Intro to EDF’s Part 2

Maiden Flight 2Welcome back. This time we are going to start off by talking about the various classes of jets. Just like anything else in this hobby there are all sorts of flavors from which to choose. The main breakdown would be foamies and wood/composite. Both types come in all sizes and models; both scale and what are called ‘Sport jets’. Most are ARFs as this hobby has been veering in that direction for some time. There are still some kits around, but one must look for them. I, personally, enjoy building. Unfortunately I don’t have many options in the way of kits for jets.

Let’s start with the sizes. EDFs are broken down into what fan size they accept. Most are in millimeters, but there are a few inch sizes out there. By far the most plentiful is the 70mm and 90mm classes. The latest in micro technology is in the 25 to 30 mm class, while at the other end we are seeing fan sizes approaching 130mm in production. Cost is not linear. The cheapest option is the 64mm foamy. The prices are higher for both those that are smaller or larger than 64mm. The 64mm won’t have jet-like speed or performance, but it will be a jet.

The 70mm class is probably the most popular in EDF as the comparative cost is reasonable. The jets themselves rarely go to $300 as most are between $100 and $200, while the fans and motors can be anywhere from $20 on up. The 2 most popular fans are  the HET and Wemo minifan. The HET cost a bit less, but both perform within a few percent of each other. These 2 fans have been around a long time and are guaranteed performers, bulletproof, and can really take a beating. Motors for either can be had for under $70, or much less depending on the source. HET motors in the 28mm (70mm fan) class are almost the standard, especially for the price.

Kobra Fan 3-18-12

One real problem with the 70mm jets is the space available for equipment, especially landing gear. Most of my 70mm stuff is either hand launched or bungee launched. The wheels end up being too small for grass, and often the structure just can’t cope with the loads a rough field imposes in both takeoff and landing.

The 70mm size jets will use anywhere from 3s 3000s to 6s 4000s, depending on the size of the battery compartment. Typically one of these will use between 500 and 1500 watts of power with just over 1000 being a fairly good compromise. Anything more starts getting heavy.

The 90mm class is right behind the 70s in terms of volume. The larger size opens up possibilities for less than ideal flying fields, and they tend to give more presence in the air. Also, with EDFs being grossly inefficient compared to propeller driven birds, moving up to the 90mm size gains some of that efficiency back. It’s better to move a larger volume of air slower, than to move a small amount really fast. Going to the 100-120mm sizes gains that much more, but the investment goes up substantially.

Fan choices for the 90mm start with, again, HET and Wemo, RCLander, Stumax, Changesun, and a few other odd balls. There is a move towards the quieter fans that have more of the ‘whoosh’ sound over ‘screaming cat’. The tradeoff is sound versus efficiency. You get one, you lose the other. An exception is the Stumax fans, but they are in a price class of their own. With the Stus you get both.

To power an average 90mm model consider the weight range. I have seen between 6.5lbs up to 12lbs. Take 9lbs as the average, and you thus need somewhere around 2500 watts to get you going. Yes there are jets flying on much less (and much much more), we are just using an example. Anyway, you need to get 2500 watts out of your power system. A 6s gets you 22 volts, so the amp draw comes in around 110Amps. That is a bit on the high side. I try to stay with 100 amps max. The price on everything starts going up faster than I like past 100 amps and 8s. So on 7s you get 96amps, and 8s gets you 83. Remember, amps=heat and wasted power.  The higher the voltage, the lower the amp draw needed to get the same result. Every little bit of efficiency gain helps here.

SEFF 2012 042Now we are getting aircraft of a large enough size to be able to mount retracts with decent size wheels. Good for grass, and with more structure, the gear stay with the airplane thru much more abuse. The 90’s are easier to see, easier to take out a distance to line up for a burner pass, etc. Landings still can be tricky, but some practice is all one needs there. The detail work on some kits out now is pretty phenomenal and they can be had for the $200-$400 range. Keep in mind this is for an ARF with a glass fuse that is painted and finished, and the wings may be built up or sheeted foam. New kits are coming out almost every day so there is no telling what may be available in the coming weeks or months.

Next in line are the 100-120mm classes. Here the selection thins out and what is out there is getting in the pricey range in terms of the jet and the stuff you have to load into it. For example a BVM Electra kit will cost $1400 alone, and that is before $750 landing gear, $1000 fan unit, radio etc. Fan choices include Dynamax (from the old glow days, same fan, but different motive power), Stumax, RCLander, Tamjets, and a few others.

Having attended a few jet meets I have seen the various sizes of EDFs and their capabilities. One would think that with a smaller field that is rather boxed in the 90mm class is at the upper end of the range that you would want to try to fly. Most EDFs are capable of at least 100mph, and some very easily can approach 160mph. If you have a total of a half mile for a field, you can see that one runs out of room REALLY quick.

Another thing to consider: Grass fields are not the place to try and fly an F-16 with scale gear. You must choose a subject that is tolerant of rough fields by its nature. Wide stance landing gear, lots of suspension travel, larger wheels, etc are a necessity. With well manicured grass, a slight wind in the right direction, and a healthy power system, a 70mm jet can get off the grass. In this case choose a jet carefully to give you every chance for success with each attempt. I have had a few that would go one day and not go the next.

The above is not intended to sway one away from jets, but rather give some ideas to ensure the best chance of success. These lessons have been learned the hard way at times. The reward for the research and work involved in setting up an EDF is seeing it whizzing by on a low fast burner pass, pulling vertical, and watching it go straight up while seemingly not losing any speed. It is well worth the effort.

SEFF 2012 037A note on speed… The primary purpose of flying a jet is speed and smooth maneuvers. A nifty little benefit on some jets is flying them at the ragged edge of a stall in slow flight. I say some jets because a highly loaded sport jet is not the airframe to try and see how slow it will go. A light, airy foamy is perfect. There are some things you can do going slow that cannot be done while on the step at speed. Add thrust vectoring to the equation and you have a whole new dimension with which to play.

Next time we will talk about radios and some of the common mixing options needed to fly some jets.

~Jay Hot Pants

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