.. in the early days it was all visual feedback, but now, I would say 50% of it is aural, every single minute change in collective stick position creates a different blade sound and you learn to recognise the subtle differences and play the sounds during manouvers. I found it fascinating when I realised I was actually responding to the rotor sound more than the movement of the helicopter.