Author Topic: Up line stability  (Read 1337 times)

Offline Mike Wood

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Re: Up line stability
« Reply #45 on: 31, July 2020, 11:14:21 pm »
Just to add something else into the mix:

In a video by Andrew Jeskey he shows how he has the model nicely trimmed, straight down, straight up,
knife edge all set up.

He then moves the battery back about two inches, c of g change will be less than that, he thinks around half an inch ish.

He flies again and now in knife edge the model pitches severely to the belly and rolls as well.

He now needs to only breath on the stick to hold inverted, the model no longer locks onto the horizontal
and is hunting and on landing it wants to balloon.

Same model just a c of g change transformed it from darn good to pretty awful.

I think the moral of the story is trying all the other things with a tail heavy model is
a loosing battle.

Mike
« Last Edit: 31, July 2020, 11:26:03 pm by Mike Wood »

Offline Peter Jenkins

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Re: Up line stability
« Reply #46 on: 31, July 2020, 11:39:03 pm »
Peter


Still don’t agree that as the speed increases the model will climb with properly adjusted down thrust, at least not within normal flight envelope we use.  I often see models fly in the way I am suggesting, but if it happens when it is out of my sight then I can’t comment but at this stage it would not affect the manoeuvre or cause extra work on the sticks.

My problem is I can’t fly accurately enough to establish if it is me or the model so going beyond the basic trimming guide is pointless.

Ashley

Ashley

If we assume a down thrust angle of 1.5 deg, and a thrust at normal cruise speed of say 1.5 Kg with full throttle giving say 10 Kg.  The aircraft weight at 5 Kg and therefore lift = 5 Kg as well.

The down force from the motor at cruise will be 1.5 x Tan 1.5 deg = 0.039 Kg
The down force from the motor at WOT will be  10   x Tan 1.5 deg = 0.26 Kg
So, downforce will increase by 0.22 Kg

Lift is given by the equation L = CL x 1/2 ρ v2 S  and will equal 5 Kg


If we now assume the aircraft speed increases from 50 mph to 60 mph as a result of the application of full power, we can calculate the lift will increase by just considering the change in the value of v2 since all the other elements in the equation remain constant.  So this gives us  the factor of 602 / 50 = 1.4

So lift will increase by (5 Kg x 1.4) - 5 Kg = 2 Kg

As you can see, with a lift increase of 2 Kg versus a downforce increase of 0.22 Kg, the net increase in lift will be 1.78 Kg.   Ergo the aircraft will climb as the speed increases if you leave the elevator control alone.  However, the aircraft will take a finite time to accelerate and hence for the increase in lift to be generated.  During that time, we want the aircraft to remain flying S&L and this is what we see - until the speed factor kicks in.

You would need a down thrust angle of just over 11 deg for a 10 Kg thrust to generate almost 2 Kg and thus balance the increase in lift. 

This assumes that all forces are acting through the aerodynamic centre.

Peter
« Last Edit: 01, August 2020, 08:09:47 am by Ashley Hoyland »

Offline Thomas David

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Re: Up line stability
« Reply #47 on: 01, August 2020, 11:10:50 am »
Hi Peter,


I agree with your point that you cannot trim a plane to be hands off in vertical up and horizontal lines at all airspeeds. However, it probably is possible (or close enough to possible to make no difference) over most of what we need for a sequence.


Your assessment is not correct though, as it does not account for pitching moments and changes in angle of attack.

Cheers,

Tom
« Last Edit: 01, August 2020, 11:14:01 am by Thomas David »

Offline brandon ransley

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Re: Up line stability
« Reply #48 on: 01, August 2020, 12:08:34 pm »
Mike. 


Your post is interesting because according to Peter Goldsmith‘s trimming notes) and I’m not saying he’s right or wrong, you need to sequence your trimming and his view is that the first thing you need to do is decide on your C of G as moving this impacts on everything else. 


So possibly a conclusion to your post is that if once you’ve trimmed your model you move the C of G (back or forward) then you will need to revisit other elements - incidences, control throws, mixing etc - to get it to fly well.


He also says changing prop makes a difference and particularly notes changing from 2 blade to 3 blade. 

Offline Mike Wood

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Re: Up line stability
« Reply #49 on: 01, August 2020, 12:39:27 pm »
As you conclude, sequencing your changes is to be thought about.

From the video I was shocked just how much the model pitches down in knife edge with pushing the c of g back,
when previously it was very good.

My current thought is that if one trims for "gravity neutral" that changing c of g is a little different to "trim for level flight"
as I think the pilot now auto adjusts the amount of elevator that is needed to hold straight and level.

I'm thinking (when I start again) to do some flights with the battery in different positions and see how it changes
inverted and knife edge and go from there. I'm pretty happy with the model at this point but could it be better?

Also interesting about changing prop, I'm wondering why going from a 2 to 3 blade prop should change things that much and
also what does that mean for contras?

Mike
« Last Edit: 01, August 2020, 12:43:43 pm by Mike Wood »

Offline Mike Wood

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Re: Up line stability
« Reply #50 on: 01, August 2020, 12:46:41 pm »
Hmmm ... so answering my own question, perhaps it is mainly the reduction of the diameter
of the prop wash with a 3 blade being smaller in diameter?

m

Offline Mike Wood

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Re: Up line stability
« Reply #51 on: 01, August 2020, 01:10:57 pm »
I should probably also add that with that c of g change the model now tucked under in a vertical down line
and also went to the belly slightly in a vertical up line.

Not sure I've ever had a model that tucks under in a down line!

Mike

Offline Peter Jenkins

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Re: Up line stability
« Reply #52 on: 01, August 2020, 01:17:54 pm »
Hi Tom


I think (?) we might all be agreeing but the way we've said things might be hiding the fact - but I might be wrong.


The point I was making was that for S&L flight, even if you adjust up/down thrust to give substantially no trim change as the power is applied, the aircraft will climb as its speed increases - maintaining the same attitude as power is added means the wing incidence is not changed so the only change in the lift equation becomes the speed squared term.  So, the point I was making was that adjusting the engine thrust line to ensure no attitude change resulting from the power change will never give you a situation where the aircraft will continue in S&L flight once air speed has risen, or fallen.   In all the trimming I have done, the aircraft has always climbed after a few seconds even after I have got the increase and decrease of power not to affect the aircraft's attitude at the time of power change.  Obviously, with decreasing power, the aircraft descends after a short period of S&L flying.


Having re-read my original post, I choose my words loosely by saying that getting a repeatable power setting allowed me to trim the aircraft to allow horizontal and vertical flight hands off.  I have always used a downline mix to get to a still wind vertical downline but as we all know you always need to correct this for downlines when there is a wind blowing and for cross winds.  So, it was lazy of me to say that level and vertical flight could be trimmed out when one is trimming and the other was arrived at by mixing.


I agree that I omitted pitching issues but covered myself by saying that I was assuming that all forces acted through the aerodynamic centre.  In reality they probably don't and one would also need to consider the effect of non-linear impact of drag increases from the wing and tailplane as speed increases/decreases as we hope that the pitching moments created by applying and reducing power have been reduced to zero, or very close to it, by adjusting the up/down and side thrust. 


In that respect, I agree with Ashley that if we aim to fly at a constant speed then adjusting the thrust line so that we don't have a significant change in pitching moment as power is applied or reduced is what we are aiming for.  That is predicated on the ability to achieve the constant airspeed which is a function of flying level and having the correct power setting at which point you arrive at the single point you have trimmed the aircraft to fly S&L.  If you cannot accurately fly level or set virtually the same power setting then the trimmed setting will not give the S&L flight you are trying to achieve without resorting to touches of elevator.  It may not be far out but it won't be spot on.


Clearly, the more symmetric your airframe design, then the easier it becomes to set up versus a design that isn't symmetric.  Looking at where current designs are going, it is clear that is the route being taken with some success. 


On CG, I agree with the comment from Peter Goldsmith's trimming notes.  CG position is the single most powerful trimming tool we have at our disposal since it alters the effect of all the forces acting on the aircraft in flight.  It follows that if the CG changes then we have to revisit all the adjustments we have made to correct them for the new conditions.  I think that is what Bryan Hebert also says but I cannot lay my hands on his tome just at the moment.


At the end of the day, what works for the individual pilot is what counts.  When starting out though then unambiguous advice needs to be there to provide the beginner to F3A with sufficient information for them to achieve their goals.  I agree with Mike Rieder, that starting with a hands off S&L will be the easiest way to help beginners to trim their aircraft - this at least keeps workload low during the S&L part of the schedule.  Once they have some success, then, as he says, you can experiment with other types of trimming.

Offline Mike Wood

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Re: Up line stability
« Reply #53 on: 01, August 2020, 01:34:31 pm »
Blimey Richard, I bet you didn't think it would lead to such an interesting discussion when you posted your original question  :)

Good question!

m

Offline Mike Wood

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Re: Up line stability
« Reply #54 on: 01, August 2020, 01:37:45 pm »
So if we can keep the aircraft flying at constant speed things will be easier.

Just wondering; if constant speed means the speed of the aircraft when viewed by the pilot/judges
then it's actual airspeed will be greater into wind than downwind? ... hmmmm?

Mike
« Last Edit: 01, August 2020, 01:39:47 pm by Mike Wood »

Offline Richard Clouting

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Re: Up line stability
« Reply #55 on: 01, August 2020, 02:32:49 pm »
Yes Mike, I’ve decided to speak to Thomas tomorrow in person!
Should narrow down the alternatives a bit.


I managed to squeeze in a training session Friday. Still haven’t managed to get to the top of a vertical up line. The airplane becomes so unstable that to complete any manoeuvre is quite a challenge.


I think cog could be influencing things.


Richard.

Offline Peter Jenkins

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Re: Up line stability
« Reply #56 on: 01, August 2020, 02:59:39 pm »
Mike


Constant speed is always constant airspeed.  To an aircraft, ground speed is irrelevant.  It is up to the pilot to adjust the flight profile so that a loop appears round and centred despite the strength of the wind.  It is airspeed that determines the aerodynamic forces on an aircraft but ground track that determines the accuracy of the manoeuvre.  There's not a lot you can do about a high ground speed going downwind but you can always cheat on the upwind leg by applying more power and adjusting the flight path to suit.  At least, I think that may have worked for me - sort of.  But there are more experienced practitioners on this forum who might have a different view and that might influence judges to award higher points than the pure physics of the situation.


Peter


Offline Mike Wood

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Re: Up line stability
« Reply #57 on: 01, August 2020, 03:12:19 pm »
Hi Peter

> "Constant speed is always constant airspeed"

Indeed that is what aerodynamic theory tells us.

Can't help thinking in judging terms it means the same controlled "all the time in the world" pace
both into and down wind. So in relation to trimming we want our aircraft to have as wide
as possible speed envelope for level flight as possible.

Mike

Offline Mike Wood

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Re: Up line stability
« Reply #58 on: 01, August 2020, 03:15:34 pm »
Hi Richard

> ... Still haven't managed to get to the top of a vertical line ...

Is it a power issue and the aircraft is just slowing down leaving you with less control?

Unlimited vertical would be being able to continue up at constant speed way past the point you need.

Another definition of unlimited vertical is to pull vertical, start rolling and have the aircraft continue
up indefinately with no slow down in roll rate or vertical ascent.

Mike
« Last Edit: 01, August 2020, 03:17:27 pm by Mike Wood »

Offline Kevin Caton

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Re: Up line stability
« Reply #59 on: 01, August 2020, 06:16:57 pm »
I’ve enjoyed following this without commenting so far, but just to add a little to the discussion - it’s normal to offset the effect of headwind and tailwind a little by applying more power than normal upwind and a little less downwind. On a very windy day though if you overdo it your plane will fall out of the sky on downwind rolls!


One area where it is necessary to fly constant groundspeed is any manoeuvre with integrated rolling and looping elements eg the P-21 golf ball and the F-21 rolling circle. Unless the plane is flown with constant groundspeed you can’t perform a constant roll rate around the integrated circular portions where these move between upwind and downwind conditions.