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May 05, 2019, 08:45 PM
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Build Log

F-14 Tomcat - Freewing twin 80mm EDF No.2.


I commenced "building" of the new F-14 Tomcat last week. It will all be done very much the same as the prior one (destroyed one!), with slight variations.

Firstly, I painted the whole F-14 in WBPU as PARTS, not after assembling it. This was to prevent the WBPU becoming glue between removable parts!

I added some Carbon Fiber "rails" lengthwise down the Fuselage under the Wing mounting area. These are approx 170mm long and run under the stock Lateral Spar, that goes from each main gear retract point from left to right, so these go approx 30mm ahead of that spar, and then the majority heads rearwards. These rails are to strengthen the 'back' of the F-14 because is is very weak in that Fuselage-Wing region. It can fairly easily break its back, and then that is as good as fatal for it. Whether you did that with some very big 'whack' into the ground, hitting the Main Gear and thus leveraging off those - like my prior F-14 had happen on its maiden flight take off "issue" case - or just over the long term of adding up any harsh landings it has, this will keep it intact better.

I again used all the new Taileron alloy pivot rods I had made up for slop-free operation, and the Corona servos etc. So the Taileron system is rock solid as per the prior F1-4.

One issue is the Wing Sweep 'retract type' servos it has. On the prior one they had some slop in their output shafts, plus slop in the "Hex to Hex" fitting of the arm to shaft because the hex machining is pretty poorly done. Thus it is not a tight Hex fit, but a somewhat sloppy fit. By doing up the nut on the arm, it clamps the arm tighter and prevents that slop - but not 100% of it.
Add that slop to the output shaft slop and you get an amount of freeplay that matters - the Wings can move fore and aft a bit, and on this new F-14 that is much worse on one Wing than the prior one had had.
To reduce this issue you can adjust the pushrods to make the Wing hit the Wing Box alloy verticals for one Sweep position - full forwards, or full rearwards. Having them hit the vertical then means they are under some preload and thus can't move. But whichever end you set that for, the other end will be left in 'free space' and then the Wing can move still. eg It moves a degree or a few degrees, according to the slop in the drive system on that side.
The second fix to solve this is to add spacers to the Wing Root so that it then also hits a Wing Box vertical at the other end of Sweep also. On the prior F-14 I needed about 1mm added to each Wing Root to achieve that. On this new one one side is again 1mm, but the other side needs more than 2mm - and that extra amount means the Wing hits it sooner than the other side Wing. This means that one Wing Sweeps rearwards a little bit more tan the other. At the Wing tip it is 20mm more - which is a notable amount.
But there is no other way to fix the problem. You cannot fix the actual Wing Sweep servo slop problems.
I had decided that the Wings Fully Forwards were more important to be equal than at the Full Sweep end. Partly as you fly around mainly with Wings Extended, but also in the "Delta Wing" Full Sweep mode it won't be too big a problem to be inequal anyway.

I have again added the A3super3 Flight Controller, so that I can set up some 'Automated' processes to make the F-14 much easier and nicer to fly.

I am running two SBECS - one to power just the Landing Gear, Wing Drive Servos, and the Lights systems. So that the RC system is isolated from those.
Just a "5amp" (supposed 5Amps, but uses a 3A spec regulator chip really!) SBEC for the above stuff, and the stock Freewing 8Amp SBEC for the RC stuff.

Pretty much everything else is just exactly the same as the prior one - the battery tray/area mod etc.

And I will TRIPLE CHECK controls each flight (for all aircraft now!) too !!!
Last edited by PeterVRC; Jun 12, 2019 at 08:20 PM.
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May 20, 2019, 11:50 PM
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Maiden - it "flies"... somewhat


I think I can say "It flies exactly the same as the first one". Which is BADLY....
It is a pig of a plane. A lot of effort to fly it around 'nicely' and to scale.

All the setup of the prior F-14, in the OpenTX (Horus X10S), worked identically as for F-14#1. Except I had fixed up the reversed Tailerons of course!
But all trims etc also worked almost perfectly just as they had been. So this is so close to being the 'same' F-14 !
Unfortunately in how it flies too....

I had forgotten to cut the nose gear oleo spring shorter, so it was too strong and makes the nose notably bob and bounce away down the runway....
This is not good for AFTER landing/touchdown as it gives it more chance to pop the nose up and lift off again.

I tested and then used the High Angle Full Span Flaps to land and that all went perfectly fine - as you would hope they could!!
I had done a circuit with them deployed, to test its speed (and loss of speed) around turns and that was all easy and fine too.
But for sure if you did end up slowing down too much you could find out how quickly it would tip stall into a likely fatal path. Just keep the speed up for sure....

I had to use High Rates for Pitch - full throws - because in turns it falls into them badly, thus falling away in altitude. Then you need a lot of Pitch Up to offset that - though I expect I also used Reduced Roll at that same time. (some aircraft need negative Roll to offset how they would otherwise fall into the turn too much). These things you do intuitively, but the F-14 just does not respond to those.... you need 'extra' focus and inputs to make sure it is turning/banking fine.
Then with the High Rates it is very Pitch sensitive with Wings Level. Even with the 30% Expo I have set.
If you need High Rates then you are pretty well stuck with its over-sensitivite issues also.

I was too busy sorting general flying, to try to get it flying nicer, but I should have focused more on flying it with the A3super3 using the Angle Mode of Control via its "Trainer" mode. Thus let the A3super Flight Controller "work out" all the required control things to make it fly around PROPERLY!!
I am thinking that a pilot flying it around manually has little hope of getting it to fly nicely like any normal jet!
So I will do that next time out!!

I cut the nose oleo spring, and also changed the nose Retract pin to a softer metal one, with a 'Ring groove' put into it so that it is held in by a 4mm grub screw in the side of the Trunion. This is so that I can fit and remove it easily, without pulling the Retract unit apart to get to the stock flanged method Retract pin.
May 22, 2019, 08:04 AM
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Strakes - screw mounted


After slightly damaging the F-14 #1 strakes, which generally can happen on a model stand, or in transport with no landing gear down, I decided to make the F-14 #2 strakes a screw on system. So you just pout them on at the flying field.
I tossed up on using some magnet system idea, but that seemed a bit hard to form a good system you could trust in holding them 100% reliably. One good bump on a grass take-off.......


....
May 22, 2019, 08:16 AM
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2nd Day out


It was a perfect Sunny, calm Wind day today, so off I went for some F-14 flying/testing!!!
This time down to the parklands and grass 'runway'.
After the maiden flight day testing High Angle Full Span Flaps to fly around with, and then to land with, I felt reasonably sure that you could even use those for take-offs without any issue - and some other people do that (not many though!). But it seemed likely to be fine....

Using the Full flaps is awesome!!
The high drag grass, which retards the maximum ground speed, does not bother the lift level it attains with the High Angle Full Span Flaps anyway! The whole take-off process is just a more scale, easier, nicer looking result!! So this a a MUST-DO thing!!
Set them up, and USE them... for all Take-offs and Landings! The drag from them is a negligible factor versus the EDFs/Thrust it has anyway. Even on grass. The greater amount of Lift far outweighs that!

And there is no need at all to have any other lower Flap range really.

With some changes to the Pitch and Roll levels going into the A3super where they are Delta Wing Mixed (sloppily), and than also changing the Ailerons to be true Ailerons instead of Spoilerons - and also with a 50% Differential on them - the F-14 was a very different flier!! WAY better!
It is somewhat of a fluke as to how great all the 'totals' of the values and control interactions turned out!!
Maybe I could tweak them even better, but at this time it all seems really great!

Though I did find out an issue - a notable and dangerous issue(!!) - with the way the RolL Rates need to change for Fully Extended and Fully Swept Wings.... and what those Rates actually do DURING the Sweep transition!! (next thread post for that!)
This issue cam close to having the F-14 fly away into the distance! Because it unfolded at a very inopportune time... a rare situation, but it just should never even be possible!! So that was fixed up....

(videos on the way)
Last edited by PeterVRC; Jun 12, 2019 at 08:24 PM.
May 22, 2019, 08:19 AM
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Roll Rate versus Wing Sweep Angle (OpenTX model file also)


I did discover another quirk of the F-14.......

It can Sweep its wings between two fixed states (derr).
The Wingspan reduces dramatically when the wings are swept, so you really need two Rates of Roll at those two end states of Wing position.
BUT..... what are you going to do for the transition period? That is about 4 seconds long.

If you use a 'Gated Mix' that is triggered by the Wing Sweep Switch then you have a number of possible ways to alter the Rates:
1) Change to a Low Rate as soon as the Wing Sweep Switch is used.
2) or WAIT 4.7 seconds to change the Rate to Low.
3) On the Unsweep Switch selection, change to High Rates.
4) or WAIT 4.7 seconds before going back to High Rates.

Keeping a High Rate all that way to swept is going to be excessive Roll control as the wing sweeps more rearwards.
Changing to a Low Rate immediately means a severe lack of Roll ability during the first portion of Wing Sweep unfolding.
Changing to a High Rate 4.7seconds AFTER triggering the Unsweep means a lack of Authority as it unsweeps.
So no form of 'instant' Rate change can cover all situations that will be encountered during Wing Sweep changes.
And I found that out big time today!!

I had not thought about it and sort of left it that "Well, fly along straight and level, then sweep the wings......", then you won't need any Roll use, and thus don't need much "detail" in the Roll Rate as the Sweep angle alters.
But then I began a Wing Sweep while heading a certain direction - towards a treeline..... and then decided I needed to turn as it went over those, or I would lose sight of it soon.... and using Roll control achieved NOTHING. For one split second it was looking like it was going to be gone... loss of control. But luckily 1second or so later it did respond!

Post flight checking showed it is because of the LOW Roll Rate for Wing Sweep, and how it does not go back to High Rate until 4.7seconds after the Unsweep command is sent. And just "45%" Taileron alone (No Ailerons used for Swept) Roll Rate achieves almost nothing when the Wings are Fully, or close to, Fully Extended!
This was why I got no Roll response for approx 1 second or so. I had only tried to Roll a few seconds after initiating the Unsweep thus there was only 1 second left to go until the High Rate came online.

Doing this "Just change Rates in one hit" whether at the start of Wing Angle change, or later on, is just a slip-shod idea really.
The truly best solution is to have the Roll Rate TRACK the Wing Sweep Angle.
There is no way to measure/know the Sweep Angle directly, but using TIME is a close enough approximation anyway. Trigger a Sweep.. then use 4.0 seconds as the Sweep Time. Set the Roll Rate to alter its Min to Max value over a 4.0 second period also and that means you are effectively Tracking the Wing Angle.
Well, that is if you have a TX system that can even be programmed to do that number of factors required..... (OpenTX can)

So a bit of programming addition tonight to fix that issue...... presto, linear Roll Rate tracking of the Wing Sweep angle.

I had also re-added 'proper' Ailerons to the F-14... not just "Spoilerons".
When the Wings are extended it needs a lot more Roll Authority.
For any Taileron type aircraft it is not a good idea to ONLY use Tailerons for Roll. They are so close to the centerline that you need a LOT of authority out of them to Roll the aircraft - versus Ailerons do that task 'easily'.
Also, if you use only Tailerons, then you are trying to make a Wing 'rotate' against its aerodynamic will. It has no intention to do anything but track dead straight. Only Ailerons will alter the Wing profile to make it 'want' to Roll. So why would you try to use Tailerons only to 'force' a Wing to roll? There is just no point. You want the Wings to ASSIST the Roll, not OPPOSE it!
Note that the full scale F-14 uses true SPOILERS to create the required 'Wings truly want to Roll also' resultant.

It is OK to do Tailerons only for when the Wings are Swept because the Wingspan is then virtually no wider than the Tailerons span anyway, so that is a very different case. The Tailerons ARE "Ailerons" then!

As for setting up only Spoilerons (not Ailerons)...... well, I thought "The Wings are high on the Fuselage, so I can add only a downwards force on one wing to form an axial Roll - in combination with what the Tailerons are creating also." This is sort of right and OK.... but strictly it needs a DIFFERENTIAL Aileron setup, if you want to have enough Aileron Roll force (authority) and remain axial also.
Spoilerons only are either too weak in authority, OR you need a LOT of Aileron 'up' Angle and then the Roll is not axial anyway.
So I changed it to use a 50% Differential full Aileron mode. (I had done this Differential Aileron change before today's flights)
This Differential Aileron addition really cleaned up the linearity and flight result.
I think this change (Ailerons not Spoilerons), combined with the Delta Wing Mix changes, really cleaned up the linear flight manner, and removed the 'dive into turns' issue.

The Sweep Angle Roll Rate Tracking will tidy up - complete - the control system to be very 'clean' and aerodynamically valid at all times too. No need to only Sweep whilst flying in a long straight line.... you can do whatever you want, any time, then, and it will operate/respond with full appropriate Control Surface Authority at all times.

----
The current OpenTX Model file for the F-14 is attached:
Last edited by PeterVRC; Jun 12, 2019 at 09:06 PM.


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