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Dec 01, 2020, 03:49 PM
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Question on Handling : Proper term


Hi,

If I set up an airplane to have 10 degrees of elevator deflection per unit CL and the airplane is flying at a CL of 1.0 and I pull up 10 degrees then I will get a change from 1 g (straight and level) to a 2g pull-up (assuming that it doesn’t stall first!).
But if I am flying at a CL of 0.5 and I pull up 10 degrees my lift coefficient increases to 1.5 and I am now pulling 3.0g

So airplanes flying fast or in low CL handles better.

My question is what is the correct aero term for this (is it "Control position gradient"? ) and what would be the formula for this

Thanks very much.
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Dec 01, 2020, 04:44 PM
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Coupez's Avatar
Sounds like you're fishing for "elevator angle per G". See Equation 26 in this paper:

http://www.dept.aoe.vt.edu/~lutze/AO...aneuvering.pdf

Note this is directly proportional to "C_w" (lift coefficient for 1G flight), so less change in elevator is required per G as speed increases.
Last edited by Coupez; Dec 01, 2020 at 04:52 PM.
Dec 01, 2020, 06:13 PM
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ShoeDLG's Avatar
The “thing” that is often used to quantify longitudinal maneuvering sensitivity is Stick Force Gradient or “stick force per g”. As you point out in your post, this quantity can be a strong function of airspeed (dynamic pressure).

Airplanes flying faster will generally be more sensitive. This doesn’t mean they handle better.
Dec 01, 2020, 06:41 PM
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richard hanson's Avatar
For those who are model flyers and like to try and duplicate full scale aerobatics,,the stick force becomes a reality
It is something the mind creates, using observed flight changes and finger positioning
Ithe muscle memory becomes developed over time and there are no text books or formulas to follow
But nevertheless, the finger stick force required, is developed ,tho it is all from the visual input
Learning to change from full throttle level flight to a stall, and spin entry with no change in altitude , demonstrates how developed the link can become as input varies throughout the approach even tho the altitude is constant tho the aoa is increasing
Fwiw"........
My next slide ,please
Last edited by richard hanson; Dec 01, 2020 at 06:48 PM.
Dec 01, 2020, 07:43 PM
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tspeer's Avatar
Both a change in speed and a change in normal acceleration result in a change in angle of attack. You can measure static stability in flight by trimming for level flight, and then seeing how much stick position or stick force is required to change speed by pitching over or pulling up slowly.

This is also the basis of the dive test for stability and c.g. location, in which the plane is initially trimmed for level flight, pushed over into a shallow dive, and then the stick released to see how rapidly it pitches back up. Or see if it will pitch back up by itself.

The c.g. location where the gradient of stick position vs speed change goes to zero is the neutral point.

Maneuvering stability is a bit different because in a turn the plane experiences a pitch rate. The pitch damping adds a stabilizing moment. This is what is measured with stick force per g. The same thing could be measured in a model by recording the stick position required to fly increasingly tight turns.

The c.g. location where the stick position (or force) gradient vs normal acceleration (g) goes to zero is the maneuver point. The maneuver point is normally a little behind the neutral point.

In our radios, stick force and stick position essentially mean the same thing, because the sticks are spring loaded. So what you feel is the spring force gradient. It is possible to decouple stick force from stick position, but that is a topic for another thread.
Dec 01, 2020, 09:55 PM
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Coupez's Avatar
Quote:
Originally Posted by tspeer
In our radios, stick force and stick position essentially mean the same thing, because the sticks are spring loaded.
This is a key difference between full scale and model flying. In a full scale plane (with unpowered controls) the surfaces feel loose and sloppy at low speed - and can be quite stiff at high speed. This is one indication of "airspeed feel" and cues the pilot about displacement required for a given response. With a model this is absent; the primary airspeed cue comes from the eyeball.
Dec 02, 2020, 12:31 AM
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richard hanson's Avatar
It is @mazing how the eye hand link can develop
It becomes extremely accurate with [email protected]
As a kid ,my controline stunters had eye and hand feedback
I learned full scale basics then and the full immersion of sensations made things easy
Fast forward to me trying to teach full scale lyers how to fly rc , even very basic trainers were too much for some because there were no inputs except visual and corrections wereout of sync
Some got tho and all was fine but they all agreed full scale was much easier as far as the flight control was concerned
Dec 02, 2020, 01:39 PM
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Thread OP
Thanks guys for the replies.
Dec 03, 2020, 08:23 AM
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exf3bguy's Avatar
Quote:
Originally Posted by Coupez
This is a key difference between full scale and model flying. In a full scale plane (with unpowered controls) the surfaces feel loose and sloppy at low speed - and can be quite stiff at high speed. This is one indication of "airspeed feel" and cues the pilot about displacement required for a given response. With a model this is absent; the primary airspeed cue comes from the eyeball.

Correct, however after a while you can develop a " feel " of airspeed not only by visual cue but by the crispness of control inputs.
Dec 04, 2020, 08:46 PM
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so 10degrees will change according to the CL chart. could you just suggest DeltaCL and then x V^2 ?

Honestly, this is something I would have asked you, and Am actually asking

Correlation of DeltaCl v deltaV
maybe a matrix for the data info would help?

One for DeltaV and one for Delta Cl.

Subed.

from Cl1 to gain 2 times the lift would suggest your cl went to 2
@Cl0.5 and 1g would suggest the speed is increased by about 1.41
but you say you increase the Cl from .5 to 1.5 with 10 Degrees pitch change, this is 3 times the lift and you got 3 times the g load I am guessing it's just an off-the-cuff data-set you threw out there.

Given an aircraft where the Values of Weight and surface area and altitude don't change.
If you flew at 1g @Cl1 in example 1 and 1g @Cl0.5 your Velocity must increase

My son (15) and I (50 yo Bus driver) came to this conclusion
Cla Va^2=Clb Vb^2
Vb = (Square root of (Cla/Clb)) x Va
velocity increase by a factor of 1.41
Sorry if this is Embarrassingly wrong .
Dec 04, 2020, 11:29 PM
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Coupez's Avatar
Quote:
Originally Posted by Bryce.R
from Cl1 to gain 2 times the lift would suggest your cl went to 2
You may have missed this part of the OP's original question:


Quote:
Originally Posted by Buwa
...(assuming that it doesn’t stall first!)...

CL = 1.0 is pretty close to CL_max for a lot of model airplanes. Elevator angle per G and stick force per G aren't very meaningful near the stall.


Quote:
Originally Posted by Bryce.R
Given an aircraft where the Values of Weight and surface area and altitude don't change.
If you flew at 1g @Cl1 in example 1 and 1g @Cl0.5 your Velocity must increase

That's true - but in maneuvering flight you don't stay at 1G. So (as an arbitrary example) you could be at 1G at CL = 0.3, and 3G at CL = 0.9 -- at the same velocity.
Last edited by Coupez; Dec 04, 2020 at 11:35 PM.
Dec 05, 2020, 12:27 AM
Registered User
Quote:
Originally Posted by Coupez
You may have missed this part of the OP's original question:





CL = 1.0 is pretty close to CL_max for a lot of model airplanes. Elevator angle per G and stick force per G aren't very meaningful near the stall.





That's true - but in maneuvering flight, you don't stay at 1G. So (as an arbitrary example) you could be at 1G at CL = 0.3, and 3G at CL = 0.9 -- at the same velocity.
That makes perfect sense, 0.3 is 1/3 of 0.9
Cl variations from .5 to 1.5 and Given the Example in the op ;
Quote:
If I set up an airplane to have 10 degrees of elevator deflection per unit CL and the airplane is flying at a CL of 1.0 and I pull up 10 degrees then I will get a change from 1 g (straight and level) to a 2g pull-up (assuming that it doesn’t stall first!
its fair to presume Cl of 2? (presumed he did not change any other variable like speed or wing loading etc)

I Get it that a lot of Airfoils have a ClMax near or below a value of 2 in low Renolds. but when he said "assuming that it doesn’t stall first"!, I assumed he was suggesting that we are working with an airfoil that would not stall (ClMax)at that angle and therefore would be able to achieve a Cl value of 2?

so the working out of 1.41 velocity increase was to clarify if he was talking about the same aircraft that is performing seemingly better with a higher velocity, and the Cl values for low and high speed flight were 0.5 and 1

the two examples of that are;
Quote:
But if I am flying at a CL of 0.5 and I pull up 10 degrees my lift coefficient increases to 1.5 and I am now pulling 3.0g
in that one he does not specifically say 0.5=1g, so I could be wrong there.
Quote:
If I set up an airplane to have 10 degrees of elevator deflection per unit CL and the airplane is flying at a CL of 1.0 and I pull up 10 degrees then I will get a change from 1 g (straight and level) to a 2g pull-up (assuming that it doesn’t stall first!).
Dec 05, 2020, 12:44 AM
Registered User
Quote:
Originally Posted by Coupez
Elevator angle per G and stick force per G
Sorry man, I think this is where I got it all wrong, I thought it was the main wing AoA change by 10 degrees and that the Cl values were for the wing.

I did see this originally (10 degrees of elevator deflection per unit CL) but then got stuck into the Cl values and the math part and forgot about the 10 degrees being the elevator.

So as the airspeed increases his elevator deflection needs to reduce by some factor to achieve the same g load



I am stepping back from the microphone now.
Dec 05, 2020, 08:51 AM
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Coupez's Avatar
Quote:
Originally Posted by Bryce.R
I thought it was the main wing AoA change by 10 degrees and that the Cl values were for the wing.
No, the 10 degrees is elevator angle change. The CL value should be for the airplane for this kind of analysis; you need to account for the contributions of both the wing and the horizontal tail.

Quote:
Originally Posted by Bryce.R
So as the airspeed increases his elevator deflection needs to reduce by some factor to achieve the same g load
No, this is maneuvering flight - we're looking at changes in G at a constant airspeed.
Dec 05, 2020, 10:25 AM
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richard hanson's Avatar
Quote:
Originally Posted by Coupez
This is a key difference between full scale and model flying. In a full scale plane (with unpowered controls) the surfaces feel loose and sloppy at low speed - and can be quite stiff at high speed. This is one indication of "airspeed feel" and cues the pilot about displacement required for a given response. With a model this is absent; the primary airspeed cue comes from the eyeball.
Not to stray from the calculated interpretation of control ....
The the eye is obviously providing the cues for correct input whe flying rc,
The intangible effect, with practice, is a feel of direct hand link to the model
It takes practice, doing same routines over and over, before the linked feel kicks in
But it does
Musicians and others who practice art , know this to be true
I remember one time at the field ,watching a flier doing practice on power maneuvers
I noted , that the engine setting as a bit lean
He said “ how can you tell that?”
Just by listening to the exhaust note during power portions of maneuvers
Same thing happens to the linked feeling in control sticks as apparant speed varies


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