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Old Oct 05, 2011, 02:53 PM
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Stealth Cheater Holes on LX F-22

Hey Guys,

I have recently installed some upgraded fans to my LX F-22.

I got the feeling they were a little starved because the static thrust on the bench was a lot more than in the plane.

I'd seen a lot of comments that cheater holes are only good at low speeds. It was speculated that at high speeds the ram air coming into the front of the inlets would actually vent some pressure out of the cheater hole defeating the purpose.

I thought about it for a little while and came up with this concept that works like a check valve, letting air in but not out, and keeping a somewhat scale look.

'Stealth' Cheater Holes on LX F-22 (0 min 19 sec)


I've had some great input and info from some very knowledgeable modelers on the build logs and I hope to follow up their questions in this thread.

Over the next few weeks I will document how they were constructed. I also came up with some great 'surgical' tools for modifying the foam.

I've also ordered a key-chain camera, on a modelers suggestion, to answer the question on what happens to the louvers as the ram air pressure increases at high speeds.

I'll be doing some tests comparing static thrust with the louvers open vs. covered and comparing the straight and level top end speeds.


Stay tuned .....

Steve
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Old Oct 05, 2011, 03:13 PM
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I like the idea of "blow in" doors. I'm assuming they are on a servo and operate in a linear fashion with throttle? another option would be to spring load them in such a manner as they open with suction pressure from the fan for maximum static, but air pressure keeps them closed at speed for maximum dynamic.

what you are seeing with the difference in installed thrust numbers vs bench numbers is true of every system out there. bench numbers are always higher than installed numbers. the inlet and exhaust ducts rob some of the thrust due to drag in the form of friction. the bigger the duct, the more wetted area there is and the higher the drag is. there's no escaping this and you can minimize it with properly designed ducts being only as big as they need to be. also, remember that your thrust losses will start to equal out at speed, because you are trading static thrust for dynamic thrust if you neck down the exhaust outlet to 85% FSA. you can increase this slightly more if you neck down the inlet area too. the trick is to match your system to what you expect in the airplane, not what you get on the bench. hope that helps.

Rich
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Old Oct 05, 2011, 03:25 PM
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Hey Rich,

I had originally considered a servo mechanism but aside from the fact that it would add to weight and complexity I wanted something that would work on providing the fan what it needed but would then "back off" as the ram air increased.

I tried using springs but I found it difficult to find/make one that would give a consistently low pulling force over a relatively long travel. I actually wound up using a rubber band. It had a nice gentle pull over the whole travel distance.

Thanks!
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Old Oct 05, 2011, 03:30 PM
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Old Oct 05, 2011, 07:09 PM
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Quote:
Originally Posted by LuvEvolution7 View Post
....the bigger the duct, the more wetted area there is and the higher the drag is. there's no escaping this ....Rich
That's why I was so surprised the LX F22 and F35 had no ducts, beautiful exterior very large open interior. The only thing that can fix that to my knowledge is a duct.

I'm kinda guessing the cheat/flap will, at full speed, still work as though it were a solid fuse wall and the poor interior will still hinder the performance. At slow speed you should note better thrust control.
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Old Oct 06, 2011, 06:56 AM
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WHA WHA WHAAAAT!!!!!!!!!!!! there's no ducts? isn't this a foamie? aren't the ducts already molded in at the factory? it's just two holes sucking air into a giant gaping void and the fan(s)? are there at least lips on the fan?
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Old Oct 06, 2011, 11:06 AM
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Very cool mod...nicely done. Clean.
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Old Oct 11, 2011, 01:10 PM
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Ducts

This airframe (LX F22 twin 70mm) has ducts...and that mod is awesome BTW...Any luck with the key chain cam and seeing if the ducts are forced shut at high speed???
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Old Oct 11, 2011, 02:11 PM
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Quote:
Originally Posted by timbersnake View Post
This airframe (LX F22 twin 70mm) has ducts...and that mod is awesome BTW...Any luck with the key chain cam and seeing if the ducts are forced shut at high speed???
Thanks Man

The Camera is in the Mail! I'm thinking I should have it in a week or two.

Last night I had a melt down on the bench of my power plug when I plugged the battery in. I had a little fireball going at my fingertips

Not sure yet what caused it, I looked and tested for a short, couldn't find one. I replaced the plugs, connected it back up with a 10amp fuse in line and everything looked fine ... except my right fan is not working

Dunno if I blew up my ESC or somehow wound up with a short somewhere. Oh well, much better on the bench than in the air I'm sure I'll get it figured out tonight.

I will at least be able to conduct the static tests while waiting for the camera
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Old Oct 11, 2011, 11:04 PM
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Fireworks

Steve---I replaced all my connectors with Dean's connectors, and I still get a big fat spark when connecting the 6S batteries. Anyway...I hope your electrical problem isn't what it sounds like it is. I've had no probs with the electronics on my F22 from BH. The landing gear was crap though, and I had to cut apart the fuse top/bottom where the factory glued it up. I fear you may have to do the same if your ESC is dead. ...It gave me the opportunity to replace a number of the crappy servos...now have Hitec Karbonites on the primary flight controls...still, a lot of work to fix a problem that should never have existed in the first place.
Good luck...I ordered the retracts from HK today...(I've been hand launching!)
Tracy
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Old Oct 12, 2011, 09:34 AM
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Great mod, will be waiting on construction pics.

Please take lots of them...

Scale ducting for EDFs is always hard, EDFs want as much as possible were gas turbines want to slow down air to prevent ram-air effect.

We need good cheater methods
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Old Oct 12, 2011, 03:10 PM
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Thanks er7771, timbersnake,

Hand Launching !! Very cool!!

Yes there is some great engineering in these LX models and then some not so great.

I'm still using the stock ESC's on this one and I have some 30+ flights on them. I'm even powering the RC Landers fans which draw an additional 10 amps over stock. Never had a problem with them before. I did blow up and ESC on my F-18 almost immediately but they appear to be from a different manufacturer.

I actually like the LX gear but I hate the grade of plastic they are made of. It has a terrible impact strength. On my A-10 I installed them with 6-32 nylon screws and that has really helped on maintenance. But I still bring a 'box'o'gear-parts' with me when I fly.

Name: 2011-10-10 21.48.04.jpg
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Description: Bar-B-Que Plug

These are 4mm bullets are are usually good for 100+ amps. I did not get the little spark when you plug in the batt - I got a BIG one

I haven't done the autopsy yet but the right ESC is dead. I have power and signal going to it, but no response. I'm guessing it did an internal meltdown causing the short and then drew heavy current until the short opened. I just have to figure out the best surgical procedure to replace it.
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Old Oct 12, 2011, 04:44 PM
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wow those are cool. I look forward to seeing how you did them.
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Old Oct 14, 2011, 04:52 PM
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Here's a quickie vid of the prototype louver I used to figure out all the angles and construction techniques.

Prototype Stealth Louver (0 min 29 sec)


For the hinge points I used some plastic tubes that were actually push rod sleeves from a Multiplex Easy Star. The larger one about 1/8" OD and the smaller diameter on fitting inside of it.

I would get every thing lined up, drill a hole for the larger tube at the hinge point. Insert the larger dia. tube into the hole going into both balsa parts and CA glue it in place. Then take a Zona saw and saw between the parts leaving the large Dia tube glued in place lining up perfectly with the other piece of tubing glued into the frame.

Then the smaller dia. tube would be the hinge pin. I would just hit the ends with a soldering iron and smash them flat to keep them from coming apart.


You can also see the rubber band that pulls the louvers shut.

I got my new ESC's in the mail today so I'll start the job of replacing them. I'll take some more pics. I actually think I'll be able to disassemble the louvers and get to the ESC's through the hole.

More to come ..
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Old Oct 14, 2011, 05:40 PM
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very inventive way of making blow in doors. love it. gonna use your idea for the blow in doors on my Sepecat Jaguar if you don't mind.

Rich
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