CMP F4U Corsair 50 Build Log (Full Version)

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w1nd6urfa -> CMP F4U Corsair 50 Build Log (12/29/2008 3:16 PM)

Hi everybody and Merry Christmas!
I decided to compile this build log, as I read a lot of complaints about CMP warbirds on this forum and RCG. The complaints are fair, if you consider these kits to be ARF, which they are NOT (compared to ARFs from Hangar 9, Top Flite, Kyosho, etc). They are more like a BC (Builder's Challenge)-ARF or FOBI (Figure Out the Building Instructions)-ARF [:o]

If you don't have building experience (at least 2-3 "proper" ARFs) then either ask an experienced builder to guide you through OR be prepared to buy a second Corsair to get it right! Also it is a warbird, not a trainer or acrobatic model, so make sure you know the right technique for take-offs, landing and flying it.
That said, it is a model that if you build it right and can fly it in an "aggressive scale" manner, it will reward you with its looks and good flying manners. The latter is probably because of its large wing area which leads to a mid-range wing loading, at least to other warbirds on the same or larger category:

................................Wing Area (dm2)........AUW (kg)...............Wing loading (g/dm2)
Kyosho FW-190 .50_______34_______________2,9_________________85
CMP F6F 1.20____________53,2_____________4,7_________________88
CMP F4U .50_____________41______________3,0_________________73

The other 2 models are also good flyers and land safely at manageable speeds, but they both employ flaps (I own the FW190 and a friend flies the F6F at our local field)

So, the story is that I first got this model last summer, made lots of mistakes building it, but it flew well regardless. This is Corsair #1 in later posts which eventually I crashed by hitting some electricity power lines on base leg approach on a cloudy winter day [:@]
This model had attracted quite a lot of attention from fellow pilots and I was fond of its looks, so decided to buy another and build it much stronger, based on the experience from Corsair #1.

There are 10 building challenges involved in making this model sturdy and long-lasting, so read on and please add your comments!
Note that I have tried to use the provided material as much as possible, improving on them where required and adding extra stuff only where needed.



w1nd6urfa -> RE: CMP F4U Corsair 50 Build Log (12/29/2008 5:14 PM)

Challenge #1: Landing Gear

OK, on to the Build Log: the first item that needs your attention is the landing gear. There are quite a few posts around regarding retracts, I decided to pass and improve the fixed gear (my experience with mechanical retracts is "mixed", but the added weight of additional hardware & battery, the complexity of rotating retracts and the sensitivity of the gull wing to belly landings make my model-survival bells start to ring!). So following my experience with Corsair #1 where the crappy CMP gear wires had to be re-bent after each landing and the plane taxied with small hops, I chose some shock absorbing Robostruts. OK, the proper ones for the Corsair would have been the #650 straight ones, but my LHS didn't have them so I settled for the #652.

Another problem is the "hardwood" base for the gear (in fact it is multi-layered ply) - on Corsair #1 I had one of the wire struts wobble, so after the crash I opened the covering on the top of the wing to investigate: the "hardwood" had a split in the middle along the ply layers so most probably if there were a next landing the gear wire would have gone through the wing!

Gave it some thought and here's the new setup (I have tried to keep the original materials as much as possible, of course one could build from scratch the whole assembly). If you keep the original hardwood gear base you need to reinforce it to make sure it won't split lengthwise: I made 6 vertical 2 mm holes near the edges and filled them with thin CA.

See the diagram below - the principle here is that the landing stress will be distributed to the whole assembly as evenly as possible.




w1nd6urfa -> RE: CMP F4U Corsair 50 Build Log (12/29/2008 5:33 PM)

Then need to align the wheel axles using a straight edge - however a 1-2 deg toe-in is recommended




w1nd6urfa -> RE: CMP F4U Corsair 50 Build Log (12/29/2008 5:50 PM)

Challenge #2: Reinforce the wing

The built-up wing is light, but also fragile in some areas, notably the "gull" part where the mid-wing spars are curved. Also the LE and TE balsa is joined in this area making it susceptible to a crack if under stress.
Corsair #1 had a wing crack on this area following a deadstick landing (I had the stock gear struts back then, so no shock absorbsion and the touch-down stress went straight to the wing).

So get some strong wood glue and use the wheel bay openings and the wing root to drop glue in this area - aim for the spars and the LE and TE joins.



w1nd6urfa -> RE: CMP F4U Corsair 50 Build Log (12/29/2008 5:59 PM)

Then I re-sealed the wheel bays using covering I stripped off Corsair #1 - you can use the plastic bays provided if you don't mind the weight and the bad looks ...

I also cut some scrap balsa to create a back cover for the square gear bay. This is to avoid any fuel oil running in the wing.




chalmrast -> RE: CMP F4U Corsair 50 Build Log (12/30/2008 7:15 AM)

I too struggled with the retracts on this bird. I first tried the mechanical's but they didn't work well. I then ordered some servo-less electric retracts (Lato-Tech) but after 8 weeks of waiting I cancelled my order. While I was waiting I went ahead & installed Robart 615's with struts. They barely fit for proper operation but they do fit.

I have 8 flights on mine now (it's an electric conversion) and I have yet to see any of the characteristics that so many complain about. Of the CMP warbirds in this size range the Corsair is probably the best, followed by the Zero, P-51, then last is the infamous .50 P-40!

My e-conversion is no light-weight either. It's well over 8.3lbs but flies well!






w1nd6urfa -> RE: CMP F4U Corsair 50 Build Log (12/30/2008 12:33 PM)

Challenge #3: Wing-fuse join

You will notice that there is little guide as to where to drill, glue the dowels etc. so a lot of measurement is required. Dry fit everything and measure 2-3 times from all angles before drilling and glassing in place.

Tip #1: To locate the holes for the 2 wing bolts start with the belly piece, drill the 2 access holes and then mark on wing and ply connecting piece. Drill with 4mm bit all the way through the wing, the fuse and ply inside the fuse. Then you have to enlarge the fuse-ply holes to 5.5mm for the blind nuts to fit.

Tip #2: You need to drill 2 holes in the LE to insert the alum dowels. Do not use the blue-gray camouflage line as a guide for the middle of the LE as I found it is not true on all places (e.g. left and right wing at root) - use geometry instead.

Wing halves: I didn't want to glue the 2 halves permanently as I might have to do some fixing in the future, but this is a personal preference and not a recommendation. I dry-fitted the aluminum tube in the 2 halves and marked the one where it fits less snuggly - I epoxied it to that half. The wing halves are held together with a wide tape (and the wing-fuse bolts and dowels of course). This has been tested on Corsair #1 and never had any issues, on the dreaded crash the wing broke at the beginning of the gull curve area near the fuse without the 2 wing halves separating.

Ply connector: This is not ..ahem.. the best quality ply you can get, it will not split in 2 but it will wear around the bolt holes, and you will definitely get fuel residue on it making it soft. So I kept the original piece but covered it in a thick layer of epoxy, removed the covering on one of the wing halves and glued it on.



w1nd6urfa -> RE: CMP F4U Corsair 50 Build Log (12/30/2008 1:32 PM)

Challenge #4: Fuse belly

I decided to use screws - I guess one could strip some covering off the wing bottom and epoxy the belly but then it will not be removable which could be useful for later attachment of bombs (see this: Corsair with bomb), separating wing halves, etc.

The challenge is where to place the screws, as most of the wing is a sheet of the lightest balsa China produces [&:] definitely will not hold a screw (I have witnessed a CMP Corsair belly flying off mid-air). So the front is OK (there is LE spar), rear is also OK but I chose to add 2 screws near the middle of the belly as on my model the belly was somewhat twisted, and you sure don't want a gap where the airstream might catch the belly and rip it off in flight. For this middle screw you need to hit it right, i.e. make a hole exactly where the main spar is under the sheet (see photos for rough guide, but measure your wing as there might be deviations).

I drilled the wing and placed 3mm pushrod tubes in the balsa for the screws to hold tight and be able to remove and re-screw as needed.




w1nd6urfa -> RE: CMP F4U Corsair 50 Build Log (12/30/2008 4:54 PM)

Challenge #5: Reinforce firewall

On Corsair #1 I had used the OS 55 AX which flew it decently, but was tail heavy so had to add ~150 g of lead on engine mount (to get to the 85mm CG stated in the CMP instructions!). This engine is now in the FW-190, so after studying the forums on this model I got a OS FS-91 Surpass. This engine at 650 g is 250 g heavier than the 55 AX and is also a 4-stroke so it will balance the model and make it more scale (remember the reason for the Corsair's gull wing was to clear the oversize props driven by the huge 2000 hp engine).

You will need to reinforce the firewall, so use some 30-min epoxy and glass cloth if possible on the firewall perimeter where it joins the fuselage. The engine mount assembly is straightforward, use the lines on the firewall, measure twice as usual dry fit before drilling. Also measure where the throttle rod will go and check that no former is in the way.

While you are adding epoxy to the firewall, you can also add some to the wing saddle part of the fuselage, as it will easliy crack otherwise.




w1nd6urfa -> RE: CMP F4U Corsair 50 Build Log (12/30/2008 5:09 PM)

Stabilizers:
Minor challenge here, you have to drill holes in the stabs for the brass tubes. Don't do it without first inserting the alum tube in the stab as a guide for drilling - you will notice that the stabs attach to the fuse at an angle, not perpendicular.
Sand the fuse stab area up to a guide line recessed in the fiberglass - the rest is elevator so no need to get the paint off.




w1nd6urfa -> RE: CMP F4U Corsair 50 Build Log (12/30/2008 5:17 PM)

Next I drill a hole in the cockpit for the pilot to go through. Why not chop the pilot figure? I also wanted to have a access hole to get epoxy on to the ply that holds the wing bolt blind nuts - a stress area for the whole airframe at high-G maneuvres, landings, etc.




CorsairJock -> RE: CMP F4U Corsair 50 Build Log (12/30/2008 5:57 PM)

quote:

ORIGINAL: w1nd6urfa
Challenge #1: Landing Gear
..................... the complexity of rotating retracts and the sensitivity of the gull wing to belly landings make my model-survival bells start to ring.........................


In my early years of warbird flying, I made many dead-stick landings with the wheels up on my Top Flite Corsair. I found the Corsair to be fairly easy to land with the gear up, and damage was limited to very minor scratches on the underside of the wings. I came to prefer landing with the gear up anytime I had engine problems, as the aircraft was much 'cleaner' and glided much better with the gear up. This was of course, on grass runways tho. I can't say the same about Mustangs: that under the wing radiator causes a few problems when landing wheels up.

As far as complexity: yes, rotating retracts are more complex than conventional retracts, BUT there are some very good options now (Lado in particular), and the Robarts ain't so bad either.
I've owned and flown about 10 Corsairs now, various sizes and scales. Only 1 had fixed gear, and I thought is looked gawd awful in flight because of it. Just my 2 cents.

If one want's to modify ANY CMP kit: my advice would be to apply a more scale authentic finish to it. CMP is famous for producing warbirds that appear to have scale finishes to the casual eye, but anybody who knows anything about WW II paint schemes can readily identfy many in-accuracies in the CMP schemes.
It's such a shame, because the fuselages traditionally have very nice scale details, but they just can't get the finish close to being right. Again, just my 2 cents.

CORRECTION: after looking at the pics in these posts, I see only a few minor discrepencies:
1) the model represents an F4U-1D which was based on the aircraft carrier U.S.S. Bunker Hill in Feb., 1945. That said: the underside should be the same color as the top side (overall glossy sea-blue). NOTE: since these carrier based aircraft were constantly exposed to salt water spray, some of the glossiness was quickly 'washed' away.
2) Nose art was strictly prohibited by the U.S. Navy, and was strictly enforced on carriers. That said: the cards emblem should not be applied.
3) the "188" which is on the cowl should be on a retangular glossy sea-blue background, and be located within the yellow band on the cowl.

Drawing is of another Corsair, based on same carrier at same time:



w1nd6urfa -> RE: CMP F4U Corsair 50 Build Log (12/30/2008 6:01 PM)

Challenge #6: Tail wheel

Another marvellous mystery to solve - here are some hints:

* The tailwheel wire is bent and inserted in the rudder
* The force of landings, taxiing etc. is therefore directly transferred to the rudder hinges unless buffered somehow
* A brass tube is provided and may be used to guide the tailwheel through the fuse
* Hint: the CMP fiberglass fuse is built in 2 halves which are joined. Test this by gently pressing the fuse side over the wing saddle and then along the bottom in the tailwheel area.
* Another hint: if you didn't use the retracts there should be 2 brass eylets left over, if you used them for retracts, you can get some from old servo grommets
* Final hint: try to find a shot of full-scale Corsair and study the tailwheel - you will notice that it is quite long so the tail sits high.

Experience from Corsair #1: I had a couple of rough deadstick landings, where the tailwheel wire would bend under force and the tailwheel would hit the rear of the fuse - it eventually cracked open.

One possible solution:

I used the eylets to be guides for the brass tube, so drill 3mm holes on the top and bottom of the fuse. I also used a 3mm washer to distribute the stress over a larger area. Glue both top and bottom with epoxy being careful not to get any glue in the eylet hole. Then fit the long brass wire which should fit snuggly in the 2 eylets. Now the tailwheel wire can go through BUT before inserting and bending the top into the rudder (point of no return!) fit a 2mm collar and then a 5mm-long piece of fuel line (or a wire spring could do) and tighten the collar so that the fuel line is under some pressure. The idea is to have some shock absorbsion to be able to buffer the landing impact on:

* the rudder hinges (undesirable)
* the fuse bottom through the silicone fuel tube
* the elasticity of the tailwheel wire itself

Also use most or all of the tailwheel wire provided, to make the tail stand high.

After completing the tailwheel & rudder you should really test this before flying as follows: hold the fuse from the engine and let the tail drop from say 0.5 meter and see what happens; if the rudder comes off you should start over [:D]
If the rudder doesn't move and the wire / fuel tube absorb, you're most likely OK (check the fuse for any cracks though!)

It is a good idea to drip some glue into the rear fuselage to reinforce it:




w1nd6urfa -> RE: CMP F4U Corsair 50 Build Log (12/30/2008 7:10 PM)

Challenge #7: Control rods

This is where you get to destroy part of your plane to make it work!
If you use the provided carbon tubes use glue on the wire-carbon join to make the whole thing rigid. This setup lasted on Corsair #1 6 months of tough flying including a couple of crashes.

Elevator rod: straightforward, but make sure the fork wires are bent identical. Also measure precisely the position of the 2 control horns on the elevators. Think that 1 mm offset will mean 1-2 mm asymmetric travel of the 2 elevators resulting in roll/snap problems in flight. Dry fit the rod for measuring but remove it from the fuse for the next step.

Rudder rod: This is the fun part. First be careful to drill the exit hole on the left fuse side only. There is a guide on the right part too, I guess intended for modders who fit pull-pull wires?
Now if you test-fit the rudder rod you will find there is a ply former blocking the way [:@]
DOn't ask how they made it this way, guess the test pilot (if there was one!) forgot to mention it to the CMPro production staff ...
Anyway, unless you are Popeye with oversize arms you should be able to carefully slide your arm in the rear fuselage with a saw blade to cut away part of that former. You are working blind here so the technique I devised is to have the rudder rod in place as a guide and then feel with the fingers where to cut (~5mm either side of the control wire). The formers have lightening holes so if you cut at the right spot you will get the desired result easily (see photos)

The rudder control horn should be as close to the bottom as possible.

... to be continued ...



w1nd6urfa -> RE: CMP F4U Corsair 50 Build Log (1/7/2009 4:33 PM)

Challenge #8: Cowl
It is confirmed by all CMP Corsair owners in this forum that the fiberglass cowl is very thin and will easily crack or break on a nose-over. The dummy engine is even worse, as it is thin brittle plastic. Some people seem to not install the dummy engine, I fly in warm climate so experience from Corsair #1 is that it is necessary to partially block the air inflow to the engine, and thus create a cooling flow.

I followed some of the ideas posted here to strengthen the cowl:
Measure where the dummy engine will go inside the cowl - it will depend on engine dimensions and geometry. What I did was measure where the cowl will go relative to the engine to end up with the prop backplate ~5mm in front of the cowl. Then drill the center hole in the dummy engine for the crankshaft and dry fit the engine inside the dummy - trim as required. Mark with pencil the dummy engine perimeter inide the cowl, then cut a 1.5mm plywood ring to go inside the dummy engine. This will strengthen the nose-over impact area of the cowl and will protect the dummy engine from breaking.
Glue the dummy engine in the cowl using a little CA so that it stays in place, and then use fiber cloth and epoxy (or polyester and hardener) to nicely cover inside of cowl and dummy engine. This will add some weight in the nose, where its needed for the correct CG. To protect the external finish of the cowl, I first cover it using a plastic bag and some covering tape:



w1nd6urfa -> RE: CMP F4U Corsair 50 Build Log (1/7/2009 5:06 PM)

Challenge #9: CG

A lot of (digital) ink has been spent on this subject in these fora ... My little experience from Corsair #1 also suggests a forward CG around 80 mm.
To fine-tune the CG you need to maiden the plane and make adjustments until happy with its handling. So off to the flying field without having fitted the cowl, as I want to make weight adjustments to measure the CG. The engine had been broken in (high rev and idle), I also added some weight to the motor mount to simulate the weight of the cowl and bullet spinner, under 3 kg AUW, CG at 82 mm.
Low and high rate throws set as per "manual", spent 1 tank taxiing and doing aborted take-offs (just enough speed until tail lifts and then cut) until I was happy with ground tracking, and then take-off. Its a sweet moment when you see the gull wings slowly lift up!

After trimming I did some CG tests: 45 degree climb, 45 degree dive, it maintains attitude with no visible nose pitch. However flying inverted requires quite a significant amount of down elev, which suggests nose-heavy? I judged that it is probably due to the non-symmetrical airfoil and didn't move the CG.

Tried a couple of slow-speed stalls at altitude, she will wobble a bit and then drop the nose (not a wing) - same behaviour as I recall from Corsair #1

On landings the low wing loading displays its advantages, she will float in and remain in control provided you don;t slow it down to a stall. The oleo struts tend to produce a small bounce if the touch down is not very gentle, but no nose-overs on the 4 landings I did (paved surface). However I did notice that the rudder authority is not as much as I would like for crosswind correction, so I will increase it.

Some photos:




w1nd6urfa -> RE: CMP F4U Corsair 50 Build Log (1/7/2009 6:09 PM)

Challenge #10: Flying

I have read many posts with descriptions of take-off stall crashes. At least most people admit that it is pilot error, not bad model design. I am not a great fun of CMPro, but IMO these models have been wrongly accused of tip stalling - most warbirds of any manufacturer and size from park flyers to giant scale will tip stall if the pilot is not careful.

If you also fly acrobatic/3D type planes (like I do), you need to mentally switch to "warbird mode". In order to develop the habits that make you a better pilot (not in the 3D sense!) a good starting point is here: Warbird technique

So forget about high-speed snaps, blenders and other extreme maneuvres that might break this airframe, and try to perfect that stall turn, do large round loops with throttle and rudder adjustments, slow rolls. Its also great fun to do formation flying with fellow warbird pilots. Just remember to keep a safe distance in the air and plan the patterns on the ground before taking off.

I hope the above info will be useful to anybody attempting to tackle this tricky subject. Any comments welcome [8D]!



chalmrast -> RE: CMP F4U Corsair 50 Build Log (1/8/2009 8:16 AM)

I'm glad you are attempting to "clear the air" so to speak about this bird. Takeoff's are a challenge in that you must let this baby gain sufficient ground speed before applying gentle elevator. As you state though, this is true with any substantial warbird!

My throws on my elevator are extremely small (probably less than 1/2") with a fair amount of expo too. Turns should be wide with plenty of throttle to be on the safe side.

My e-conversion came out heavy at 9.0 lbs even. Still flies great as seen in this recent video!

http://www.youtube.com/watch?v=yj2tJr-ps6k



w1nd6urfa -> RE: CMP F4U Corsair 50 Build Log (3/3/2009 11:04 PM)

Well, after 20+ landings the CMP "hardwood" (actually multi-layer ply) gear base split, and I had to build a proper base from a piece of spruce. This was shaped as a "T" to fit in the wing and was screwed to the ply base:




sparky251 -> RE: CMP F4U Corsair 50 Build Log (3/27/2009 7:05 PM)

Great build thread,I just ordered mine. Ill be using this info quite a bit. B



w1nd6urfa -> RE: CMP F4U Corsair 50 Build Log (3/30/2009 8:53 AM)

Good luck Sparky,
ask questions as you go along, I'll be glad to help

mine flies almost every weekend, it has just a few scratches on the wingtips from the occasional hard left turn when I reach the end of the runway on landing ;-)

I'm considering retracts next ...



sparky251 -> RE: CMP F4U Corsair 50 Build Log (3/30/2009 4:12 PM)

Great,Ill keep in touch.



jeffk464 -> RE: CMP F4U Corsair 50 Build Log (3/30/2009 8:35 PM)

How was the 91fs for this model?



chistech -> RE: CMP F4U Corsair 50 Build Log (3/31/2009 5:20 AM)

I had this plane with a TT 91 FS and it flew awesome. The Thunder Tiger is the heaviest and it helps with the balance. It had the mechanicals that I had reworked so they would function more reliably but it's problem was that it would constantly flip over on it's tail. The grass field being real rough was a big contributor to that. My buddie stalled it on take off and put it litely in. I still have it but I stripped it out. It needs to have a little glass work done to the fuse around the wing bolt area, it has a slight crack in the wing leading edge, and it needs a new cowl. If some one is interested in it for cheap, PM me. It still has the mechanicals that will work ok.



w1nd6urfa -> RE: CMP F4U Corsair 50 Build Log (3/31/2009 8:37 AM)

I'm very happy with the 91 FS Surpass II (pump model), very reliable and fits the Corsair perfectly using a 14x7 2-blade prop. On Corsair #1 I used the .55 AX which was OK but needed a lot of lead in the cowl to balance (and the high pitch noise of course). The .91 is heavier, and on Corsair #2 i have moved the engine further forward (140mm firewall to prop backplate) so no lead.

The pump FS is very reliable, my favourite maneuvre is a vertical climb up to stall point, stall turn, left aileron roll going down then snap and go into flat spin (kind of blender I guess). I usually pull out of this a couple of mistakes high for fear of a dead engine, but good old 91 has never quit on me

I also like the gradual response of the throttle, as it avoids extreme torque effect (e.g. on an aborted landing)

Altogether a good combination



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