Adverse Yaw

Being new to the world of EDF jets, I need a little info. Having flown both IC and Electric prop planes for years, I am aware that you can obtain different power levels with larger or smaller engine / prop combinations. Also that two blade props are generally more efficient than 3 or 4 blade props. Now, do EDF units differ in the same way? Obviously the faster the engine will tun the more power but what about the fans themselves? Is it a case of more blades equals more thrust? And what about plastic fans compared to aluminium ones. Any and all advice will be very much appreciated.

erh7771

My Feedback: (30)

My Experience

Blades

More blades usually more static thrust (torque) because flying them with less than 90% FSA (or thrust tube closed down more than 10%) usually degrades performance....put those things in your airliners, A-10s or anything you want some good static thrust on and don't mind hitting speed wall which IMHO isn't that bad most of the time

Less blades = more tip whine (dentist drill sound) UNLESS they are REALLY REALLY well balanced. They also provide more efflux also so if you have a fairly light plane you can get it to higher speed over longer range (longer field) then this is your best bet.

Plastic vs Metal rotors

Plastic rotors (the blades) have held up pretty good so far, a wemo 90mm fan can be pushed into the 5000watt range and fly Skymaster F86's etc. with authority (subject term).

Of course the smaller the fan the less efficient on static thrust you get, a bigger fan you usually get more thrust for less watts.

Metal rotors can turn more RPM and provide better cooling but I'm seeing that out of the plastic ones so far. I'm a static thrust guy, if it aint putting out the static thrust it should be putting out efflux (air speed)...I see neither stat I'm not paying attention to it

The METAL fan "shrouds" (not the blades) provide decent heat sink at the cost of weigh but I've only seem them add a couple of ounces total.

Check Jack out at EDFHobbies for power systems...

Anything larger (big 6 - 8 foot jets) there's scubler, BVM, et al

buckarooo

My Feedback: (7)

Hello adverse yaw, My buddy gave you good answers, he knows EDF well. I'll chime in on a few generalities that ERH7771 hasn't already mentioned. In ducted fans. diameter is what regulates thrust. The larger the fan unit,the more thrust and the more power needed to make it work. Efflux or exhaust velocity is a trickier matter. It is more regulated by rotor and stator design, intake design, outlet tube diameter and power. Bigger heavier airframes need bigger fans. High thrust will help you get airborne and get good vertical performance. High efflux will speed you through the air. Some units give you both but generally speaking its the airframe that governs speed. A sleek low drag sport design will fly faster than a say a scale F-15 because of the drag factor.

One of the new factors effecting the EDF world are high blade count rotors (10-12 blades) They make a more realistic jet sound and are quieter than the old 4 and 5 blade rotors, however they are not quite as efficient so you run them at slightly higher power levels to make up the performance difference. Metal housings are made to dissipate the motor heat and also take the worry out of a plastic mount overheating, going soft and failing. There is a bit of a weight penalty but you treat that as just another design parameter for your project. As with all electric flight you can make power two ways. Low volt, high KV, higher current or High volt, lower Kv lower current. I always prefer the highest volts I can use with lower Kv's to achieve the desired RPM. Lower currents = less heat= less demand on the battery and ESC and more reliability. In EDF there are many ways to achieve the desired results. That's why we read forums and gather others experience's to help us make decisions. Plan your power system based on your airframe and your desired results, dive in and have fun. Its a great time to be a jet modeler.

Whats happening ERH. What you flying lately? I still haven't flown the Byron powered F-15. Work has been to darn busy. I need to order some batts. and its ready to go. I also have a CMP L-39 all done up. I put the dog on for her, retracts. gear doors and Shulman brakes. I am waiting for the fan unit now and it will also go this summer. Ah that is when ever we get summer. Its been cold and rainy all month. Take care.

Good luck Adverse yaw, keep us posted.


Dean

Adverse Yaw

Thanks to both of you, it's all starting to make a bit more sense to me now.