200mph with 70mm - Page 7 - RC Groups
 Nov 02, 2012, 08:44 AM Team White Llama! My schubies are the only things that held together at that power level. I would recommend staying away from cutting a 90MM rotor down to 70MM. Basically the center hub will be around 36mm(the size of a 90mm motor), thus you get less FSA. Basically means the fan will have higher efflux, but less static thrust. My DS-30's at 3KW routinely pushed +240mph calculated efflux speeds. Your real enemy at +180mph is going to be drag, not efflux(assuming you are pushing +3500 watts), and you are gonna need thrust to get over that. Thrust means FSA. It's much easier to get more thrust through a bigger fan(higher FSA), than through higher efflux. Case in point, 70mm fan@3000 watts= 7 pounds of thrust. 90mm fan@3000watts = 8.5 pounds thrust.
 Nov 02, 2012, 08:52 AM Victim of C.D.O. EDIT: Pete just talked about a lot of this while I was typing... So if you turn the 9 blade at 55K, it will pull more power than a 5 blade turning at the same 55K? That's something good to know. I for sure don't want to waste watts. Hate that. I also want to make sure that we don't confuse RPM with efficiency. If the 9 blade pulls more power than the 5 blade at the same RPM, but it puts out more IN-FLIGHT thrust than the 5 blade at the same watts, but lower RPM, then the 9 blade would be the winner. Stone, do you think a 5 blade will put up higher aircraft speed when at the same power level as the 9 blade? The 5 blade will for sure be at higher RPM. But I want to make sure the thrust comes along with those higher RPM. Last edited by murdnunoc; Nov 02, 2012 at 08:57 AM.
Nov 02, 2012, 09:01 AM
Team White Llama!
Quote:
 Originally Posted by murdnunoc So if you turn the 9 blade at 55K, it will pull more power than a 5 blade turning at the same 55K? That's something good to know. I for sure don't want to waste watts. Hate that. I also want to make sure that we don't confuse RPM with efficiency. If the 9 blade pulls more power than the 5 blade at the same RPM, but it puts out more IN-FLIGHT thrust than the 5 blade at the same watts, but lower RPM, then the 9 blade would be the winner. Stone, do you think a 5 blade will put up higher aircraft speed when at the same power level as the 9 blade? The 5 blade will for sure be at higher RPM. But I want to make sure the thrust comes along with those higher RPM.
The number of blades, and the pitch of those blades, really only determines when he fan loads up, that is assuming the blades themselves are aerodynamically sound. At 70mm, there is no "unwinding" effect, you can tell this by dopplering your setup on the bench, then calculating the rpm based on a doppler as it passes (average between the low line and the high line).

Basically, in terms of thrust, when it comes to EDF's, FSA(Fan Swept Area) is really all that matters. If a 2 blade and a 10 blade are put side by side, and both are designed decently well, and both have the same FSA, and both are running at 3000 watts, both will produce the same thrust and efflux.

The difference is, the 2 blade might be turning 80,000RPM, and the 10 blade might be turning 40,000 RPM.

I know the DS-30 rotors are expensive, but they really are worth it. I still have a couple, even though I almost exclusively fly multicopters now.
 Nov 02, 2012, 09:07 AM Victim of C.D.O. Hmm... That's a well-written "nutshell" Pete. Thanks So basically any fan that will safely hold together, as long as it's decently designed, will produce the same flight speed on the same watts as another decently designed fan. That the nutshell?? If that's the case, then I like the lower RPM setup, since it's easier on the ESC with my 6 pole motors.
Nov 02, 2012, 09:11 AM
Team White Llama!
Quote:
 Originally Posted by murdnunoc Hmm... That's a well-written "nutshell" Pete. Thanks So basically any fan that will safely hold together, as long as it's decently designed, will produce the same flight speed on the same watts as another decently designed fan. That the nutshell??

assuming they have the same FSA, at 70mm my experience has shown me yes, that's the nutshell.

Obviously though, there is a point at which to much pitch is to much. Case in point is the old haoye fans. They copied a BVM rotor, but the pitch was changed, they were pitched much to high at the blade root. This caused them to be really inefficient with most power setups (lee liddle found a good one for the 4.5" fan though). They loaded up at low RPM, but they just mixed air up, instead of increasing it's velocity. Obviously we can't have a 70mm fan that spins at 10RPM and makes 200mph efflux . The air would just go around it.

That said, these days there are alot of good 70mm fans on the market. The wemo and HET 70's won't take much more than 2000 watts (sometimes not even that much.), DS-30's are good up to 3000 watts and maybe more(right out of the box, I might add). Gary's fan's I haven't had a chance to test, but gary knows how to make things go fast so if he vouches for them, they'll be up to it. Also the aluminum heatsink effect can never be over stated. That is a really good benefit of the alloy fans.
 Nov 02, 2012, 09:32 AM Victim of C.D.O. Well there's no way that 2000W will do 200mph unless the airframe is an EDF hotliner (hat-tip to Jwmflying14) So running an out of the box plastic 70mm is not an option. I didn't think they'd be good for it from the start. I'd like to get some testing underway before I shell out \$100 for a DS-30 rotor that MAY hold up at 4KW. That leaves the alloy fan for ready-to-run 70mm fans. When it comes to cut down fans, I do for sure see what you're saying, Pete, about the 36mm hub wasting some power by unnecessarily decreasing FSA. The only plastic rotor I've seen that still has a 28mm hub is the E=flite 80mm fan. That one runs at 52K and 2300W at full size, and hasn't been run faster to find its failure point. Surely it would take 60K and over 3kW when cut to 70mm?? That's the setup I had on the Spear's attempted maiden, and it ran 58K in a static runup, but the fan broke on the post-launch crunch. Might the cut-down E-flite fit the bill for low blade count, high rpm? Or is there something I'm missing in this fan's design?
 Nov 02, 2012, 09:52 AM Team White Llama! It's possible that it might hold up, but my money is on the full carbon rotors. Lemme put it to your like this. On one of my +3000watt tests, the shroud distorted and the rotor hit the shroud. Most fans will explode if they really dig in, like this one did. Instead, it bent my motor shaft The full carbon rotor will just fine (although a PITA to get off! haha!). You might be able to get away with plastic, but alloy or carbon would be my choice. You are a smart guy though, if a cutdown eflite holds up, let us know. Could be a nice high power/lower cost option.
Nov 02, 2012, 11:58 AM
Need 4 Speed!
Quote:
 Originally Posted by murdnunoc Hmm... That's a well-written "nutshell" Pete. Thanks So basically any fan that will safely hold together, as long as it's decently designed, will produce the same flight speed on the same watts as another decently designed fan. That the nutshell?? If that's the case, then I like the lower RPM setup, since it's easier on the ESC with my 6 pole motors.
I wouldn't put my money on this theory!!!
Latest blog entry: original Y/A F-18 Kit
Nov 02, 2012, 12:00 PM
Registered User
Quote:
 Originally Posted by pdawg i wouldn't put my money on this theory!!!
+1....
 Nov 02, 2012, 12:11 PM Team White Llama! That has been my experience. Assuming both fans are designed well, and both motors are around the same efficiency, and both have the same FSA, they will produce near identical numbers. Feel free to prove me wrong.
 Nov 02, 2012, 12:20 PM Registered User So your saying, same motors, same shroud, and a well design 4 blade high pitch and 10 blade fan will produce the same numbers? Wattage would go through the roof on the 10blade. It is a very similar scenario to a 4 blade prop, it is more efficient for thrust and less efficient for speed. A 2 blade prop with 10 inches of pitch will be fast than a 3 blade prop with 7 inches of pitch. HOWEVER the 3 blade prop with 7, will most likely produce the most THRUST. An EDF is a prop in a tube, the ducting allows the unit to spin at extreme RPMS more efficiently, however pitch principles cary over just as a prop does.
 Nov 02, 2012, 12:37 PM Team White Llama! I'll put some number to mine, from Carl Rich's testing thread: 4S Wemo Mini rotor 16.76v-15.51v, 54.36A, 47.3oz T, 850W, 154mph efflux, 45,225rpm, 2898Kv under load 4S E-flite Rotor 16.71-15.4v, 57.6A, 49.6ozT, 889W, 44,475RPM, 161mph efflux, 2910 kv under load Pretty identical numbers, close enough to call even. Now 5S 5S Mini Rotor 20.97v - 18.8v, 77.65A, 67.8oz T, 1475.5W, 186mph efflux, 53,350rpm, 2814Kv underload 5S E-flite Rotor 20.95v-18.63v, 81.5A, 69.5ozT, 1531W, 51,900RPM, 190mph efflux, 2763Kv underload Same thing, Eflite rotor pulls 6.5% more watts, gives 2% more efflux, and 2% more thrust. Your biggest variable is actually the motor. Here's 2300W on an outrunner: 6S Mini rotor 25.15v-20.91v, 106A, 72ozT, 2316W, 55,250rpm, 202mph efflux, 2581 underload kv and 2150W on an inrunner: 6S Wemo rotor 25.09v-21.69v, 97.7A, 95.6ozT, 2151W, 59,350rpm 224mph efflux, 2714kv underload. Initial no load testing showed kv closer to 3200 The inrunner destroys the outrunner in terms of efficiency at this load.
Nov 02, 2012, 12:43 PM
Team White Llama!
Quote:
 Originally Posted by Jwmflying14 So your saying, same motors, same shroud, and a well design 4 blade high pitch and 10 blade fan will produce the same numbers?.
No, I'm saying same WATTAGE, with appropriate motors. I.E., two efficient inrunners that produce that same wattage. So 2000 watts on the 4 blade, and 2000 watts on the 10 blade will produce the same numbers, If both fans have the same FSA, and if they are both use a good design, with efficient motors. My example above this should show that the MOTOR selection is the most important part of an efficient system. If you select to small of a motor and just "cool it well", it's going to be burning watts off as heat. THat's why the outrunner pulls 150 more watts, while making less RPM, thrust, and efflux.

Quote:
 Wattage would go through the roof on the 10blade..
You misunderstand. I'm not saying the same KV motors will produce the same under both loads. I'm saying 10 blades just load LOWER than 4 blades do, usually(that's also dependant on pitch!). Basically it takes 3000 watts to spin a 10 blade to 55K, and 3000 watts to spin a 3 blade to 65K, but assuming both are using efficient motors and have a good design, the end thrust/efflux will be the same. Aslong as FSA is the same, in turns of power used, your numbers will be the same, if you use motors appropriate to the load.

Quote:
 It is a very similar scenario to a 4 blade prop, it is more efficient for thrust and less efficient for speed. A 2 blade prop with 10 inches of pitch will be fast than a 3 blade prop with 7 inches of pitch. HOWEVER the 3 blade prop with 7, will most likely produce the most THRUST. An EDF is a prop in a tube, the ducting allows the unit to spin at extreme RPMS more efficiently, however pitch principles cary over just as a prop does.
I'm not saying the same motor, I'm saying the same POWER. EDF's only care about FSA. Pitch only determines the KV of the motor you choose. It determines WHERE the fan loads.
Last edited by gundamnitpete; Nov 02, 2012 at 01:05 PM.
 Nov 02, 2012, 01:17 PM Need 4 Speed! Pete, I would just like to point out that your numbers are based on static testing which does not accurately simulate a model traveling at high speed. This is a common but big flaw when estimating high speed performance. Some of my fastest power systems intended for the Electrolyte actually performed poorly in static testing. Latest blog entry: original Y/A F-18 Kit
Nov 02, 2012, 01:28 PM
Team White Llama!
Quote:
 Originally Posted by pdawg Pete, I would just like to point out that your numbers are based on static testing which does not accurately simulate a model traveling at high speed. This is a common but big flaw when estimating high speed performance. Some of my fastest power systems intended for the Electrolyte actually performed poorly in static testing.
Intake and exit variables make testing on a unified platform basically impossible. And even if one used a unified platform, the results would be meaningless when you used that power setup in a different aircraft with different intakes and thrust tube. Either way, you can use a simple doppler program to see that fans don't "unload", or you can look at the plenty of eagletree data points about it.

Unless you have data to the contrary?