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Friday, November 19, 2010

125 MPH for $250

I've been wanting to buy a warbird with a big engine compartment to install my single-shaft, double-brushless motor concept (post coming soon).  I figured this project would be fun and faithful to scale--a monster engine turning a 4-blade buzz saw, a strong airframe, and at least 100 mph flat out.  As it came together, I also realized this was also shaping up to be a pretty cheap model, and even faster than planned.

I decided on an F8F airframe.  Even though it's a radial, the nose is plenty big and the real Bearcat's blistering 424 MPH top speed proves this little bullet is more than up to the task.  More significantly, the Bearcat's small proportions and beefy airframe provides a strong foundation upon which to build an RC powerhouse.

This seemingly amazing buy from Hobby King, further discounted to $80 via a pop-up promotional code:


Too good to be true?   No way, this kit far exceeded my expectations, the build quality and strength is formidable.  The entire airframe came assembled in interlocking, laser cut plywood.  This amazing find is 90% sheeted in balsa (Fuselage: 100%; Wing 80%), then covered in high-strength film.  The 35" wing, almost fully sheeted in wood, is one of the strongest I've seen in an RC kit.  This was the perfect choice for my WWII era demon.

My current iteration is a hybrid test platform, more traditional than its final destination.  It weighs in at 34 oz with its chunky 4S battery, a hefty 16 oz/ft^2 wing loading, a 14.8V .10 size motor driving a GWS 10x8x4 prop, a 60 Amp ESC (draws 61 Amps @ static full throttle), and a thifty OrangeRX 6ch Spektrum DSM2 RX (replaced with Spektrum AR500 due to consistent link dropping).  A functional side scoop keeps ESC temp in check.

Total cost?

$78 (with the usual HK "you've been looking at this a while" discount) for the Bearcat + $25 Temp Motor + $9 4S battery pack + $84 E-flight 60A  Pro ESC + 3 $15 Hitec MG servos + $8 RX = about $250.  Thrust to Weight Ratio?  Approaching 3:1.


With its scale-look 10x8x4 prop (shown) the little F8F can pull a 3 lb barbell, straight up, along with its own weight.  With a speed prop, 125 mph is well with reach.



Airborne telemetry shows some solid numbers:

With a 4S, full speed runs with a 10 x 8 x 2-blade draws about 600W.  Max static thrust pulls 1000W.  Not bad for a 35 inch wingspan, 30 oz'er.  

Flying Impressions:

This plane is a bullet (minus the stability of a bullet).  I need to tone the throws down some more.  Particularly, the elevator is touchy, probably due to the short distance from the CG.  The plane also has a strong desire to climb straight up under full power.  Balance seems ok in flight tests, and a bit tail heavy in others, but wow this thing rockets skyward.  I adjusted the thrust angle more downward, it doesn't seem to care much about that.  Makes me wonder a little about the fundamental angles of the wing and horizontal stab.  Maybe a THR>ELEV mix is in the cards, but I hate to resort to that.  More testing soon...

Notes from Dec 26, 2010 - Before taking the runway, a routine flight control check caught a dead left aileron.  I'm not sure how or when it happened.  Removing the wing revealed one of the threaded rods had pulled through the clevis and detached.  While fixing it, I decided to take the opportunity to add a bit of washout via slight permanent spoileron.  TP stall characteristics were noticeably improved, with nice symmetrical breaks and no tendency to dip a wing or spin.  This allowed me to flare the landing a bit more without planting a wheel.  I'll do more testing to find the right amount, for now I have each aileron displaced upward by one trailing edge width (about 1/16").

I tried an experimental THR>ELEV mix, but it was too much and pulled down under heavy throttle.  Overall though, the plane still feels reasonably well balanced over a wide range of speeds.  I'm leaning towards tail heavy as the culprit for the pitch hunting.  Next time out, I'm going to add some weight to the nose to assess results.

Dec 30, 2010

Found the high power climb culprit -- unwanted (or perhaps neutral) flaperon.  Screwing in a tiny dose of  permanent spoileron has completely fixed the handling of this model, making it beautifully stable and arrow-straight at high speeds. Vertical is unlike anything else I've seen or flown, it tears skyward with no respect for gravity.  

The only issue is seeing/orienting the solid blue paint scheme, going straight up turns the F8F into a dot in record time.  It is so fun to see a plane vanish in a second or two, then reality sets in that the plane vanished in a second or two.  Thrust is so strong that you can see the bullet hit terminal velocity, clearly prop limited, like it is attached to the throttle stick. By the time the stick hits your upper stop, so does the plane's speed, though it does take some room (like a small state) to slow down.    In fact, this plane rockets to top speed and disappears so fast, I'm contemplating adding streamers to the wingtips to help with orientation. 

Still a touch tail heavy with a 1300 mAh 4-cell.  The plane doesn't want to come down as it slows down, the tail droops as throttle comes out and up it stays.  You have to push it down final.  I've got a 2200 mAh up to 50C 4-cell on the way to fix that.  

First RADAR gun speed trials are scheduled this Monday.  Will start with a 100 mph setup, I want to take it slow.  If all goes well and the weather cooperates, we'll shoot for a 125 mph RADAR pass on video when the heavier battery arrives, hopefully late next week.

Jan 6, 2011

Tenergy battery company defaulted on the my order, noting they could not honor the price listed and  ordered from their website, paid for, and shipped (complete with a Fedex tracking number showing delivery today--all a scam info.  Amazing.)  So the RADAR test is delayed until I get the fast 4S I need to blow through 125 mph from a real company.
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