Where can i find information on scratchbuilding edf's. What kind of calculations are used to figure out how much power is needed, what size fans, what motors etc for a given size/weight of airframe? I'm interested in building a fairly large Vickers VC10 with 2 or 4 fans.</p>

If it were my project, I would decide what wing span I could live with based upon the carry capacity of my car. Personally I am limited to a WSof about 54 inch max if I want to carry the plane in one piece. A major consideration at this point is what air foil to use. My personal preference is the Clark Y which developes high lift a low speeds.

Once the wing span has been established I would calculate the area of the wing. I have scratch built enough planes to know, based on my building techniques (all built up balsa), the empty air frame will weigh approximately 1 ounce for every 10 sq. inch of wing area. Thus a wing that has 400 sq inch of area will be supported by an empty airframe that weighs about 40 ounces.

Then I would add the weight of a "typical" power system (fan/motor/battery/ESC) plus receiver and servos to arrive at the total all up weight. For good EDF performance a usable rule of thumb is 150 watts per pound of AUW. So for an AUW of 56 ounces (3.5 pounds) I would plan on a system that can deliver 3.5 * 150 or 525 watts.

I like 4S lipo batteries and I aim for a 5 minute flight. Using a nominal 3.7V per cell, a 4S is 14.8V and the current required would be 525/14.8 or ~36 amps. A 5 minute flight * 35 amps is 175 amp-mins. 175/60 =2.9 amp-hour x 1000 = 2900 mAmpHr pack. I would select a 3300~3500 mAHr pack.

Next I would get the specs on decent 4S lipo packs (no el cheapo packs need apply) that have a "C" rating of 25 and get the weight. Similarily get the weight of whatever fans I plan on using such as Wemotech MF480's or WM400's, as well as the weight of the motors, actual weight of servos and receiver. These are all available from the internet.

Recalculate the AUW and redo the power calculation. I prefer 70 mm fans and the web sites of either of the two mentioned will have thrust data available. If the fan vendor doesn't have thrust figures, I'd find a different vendor.

My Twin Biz Jet (380 sq. inch wing area, 45 inch wing span) uses a pair of Wemo MF480 fans driven by HET3S motors controlled by 36 amp speed controls powered by a single 4Sx3300 TP LiPo. At full throttle the motors draw 29 amps each or 58 amps total. With a 3300 mAHr LiPo, the combined power is 840 watts. With an AUW of 60 oz (3.75 lbs) the power loading is 250 watts/lb, well above what is needed for good performance. The total static thrust developed is 52 oz. so the thrust/weight ratio is 52/60 or .87. This is not enough for true vertical performance but is good enough for 70 MPH level flight. With throttle management, 6~7 minute flights are easy.

All of the above calculations will give you a starting point. You may want to use a bigger pack to help in balancing the plane properly as there is no sense in adding lead balast.

Hope this helps.

Here is a scratch built EDF i just made.

Here is a link to a video of it flying.
http://www.youtube.com/watch?v=lQXlZfGaalE

I'll give you the plans of my jet for free if you want (I'm stil working on them). I can also help you with yours. Gice me a week or so and I'll try and do some research on the jet you are trying to make.


"Mikey"
MikeysRC.com

Good info. Thanks.

I'm thinking about making it IMAA legal which would be 140 inches combined length and width. Is this even feasible as an edf? Here is some info on the VC10:http://en.wikipedia.org/wiki/Vickers_VC10I also have a 3 view in eps vector format so Ishould be able to print sections at the finished size fairly easily.

Just a quick look at the VC10 indicates a 67 inch WS x 72inch length does the trick at 1/26 scale. Guessing an average chord of 9", which results in about 600 sq. inch wing area.
I would guess 60 oz empty airframe plus 20% for heavier construction (more lite ply, bigger longerons, etc) - say 75 oz for the bare airframe, perhaps 120 oz. AUW.

If all about right, the VC10 would have a wing loading of 29 oz/sq.ft. Certainly not unheard of but she would require langing gear or a bungee launch. Leaning away from true scale, a larger average wing chord of 10 inches, ups the wing area and lowers the wing loading. Everything becomes more favorable - stall speed drops, landing speed decreases, etc.

With a decent thrust/weight of 0.5, the static thrust would be only 60 oz. The two MF480's on my TBJ develop 52 oz static. Four MF480's would give you almost .8 thrust/weight

I would say this is a viable project but AUW will be the issue and will require careful detail to construction. Yuor choice of air foil will make or break this.

Go for it, post lots of pix.