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Sep 19, 2007, 11:23 PM
Lee Liddle
Knife Liddle's Avatar
Thread OP
Build Log

Quest 200mph, ...maybe 250mph


Note: I said journey towards, not to. 200mph seems to be to RC EDF pilots like the sound barrier was to full scale aircraft in the 40`s. Some kind of wall out there. Those who get too close instantly fry motors/ esc and batteries. Sometimes airframes survive.

This is my first scratch built EDF airframe. Goals were light weight, low drag and strength enough to handle a 1200w powered mini fan.

The wing was framed in 3/8" balsa, sanded to an airfoil shape and then sheeted with 1/16" balsa. The fuse, which is little more than a cowl for batteries and rx, has a floor of 3/8" balsa which was tied into the wing structure, then an upper deck shaped from 25lb extruded polystyrene (blue insulation foam. Then the whole thing was covered with monokote.

I test flew it without motor or batteries as a glider to sort out the CG, then positioned everything to achieve that balance point. That turned out to be a good method because the first flight was successful.

I have since added the cowl to the fan unit and have sorted out some programming issues with the esc, so it`s ready for flight #2.

My plan is to verify speed with an Eagle Tree unit in conjunction with radar and Doppler sound analysis.





The power system is a mini fan, an HET 2W-25, a Tower Pro 65a esc and an 8s a123 battery (22.4v) (up to 60a).

The system is producing 1600g of thrust at 1100w and 50,200 rpm.
I don`t have the Eagle Tree yet so I cant measure the E-flux speed, but is seems pretty fast and light stuff in the shop has been rearranged pretty well.

I plan to fly it again in the morning before work, and if all goes well, post some video this weekend.
Last edited by Knife Liddle; Jul 13, 2015 at 01:28 PM.
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Sep 20, 2007, 01:29 AM
SoCal, Year Round Flying!
bmiller's Avatar
My VERY rough calculations show me you need,
58,000rpm with a 80%FSA exhaust duct for the efflux to reach 324kph(200mph)
50,000rpm w/80% is around 280kph(?mph) efflux
54,122rpm w/75% is 325kph(200+mph)efflux
50,208rpm w/70% is 324kph(200mph)
Mind you this is the efflux speed, not the speed of the aircraft. also it does not take into account ducting losses or fan efficiency in-flight.
80% is an ideal exhaust size for the minifan under most conditions.
70% may create a stalled fan situation and almost certainly a loss of efficiency but I'm not sure of if the speed of the incoming air (ram effect)will help combat this.
Bruce
Sep 20, 2007, 01:55 PM
Lee Liddle
Knife Liddle's Avatar
Thread OP
Thanks Bruce, all of that is good info. I`ve added a cell,,, 9s now,, and the rpm is up to 53k. Sounds like if I reduce the exhaust some, I can get near 200mph efflux speed.

To be clear, this is just an early step on what I`m sure will be a long trip.. I don`t even know if a 70mm fan can push a plane past 200mph today, given the available 28mm motors. But I want to push the 70 as far as it will go, then take what I`ve learned and apply it to a good 90mm system. Right now my P/W ratio is about 1.2 to 1, so I can afford to reduce the exhaust size if it does not stall the fan.

Is there a way to tell when the fan is stalling? Sometimes at high rpm the fan starts to "sputter" or jerk. At first I thought the impeller was rubbing, but then I was able to correct the problem by regarding the timing advance to 3 degrees and increasing the PWM on the ESC to the maximum (32kpm) I think.

It`s much easier and cheaper to do R&D at the 70mm size.

Thanks again for those numbers. They will help me dial in, until I get an Eagle Tree.
Sep 20, 2007, 07:42 PM
Registered User

Hey where did Saucerguy go??? 300 mph target speed


Saucerguys 300 mph thread seems to have died out. He wanted to make a 20 ouncer go 300 mph.... ( Prop Jets thread..."Fastest Design" )

From what I have seen 200 mph seems to be about it for most powered models..

I just wanted to see the 300 mph prop and motor, not even mentioning the battery.

If I felt like I could control it id make one, but at that speed - one flinch or glitch or any sort of distraction or any other sort of "thing" and your plowing into something. If I could find somwhere that had no chance of hurting anything or anybody ( Very Worried abut that) then id have a go.. Salt Flats perhaps.

I think I am ok with my 18 ounce hand launch powered gliders ripping the wings off going straight up....

Looks like your design is along the right train of thought.. Vector II seems to be a fast or fastest Fan Jet out there.... Close to 200 mph....

I think your going to need a stronger airframe for the speeds though. Id like to see you get there (200 mph ) Good Luck.

Rick
Last edited by rzzzzz; Sep 27, 2007 at 08:17 PM.
Sep 20, 2007, 07:56 PM
Dont forget the velcro straps
corsair nut's Avatar
what diameter in inches is 80% for the 480 fan? im running about 2.2" exit diameter, seems to be about nice for acceleration/speed. getting 100mph on 480 watts, about 39,000 rpm.
Bmiller, what program do you have that is telling you these #'s? im very curious!
Sep 20, 2007, 09:01 PM
Lee Liddle
Knife Liddle's Avatar
Thread OP
Looks like 80% would be 55.2mm and 2,2 inches is 55.88mm, so you are just a hair under 80%. Right now I`m at 59mm which is 85%. So the next step for me is to reduce mine to about 52mm or 75%.

I found a bad cell in my 9s 123 batt this evening, so after replacing it my numbers are quite a bit better.

With a fresh charge I`m getting:

1400w
57a
25.2v
55k rpm
1900g thrust
1400g AUW
T/W = 1.35/1

I don`t know how long the motor will last at this level, but I`m only going full throttle in 10 to 15 sec bursts. So we`ll see.

I`m going to fly it like this one more time before I reduce the exhaust size.

It was pretty darn fast even with the bad cell. Can`t wait to get and Eagle Tree system to measure airspeed.

I finally got the cg and trim set so that there is very little difference between power on and power off. (three flights) It does take some planning to get it to slow down enough to land.
Sep 20, 2007, 09:11 PM
Lee Liddle
Knife Liddle's Avatar
Thread OP
Quote:
My VERY rough calculations show me you need,
58,000rpm with a 80%FSA exhaust duct for the efflux to reach 324kph(200mph)
50,000rpm w/80% is around 280kph(?mph) efflux
54,122rpm w/75% is 325kph(200+mph)efflux
50,208rpm w/70% is 324kph(200mph)
Mind you this is the efflux speed, not the speed of the aircraft. also it does not take into account ducting losses or fan efficiency in-flight.
80% is an ideal exhaust size for the minifan under most conditions.
70% may create a stalled fan situation and almost certainly a loss of efficiency but I'm not sure of if the speed of the incoming air (ram effect)will help combat this.
Bruce

Bruce, now that I have a solid 55k I`m going to try 75% and according to your calculations, maybe 200mph + for efflux.

One question. How can I tell if the fan blades are stalling?

I know that what happens on the bench may not be a problem in the air because of "ram air", but I`d like to know if I`m getting stall on the bench.

Also I take it that when you say stall, you are describing a situation where the fan basically runs out of new air to push. This would be like a boat prop "cavitating, right?
Sep 20, 2007, 09:23 PM
Lee Liddle
Knife Liddle's Avatar
Thread OP
Quote:
I think your going to need a stronger airframe for the speeds though. Id like to see you get there (200 mph) Good Luck.

Rick

Thanks Rick, Yes, stronger is on the way. Although I am surprised how stiff this wing is. The plan for the next one is a solid balsa wing with 1/8" fiberglass rods in-bedded top and bottom, then the entire wing glassed and finished out very smoothly. I`ll post soon on #2, and now that I know it will fly, I`ll document the build for those intrested.
Sep 20, 2007, 09:43 PM
EDF rules... :)
AirX's Avatar
Did you calculate the frontal area of your airframe, you might find that calculating the drag will give you an idea of how much power it will take to arrie at the fastest speed the fan is capable. For example, if your aiframe is 50in^2 frontal area and the thrust force from the outlet can be plugged into a drag equation. This would get you into the ballpark.

Eric B.
Sep 20, 2007, 11:59 PM
SoCal, Year Round Flying!
bmiller's Avatar
Quote:
Originally Posted by Knife Liddle
Bruce, now that I have a solid 55k I`m going to try 75% and according to your calculations, maybe 200mph + for efflux.

One question. How can I tell if the fan blades are stalling?

I know that what happens on the bench may not be a problem in the air because of "ram air", but I`d like to know if I`m getting stall on the bench.

Also I take it that when you say stall, you are describing a situation where the fan basically runs out of new air to push. This would be like a boat prop "cavitating, right?
55,076rpm w/75% exit size= 328kph

Stalled condition....Yes somewhat as you described but I like to think of it as trying to force more air thru a straw, ducted fan designs are not compressors so can only build up very very slightly higher air pressure on the exhaust side of the rotor, if the air can't get out the back because the exit is too small, it will flow right back thru the rotor, hence the cavitation.
One of the EDF Gurus is really needed to answer this question accurately.
And especially when its inflight.
I doubt your fan is stalled on the bench. Your thrust/rpm numbers look great plus the pod type mounting of your fan, the ducting is short and efficient.

CN, I used an online EDF calculator and just played with the kv and duct size to achieve the various minifan rpm/efflux speeds.

Bruce
Sep 21, 2007, 10:41 AM
Registered User
ChrisAttebery's Avatar
I've run 52mm at 800W in a Pyranha with no problems. I know Herb ran his as low as 47mm.
Sep 21, 2007, 06:30 PM
smug in granny panties
monkamarm2000's Avatar
i usually run 49 or 50mm.
Sep 21, 2007, 07:11 PM
Lee Liddle
Knife Liddle's Avatar
Thread OP
Thanks everyone for the info. I`ll reduce the exhaust to about 50mm,

I have a combat contest tomorrow so it will be Sunday before I will be able to fly it.

The battery is performing very well with balanced cells. Fresh charge thrust holds at 2000g for a 10 to 15 sec burst! The vertical on this thing is incredible. Straight from the bungee into an accelerating 90 degree climb is pretty cool to watch. The climb only lasts a few seconds though, otherwise it would be out of sight.

It`ll be a shame to give that up, but the goal is speed, so speed it will be.
Sep 21, 2007, 07:24 PM
Lee Liddle
Knife Liddle's Avatar
Thread OP
I`ll be starting #2 on Sunday and will document the build.

Changes for #2 will include:
1) Solid balsa wing with 1/8" carbon rods top and bottom.
2) Fully glassed wing and vertical
3) Fuse will be a fiberglass cover bonded to the wing
4) Elevon pushrods and horns enclosed in the fuse
5) Hatches for access to radio and batteries
6) Better cooling for the batteries
7) Batteries will load from the front through a removable nose cone, ie Blade II
8) Streamlined tips on the wing and vertical
9) Better streamlining around the motor pod
Sep 22, 2007, 08:08 PM
Lee Liddle
Knife Liddle's Avatar
Thread OP
After my combat meet today (BTW I scored well enough today to move into 1st place in the National Points Race...yeah baby) I saw a fellow combat pilot who had a nice digital camera, so I got him to shoot some video of my jet.

The exhaust is still at 59mm, but I did the bungee into the full vertical for him, then did several good passes which should be good for a Doppler if someone wants to do it.

The wind was at 90 degrees to the runway, but I did passes in both directions, so we`ll see. The video looks to be of good quality so I`m looking forward to him e-mailing it to me. I`ll need some tech support to post it, since I haven`t done that before.

Rest tonight. Tomorrow, downsize the exhaust and fly again.


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