I think there must be a relatively thing going on, on my monitor. On your side (of my monitor) planes fly with postage stamps for wings. On my side I need big honk'n wing area and something slow for my skill level (thinking about claiming I'm a novice but that might jinx my whole world). I hope to go flying today... where is that wood table to knock on?
So when I brave an EDF plane, I'm thinking of an F-104 sibling... maybe a U2.
Goes well with my black site background.
So... questions
- Do you always put the EDF in the same relative spot (front/back) or do you move it around based on plane?
- Have you ever done any tests to see if there are any advantages one way or another... EDF at the rear and sucking through the fuselage, or EDF at the front and blowing through the fuselage?
- I was thinking of putting the EDF under the tail surfaces (U2) and although the primary inlets would be the scale ones, that it could also have spring loaded inlet doors just in front of the EDF (still under the tail surfaces) that would open up when the vacuum pressure was high like at take-off? Any thoughts / insights?
- What kind of thrust loss do you get from your EDF on a stalk test versus being in a scale body like this?
- For this '21, where are you going to put all the hardware? It seems to me you have one long tube that needs to be un-obstructed and you don't really have any place to put it. Or are you talking a fairly large plane where the battery can sit up in the turtle back and be out of the flow?
- What kind of thrust to weight ratio are you typically dealing with?
- I have a pair of cheap 70 EDF's (4S)... Do your designs scale (say 70/64 worth) or is the internal structure optimized so tightly for the 0.4 mm walls that scaling of 109% would fail?
Well-designed wings will generate a ton more lift than most people expect, and deltas basically never stall so you can high alpha them in to land. My F-104 has a stall speed not that much faster than a scale warbird with the flaps down, and this one will have a stall speed of about 25mph When I'm talking easy to fly, I'm talking easy to fly for model of a 1950s/60s supersonic fighter. This plane is by no means a trainer, but it might be a second EDF (I'll let you know once I fly it). I'm actually planning on taking my 50mm F-104 and changing some things around to make a printable U-2 as well since the early U-2s were essentially heavily modified F-104s anyway.
Now for the questions:
1. EDF placement varies. Mostly I try to position it to attain a good CG with a reasonable battery position
2. EDFs are most efficient with a short thrust tube, so it's good to place them as near the back as reasonable. That said, efficient ducting is much more important, so avoiding sharp corners and sudden changes in area will go a long way. The outlet should typically be about 90% FSA so there needs to be some length for the area to decrease smoothly. The ducting in this MiG 21 is terrible and should not be used as an example. The ducting in my F-104 is better, but still has room for improvement, largely because of trying to package 6ch worth of electronics into a 50mm EDF. The ducting in the FT Viggen is the golden standard for foam board, in my opinion.
3. In the U-2 I would generally expect the EDF to go about halfway between the wing and tail or maybe a bit more forward, largely for CG reasons. So long as the plane will balance there is no reason it should not go more aft, but under the tail planes is probably too aft (see 2). Spring loaded cheater inlets work and look really scale compared to other types of cheater inlets. I personally find them too mechanically complex for too little benefit, especially when NACA inlets work efficiently through the whole flight, but especially on larger planes it's a matter of personal preference.
4. Losses can be anywhere from near zero to as high as 50% with very poor ducting. I expect the MiG 21 to lose about 10-15% thrust when static and more when at high speed because of the duct design. In contrast, my F-104 lost a similar amount of thrust when static, but would actually gain some thrust back past about 80mph (if it could get that fast in the first place) because of much better designed ducting. When I've really messed something up, I've seen losses as high as 40-50%, but that is rare, even with horrible ducting design.
5. Most of the hardware sits on top of the duct on a platform under the canopy. You can kind of see where the canopy will be accessed in the pictures I posted. There is barely room for a 2200 mAh 4s pack, a receiver, one servo, and the mechanism to steer the nose gear. The nose gear strut and wing spar extend through the duct and will just result in some losses. The elevon servos aren't modeled yet but will go right below the horizontal stabilizer in the fuselage. I expect the EDF on this plane to mostly breath through the cheater inlet anyway since the nose inlet is far too small for the size of EDF and is mostly there for aesthetics, so the losses from the strut and spar protruding through the inlet should be minimal.
6. Thrust to weight ratio varies hugely. I have an old Phase 3 U-2 that can't have a TWR more than about 0.3, while my 80mm EDF speed-optimized F-104 is a bit over 1. This plane will have a TWR of about 0.8 and my 50mm F-104 flew with a TWR of about 0.6 on 3s, although that should exceed 1 on 4s.
7. This model should probably scale up just fine. There isn't that much to go wrong with the print. The only concern would be the wing structure printing separately from the wing skin since there are some small clearances to make sure the skin turns out smooth, but that shouldn't be too bad if you turn off retraction and print it one part at a time which is the recommended method anyway. I tried slicing the nose in Cura scaled by 109% and it didn't seem to cause anything to go wrong. I cannot speak to how the structures would stand up on a larger model, but I don't think anything would go too badly wrong there either. You would definitely need some shims for the servos and some of the non 3D printed parts (CF rods mainly) would be weird sizes like 11mm as opposed to 10 for the wing spar. Other models could have substantially larger problems especially when scaled up by more than you're talking about.
