|Wing Area:||612 sq. in.|
|Wing Loading:||28 oz/sq. ft.|
|Servos:||Expert SL451, plus a CS100 retract servo|
|Gearbox:||MEC SuperBox (60/12)|
|Prop:||APC Sport 15x11x4 trimmed to 12.5"|
How do you pick your next model? Usually something catches my eye; I saw it at the field, in an interesting ad, or maybe a photo of a full scale. In this case it was two of those!
A while back, I threw the “Hobby People” sales catalog in my briefcase, and then flipped through it during lunch the next day at the office. Something about the ModelTech Mustang caught my eye; a quick glance at the photo hanging in the hall confirmed what I thought - it's a model of my boss’ plane! Suddenly the ModelTech lept to the top of my model list! Mr. “Bill” Bruggeman flies Short-Fuse Sallee out of Janes Field/Blaine Airport in Minneapolis, MN to shows and events during the summer – wow what a sight to see; and, oh that sweet sound.
Watching Short-Fuse Sallee taxi away and take off, behind that big beautiful 4 blade, got me to thinking – no two bladed prop on this conversion! After all, one of “electric’s” advantages over glow is the ability to more effectively match a prop to the mission.
As with most conversions, the power system needed to be configured, or at least envisioned, early in the process. Step 1 – how many cells? A lot of “Forty” ARF & kit conversions have been done over the past few years (I’ve done 3 Forty ARFs prior to this, and a couple since); cell counts have ranged from 9 to over 20; the average being about 15. So I did my initial guesstimate for a cell count range of 14 to 18; thinking 16 – about 560 watts at 35 amps. After doing these mental calcs I decided to “think on this in the background” while I built the kit. After all, it would not be too hard to get a motor and speed control. Also, I’d have a better idea of the weight; I never seem to get good results adding up the weight of the parts (even adding in a pack of epoxy and bottle of CA) to predict the all up weight (AUW) of a conversion.
More thoughts about the motor/power system, looking at the kit I noticed that the engine mounted to “hardwood beams”, built into the nose, under the fuselage top, and behind the spinner-ring; something a bit different. I wondered how this would affect things.
First I checked a generic “05/550/S500” ferrite can motor and found that it just slipped between the rails. I put a clam shell mount on the motor and it no longer fit nicely. The rails needed some modifications to get the spinner centered. This made a direct drive solution a challenge. Also, it was a bit tight (only 3.6") between the firewall and spinner ring; not a lot of room for a planetary gearbox. An off-set drive system looked like the best choice. I tried a long “05 can” motor on a Master Airscrew gearbox and it slipped right into the motor space.
I weighed Short-Fuse Sallee at the point where it could go either glow or electric – add the tank & fuel, engine servo, and engine for glow; or flight battery pack, speed control, and motor/gearbox to fly electric. 75 ounces, a bit heavier than I would have guessed; but that included retracts with servo and prop & spinner -- use 18 cells. My estimate at this point for the ready to fly weight: glow - 112 ounces (7 pounds, dry - w/o fuel) and electric - 115 to 140 ounces, depending upon the “cells” and motor choice.
Checked my guesstimate against some prevailing rules of thumb using 8 pounds (the mid point of the weight range), yields: 34 sq. in/cell, 80 watts/pound, a wing loading of 30, and with a wing cube loading just under 15. I compared this to the Mustang conversions I found during a literature search, and it put my Mustang on the high end (near some of .60 conversions) rather than the low end (10 cell & slopers), but manageable.
With the cell count set, I started looking at motors and drive systems. Direct drive and planetary solutions fell out as described above; so I started looking to 600+ watt offset solutions. Belt and gearbox systems were surveyed; the belt drives looked to be more of a challenge for this conversion, the gearbox easier.
Model Electronics Corporation (MEC) made some very nice high watt light weight gearboxes; the Monster Box and the Super Box (2oz.) which fit – “between the rails”. Other features of the MEC box were that it fit any “05” with the standard 1”/25mm bolt spacing and the gear ratio was easily changed, even at the field. Dozens of ratios are available by changing either (or both) the spur or pinion. Their boxes used a Delirn spur gear (40, 50, 60, or 70 tooth for the Super Box) and metal pinions. MEC has pinions from 10 (1/8” press on) to 29 tooth (T); most Local Hobby Shops have 12 to 30+ tooth gears (standard 48 pitch) for 1/8” shafts available in the R/C Car area. “Bored out” pinions (3/16” & 5mm) were also available from MEC though only for 14T and higher.
With all that gearbox flexibility, just about any 18 cell “05” motor could have been used, all the better if it had an 1/8” shaft.
Motor technology, availability, and price have recently been moving in our favor; a brushless motor now costs about the same as a traditional Cobalt brushed motor. Speed control prices are falling as well; so with the efficiency and life advantages of a brushless motor it would have been hard not to go brushless.
Aveox came through with a surprise, a motor about the size of a long can S400 and an 1/8” shaft; also quite light at 5.6 oz. What about the “05” mount? A fix for that as well; an adapter plate, part number 9500029-00, allows the 1” diameter motor to bolt up as an “05”. That’s a lot of power for such a small motor? Aveox’s 2700 series of motor incorporates carbon fiber wrapped high temperature magnets, running at better than 90% efficiency. Compared to either of the classic solutions, Cobalt or brushless, the 27/39/2 wasted less power such that the motor had less heat/watts to dissipates per square inch or per ounce – it could handle the power. The speed control was also from Aveox, their new SH-48, good for 10 to 32 cells and 40 amps continuous.
Cell count, gearbox, motor selected; now for the prop. Should I use an 11 inch prop as recommended for the 2 stroke, or maybe a 13” like the 4 stroke? Bigger, smaller; how about a 4 blade, sure looked good on the full scale. I set Short-Fuse Sallee on the mains with the tail raised so she sat level (lift off position) and noted that the largest prop that could be turned was 13”; without much clearance, just like the full scale P-51. 630 watts in, max of 13 inches to get the watts out?
Lots of “prop myths” abound; trimming/cutting a prop changes its pitch, pitch speed limits flight speed, less blades are more efficient, to site a few. Most were developed based on tests with engines that mask the prop’s performance with its unique influences; such as temperature & altitude effects and torque versus rpm curves. Kind of confusing, so I called APC and spoke with Fred seeking some advice. First myth killed, Fred said trimming a prop doesn’t change the pitch since its on a helical; OK, but I know that an 11x6 trimmed to 10 inches loads up more than a 10x6 – the blade geometry is different! The blade geometry changes greatly between APC’s various lines of props (Slow Fly, Electric, Glow, Pylon, etc), each being designed for a specific power source. Some 4 blade props are on the way and there's still lots of confusion in my mind. I subscribe to a simple prop matching philosophy – put on the largest diameter you can turn and then pitch it for the amps you want! It seemed to work fine.
With the 4 blade 15x11 in hand, I compared the blades to some of my “Es”; but in the end I just trimmed it to 12.5” where it looked best against the full scale proportions. It was time to run it up.
During the first low power runs (up to 250 watts in) the 4 blade seemed to load up the motor like an APC“E” 15x10; not far off the multi-blade rule of thumb where D of the 4 blade is 87% of the two blade – 12.5 divided by 0.87 = 14.4. I measured thrust to be the same as well. Now for some watts – 18 cells and 33 amps, its loaded up like an APC“E” 16x10. Using integrated measured velocity as a thrust comparison, the 4 blade did more than the 16x10; and its 10 mph faster too. My Kestrel 1000 measured 40mph off the back side of the four bladed prop and pitch speed for that rpm calculates to 60mph; Short-Fuse Sallee has been clocked at 73mph in level flight. The 4 blade also ran a lots smoother than a two blade and looked great spinning those yellow tips!
18 Cells can be an expensive “custom” proposition. The 1st E-flight pack I ever bought was from a big name battery source, and was welded. The 2nd pack was a custom soldered and zapped pack, which worked as advertised. But mostly I’ve used packs from a handy source, the local hobby shop, typically welded 6 & 7 cell (RC Car packs “Two Six Packs”), with an 8 turning up every once and a while. All have worked fine for the mid 30’s; I wouldn’t push a welded pack over the low thirties as the tabs get hot and can melt the plastic end caps & shrink wrap.
If left to my own devices, I probably would have gone with 3 six packs; but Pete at MEC gave me a different idea, how about some Zapped cells in Solderless Power Tubes (SPTs)? I had several packs (both welded and soldered) with bad cells staring at me, easily fixed with some solder; hmm… lets give them a try. I got some cells and SPTs, worked up a configuration for one of my other Forty ARFs as a test bed, and a set for the Mustang. At this point it was really hard to go back to my other packs; Pete’s Zapped NiMhs are great! They offered about 3 to 4 more amps than a welded pack of NiCds. Some of those extra amps come from the Zapping, and some more is from less loss between the welded cells; as good end-to-end as soldered, probably not for F5B, but plenty good for me. In addition to zapping, MEC also offers Pasting; sliver paste is used between the cells which boosted a pack’s performance by another 4amps. I measured a gain of better than 7amps between Zapped and Pasted cells in SPTs over welded NiCds via my Whatmeter on my 12cell 40 ARF!
Cooling Air: The only wood that was cut different from the glow version was a hole in the aft black stripe on the bottom of the radiator scoop (not too far from full scale’s exit ramp) and some ¾” holes bored through the firewall. This allowed cooling air to pass through the fuselage over the speed control and then past the battery pack(s). A small fan/blower was used to test the flow path, and was also used to cool the packs post flight prior to re-charging.
Motor & Gearbox: A S400 clam shell clamp and some hardwood blocks were used to hold the motor off the beams as shown in the picture. Just as was done with a glow engine, the spinner back plate was used to make the final alignment. Washers could be used for a minor adjustment when changing the gear ratio. The second picture shows the completed motor/gear box installation.
Radio Gear: The rudder and elevator servos were installed using 3/8”x1/2” balsa spanning the width of the fuselage at their intended location, rather than the typical plywood servo tray. This provided more room for fore and aft movement of the battery to get the proper CG. The receiver was placed below and beside the elevator servo and was powered by another clever device available from MEC – UBEC, Ultimate Battery Eliminator Circuit by Kool Flight Systems. The UBEC was a special purpose designed switching regulator (frequency chosen not to interfere with ours) that was good for 29 cells. This photo shows the harness and schematic I used with the UBEC.
This arrangement afforded a lot of capability: RX on/off, battery charging, motor arming, and installed current measurement without disconnecting any wires. Power was only applied to the speed control (motor) when something was plugged into to the Sermos fuse holder, a key measure of flight line safety.
To measure current, the wire loop pictured was used as a 1 milliohm shunt, so that each millivolt read via a voltmeter equals 1 amp. Either a solid wire link or a fuse (40 amp pictured) was used to arm the motor/speed control just prior to a flight.
The RX switch, charging jack and fuse holder were mounted on the starboard side of the fuselage as pictured here.
Battery/flight pack(s): The 18 Zapped cells were split into an 8 and 10 cell packs, weight – 40oz complete. A battery tray was made from 3/8x1/2 balsa spanning from just ahead of the servos to the wing mount former. The final pack(s) location was chosen for the proper CG. Interestingly enough, Short-Fuse Sallee balanced well w/o the packs; so they straddle the CG The photo shows the pack(s) installation, and also shows the servos, UBEC & speed control along the starboard side fuselage top.
The 1st moment of truth...
Other static Whattmeter data: 20 volts, 700 watts, and after 1000 mah the motor is warm, not hot, keep your finger on it as long as you’d like.
Winter came early, delaying the maiden flight; mostly pushed in by some strong winds, so Short-Fuse Sallee spent a few weeks just making trips to and from the field. Fortunately, we’ve only had a couple of light snows; but the runway still had snow on it when the time came.
The snow was light and fluffy, so even at 1” it didn’t present much of an issue; Short-Fuse Sallee was off the ground and headed skyward in less than 75’!
I flew a couple of circuits around the field at about 2/3 throttle with the wheels down; then wheels up. I pushed the throttle forward and she quickly accelerated, and climbed nicely. I found the Sallee to be very responsive and quite light on the stick.
She slowed fairly well on landing; I used another one of those electric tricks, a big free wheeling prop (brake disabled) kind of acted like flaps. Touchdown and rollout were very nice. Short-Fuse Sallee easily handled the 8 pounds & a few ounces; never felt “heavy” and floated rather well when the prop was just turning over.
While getting ready for our annual E Fly-In, I decided we here in Minnesota should see what the fuss was all about with the new "big" LiPoly cells. A set of Thunder Power TP7800-5S4P (2 & 3) was soon on the way.
A quick calculation suggested that the pinion needed to be changed from 12 teeth to 13, since the cell count was now going to be about 15 instead of 18 - a very nice feature of the MEC Superbox. A quick run-up proved that the pinion change kept the amps right at 33, so I lost no extra watts than needed when reducing the cell count.
Performance with the LiPos was fantastic! (FLIGHT VIDEOS!) They also allowed Short-Fuse Sallee to lose a little weight, 12oz (3/4lb) -- down to glow weight! -- at 7.5#.
But the best feature of the LiPos was peace of mind and convenience. I know for sure that I’ve got plenty of capacity left if I need to “go around” after my normal 6-8 minute flight of strafing runs, victory rolls, and aeros. She still had enough for a 3rd flight from the charge I put in when I got home from my last day of missions.
Your boss's aircraft, how strange and cool!
That thing sounds awesome on the video. Apparently a large part of the sound quality of a real P-51 must be from the four blade prop. You can really hear the prop on the model. It just needs the bass note of the exhaust to get the rest of the way to scale sound.
I've noticed that large electric planes typically sound more scale than gas powered planes. Gas models don't sound as good because the overpowering model engine exhaust drowns out the proper sound of the prop.
Sweet aircraft. I'll have to start saving money for something like that.
I'm getting ready to convert two old Dynaflite .40 size funscale kits, the P40 and P51. Can't force myself to buy Lipos yet, have a 16 cell 1950AUP pack and AF 25 geared. The old Dynaflite kits were very light with copious wing area and high lift airfoils that flew like trainers on glow. I'll be happy with 5-6 minute flights given the "knee knock" factor besides with my 66 year old bad knees etc, I can't stand for very long anyway.
I'll be looking for recommendations on available 4-blade props/cut-down mods.
Using the data from the AF site you’ll pull 29ampd w/11x7; that’s 4minutes. Or, 35amps w/12x8; 3.3 minutes.
With throttle management (w/ a warbird??) that might be getting close to 5+.
Without a variable ratio drive (i.e. your geared 25) you won’t be able to consider a 4blader; go with AF’s suggestion (or APCE equivalent for 33amps).
Short Fuse Sallee (2) w/AXI, VarioProp, and more Watts!
I Retried Short Fuse Sallee at the end of the flying season last year, it was always a thrill to fly!
Pulled out the RC stuff (etc.), cleaned it up a bit, rigged the prop to spin, and gave it to Mr. Bruggeman to hang on display.
As the winter hung on, I got the urge to do another Warbird and looked at several, the H9 Corsair & ModelTech P-47 were at the top of the list.
But, I decided to do a SFS-2!
SFS-1 looked good & flew great; but, wasn’t blistering fast – 73mph straight and level, 80 via a strafing dive – measured by radar and sound recording/Doppler analysis.
Not bad for the 33amp limit of the motor and 5S!
I wanted to see if I could get more speed for SFS-2.
Don’t have a need for 2S packs; but, do have a few 3S packs, so I decided to go for a 6S set-up.
Also, I still wanted to have a 4 blader!
So, the search for a motor began, along with a study on how to use a VarioProp ( http://www.ramoser.de/ ). Motor selection was far easier than the later!
For the motor I thought why not give up the gearbox and try an Outrunner as this seems to be the technology of choice these days. Though, I did look at some motors that come with planetary GBs too.
Based on my calculation for 6S and a big 14” four blader, I needed to find an Outrunner with a Kv of about 300 – and it has to be able to fit under the hood.
Two Outrunners have the required Kv: the AXI 4130/16 (Kv = 305) & E-Flight BL 110 (Kv = 295).
I also like to get my stuff via my Local Hobby shop and they had these two motors! So, I got to look at them up close and personal and see if they’d fit, as I was also picking up SFS-2.
The AXI is long and skinny; while the E-Flite is short and fat (& 3oz heavier) – won’t fit – got the AXI.
The chart below shows the electrical comparisons of the two; it includes Kt & Km which are be used to describe their electrical size.
Don’t know exactly why, must be the difference in winding configurations; but, all of the parameters are quite different except for Kv & Rm. Rm is always questionable -- are they listing a single winding or two in series as they should! Also, wonder what voltage was used to get Io?
The difference in weight and size of SFS-1’s set-up and the AXI is striking:
Aveox 27/39/2 - 1” dia. & 5.6oz (add 2 for GB = 7.6 total)
AXI 4130/20 – 1.9” & 14.5 (double the weight)
Because of the extra weight in the nose, and based on SFS-1 I decided to put the tail feather servos in the tail, to keep the flight pack(s) near the CG. The servos are shown in the tail and the LiPo packs in the fuselage very nearly centered on the Fuselage’s CG – note the wire rod.
As posted here ( https://www.rcgroups.com/forums/showthread.php?t=672852 ) I always find the CG of the Fuselage after balancing with the wing off; lets me finalize the position/mounting of the flight packs as well even being able to checkout and accommodate different pack types and geometries. The picture below shows the NiMh packs that were 1st used in SFS-1 – nearly centered on the rod allowed me to go LiPo very easily – used a 2S & 3S pack of either TP7800s or TP4200s.
Motor Installation – a lot of thinking and work!
I picked up the AXI Radial Mount System (RMS) when I got SFS-2 and the 4130; had planned to mount the motor off the firewall using nylon spacers as shown in the pictures below.
Here’s a reply I got from VarioProp:
“Dear Mr. Blanner,
For your P-51 I would advise not to take this aluminum shaft. Please mount the motor on a front plate. Because fixing the AXI with a back plate is not stable.
Ugh, I can’t really speak to “stability”, but the hole in their prop hub is 8mm while the RMS prop shaft is a lot bigger – 12mm. I actually had asked if either VarioProp or AXI could make a RMS prop shaft at 8mm?
OK, so now I have to hold the motor from the front of the plane!
The Aveox mounted easily in SFS-1, as shown in the photo below, made use of the kit’s “rail mounts”; I had planned to cut them out to use the RMS.
There’s no way the front ring could be used to solely hold the motor, and beside that I had to “hog out” the rails (spaced at 1-3/8”) for the 2” diameter AXI.
Hoped I could use RMS parts and have the nylon spacer go past the motor – see photo w/motor nested inside the spacers!
Well, the RMS isn’t big enough. E-Flight sells the cross ( + ) portion of their RMS separately, I hoped that it might do the trick – NOT, just like AXI’s.
Maxx Product motor mounting system for their Himaxx’s Outrunners does what I wanted ( http://www.maxxprod.com/mpi/mpi-29b.html#Ring ); photo below compare’s the MPI & AXI crosses ( + ).
Before cutting out the rails, I thought I’d try and make a mount that would bolt to the rails and interface with the front ring. So, I laid out the parts using a CAD Lite program called Draw-It ( http://www.postersw.com/ ); it worked very well.
The aluminum bracket turned out pretty nice; so. I’ve used it and can always cut out the rails and use the MPI mount if needed. The hard wood along the sides of the mount were added just for a bit more security.
Its obvious that you need a hub, some blades, and a way to mount it to a motor – and Oh Yea a P-51 spinner too!
My motor shaft is 6mm, they have a collet adapter that fits; actually two – the “long” lets you use a spinner back plate (about 5mm thick) and still use their trimmed washer (access to the pitch adjusting screw).
For the spinner, and not too obvious, the front end of the collet accepts a 4mm screw – just gotta find one long enough!
The other photos below show all of the parts are laid out and a progression of mounting the prop and spinner.
Note: as with SFS-1 I used a CB Tatone white P-51 spinner a lot more to scale than the stock chrome odd shaped spinner – got 2 anybody need one? Also, the plastic spinner is a lot easy to cut for the 4 blades. The white tape on the back plate and the black mark are used to key the spinner and back plate for good a balance.
Ran her up on a set of TP8000s using a Castle Creations P-60; with the prop at max pitch got 51amps – a bit more than I expected.
Had intended to use the Aveox SL-48 (run the motor great); but its limited to 40amps continuous. But, I really wanted to get to a lot of watts; so I’m not going to break up the Aveox set.
Got a really good deal from Hobby Lobby on a Jeti Opto 70 w/programming card; but the pilot figure that all that I really wanted was out of stock – all that is left to do is to install pilot and glue on the canopy.
Putting an Outrunner that could use a big 4 blalded prop into SFS-2 was a lot of work! And a lot of added cost.
The Varioprop is a very nice system; but it too required a lot of “learning” and is expensive! A set of blades is about the cost of a typical multi bladed prop; and, then you need at least a hub - about twice as much as the blades. Just for fun I aslo got a 2nd set of larger diameter blades; plan to try them pitched less to pull the same amps and see which gets me more speed – either will give me a lot more watts than SFS-1
This is a great 5S conversion! Use what ever motor suits your tastes, and you will enjoy a very nicely priced and good quality P-51.
Will post more after the maiden flight.
Last edited by jrb; Apr 21, 2007 at 09:03 AM.
Just swapped the AXI (Kv = 305) to E-Flite Power 60 BL (400); and will be flying via A123-6S.
This shed 18oz compared to using two TP8000-3S4Ps.
Only ever used just a bit over 2000mah for a typocal 6minute sortie.
Also, upgraded the arming/charging system to my now standard APP/Sermos block, which allows a 20amp recharge of the A123s via my MasTech -- 7minutes!
With prop "fully" pitched (14.5") the new combo pulls 60amps; will likel lower the pitch.
Also, I'm using the a mix switch & throttle curve function for an in flight throttle limit works well!
Quick test last night of my Six Packs.
200mah out @ 2amps via my light load, then charge via TP-1010C & TP210V @ 8.25amps.
Hooked up to my BA resistor; 40amps intial, about 37 in 10seconds when displyed slowed & reading was taken.
0.2 volts diference from best to lowest; order of finish: black/Robotic Power Solutions (P-51), white/Radical RC (Sundower 50), MEC loaded SPT, recently tapped (FreeStyle), and last self loaded w/o taps SPT (SNJ).
PS: 1/4 Cub use two DeWalts (just w/o plastic) for A123-10S2P; 1/5th is A123-5S2P -- they go for nearly 20 minutes, charge in same langth of time.
Hello, jrb. I'm nut sure if you still visit this thread but I thought it worth a shot to ask a question. I'm putting an AXI 4130/16 on a Royal Kits P-51 with a 4-blade VarioProp (identical to your setup). My concern is the comment from Christian regarding stability in the redial mount configuration as this was my plan. I may be asking you to speculate but do you happen to know why? If not, was this ultimately the reason why you didn't? Further, I'm not a big fan of collet-based prop adapters. as I've seen them depart from the plane. Have you had this problem with your P-51?
I've had no issues with my mount and using the collet adapter.
Even changed from the AXI to an E-flite Power 60 BL on 5S LiPo.
I've notice noisy bearing with 2 bladed props when mounting the other way around.
I also put the AXI "fan" on to aid in cooling; can only be used w/collet.
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