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Old Jan 07, 2010, 04:06 PM
Da' Cajun
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Lake Charles, Louisiana
Joined Jun 2009
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F-84F Thunderstreak scratch build ***maiden flight video***

The P-80 is gonna have to wait. I ran across these plans last weekend and have been wanting this jet since I was a kid.
More freeflight to RC headaches but this time around I'm a little more street wise about it. I learn a lot building the Hun and this one should come out more gooder ( = lighter ).

Same 64mm fan I was going to use in the P-80. I scaled it up to match my Alfa F-86 in size. Same wing span and fuse width but it's 8" longer over all.
Should I be worried about duct length on a fan that size?
I haven't had a chance to see what the intake opening is drawn at. I'd like to have 90% intake & around 85%(?) at the rear. I thought I read somewhere that long ducts on small fans is trouble.

My balsa order just showed up so the wood chips are gonna fly this weekend.
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Old Jan 07, 2010, 04:49 PM
Big gov never Works
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Skunk Water, Rhode Island
Joined Jul 2002
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I would try and achieve 100% on the intake, J. Are you figuring the FSA without the mtr holder? If your going strait from the outside of the shroud, same dia, your about 120%.

I based most of my modeling life, converting/enlarging rubber scale for RC. You just have to avoid and watch for a few things.

Fuzz
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Old Jan 07, 2010, 05:50 PM
DELTAS RULE
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tehachapi, CA
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youll want a 2" dia tail pipe and atleast a 2.25" dia inlet. the length wont hurt much.
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Old Jan 07, 2010, 07:02 PM
EDF rules... :)
AirX's Avatar
Joined Nov 1999
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Quote:
Originally Posted by Boogie_ View Post
I thought I read somewhere that long ducts on small fans is trouble.
You will see some losses but generally you will get more performance without going with a cheater. I have a 40mm F86D that pushes 5-6 oz on test stand and in my F86D I still get 4 oz. There will always be duct losses but the duct can play a good part in pressure/flow recovery so that you will regain some of the lost performance.

Eric B.
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Old Jan 10, 2010, 08:38 PM
Da' Cajun
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Lake Charles, Louisiana
Joined Jun 2009
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Nothing happening on the F-84 this weekend. My work area at home is up stairs in the attic. It's really more like a normal upstairs than an attic but he only heat is a gas heater and I didn't feel like burning a bunch of gas for just a few hours of building. They don't build houses in the deep South to stand up to much cold weather like we had this weekend

Annnywaaay,
On these jets, is a flat bottom wing a liability? should I go with a semi or full symetrical air foil?
It's so thin I don't see where it can be so bad, it's not like it's a Clark Y. I understand about the flat bottom making more lift as the speed goes up but is that it?
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Old Jan 10, 2010, 09:23 PM
DELTAS RULE
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tehachapi, CA
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keep it thin, and youll be fine. its a small model, so if anything, it wil help out. just make sure you build in a little washout.
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Old Jan 10, 2010, 09:25 PM
EDF rules... :)
AirX's Avatar
Joined Nov 1999
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My first electric jet was Kevin Cox's F-18, it was desinged arround a thinned flat bottom airfoil. The plane needed a little trim to cover WFO and landing speeds but it flew fine to me.

MH33 or MH43 work good. I have used both of those. They are a little faster than a thinned down Clark-Y and they give decent warning at stall speeds. They need a little twist in the wing to make them work at thier best.

Cheers,
Eric B.
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Old Jan 10, 2010, 10:40 PM
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USA, TX, Round Rock
Joined Dec 2006
440 Posts
I purchased a set of Walt M. F84-F plans from Tempest who posts here. They were drawn to use a RK049 fan way back when. I will be following this post. I probably will use a 70MM fan. It looks interesting. BTW, a guy in our club had several hundred hours flying the full scale and has a beautiful turbine powered F-84, he made all the molds himself and is a master builder.
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Old Jan 11, 2010, 04:28 PM
Da' Cajun
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Lake Charles, Louisiana
Joined Jun 2009
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Cool.

What about the scale anhedral? good or bad?
I would love to have in there just for the cool factor. The plans actually show some dihedral.
Several of my prop planes (& the F-100) have flat wings. Will anhedral make it less stable than that?
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Old Jan 11, 2010, 04:43 PM
Grumpy old git.. Who me?
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Aberdeen
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Originally Posted by Boogie_ View Post
Cool.

What about the scale anhedral? good or bad?
I would love to have in there just for the cool factor. The plans actually show some dihedral.
Several of my prop planes (& the F-100) have flat wings. Will anhedral make it less stable than that?
Anhedral is de-stabilising so yes it would give less stability than a flat wing. Anhedral should be fine if the model RC though as you dont need lots of stability. The plan you have is for freeflight so it needs plenty of stability, hence the dihedral.

Sweep back actually works like dihedral. The relationship is complex but as a rule of thumb about 5 degrees of sweep = 1 degree of dihedral. That's why swept wing jet fighters often have de-stabilising anhedral added, otherwise the sweep back would make them too stable and so less manouverable.

Steve
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Old Jan 11, 2010, 04:58 PM
Grumpy old git.. Who me?
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Quote:
Originally Posted by corsair nut View Post
youll want a 2" dia tail pipe and atleast a 2.25" dia inlet. the length wont hurt much.
That seems kinda small for a 64mm fan.. about 70-75% FSA on the outlet, maybe 90% on the inlet.

I'd expect 90% FSA outlet (56mm) and 100% FSA inlet (60mm) would be more optimal, assuming the design can accomodate these diameters, which may be a problem because the tail pipe especially is pretty narrow on the F84F
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Old Jan 11, 2010, 05:51 PM
DELTAS RULE
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tehachapi, CA
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remember, were talking about FSA, not % of diameter. a 64mm fan is only about 2.5" dia. 2" has worked exellent on more than 1 64mm setup. but thats just personal experience.
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Old Jan 12, 2010, 01:01 AM
Grumpy old git.. Who me?
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Aberdeen
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Originally Posted by corsair nut View Post
remember, were talking about FSA, not % of diameter. a 64mm fan is only about 2.5" dia. 2" has worked exellent on more than 1 64mm setup. but thats just personal experience.
It was FSA I calculated, assuming a hub diameter of about 25mm.. ..

For instance, assuming 25mm hub and 90% FSA:

Outlet = sq root (64^2 - 25^2 x 90/100)

Outlet = 55.9m

Smaller hubs would lead to slightly larger nozzle outlet requirement...

How did you calc your diameter or was it just based on experience?

Steve
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Old Jan 13, 2010, 12:33 PM
Da' Cajun
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Lake Charles, Louisiana
Joined Jun 2009
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Thanks for the info guys. I was working on the intake/exhaust this morning. I figured the FSA on my fan is 4.02 sq/in.
The tail as drawn is 2" dia., that's 79% FSA there abouts. If I need a couple more % no problem with keeping it scale looking.

The nose opening is too small as drawn. I think it's 3.1(?) sq/in. (I'm at work and my notes are at home)
I worked it up to 3.93 sq/in (98ish% fsa) that's as large as I want to go and still look OK. I don't think it will have "fish mouth" like the Alfa & Freewing Sabres. It's only like 1/8" bigger all way around.
My question is this: What would be the optimum intake opening to match the exhaust I've got?

Another thing I've thinkin' on is the full flying stab. Yes or no?
It's up plenty high to stay out of the grass but I just don't see how on a model this small the pivot can be made strong enough not to wobble and still be free enough not to kill the servo. The fin is gonna be 1/8" thick, 3/16" tops.
Maybe some kind of thrust plate on both sides of the stab pivot rod that would wear on the fin. I don't know if thrust plate is the correct term but here in the automotive world it best describes what I'm thinking of.
Maybe it's not worth the trouble.
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Old Jan 13, 2010, 01:17 PM
DELTAS RULE
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tehachapi, CA
Joined Jan 2006
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i tried a few differnt sizes, and went with what works best. heck, we use 2.05" dia on 70mm fans...and it very extremely well, especially with the higher power setups.



Quote:
Originally Posted by JetPlaneFlyer View Post
It was FSA I calculated, assuming a hub diameter of about 25mm.. ..

For instance, assuming 25mm hub and 90% FSA:

Outlet = sq root (64^2 - 25^2 x 90/100)

Outlet = 55.9m

Smaller hubs would lead to slightly larger nozzle outlet requirement...

How did you calc your diameter or was it just based on experience?

Steve
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