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Old Aug 27, 2008, 06:14 PM
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Adjustable thrust outlet

So I have a Fly Fly Mirage 90mm on the way and was reading about the benefits to have a smaller (faster airspeed) or larger (easier launch and better low end) thrust outlet. So has anyone experimented with an in-flight adjustable outlet?

I am thinking of a couple different ways to try this, but was wondering if there was any info out there on this. Anyone have any ideas?

John
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Old Aug 27, 2008, 06:29 PM
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back in the glow days i saw some BVM planes had some nice turkey feather adj with a air data probethe planes were wicked fast back in the day don't remember who made them.
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Old Aug 27, 2008, 07:27 PM
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I have an adjustable set in the back of my YA F16. I don't know who made then, but I can remember seeing them reviewed in MAN years ago.
They are two concentric fiberglass tubes about 4" long, split with finger cuts 3 1/4" long and about 3/4" wide, somewhere around 10 fingers to each cylinder, and bound at the base. Fingers of the first tube overlap cuts in the one below. Each finger has a carbon fiber spine about three inches long from the bound base almost to the tips. About halfway out on each finger is a guide hole under the carbon fiber spine. Two loops of fishing leader go completely around the tube and overlap in the last spine so that pulling on the leader causes cantraction.( Each leader is opposite the other so as not to cause misalignment when the leaders are pulled). The whole thing weighs in at about 3 oz. less the servo or servos used for operation. The thrust tube attaches to the base at nearly 100% fsa and can be adjusted down by the feathers to 70%. Fan pressure opens feathers up when contraction is eased.
This is a pretty piece of engineering and works well on reducing the exit opening, but I have yet to test it in the air.
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Old Aug 27, 2008, 07:33 PM
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Quote:
Originally Posted by PlumberJohn
I have an adjustable set in the back of my YA F16. I don't know who made then, but I can remember seeing them reviewed in MAN years ago.
They are two concentric fiberglass tubes about 4" long, split with finger cuts 3 1/4" long and about 3/4" wide, somewhere around 10 fingers to each cylinder, and bound at the base. Fingers of the first tube overlap cuts in the one below. Each finger has a carbon fiber spine about three inches long from the bound base almost to the tips. About halfway out on each finger is a guide hole under the carbon fiber spine. Two loops of fishing leader go completely around the tube and overlap in the last spine so that pulling on the leader causes cantraction.( Each leader is opposite the other so as not to cause misalignment when the leaders are pulled). The whole thing weighs in at about 3 oz. less the servo or servos used for operation. The thrust tube attaches to the base at nearly 100% fsa and can be adjusted down by the feathers to 70%. Fan pressure opens feathers up when contraction is eased.
This is a pretty piece of engineering and works well on reducing the exit opening, but I have yet to test it in the air.

I'd love to see a pict of it if you get the chance.
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Old Aug 27, 2008, 07:49 PM
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Look at the plug nozzle concept used on the ME-262.
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Old Aug 28, 2008, 04:35 AM
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This idea interests me a great deal as I have been coming up with ways to do this in my head at this point. The simplest way that I had come up with at this point is a system similar to the one PlumberJohn mentioned. Shame someone beat me too it years ago.

It may sound complicated but its really quite a simple design. Plus has the potential to give a aircraft a wide operating envelope. I personally look at it as squeezing every last bit of performance out of a given setup. I do not have a current cadidate airframe wise but is something I am going to be playing with in the future.

Jason
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Old Aug 28, 2008, 10:04 AM
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Jason,

I have been trying to work this one out in my head for some time also. I even built a cardboard mock-up. I hope that everyone keeps sharing data regarding this idea.

Regards, Bill
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Old Aug 28, 2008, 12:12 PM
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Problem is it has no practical application in EDF. When you squeeze down your tail pipe you raise the load exponentially. every mm can raise the load a lot and thus when you select tail pipe size your going for the best balance of static and efflux that wont over amp everything. So you say no prob I will just adjust my kv and or cells and wind so it doesnt blow when pinched. Which you can, but now you have so much less load when its large that you no longer generate watts or static. Not ot mention now how much is efflux being harmed if the setup isnt smooth as a babies bottom. And really at the end of the day why make an elaborate setup that adds weight and complexity to get static thats gone few seconds after wheels up anyway.

Barry
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Old Aug 28, 2008, 02:05 PM
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I think that it's well worth a try if you have an airspeed logger that you can play back after a flight. Does the Eagle Tree system have such a recorder?

Then you could set the plane straight and level at max throttle on one turkey feather setting for a couple of passes, cntract the outlet for another two passes, and repeat with further contractions another few times. Then analyse the results to see if closing down did make the plane significantly faster, and what the optimum contraction was.

However, one variable that could distort results is battery temp. The further into the flight, the hotter the batts and the faster the model. Hmmm....

Gordon
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Old Aug 28, 2008, 02:29 PM
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Quote:
Originally Posted by PlumberJohn
I have an adjustable set in the back of my YA F16. I don't know who made then, but I can remember seeing them reviewed in MAN years ago.
They are two concentric fiberglass tubes about 4" long, split with finger cuts 3 1/4" long and about 3/4" wide, somewhere around 10 fingers to each cylinder, and bound at the base. Fingers of the first tube overlap cuts in the one below. Each finger has a carbon fiber spine about three inches long from the bound base almost to the tips. About halfway out on each finger is a guide hole under the carbon fiber spine. Two loops of fishing leader go completely around the tube and overlap in the last spine so that pulling on the leader causes cantraction.( Each leader is opposite the other so as not to cause misalignment when the leaders are pulled). The whole thing weighs in at about 3 oz. less the servo or servos used for operation. The thrust tube attaches to the base at nearly 100% fsa and can be adjusted down by the feathers to 70%. Fan pressure opens feathers up when contraction is eased.
This is a pretty piece of engineering and works well on reducing the exit opening, but I have yet to test it in the air.
Hm.. could 3 wires and 3 servos work for ovt?
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Old Aug 28, 2008, 02:34 PM
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Nice avatar there Gordon

Anybody that has experimented with outlets a bit knows what horrible losses you run into as soon as you clumsily touch the exhausts: small roughness, folds, glue dots, or any sort of minor turbulence in the tube end eats up tons of fan and ducting efficiency, not just one or two percent. If you do not believe me, try it out with an edf with really good ducting

From my drawer, ca. 2001:

.
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Old Aug 28, 2008, 03:03 PM
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That saved me searching back through my photo archive Herb

Regarding surface roughness in the ducting robbing efficiency, when I sprayed my Hawk I wasn't careful enough in masking off the intakes. Spray mist settled all the way down the inside of the ducting and I only discovered the resultant sandpaper-like surface by accident. It took 30 minutes with a rag soaked in thinners and tied on a long stick to clean out the ducts, taking great care not to mark the exterior finish of the model.

You can be sure that I'll be more assiduous with duct masking in future

Gordon
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Old Aug 28, 2008, 03:28 PM
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well I don thtink you really need to log all that much. just setup a system and leave a watts up in there and take reading with small inlet, then open it up and take another reading in flight. Take those and you see same setup with higher load and effect's, then lower load and its effects. then throw all that way and buy another setup thsats made to work the other way around, has enough static when open, aqnd blows up or heats up so bad from load and batteries go to or above c rating to depress like crazy and take those readings. Or buy another system between the 2. So one that doesn't take advantage of good static, nor takes advantage of higher efflux. Then give it a bad exhaust path to mess it up a tad more, then log that flight! lol
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Old Aug 28, 2008, 04:18 PM
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I started with ICDF planes which are large and for the most part fairly heavy. When Bob Ruff put some of his adapters for sale on E-Bay to convert a Byron fan to electric I got hooked on converting as many as possible of my ICDF planes to electric as well. The average weight of these planes is above 11#, so I need a lot of power to horse these things in the air. I'm also addicted to speed, so when I do get them in the air I like to go as fast as possible. Without adjustment of some sort the only way to go faster once you are on step with a tailpipe diameter set for power is to lean out ( read that take a chance on a blown engine ). If one tailpipe diameter will get me in the air, and another will give me more speed, the only way I know of changing in the air is with the turkey feathers.
I think that these should work just as well on electric as well as they do on nitro, I just haven't built another set yet to try it out.
The turkey feathers reduce the diameter about 2 cm, and you don't have to use all of it, so you could play with it in flight
It's a novelty for sure, but may just serve a good purpose. I won't know until I try, which is why I try. Ain't experimentation fun?
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Old Aug 28, 2008, 07:39 PM
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well yeah with i.c it made total sense, but in E you can have 2mm alone make a diff between 1/4's of pound thrust and or 5 to 10 amps in load pending on how hot you were to begin with. So now you think hey just lighten the kv for squeezed fast flight, and you can do that. but now you lost 1/4 to 1/2 pound of static in this compromise, and added weight and complexity to the plane. So you wanted more static for take off and traded the static you already would'a had in stock form. And now you also get poor efflux cause every one of those added wall shapes is an efflux killer
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