olnico

Hello guys,

I have a problem here. I am doing the maths on my large F-16 to check the dynamic balance requirements.
My stabilator has the following dimensions:

Root cord=46cm
half span=33,5 cm
tip cord=17,5 cm
Leading edge swept by 28,5 cm

Position of the pivot point= 21 cm

Could you guys verify if you have the same dimensions on your plane and particularly the position of the pivot point ?

Here is the 25% MAC simulation I get for this plane:



I get the 25% MAC at 20,5 cm from the root tip. This means that the stabilator would be unstable.

Could you guys also verify my computations please ?

The file is here:
http://www.geohei.lu/olin/data/model...stabilator.xls

olnico

The initial link to the spreadsheet was wrong.

I have a pre launch kit and I need to know if the regular customers have a different setup.

The pivot point placed at 21 cm is at 26,2% MAC according to the spreadsheet. Could you guys also verify this?

Thanks

Eddie Lozano

I THOUGHT YOU ARE THE EXPERT ON THIS KIND OF STUFF !!!!!

olnico

The question is not who is an expert or not.

I need two types of information here:
Does everybody else have the same stabilator setup as mine with this plane ?
Could other people compute the pivot position using a different tool ?

What I found is pretty concerning and I need a computation from a different source before making any conclusion.

Kmarks

My Feedback: (36)

Olivier,

Here is a photo with measurements of a spare stab I have. I hope you can see the measurements. They are measured top the center of the steel shaft and edges of stab. You are the man on calculations, I have never used that program.

Just a few Key Points.

1. The photos that showed a lot of missing support on the FEJ F16 stab holders, well it is this bird and I now have 22 flights on it.
2. I did indeed remove the airbrakes and apply lots of epoxy and carbon fiber to stregthen that area.
3. Both sets of stabs are built with the shaft in the same place.
4. I am not sure of the speed this jet flys at and I know a larger jet "appears" to go slow, but it is no where close to the speed of a Sky Master F16 with P200.

Please inform us of your findings and any corrections that may need to be done.

Thanks

Kevin

olnico

OK same measurements as mine within a few mm.
Kevin, is your plane a pre-launch one or a regular production one ?

rbxbear44

My Feedback: (86)

Oli ma man!!!

Must be snowing over there again and you are cooped up

I checked your R,T and S measurements and confirm them as what I have also.

Now, the pivot point from Chord tip is 21.3cm on my bird.

I did "balance" my Elevs

Rex

Kmarks

My Feedback: (36)

Olivier,

I believe both sets I have are regular production stabs.

Please explain what is the problem or concern here, and the cause and effect of such positioning. I want to fully understand what occures when the pivit shaft is placed in different locations.

Thanks

Kevin

Bob.R

My Feedback: (2)

I obtain the same numbers with graphic method.

olnico

Kevin,
If the pivot point is placed aft of the Center of Lift ( CL ) then the stabilator is in a dynamic divergent balance state. Ie in flight it will try all the time to do a 180° turn on its pivot shaft.
Let me explain that.

Imagine that you increase the AOA of the stabilator and leave it alone. The lift will increase accordingly. As the CL is forward of the pivot point, the lift force will tend to move the stabilator to higher AOAs, then this will increase the lift again, etc...

If you have a very unstable condition, the stabilator could blow out in flight manoeuvres: the torsion force exerted by the aerodynamic loads against the servo could overcome the structural resistance of the control when deflected at significantly high AOA.
Another threat is the flutter. It is a combination of this unstable condition and of the slop that makes the control bounce against its slop limits around neutral. If this resonates with the aerodynamics, then your stabilator will almost certainly blow away as well

I will publish a complete technical article about this in the June/July RCJI publication...

The F-16 has a specific geometry with the outer rear of the stabilator cut and the trailing edge perpendicular to the flight line.
I would be very interested to get someone to compute the 25% MAC with a different method.

ianober

My Feedback: (12)

Oli, you can use this online calculator, theirs should be pretty trustworthy I would hope:

[link]http://www.nasascale.org/howtos/mac-calculator.htm[/link]

rbxbear44

My Feedback: (86)

Oli,
With me balancing the Elevs...will that have any negative effect on the dynamics of this whole issue???

Rex

Jaws3D

My Feedback: (5)

Here are other formula's I obtained from Aircraft Design: A Conceptual Approach by Daniel P. Raymer

C = (2 / 3) * R * (1 + t + t^2) / (1 + t)
Y = (b / 6) * [(1 + (2 * t)) / (1 + t)]

Where:

C = Mean Aerodynamic Chord (MAC)
R = Root Chord
T = Tip Chord
t = Taper Ratio = T / R and t^2 = t squared (t * t)
b = Span = 2 * Half Span
Y = Distance from root chord to C
S = Sweep Distance

You will then need to use a little trig based on your leading edge sweep angle to determine the 25% MAC from the root leading edge at the root - Pivot Point distance @ Root Chord.
Using these formula's I obtained the same numbers as you did.

t = 17.5 / 46 = 0.38
C = (2 / 3) * 46 * (1 + 0.38 + 0.144) / (1 + 0.38) = 33.88
Y = (67 / 6) * [(1 + 0.76) / (1 + 0.38)] = 14.24

Sweep Distance @ MAC = (S / b / 2) * Y = 28.5 / 33.5 * 14.24 = 12.12
Pivot Point distance @ Root Chord = 12.12 + .25C = 12.12 + .25 * 33.88 = 20.59

Kmarks

My Feedback: (36)

Ok Jaws3D,

So are you saying the pivit rod based on your calculations should be at 206mm from the leading edge of this elevator. The placement currently is 213mm which is really close. If this is accurate I think we are OK.

I now have 22 flights on my FEJ F16, anyone else have as many or more. I have not experianced any problems with the stabs. I preflight each moving surface before each flight and all has worked perfect so far. (except the maiden that ended in a flame out)

Also, there was a thread a while back comparing the Skymaster F18F to a Tams F18F. Both kits were being built side by side. The stabs were compared and the pivit rods were in completly different area's. From this thread is seems that if the pivit rod is not dead nuts on the jet will end up malfunctioning and crashing. Well I have not seen anyone in years complain about there Tams or Skymaster F18F crashing due to this concern.

So with that example, there must be allowable fluctuations to exist that are still within working standards.

I'm just going to fly my jet and not worry about this for right now.

Best Regards

Kevin

Jaws3D

My Feedback: (5)

Hello Kevin,

No, my intent is to not make a statement as to where the pivot point should go and apologize if it reads that way. I was just replying to Olivier's request for someone to calculate the 25% MAC via a different approach for the dimensions he supplied for his stab.

I am very interested in this because I am an owner of a Jet Legend F-20 that I am currently assembling. As you may know, there is an RCU thread that has been discussing the proper position of the pivot for the JL F-20 stab. Since that post, I began researching liturature about stabilators. As a side note, our club Palos R/C, has MAC and C.G. calculators on our web site that are very similar to Olivier's spreadsheet. Although I did not write these calculators, I do want to help out here where I can so that I may learn something about stabilators for R/C turbine aircraft and to help me determine the airworthiness of my JL F-20.

I did post the dimensions of my JL F-20 stab in that RCU thread so that folks could determine the MAC and then determine where the proper location for the pivot point should be. Just as you have had successful flights with your FEJ F16, I have been in contact with JL F-20 owners who have successfully flown their jet. That is what makes these RCU posts so important to me.


Best regards,

olnico

I have just cross checked the computations with the graphic method as well and it is right. So my spreadsheets works in all the cases.

What we have here is a quite different case from the JL one.

The pivot of the F-20 locates at 30% of MAC ( apparently, I have not checked it but other have). This is way too far from the 25% neutral limit.

Here we have a pivot point that is slightly on the unstable side. I believe that it will be fine provided that you guys make sure that THERE IS NO SLOP IN THE CONTROL.
The best thing here I believe to get rid of the brass tube slop is to insert as much thick grease as possible in there and to control the slop after each flight. Also liaise with Kevin to make sure that your setup is the same as his since he has proven 20 subsequent event free flights. Others like Brosi in Germany have done it as well.

I think as a general comment that it is really time for the manufacturer to think about what they are doing in terms of aerodynamics and aerostructures. With the way the Civil Aviation Authorities are looking at us in Europe and USA it is really time for the manufacturers to step up a little bit.

Thank you everyone for your help on that matter. Thank you Kevin for your feedback as well.

olnico

For information, the pivot point of the 1/7th scale F-18F locates at 17% MAC like it should do.The pivot system is a full aluminium structure with dual bearing and completely slop free operation if the bearings are sealed with the proper Loctite product.
This plane has proven many successful hours of operation and was designed one good year before the big F-16...

invertmast

My Feedback: (23)

Oli,
what if the pivot was located at roughly 22.5% MAC? From my research it seems that this is still within the acceptable range of 17-25% MAC for the pivot location, so i'm assuming that the stab will be slightly more unstable at this position than if it was positioned at 17 or 20% Mac... is my understanding correct on this?

rbxbear44

My Feedback: (86)

LET's FLY![>:]

mr_matt

My Feedback: (10)

But Oli with all respect, in this day in age of 400 ounce servos, why cut it so close? Move to 18% MAC and never have a worry and only increase the servo load maybe 15% (a guess)

What do you think?

olnico

Of course Matt,

17% is a good value but there is no real limit apart from the aft one. If you compute that the power requirements can be met at 15% or even 10% why not ?

The servo load will be increased by an amount depending of the control half span. In fact the servo load is roughly equivalent to drive a control surface that has the control half span along the swept line and which cord is the distance { 25% MAC; pivot point }

olnico

Thomas, in theory the CL does not move when changing the AOA. That is for NACA 00x series profiles. If you make a very accurate NACA 00x stabilator profile with a very good structural rigidity, you'll be fine.
If the CL of you surface stays at 25% MAC at all times, your stabilator will be stable at all times.
You can always "wind tunnel" a half stabilator from your car. But as a matter of precaution I would strongly recommend you to go with the 17% figure.

olnico

Good luck then.

olnico

Matt also keep in mind the AMA requirements:
http://www.modelaircraft.org/files/520-a.pdf page 3

Min torque = 6 x surface x servo arm/control arm

On a setup like the one recommended by FEJ ( which is wrong once again ) servo arm = control arm. so the formula becomes Min torque = 6 x surface ( in sq in )
You get over 1000 oz-in with a 1/7th scale F-18F and close to 2000 oz-in with the 1/6th scale F-18F...

Using a program to demonstrate a finer simulation could save people from an impossible requirement and impossibility to get the plane signed off.

mhm21

Well, to chime in here, I ran your stabilator in XFLR5 to see what it had to say about the situation. It looks as though 20.5cm is just slightly on the unstable side of things. The calculated center of pressure for this particular geometry is 20.2 cm aft of the root leading edge.

The simulation parameters are as follows:

Geometry is consistent with the above dimensions
Fully viscous solution
NACA 0010 foil
Vinf = 100 mph (rough guess, faster is better this corresponds to a root RE of 1,370,923 and a tip RE of 521,547

Anyways, the attached graphs show a negative pitching moment coefficient (stable) for pivot positions below 20.2cm. Any further aft and you are behind the CP.