Official Freewing MiG-29 Fulcrum Twin 80mm Thread

[Jdcrow]

Let me be a bit nitpicky here. I see the word 'tail heavy' being used way too often and too lightly and it can misdirect people.
If the plane CGs behind factory markings, that DOES NOT make the plane 'tail heavy' (unstable). You can say 'tail heavier' which is correct.
Factory CG is mostly always a notably 'nose heavy' (stable) starting position. Neutral CG is often several centimetres behind that mark.
A plane only becomes tail heavy when your CG exceeds that neutral point, not when the CG falls slightly behind factory reccommendations. ;)
Again, you can say I am nitpicky but it's an important remark in my mind.

I am NOT trying to be confrontational here (I know the internet can make some comments sound the bad way), I'm just trying to help with 'best practices' and rigorous wording, so that the community eventually gravitates towards better manners and less ambiguity.

just call me simpleton, but i conceed, there are degrees of heaviness. until it reaches SaturnV launcher status.

[Airguardian]

Well, there's the problem...
Things like this graphic poster are really confusing / ambiguous (short of saying outright false) and don't really help in making people do proper well-informed decisions when setting up their aircraft.

Not to mention that the concept of a CG being 'good or bad' is highly subjective

Here tail heavy is referring to a balancing act of the jet on your fingers and whether the CG ended up ahead or behind of them.

This has nothing to do with the actual stability of the aircraft (Nose heavy and tail heavy are actual technical terms).

When people use the same term to describe different things, confusion ensues. And we really do not want to be confused when setting up the CG of our model! :)

It is a personal battle of mine, hope you don't mind!

[Evan D]

Most people understand what that picture is saying though. Even though you are technically correct your explanations are confusing those people...

[leithalweapon]

Got mine in the air today with the new servos. Barely got off our grass field. Had it in the air less than 1 min at about 3/4 throttle. What happened next looked exactly like the crashes on video. The nose pitched down and I had no elevator control. At first i chopped the throttle thinking it was toast, but at the absolute last second I got the elevator contol back and was able to pitch up enough to avoid the gound. Slammed the throttle wide open and no response from the motors for a few seconds. Didn't seem to lose aileron control. So I have no idea what happened, but I am using an Admiral 10ch receiver which was probably a bad idea. Originally I had ordered 2 Admiral sattelite rxs, but they were both bad.
One other thing, I will never fly with 30% expo again.

[Airguardian]

Most people understand what that picture is saying though. Even though you are technically correct your explanations are confusing those people...

Sadly, I've run across many that indeed do not.

Here's a short attempt at making a not-confusing explanation:

Once your jet is trimmed for level flight at half throttle, gain altitude, cut throttle, and when the plane slows down enter a 45º deadstick dive.

If the plane pulls out on its own back to level flight, it is nose heavy.
If the plane keeps track at 45º, it is neutrally balanced.
If the plane pitches progressively down as it gains speed, it is tail heavy.

Yes, that may sound opposite to what one would expect but it is not.
People will often assume that a plane is tail heavy because it raised the nose during such a flight condition and it is the exact other way round.

The key thing to understand here is that CG affects how much trim you end up using on the elevator/stabs.
The more nose-heavy, the more pitch up required, and pitch trim shows up with increasing speed (hence the dive).
You cut throttle so that thrustline does not affect this test.

Can't really make it any more compact or straightforward. I can explain it further, but then people may indeed get confused if not patient enough, lol.

What happened next looked exactly like the crashes on video. The nose pitched down and I had no elevator control.

Well, can you tell us more about your setup, throws?
Was it by the book, were there any noticeable changes that you had made?
Were you on low rates and nose heavy at that point?
(You can consider factory reccommended CG as nose heavy)

At first i chopped the throttle thinking it was toast, but at the absolute last second I got the elevator contol back and was able to pitch up enough to avoid the gound. Slammed the throttle wide open and no response from the motors for a few seconds. Didn't seem to lose aileron control.

That's weird. I'd consider the brownout but for your comment about the ailerons. Are you SURE that you still had ailerons?
If so, when you say the EDFs took seconds to power back on... was it really seconds or more like 'fractions of a second'?
When we are under stress in 'danger' conditions, time perception can warp a bit, you know... :p
Sometimes when suddenly slamming thrust wide open, ESCs can take a moment to react.

One other thing, I will never fly with 30% expo again.

And, why would that be?

[Jdcrow]

Well, there's the problem...
Things like this graphic poster are really confusing / ambiguous (short of saying outright false) and don't really help in making people do proper well-informed decisions when setting up their aircraft.

Not to mention that the concept of a CG being 'good or bad' is highly subjective

Here tail heavy is referring to a balancing act of the jet on your fingers and whether the CG ended up ahead or behind of them.

This has nothing to do with the actual stability of the aircraft (Nose heavy and tail heavy are actual technical terms).

When people use the same term to describe different things, confusion ensues. And we really do not want to be confused when setting up the CG of our model! :)

It is a personal battle of mine, hope you don't mind!

i think you are more focused on Dynamic Center of Gravity (affect in motion) Vs Static Center of Gravity (used for initial setup). im just referring to Static balance for setting up the aircraft.

[GliderGuy]

Got mine in the air today with the new servos. Barely got off our grass field. Had it in the air less than 1 min at about 3/4 throttle. What happened next looked exactly like the crashes on video. The nose pitched down and I had no elevator control. At first i chopped the throttle thinking it was toast, but at the absolute last second I got the elevator contol back and was able to pitch up enough to avoid the gound. Slammed the throttle wide open and no response from the motors for a few seconds. Didn't seem to lose aileron control. So I have no idea what happened, but I am using an Admiral 10ch receiver which was probably a bad idea. Originally I had ordered 2 Admiral sattelite rxs, but they were both bad.
One other thing, I will never fly with 30% expo again.

Hi LW...I'm gong to get on my soapbox again about common mode radio interference produced by HIGH CURRENT ESCs. What you experienced sounds so much like what happened to me when i lost an A-10 and an F-4...both HIGH CURRENT ESC birds. When I mentioned this to a HAM radio guy, he asked me if I'd ever had an issue with the P-38. I answered "NEVER"...the P-38 has never had any sort of control glitch. He said, "High current ESCs have the characteristic of producing common mode radio interference. This interference is enough to upset the pointer in the look-up table being used by the receiver and transmitter to implement the frequency hopping, spread spectrum operation. Hence, causing the loss of control you have experienced with the F-4 and A-10."

His suggestion was to add ferrite RF chokes to all the wires coming toward the receiver side of things from the ESC. Keep these RF chokes as close to the ESC as possible because any length of wire acts like an antenna and transmits the RF noise.

The proof is in the results. Since I added RF chokes to my HIGH CURRENT (only) birds well over a year ago, I have not seen any form of glitch or loss of control. I AM A BELIEVER!

Enter the word "ferrite" in the search window of Hobby Squawk, and you can find more information. Here is where you can get them.

• From Amazon.... Cedmon 20 Pieces Clip-on Ferrite Ring Core RFI EMI Noise Suppressor Cable Clip for 3mm/ 5mm/ 7mm/ 9mm/ 13mm Diameter Cable, Black

A lot of folks attribute your experience to a brown out, but these high current UBECs really should be able to handle the loads we place on them w/o any problem.

Think about it....try adding the RF chokes to the ESC wires coming forward from the MiG's ESCs. There's plenty of room on either side back by the rear end of the rear battery. And...for good measure, I added another RF choke to the throttle wire which already has a green ferrite ring RF choke. Can't hurt.

Offered for your consideration and thought, as always.

-GG

[xviper]

Got mine in the air today with the new servos. Barely got off our grass field. Had it in the air less than 1 min at about 3/4 throttle. What happened next looked exactly like the crashes on video. The nose pitched down and I had no elevator control. At first i chopped the throttle thinking it was toast, but at the absolute last second I got the elevator contol back and was able to pitch up enough to avoid the gound. Slammed the throttle wide open and no response from the motors for a few seconds. Didn't seem to lose aileron control. So I have no idea what happened, but I am using an Admiral 10ch receiver which was probably a bad idea. Originally I had ordered 2 Admiral sattelite rxs, but they were both bad.
One other thing, I will never fly with 30% expo again.

I fear that the well posted control box ("blue box") situation may be coming back and rearing its ugly head again. I'll be by-passing the BB for the main control surfaces just to be on the safe side.
Having expo is not a bad thing. You still have the same throw at max stick.

[Skosh25]

Got mine in the air today with the new servos. Barely got off our grass field. Had it in the air less than 1 min at about 3/4 throttle. What happened next looked exactly like the crashes on video. The nose pitched down and I had no elevator control. At first i chopped the throttle thinking it was toast, but at the absolute last second I got the elevator contol back and was able to pitch up enough to avoid the gound. Slammed the throttle wide open and no response from the motors for a few seconds. Didn't seem to lose aileron control. So I have no idea what happened, but I am using an Admiral 10ch receiver which was probably a bad idea. Originally I had ordered 2 Admiral sattelite rxs, but they were both bad.
One other thing, I will never fly with 30% expo again.

Did you use the Y-harness that came with the upgrade servos to bypass the MCB-E and go direct to the receiver?

[GliderGuy]

I'm going to jump in to the CG discussion, but from the standpoint of an ASEL and sailplane pilot's point of view. Hopefully, providing some addition insight to the topic at hand.

Aircraft mfg's include a CG range in the pilot operating handbook (POH). This range is determined through an FAA required test program with highly experienced pilots, and the limits are set at the points (forward and aft limits) when a reasonably/average Joe type pilot can handle the flight characteristics. In all the FAA training material and flight training course material, the texts state the following (note...definition of balance point is where the aircraft sits level when your fingers are placed on this point):

• FORWARD CG / As the CG (balance point) of the aircraft moves toward the forward limit of the published CG range, the pilot will experience an aircraft that becomes MORE stable. It will be less responsive to pilot inputs to the elevator, and the pilot may find that he cannot raise the nose for a flare/landing at normal approach speeds when the time comes. Flight with the CG forward of the forward limit may make raising the nose enough to execute a landing safely impossible. I might add, however, that having a CG near the forward limit will help ensure the nose drops when doing stalls and spins. Read....a more safe flight condition results when the CG is in the mid-range of the CG range or towards the forward limit (note...not forward of the forward limit). The down side is...you gotta hold up elevator to keep the nose up, and this causes drag.
• AFT CG / As the CG (balance point) of the aircraft moves toward the aft limit of the published CG range, the pilot will experience an aircraft that becomes MORE UNSTABLE. It will become pitch sensitive and the pilot must take care not to input a lot of elevator movements. As long as you are loaded and balanced either on the aft limit or forward of the aft limit, this increased pitch sensitivity does not exceed the average Joe's piloting skills and ability to handle it. As one loads the aircraft with more weight toward the tail, the aircraft may become unmanageable. Or...worse yet, if you happen to stall, the nose won't fall down and if you spin...spin recovery may become impossible. This is why during FAA flight certification, the FAA requires a tail-attached spin recovery chute for the aircraft. The test pilot can deploy the spin recovery chute to lift the tail and to get the nose down. LOL...old style thrust vectoring in a sense. On the upside of things, as one loads an aircraft so it balances toward the aft limit of the CG, less and less elevator is required to keep the nose up during cruise. This equates to reduced drag. Competition sailplane pilots always load their birds to balance on the aft limit. Many high performance sailplanes have a water tank in the tail to help the pilot make CG adjustments toward the aft limit. This water can be dumped (and should be dumped) prior to entering the landing pattern.

In summary, models and real aircraft "should' behave according to the above. As the modeler moves the balance point more and more forward, you'll get a more stable bird. Go too far forward with the CG, and you might be picking up the pieces of a lawn dart from the approach end of the runway. I once flew a 2-place sailplane with a fairly large fellow up front, and I forgot to remove the ballast from the front (I was in the rear seat). Where I normally approach at 55 mph in that sailplane, I was on final at 70 mph just to have enough elevator authority to keep the nose up. Lesson learned! The tension on the tow rope on take-off helped lift the nose, so I didn't notice the "problem" until i released the tow rope. UGH!

In my earlier post about the MIG, Post #1328, I relate a story about a flat spin in a sailplane with an aft CG. Please feel free to read that post and learn why I shy away from aft CG flight...especially when flight is with a CG that is aft of the aft limit. For modelers, we must experiment and determine our own forward limit and aft limits of the CG as determined by our individual pilot abilities.

For your kind consideration and thought, as always.

-GG

[leithalweapon]

Hi LW...I'm gong to get on my soapbox again about common mode radio interference produced by HIGH CURRENT ESCs. What you experienced sounds so much like what happened to me when i lost an A-10 and an F-4...both HIGH CURRENT ESC birds. When I mentioned this to a HAM radio guy, he asked me if I'd ever had an issue with the P-38. I answered "NEVER"...the P-38 has never had any sort of control glitch. He said, "High current ESCs have the characteristic of producing common mode radio interference. This interference is enough to upset the pointer in the look-up table being used by the receiver and transmitter to implement the frequency hopping, spread spectrum operation. Hence, causing the loss of control you have experienced with the F-4 and A-10."

His suggestion was to add ferrite RF chokes to all the wires coming toward the receiver side of things from the ESC. Keep these RF chokes as close to the ESC as possible because any length of wire acts like an antenna and transmits the RF noise.

The proof is in the results. Since I added RF chokes to my HIGH CURRENT (only) birds well over a year ago, I have not seen any form of glitch or loss of control. I AM A BELIEVER!

Enter the word "ferrite" in the search window of Hobby Squawk, and you can find more information. Here is where you can get them.

• From Amazon.... Cedmon 20 Pieces Clip-on Ferrite Ring Core RFI EMI Noise Suppressor Cable Clip for 3mm/ 5mm/ 7mm/ 9mm/ 13mm Diameter Cable, Black

A lot of folks attribute your experience to a brown out, but these high current UBECs really should be able to handle the loads we place on them w/o any problem.

Think about it....try adding the RF chokes to the ESC wires coming forward from the MiG's ESCs. There's plenty of room on either side back by the rear end of the rear battery. And...for good measure, I added another RF choke to the throttle wire which already has a green ferrite ring RF choke. Can't hurt.

Offered for your consideration and thought, as always.

-GG

I think your right. No chokes on the esc's or the castle bec. Yes it did seem, in hind site, that I was getting control back and then losing it again. I guess it could be possible when I cut the throttle the interference stopped. The chokes would have been a cheap part to include in the kit and definetely should be pre installed.
I learned to fly 40 years ago without dual rates or expo. Now I use very little. Usually less than 10%. At 30% the controls feel mushy to me.
Everything on the plane is stock except the bec. Set up per the manual.

[Airguardian]

In summary, models and real aircraft "should' behave according to the above. As the modeler moves the balance point more and more forward, you'll get a more stable bird. Go too far forward with the CG, and you might be picking up the pieces of a lawn dart from the approach end of the runway.

I couldn't have worded it better!
Absolutely agree with all you said there.

Only thing I would add is that aircraft stability works slightly different for model aircraft and fullscale 'direct control' aircraft, as on the later, the center position of the control surfaces when flying hands free is variable and depends on aerodynamic loads and tab trims, something that we don't have on our smaller models, which have a fixed center or neutral position for all surfaces when sticks are neutered.

Bigger planes (airliners, jets) may become again more similar to our scale models as the surfaces are usually not 'free-floating' when hands are taken off the stick, as there are hydraulic or electro-mechanical systems of sorts controlling them.

In any case, most of it still applies the same to both, once you take account of that.

i think you are more focused on Dynamic Center of Gravity (affect in motion) Vs Static Center of Gravity (used for initial setup). im just referring to Static balance for setting up the aircraft.

Technically I'm talking about both.
Well, Center of Gravity itself doesn't really move around unless you have a fuel tank or drop payload through the flight, so I guess you are referring instead to static and dynamic stability.
In that regard, plane STATIC stability (the one we are talking here when debating about model CG placement and stab trim) has been traditionally determined by what is called 'static longitudinal stability', rather than dynamic. The dive test I described above is aimed at sorting out visually what the 'longitudinal static margin' is for the plane (in other words, if it is nose or tail heavy, stable or unstable and by what amount).
The 'math-monkeys' do this (calculate the static margin or degree of nose/tail heaviness) by watching a graph slope after mathematically modeling the plane (typically using computers), but the 45º dive test I described above is the most visual and precise way that I know of that translates that virtual-world 'graph slope' analysis into a real world empirical test that is rather easy for people to conduct and understand after they have thought it out a bit. Dynamic stability is a whole different beast and I am not getting into that now! (Although of course, CG placement and trim also play their part).

[leithalweapon]

Did you use the Y-harness that came with the upgrade servos to bypass the MCB-E and go direct to the receiver?

No. Wires are all going thru the MCB as per the manual. I second guessed myself a few times on this plane. Didn't like that the servo wires are run through the MCB and that there are no RF chokes on the esc's, but that's how MRC designed and sold it so I ran with it.

[GliderGuy]

LW, I’ve flown my MiG 50+ flights wired up stock but with RF chokes added and haven’t had any electronic issues. The RF chokes, in my opinion, shouldn’t be the responsibility of the manufacturer because “maybe” only certain brands/equipment set-ups are sensitive to the interference. I fly a lot of flights, so the odds are that if something can go wrong, I’m gonna see it. The average flyer might not ever see an issue that bites me.

I had many, many flights on the A-10 and F-4 before I lost control of them. That being said, a glitch on each and the loss of 2 birds was an end result I want to avoid.

So, I am playing it safe with my high current birds and adding RF chokes to push any chance of a glitch far out in time. Once was too many times to have a bird go down.

I did receive the upgrade elevator servos today, and I will use the Y extension to bypass the blue box. Why? Fewer connections to potentially go wrong...to lower the odds. And lower odds of a glitch is what I need.

-GG

[janmb]

FWIW, I have achieved 7min flight time on the Su-35 with my setup when doing formation flight instead of acro. And that's improvable as formation flight requires a lot of throttle up-down, whereas for a pure cruise flight you can leave constant rpms and consume less power ;)

But yeah, you are comparing a short aspect ratio tailed delta to high aspect ratio straight wing jets... not really fair!

But to achieve that, you run fans that are suited to kill someone with sound waves at over 10 miles range... 😁

[janmb]

Most people understand what that picture is saying though. Even though you are technically correct your explanations are confusing those people...

The problem is that the figure only tells you whether or not the CG is forward, at or aft of wherever you happened to put your fingers. It says nothing about the qualities of that chosen point.

[janmb]

Got mine in the air today with the new servos. Barely got off our grass field. Had it in the air less than 1 min at about 3/4 throttle. What happened next looked exactly like the crashes on video. The nose pitched down and I had no elevator control. At first i chopped the throttle thinking it was toast, but at the absolute last second I got the elevator contol back and was able to pitch up enough to avoid the gound. Slammed the throttle wide open and no response from the motors for a few seconds. Didn't seem to lose aileron control. So I have no idea what happened, but I am using an Admiral 10ch receiver which was probably a bad idea. Originally I had ordered 2 Admiral sattelite rxs, but they were both bad.
One other thing, I will never fly with 30% expo again.

Ouch! Glad you saved out - hope you can get it sorted. Sounds most of all like radio glitching to me, but obviously hard to say.

As for expo, that depends entirely on the model, the chosen rates, and how sensitive you like things. Nothing inherently wrong about running 30% expo (or a lot more for that matter) on a jet like this, but again, depends directly on the rates you use.

[janmb]

I'm going to jump in to the CG discussion, but from the standpoint of an ASEL and sailplane pilot's point of view. Hopefully, providing some addition insight to the topic at hand.

Everything you write here is correct, but I would advise that full size, manned aircraft CG considerations vs what you want in a model are two very different things. At least for a scale model of a fighter jet, where willingness to maneuver hopefully is something most pilots want and appreciates.

Also, in terms of safety, we can afford having far more "dangerous" CGs that you would ever accept for a full size aircraft. With the massive benefit of having models that don't fight maneuvers and don't fly with a lot of unnecessary trim drag.

[TS8103]

I want to partially bypass the blue box for the ailerons. Does anyone have a schematic or diagram on the balance leads. I would like to find out which wire in the balance lead is the signal wire for the ailerons. Any help would be greatly appreciated.
thanks,
Tone

[GliderGuy]

Everything you write here is correct, but I would advise that full size, manned aircraft CG considerations vs what you want in a model are two very different things. At least for a scale model of a fighter jet, where willingness to maneuver hopefully is something most pilots want and appreciates.

Also, in terms of safety, we can afford having far more "dangerous" CGs that you would ever accept for a full size aircraft. With the massive benefit of having models that don't fight maneuvers and don't fly with a lot of unnecessary trim drag.

Janmb - Right on! The fun of models is that your butt is not on the line and having a CG set for an unstable condition makes for some impressive acrobatic flying. Go too far with the CG either way, and you've lost only a model and not a life.

i was at an aviation conference years ago where Chuck Yeager was the featured speaker. His talk was on modern fighter design for instability and hence fantastic maneuverability. He detailed how the new fighters have fly-by-wire systems which can react to the inherent UNSTABLE design's flight characteristics much faster than a human can. He stated that the guidance is to punch out if these systems go down because a human cannot react fast enough on the controls to keep the pointed end forward and wheel side down.

Hopefully, the readers now have a good understanding of CG and the effects on handling and maneuverability. I still await the results of you brave souls experimenting with the MiG-29 in the falling leaf situation I found myself in and was almost unable to recover from. I'd love to learn of a sure-fire method to recover from that situation. Remember...I am balanced on the CG marks.

-GG