Official Freewing MiG-29 Fulcrum Twin 80mm Thread

[rifleman_btx]

Well yalls way is better than mine. I just look at the damn thing. If it hits at about the the thick point of the wings maybe just abit aft, then I feel like it has a chance, so far it's worked out pretty well for me.

[jetpltrich]

Ooooh - I'm impressed.

BTW, you aren't the only technically qualified person on this list.

This evening I took the opportunity to take a close examination of this CG issue. Admittedly my previous ascertain of the CG being close to right appears to be in tatters. I needed accurate measurements versus the cursory measurements I took initially so I took the right wing off and measured it up and laid those measurements out on large sheet of paper and the results have concluded the following. Within a millimeter or two the MAC is 335mm, that MAC is 165mm from the wing root. If we dive into some CG percentages, for example 5% of MAC, that’s almost 17mm aft of the factory CG marking. 5% is a little shallow for me, prefer 17-20% and have built some models with 25% and have been happy.

[GliderGuy]

Excellent work JPR…

I added washers under the chute area and filled/painted over them. My CG is now 10 mm aft of the mark. This means I carry about 3 mm of FFS leading edge trim below the fuselage line.

Using the Admiral Pro 6000 mid and aft bays. The receiver is in the front bay.

Reminders from prior posts…
1) Deep stall / high alpha recovery requires shutting the power off….thrust line thing.
2) Don’t fly fast with flaps down. A potential FFS servo overload/servo stall may lawn dart you…Airflow over FFS loading thing induced by the flaps as captured on rear facing cameras.
3) Strongly suggest RF chokes as close to the ESCs as possible. High current ESCs splay out a lot of RF noise which may trip up the RX/TX handshake.

I made a bunch of other mods* as we were historically trying to figure out the lawn darting well over a year ago. Most were easy and couldn’t hurt. Many may not have been needed. But…happily still burning holes in the atmosphere after several thousand flights. Knock on wood…

-GG

*Most significant = high torque FFS servos and heavy duty FFS hardware + larger diameter wire feeding the FFS servos to minimize voltage drop.

[Hugh Wiedman]

Excellent work JPR…

I added washers under the chute area and filled/painted over them. My CG is now 10 mm aft of the mark. This means I carry about 3 mm of FFS leading edge trim below the fuselage line.

Using the Admiral Pro 6000 mid and aft bays. The receiver is in the front bay.

Very Interesting. I've got mine 15mm aft of book and happy with it there. The odd thing is my battery placement compared to yours. I'm using the 614 gr SMC 6200's, one as far back in the aft tray and the other as far back in the front tray, with only the Castle 20 amp BEC in the middle tray. The receiver is under the floor boards in the aft tray and the satellite receiver is all the way in the front. Those Admirals are only a few gr heavier than the SMC's at 836 gr (so only an additional 44 gr spread out). Surprised I'm able to get that CG without using the middle tray like you did. I do have the added weight in the tail of the TV nozzles and the AB's, and maybe the extra paint and clear coat gave me additional tail weight. I've also flown it on the SMC 5300's that weigh only 684 gr each, but need to put the batteries all the way forward in each of the same bays. With those, I have a lot more leeway to go further back on the CG (not planning that at this moment) but with the 6200's located where they are, no options to get any further back than the 15mm. It seems a little more "nibble" with the 5300's, but love the 5 minute flight time with the 6200's and I'm not that good of a pilot to really notice much difference except getting off the grass in about 15 less feet.

[kallend]

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.
. I do have the added weight in the tail of the TV nozzles and the AB's, and maybe the extra paint and clear coat gave me additional tail weight.

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You can add tail weight and improve the appearance at the same time with 3D printed TV nozzles:

https://www.thingiverse.com/thing:4769774

[GliderGuy]

Hi Hugh, As I recall…I added 4 or 6 (about 1 inch or 1.5 in diameter) steel washers in the area between the nozzles directly under the chute bay. I did not weigh them. Kept adding washers until I got the CG back.

The only other things I did that might shift the CG back were 1) Heavier wires driving the FFS servos and 2) added some carbon sheeting to the rear fuselage area supporting the FFS mounts…negligible weight.

The MKS HV-69 servos are the same size as the 17g Freewing servos…not much added weight there. Finally, I have two small ferrite chokes by each FFS servo…not much weight there. I moved the BEC to the shoulder area by the back bay. The rear of the Front battery butts up against the blue box….middle bay. Had to trim a little foam from the lid. I also have the Futaba SR-10 servo reverser in the front bay…not very heavy. It has a ferrite choke and I have another small choke by the green RF choke/ring at the Futaba receiver….not much weight there, either.

I refreshed my memory just now…I measure 13 mm aft of the CG mark right side up and wheels down with all the missiles installed. With this CG, it recovers nicely from high alpha deep stall when the throttle is cut, so I plan to leave the CG where it is. Its quick high alpha recovery response saved my rear a couple of times.

Overall 2 coats of Spar Urethane…and did not remove any of the stock wiring other than the two wires going to the rear FFS servos.

Bottom line, my major mod for CG was the washers.
-GG

[Airguardian]

From my calculations the CG reference points that the manufacturer has marked on the wing seems to be pretty close to accurate. What one can’t ascertain is the fuselage is a lifting body and thus this makes that calculation diluted. With that in mind and the batteries as far back as I can get them she was nose heavy.

Yeah... not quite (as you found out yourself later on!)
But you are totally right. If airfoil shape and fuselage lift were not accounted for, the value you got is only worth a first order approximation, so to speak.
I'm assuming you refer to the geometrical method where you use root and tip cords, place them in reverse and trace a few lines to get a CG 'good spot approximation'.
There are more advanced methods to get an idea of where CG should be. I personally often use XFLR5 which is a free software that allows you to account for complex wing geometries and airfoils using potential-flow theory. I made a 'rough' model of the MiG-29 on that software and I seem to recall from memory, my prediction (or rather, the SW's prediction of Neutral Point was like 4 or 5 cm behind manufacturer recommendations. That's not to say that this is correct either. The software has its own limitations, my model of the plane was rudimentary and the airfoils used, only approximate. Then again, with some exceptions, you also typically don't want to fly with your CG on the NP itself but rather set up the plane slightly nose heavier to build a bit of positive static margin (longitudinal stability). Flight testing demonstrated that this plane flies well at around 15-20mm aft of CG marks, which still does feel quite a bit nose heavy, but with the silly stock thrustline I wouldn't advise anyone to explore any further aft unless TV nozzles are installed and thrustline offset fixed. Otherwise the plane will become quite a handful to fly. Using a well-set-up gyro also allows to further relax the static margin and set CG even more backwards. I haven't tested that yet on the MiG but it's on the list of 'to-dos'. :)

these formulas to find CG locations don’t change from small to big. Wing shape does change ones perceived idea of where the location should be and so does airfoil shape.

That's correct, but those formulas (again I assume you talk about the geometrical method based on wing planform) are only good for an initial estimate and work better on more traditional designs, where the body of the aircraft does not account for a great deal of its lift as it does on the MiG. That's not to say you shouldn't use it, I think it's a great exercise and by comparing these results to other estimates and actual flight testing you can develop a sense for the usefulness of the info they provide.

How you achieved your conclusion I’m “reinventing the wheel” eludes me.

Well, for a fact we are not talking about a plane that is yet to fly but one that has thousands of flights accumulated collectively over all the users (Glider Guy roughly accounts for half of those though, ROFL) in here and RCG who provided feedback. So it is KNOWN what CG provides a good static margin (stability) and what is excessively nose heavy. The exact location of the NP is probably unclear yet but I'm willing to bet it's gotta be around 35-50mm aft of the CG marks (and again, I'm not advising anyone to place their CG there, just to make things clear!).

You and I both have no idea what the manufacturers test criteria was when they tested this aircraft. We also have no idea the root cause of these “lawn darts” accidents, only pilot reports. No onboard data recorder to look at. With that conclusion we are at a loss and become the test pilots and though I am very well aware of the CG point location change it doesn’t mean one has to follow it. As I mentioned before I calculated it and it was pretty close to the manufacturers.

Oh, I think we have a pretty clear idea of what the manufacturer's test criteria is. As far as they have disclosed, they give the recommendations that they think will suit a majority of pilots and will result in less crashes according to their data and user feedback through customer service tickets. That's often 2cm too-nose-heavy for the immense majority of their EDF fighter fleet (with some notable exceptions on both ends of the spektrum). I personally totally disagree with that way to proceed but it's their call. Making planes excessively nose heavy is a danger of its own and I've nearly lost some birds during maidens as a result of trying to follow with manufacturer recommendations. Not a fan (but I will still buy their planes and recommend them).

I also think it's pretty clear what caused the lawn darts with a high degree of certainty. Me may not have isolated a single exact cause but we know which the main culprits were (most likely a combination of several things), and there's enough material evidence such as on board and ground video and picture plus tests independently carried by forum users, some of them replicated by other users.

This evening I took the opportunity to take a close examination of this CG issue. Admittedly my previous ascertain of the CG being close to right appears to be in tatters. I needed accurate measurements versus the cursory measurements I took initially so I took the right wing off and measured it up and laid those measurements out on large sheet of paper and the results have concluded the following. Within a millimeter or two the MAC is 335mm, that MAC is 165mm from the wing root. If we dive into some CG percentages, for example 5% of MAC, that’s almost 17mm aft of the factory CG marking. 5% is a little shallow for me, prefer 17-20% and have built some models with 25% and have been happy.

I commend you for taking the time to re-evaluate your own findings. If more people acted like that we'd live in a better world, overall ;)

Notice that your new calcs closely reflect what has been more or less acknowledged by a majority here... 15-20mm is the sweet spot as long as you keep the stock fixed nozzles... that is, it's still a nose-heavy setup but not as ridiculously nose heavy as the manufacturer suggested which was akin to flying with air brakes deployed (in trim drag) and causing severe strain on the stab servos since their design was a bit marginal in several aspects (mainly the initial servo and linkage selection) and the choice of not implementing the scale hinge axis for the stabs led to a bad mechanical handicap for the servo.

[GliderGuy]

Update to post 5187…

I’ve now made over 200 MiG landings on the replaced plastic gear mount (I am on vacation…flying lots).

After 4.5 hrs of mixed MiG, F-4 and P-38 flying today, I gave the main gear strut a strong wiggle with my hand just now. It didn’t budge at all….the non-replaceable, yet replaced, plastic mounting box is still rock solid!

Should your main gear mounting box ever crack and you need assistance with the replacement of the non-replaceable plastic mount, I will be happy to go back over the tips with you which I posted earlier.

You might consider getting a set on order (link below). The main boxes will most likely crack….eventually. Due to the weight of the MiG, any attempt to fix a crack probably won’t hold up. Replacement is the only sure fix. You’ll need a Dremel tool.

https://www.motionrc.com/products/fr...unt-fj31611094

Also to have on hand….order a thin 3 mm carbon sheet, and a small diameter carbon rod or rectangular tube 8 to 10 mm, too. A strong/high PSI rated epoxy (3000 psi min, 5000 psi is better) is also needed to secure the new plastic box and carbon reinforcements you will be adding.

Hopefully, you can avoid having to do this by making perfect landings EVERY time.

-GG

[xviper]

I’ve now made over 200 MiG landings on the replaced plastic gear mount (I am on vacation…flying lots).

Hopefully, you can avoid having to do this by making perfect landings EVERY time.

-GG

Since about the middle of December when you replaced your gear mounts, you've landed this plane over 200 times. You're on vacation and flying lots. I'm retired so I'm on "vacation" all the time and I won't put 200 flights on this or any plane I have during any of their lifetimes with me. Unless I schmear it into the ground on a really hard landing, I think I can forestall this repair. Of the 50 or so planes I have, I do have a few faves that get flown a bit more than the others but even those won't likely see 200 flights before they're gone from my fleet. You're kinda like those machines in large manufacturing plants that jiggle or thump or rattle a part or a whole product to see how many cycles it takes to break something.

[jetpltrich]

Yeah... not quite (as you found out yourself later on!)
But you are totally right. If airfoil shape and fuselage lift were not accounted for, the value you got is only worth a first order approximation, so to speak.

I ate some humble pie on this one. Curious to explore the minds of the MiG engineers.

An additional factor at play with the CG that I hope others have taken into consideration is the gear down vs. up. Up nose heavy.

She’s all ready to go now just need to find the time.

[xviper]

I ate some humble pie on this one. Curious to explore the minds of the MiG engineers.

An additional factor at play with the CG that I hope others have taken into consideration is the gear down vs. up. Up nose heavy.

MiG engineers or Freewing MiG engineers? I admire someone who can admit they weren't quite correct about something, unlike a few whose arrogance refuse to ever let them be wrong. Either that or they're well past their "best before date" and don't know it.

I generally balance my planes wheels up as that's the way they fly the majority of the time when being operated in the air. (Who really cares how balanced they are when rolling on the ground?) Wheels are only down during take off and landing, when you're "on the sticks" anyway as it needs more inputs for a rapidly changing flight environment. Personally, I don't see the point in balancing wheels down since they're only down for maybe 5 seconds after lift off. As soon as there's air under the tires, I throw the switch. If it crashes right after lift off, that's one less major thing to get damaged. (I don't understand why some people fly most of their flights with the wheels down, but if you do, then balance with wheels down.) Then the wheels come down in preparation for landing maybe 1/2 a circuit before the final turn to the runway.

[GliderGuy]

Since about the middle of December when you replaced your gear mounts, you've landed this plane over 200 times. You're kinda like those machines in large manufacturing plants that jiggle or thump or rattle a part or a whole product to see how many cycles it takes to break something.

By the way…I replaced the cracked right main gear mount, only. The left one hasn’t cracked, yet. I fear that operations off grass may cause cracks with a fewer number of flights. So…I wanted to give y’all that operate off grass the heads up for when it happens.

I know…it seems unreasonable. But today in 4.5 hours I made 34 flights. Generally, I average 8 minutes for take-off, time in the air, to landing and ready for the next take-off. I almost always run through 32 batteries (2 sets…16 = 1 set) when I go out... 2 per MiG flight or 1 for an F-4 or an AL37 flight. The P-38 is a 2-per flight, and I generally do mixed bird flying. By the time the 3rd P-38 flight is done, the “first used” 6S batteries are ready to go for another “set” of EDF flights. P-38 flights follow the EDF flights to give time for the first-used 6S charge cycle to complete.

Philosophically, as I get older I am less inclined to let any good opportunity for flying go unused. No telling when the day will come when I can’t go fly. And to that, I raise a toast to the man upstairs who owns that calendar.

-GG

Aside: To support this almost continuous activity, I limit consumption to 4,000 mAh max and more typically 3800 mAh. At my 4.3 A charge rate, charge time for the 6S batteries is typically less than an hour. I use 4 min for the AL37 and MiG flights and 5 min for the P-38. The F-4 is 3:30. I do the last 1:30 at 50% throttle to conserve so the charge time is reduced.

[xviper]

By the way…I replaced the cracked right main gear mount, only. The left one hasn’t cracked, yet. I fear that operations off grass may cause cracks with a fewer number of flights. So…I wanted to give y’all that operate off grass the heads up for when it happens.

I know…it seems unreasonable. But today in 4.5 hours I made 34 flights.

As I get older, I am less inclined to let any good opportunity for flying go unused. No telling when the day will come when I can’t go fly. And to that, I raise a toast to the man upstairs who owns that calendar.

-GG

You are quite remarkable and I'm very envious that you have such dedication to this hobby. I'm already "older" but I guess I have other things that take my time during each day. When weather permits, I go flying but only in the mornings (sun not in my eyes, low winds, not crowded at the field) for about 3 to 4 hrs. I like to be done by lunch time as I don't like to take my lunch. The rest of my day starts with lunch. Evenings are rare as the sun is in my face and flying can be tricky.
On what I call a "busy" flying day, I take 4 to 5 planes. Most of my planes are on the largish size. I used to take up to 7 planes when they were much smaller. I don't like to charge batteries at the field, so I take 2 batteries for each plane. Fly them in rotation, so in total, maybe 8 to 10 flights and it's time to pack up and go home. Sometimes, getting 8 flights in is difficult as I get yapping with my flying buddies or take time out to help another flyer with a difficult issue. As you can see, each day I fly, each plane gets 2 flights and each day usually sees a different set of planes. Now you understand why it's highly unlikely that I'll get 200 flights on this or any of my planes. (Yeah, I need to get rid of some planes - some may call it "death cleaning" - but for now, I like them all.)
I've flown my MiG off grass one day (2 flights) and they were both picture perfect. Greased landings on what can sometimes be a bit lumpy, but never any gear damage. I generally fly off GeoTex - also a bit lumpy as the season gets on.

[GliderGuy]

Xviper, You appear to be dedicated! Thanks for sharing, and yes…200 flights will take you a bit of time.

Enjoy life and Happy New Year!

-GG

[jetpltrich]

MiG engineers or Freewing MiG engineers?

MiG engineers.

[xviper]

MiG engineers.

I sorta doubt any real MiG engineers were involved in the model's design. I'm sure there are many attributes of designing an RC model that a real MiG engineer couldn't relate to and visa versa. They are likely two different, though related, mindsets.

[jetpltrich]

I sorta doubt any real MiG engineers were involved in the model's design. I'm sure there are many attributes of designing an RC model that a real MiG engineer couldn't relate to and visa versa. They are likely two different, though related, mindsets.

In 1991 I was flying an Mu-2B for the FRB. My route had me go through Cleveland’s Burke Lakefront airport at around 2am and there parked on the ramp with a guard keeping watch over it was a MiG-29 which flew in for the Cleveland air show on its Friendship Tour. Once I was unloaded I made a B-Line to the MiG-29 and asked the guard who was sitting in a chair with his AK and smoking a cigarette if I could look at it. He took a long inhale from his cigarette, exhaled, and said in his broken English yes you may and waving his hand to proceed. I basically did a pre-buy on the plane and thanked the guard and flew off to Chicago. The MiG-29 in the late 80’s and early 90’s was all the buzz and I got to get really close. What did I think? Crude but effective and it leaked fuel. So I’m interested in picking their brain more so on the design than the CG issue but will never get too and it’s an old design with new Sukio’s eclipsing it.

[Elbee]

Got my MiG 29 a few weeks ago and have been slowly getting it ready for flight.

JR, Welcome to "The Squawk". I am tardy sending you a hello. Great work on your MiG. Looking forward to more photos of your modifications and build. Best, LB

[jetpltrich]

JR, Welcome to "The Squawk". I am tardy sending you a hello. Great work on your MiG. Looking forward to more photos of your modifications and build. Best, LB

Why thank you very much sir. Got my CG nailed, weight added to the tail, very satisfied. Just did four full power takeoff runs lifting the nose and aborting on my runway on my property... the plane has a nice even nose rise and I’m pleased with that. It’s a perfect day to fly it but with 110’ pines surround the property, although I’m comfortable with my Venom, I’m taking it to the nearby RC field which is wide open and I don’t have to navigate a new plane through the pines. BTW, is anyone running a gyro?

JPR

[Evan D]

Not me…