My Enterprise-E finally had its maiden flight today, and all went very well. The electric power system seems appropriately fitted and provides way more than enough thrust. There is a lot of control surface throw available so the first flight was a bit shaky for the first few times around the circle, but the craft settled down after I got accustomed to it. Three flights were put in and I brought her home unscathed - that's success in anyone's book!
January 3, 2015 Update:
A website visitor wrote to ask for details on the outboard wingtip weight box and trim tab (see new photo below, left). Also, I forgot to mention that I always remove the pins from hinges during installation and replace them with one full-length piece of music wire (see pic). Doing so makes covering much easier whether using Monokote or dope, since hinges can be aligned and glued before covering with wire in place. The wire is removed and surfaces separated for covering and then re-installed after covering. It adds very little weight and has the benefit of filling the hinge line gap for aerodynamic efficiency.
I wrote this article to describe my experience building BJM Enterprises' Enterprise-E electric control line stunt model. For now, photos are being posted mainly to get that part of the job done ahead of time. The model is very well designed and all of the laser-cut parts fit perfectly. Wood selection is very good for the most part. The only exception is that my fingers easily punched through the 1/16" sheeting for the fuselage bottom in front of the wing when handling it with the motor, battery, and ESC installed, so I replaced it with hard 3/32" sheeting. The provided building instructions are at best guidelines with no pictures or illustrations, and some parts of them referred to an old version of the kit that had built-up tail surfaces (now solid). I posted lots of photos of the building process below. This is definitely not a fledgling builder's kit, but then the Enterprise-E is a high performance model targeted to the experienced builder/flyer. Bill Mandakis, BJM's owner, is very willing to answer questions and make recommendations.
The recommended motor (KDA-A30-12M) does not fit in the location shown on the plans and had to have its centerline shifted down about 1/4". The recommended battery (11.1V, 3,3000 mAH LiPo) was a very tight fit and required hollowing out the top hatch in order to clear the outline while also providing a little clearance for airflow around the battery. I see that a different power package is now recommended by BJM, so maybe those components fit within the plan outlines. Also, while a rear motor shaft ball bearing was supplied, there was no bulkhead for it in the kit or shown on the plans so I designed my own. Bill says that new kits will include a redesigned fuselage front end that uses a laser-cut plywood structure for mounting the motor and bearing.
Details of the pushrod setup were not provided. I attempted to build everything exactly per the plans and instructions, but the supplied 1/8" music wire that was meant to connect to the flap and elevator horns proved to be too difficult to use in the confined space at the rear of the fuselage. In fact using the 1/8" music wire for the elevator horn connection would have required cutting a slot through the fuselage side and fitting a blister bump on the outside to contain the pushrod (not shown on the plans). Instead, I used a more conventional connection with standard pushrod material and an "L" bend with a nylon retainer clip on the elevator. I left the last 1½" of bottom sheeting off the very rear of the fuselage to permit easy access to the linkage. A carbon rod was used between the metal wire at both ends of the pushrod. The provided 1/8" music wire was used at the bellcrank end of the flap pushrod, but then standard pushrod wire with a threaded metal clevis was soldered to it and used at the flap control horn end.
A few pieces of balsa were missing from my kit, and BJM offered to provide them, but it was minimal so I just used what I had on-hand. There were some hardware items that, per the labels on the kit box, should have been included but were missing. Again, Bill offered to provide them, but I had what I needed on-hand and provided them myself. The retail value of the missing items probably would have totaled somewhere around $10, but the main drawback is the inconvenience.
My forward hatch hold-down method uses a pair of 1/16" music wire pins passing through metal tubing. It is fast and easy and weights almost nothing. A slight kink is bent into the ends of the pins to make sure they stay in place when pushed all the way in. Super magnets are nice, but they can be a pain, as can a threaded bolt (as shown on the plans). Note: For even easier access, I later replaced the wire and tubing with a couple rare earth magnets.
Please note that methods and materials shown in my photos might be different from those ordained by BJM Enterprises and/or RSM Distribution. I have developed my own preferences over the years. Any questions should be directed to those two parties.
The weight prior to covering (only the frame, no landing gear, motor, battery, or ESC) was 10.7 oz., and the final all-up, ready-to-fly weight tips the scales at 34.1 oz. With an advertised wing area of 357 in2, that works out to a wing loading of 0.0955 oz/in2 (13.8 oz/ft2. The MotoCalc analysis warns that at full throttle for an extended period the will likely overstress the motor (see full MotoCalc report at bottom of page), but I did not have the exact specifications for the motor, ESC, or battery for an exact analysis. RSM Distributors says the provided .25-size brushless motor is very efficient and should have no problem with their in-house tested setup (RSM25SYS) *** Note: They replaced the original ESC with a Mantis 45A model to make sure it would handle the power.
My Enterprise-E is now ready for its maiden flight. The center of gravity (C/G) is slightly forward of where it is indicated on the plans. Hopefully, I'll be back soon with news of a successful experience!
By Kirt Blattenberger
Completed Enterprise-E (top)
Completed Enterprise-E (side)
Ready for Covering: Top
Ready for Covering: Side
Elevator Control Horn Pushrod Connection
Flap & Elevator Pushrod Connections
Velcro Battery Restraint
Motor Rear Bearing Bulkhead Completed
Squaring the Firewall
Landing Gear Mount Epoxied w/Gussets
"Needed to Complete" Label
Sheer Webs for Wings
Clearance Notches in Ribs for Bellcrank Leadout Wires
Leadout Wires Ready to Be Crimped
Bellcrank Leadout Wire After Crimping
Completed Enterprise-E (bottom)
Enterprise-E Motor, Battery, & Electronics
Wing Star Layout
Ready for Covering: Bottom
Hatch w/Latch Tubes
(these have been replaced w/magnets)
Turtledeck Sheeting Installation
Tailwheel Mount Being Epoxied
Clamping Tailwheel Mount
Motor Rear Bearing Bulkhead Prior to Bearing Insertion
Motor Rear Bearing Bulkhead Installed in Fuselage
Enterprise-E Kit and Motor Boxes
Enterprise-E Kit Box Label
Balsa Included in Kit
Hardware Included in Kit
Enterprise-E Kit Contents
Flap and Elevator Control Horn Locations
Setting Position of Rear Rib Shim
Retaining Washer Soldered onto Flap Pushrod
Adjustable Leadout Guide
MotOpinion - BJM Enterprise-E
800ft above Sea Level, 29.92inHg, 56°F
Motor: Hacker A30 12M; 1370rpm/V; 2.7A no-load; 0.023 Ohms. Battery: FlightPower 3300 (20C); 3 cells; 3300mAh @ 3.7V; 0.0037 Ohms/cell. Speed Control: 45A ESC; 0.01 Ohms; High rate. Drive System: Master Airscrew 10x5; 10x5 (Pconst=1.31; Tconst=0.95) direct drive. Airframe: Enterprise-E; 357sq.in; 33.9oz RTF; 13.7oz/sq.ft; Cd=0.061; Cl=0.53; Clopt=0.66; Clmax=1.24. Stats: 215 W/lb in; 162 W/lb out; 17mph stall; 24mph opt @ 51% (54:05, 100°F); 26mph level @ 56% (47:48, 104°F); 2536ft/min @ 90°; -229ft/min @ -6.3°.
Possible Power System Problems:
The full-throttle steady-state motor temperature (359°F) is extremely high, which will likely damage the motor unless full-throttle is used very sparingly (even then, damage is possible). Current can be decreased by using fewer cells, a smaller diameter or lower pitched propeller, a higher gear ratio, or some combination of these methods.
Power System Notes:
The full-throttle motor current at the best lift-to-drag ratio airspeed (43A) falls approximately between the motor's maximum efficiency current (34.6A) and its current at theoretical maximum output (222.8A), thus making effective use of the motor.
The static pitch speed (53mph) is within the range of approximately 2.5 to 3 times the model's stall speed (17mph), which is considered ideal for good performance. With a wing loading of 13.7oz/sq.ft., a model of this size will have trainer-like flying characteristics. It would make an ideal trainer, for use in calm to light wind conditions. The static thrust (55.2oz) to weight (33.9oz) ratio is 1.63:1, which will result in extremely short take-off runs, no difficulty taking off from grass surfaces (assuming sufficiently large wheels), and vertical climb-outs. This model will probably be able to perform a hover or torque roll. At the best lift-to-drag ratio airspeed, the excess-thrust (37.8oz) to weight (33.9oz) ratio is 1.11:1, which will give very steep climbs and incredible acceleration. This model can easily do consecutive loops, and has sufficient in-flight thrust for any aerobatic maneuver.
This analysis is based on calculations that take motor heating effects into account. These calculations are based on mathematical models that may not account for all limitations of the components used. Always consult the power system component manufacturers to ensure that no limits (current, rpm, etc.) are being exceeded.
Posted November 28, 2014