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Old Sep 10, 2012, 11:32 AM
Ride, Surf, Fly, Sleep
Joined Dec 2007
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Originally Posted by Nathan Schmoekel View Post
Round here in DLG land most people seem convinced that the tail must mount above the boom in order to avoid the down wash of the wing..... we prefer to surf it!!! Also for launch rotation the elevator below is in clean air, which we think helps it's inverted foil lift down and rotate the plane with less preset drag.
Hand them Model Aircraft Aerodynamics by Mark Simmons (old but the most comprehensive and professional work I've seen for the basics), and then point them in the direction of a decent freeware CFD modelling programme (e.g. OpenFOAM). It's all in the equations, but with CFD you can see it in 3D with pretty colours and everything

The only issue with stabs in that position is damage - with one hand physics gives, with the other it takes away......you'd think it'd be more generous being a fundamental concept in a position of responsibility....darn universal laws
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Old Sep 10, 2012, 12:30 PM
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United States, CA, Tehachapi
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I'm just curious what "T-drag" is...
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Old Sep 10, 2012, 12:52 PM
Ride, Surf, Fly, Sleep
Joined Dec 2007
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Originally Posted by bwill6 View Post
I'm just curious what "T-drag" is...
"T-flow" - Transitional Flow - where turbulent flow progresses to lamina flow - behind a model wing this can be a huge area, and you do not want to sit anything in it as it's v hard/impossible to design for.

The attached should give you an idea of why you might want a stab below the boom - you can clearly extrapolate the turbulent and transitional areas from the Uc, as well as the lamina flows. Specifically note the region where Uc goes from high -ve to high +v over a very small distance....
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Old Sep 10, 2012, 04:45 PM
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One thing I noticed with the Pod and Boom of the FF is that the Pod can be rotated if one desires a side hatch as opposed to the top.

Nathan, Paul and Quincross will you please post more pictures of the gear configuration in the pod, how the aileron push rods have been mounted/installed, and the how the wing is seated on the boom/pylons.

Nathan...are you using a rudder.....I noticed 4 servos in your picture(post 12)

Michael
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Old Sep 10, 2012, 05:38 PM
G_T
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Quote:
Originally Posted by quincross View Post
"T-flow" - Transitional Flow - where turbulent flow progresses to lamina flow - behind a model wing this can be a huge area, and you do not want to sit anything in it as it's v hard/impossible to design for.

The attached should give you an idea of why you might want a stab below the boom - you can clearly extrapolate the turbulent and transitional areas from the Uc, as well as the lamina flows. Specifically note the region where Uc goes from high -ve to high +v over a very small distance....
Of course, one needs a larger horizontal stabilizer if it is intended to be in the -ve region. It suffers a minor double whammy from loss of flow velocity which also gives a loss of Re. Having to be bigger equalizes the Re issue and the flow velocity issue, leaving larger mass. To each his own though. There are always tradeoffs.

Gerald
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Old Sep 11, 2012, 01:42 AM
Ride, Surf, Fly, Sleep
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Originally Posted by G_T View Post
Of course, one needs a larger horizontal stabilizer if it is intended to be in the -ve region. It suffers a minor double whammy from loss of flow velocity which also gives a loss of Re. Having to be bigger equalizes the Re issue and the flow velocity issue, leaving larger mass. To each his own though. There are always tradeoffs.

Gerald
You're right, everything's a trade off sadly - but in most of my smaller thermal glider models have had a significant number of senarios where the flow has been at or below the critical Re. It can't be helped at this scale, whatever peeps say. So at this scale it becomes more about a predictable regime for your flight envelope.

In general (if that's actually possible to say ) for my 1.5m to 2m thermal glider designs using variable camber/flaps:

T tail - mostly lamina, good predictability for design, easy to trim
High mount X tail (>2/3) - similar to T above
Boom/Standard tail (~inline with wing) - Unpredictable design and difficult to trim
Low mount - worse than X but better than Boom/Standard

And then you have the practicalities of design - which are usually best when your efficiency is worst (e.g. T-tails add weight, low mounts get damaged etc.)....you can't win
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Old Sep 11, 2012, 11:47 AM
Ride, Surf, Fly, Sleep
Joined Dec 2007
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Quote:
Originally Posted by relaxnfly View Post
One thing I noticed with the Pod and Boom of the FF is that the Pod can be rotated if one desires a side hatch as opposed to the top.

Nathan, Paul and Quincross will you please post more pictures of the gear configuration in the pod, how the aileron push rods have been mounted/installed, and the how the wing is seated on the boom/pylons.

Nathan...are you using a rudder.....I noticed 4 servos in your picture(post 12)

Michael
Haven't got that far - but when I do I'll post
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Old Sep 11, 2012, 12:23 PM
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Nope, just 3 servos ended up going in. During the preliminary parts shuffle I wanted to account for adding a rudder if needed, BUT SO NOT NEEDED!!

I'll shoot a few pics of my final set-up later..... if I remember

The HS-35 servos need to come out. They are analog and the center hold on the ailerons is just not quite good enough. Great little elevator or rudder servos though!
I have a pair of JR 290g that are going in there this week.
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Old Sep 11, 2012, 12:28 PM
G_T
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If it will not hold center well enough for aileron it is nowhere near well enough for elevator. I'm assuming you are talking about drift, or double centering, or something of the sort, rather than holding torque.

Gerald
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Old Sep 11, 2012, 12:50 PM
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Thanks Gerald, good advise.

In my case it's the fact that analog servos "rest" at center when they are not loaded. Due to that, when the ailerons try to flutter the servo is one step behind trying to re-center.
Digitals "actively" hold their center position and have a much faster reaction time to outside forces.
I tried the HS-35 to see what I could get away with... guess I found the lower limit The HS-5035 digitals would probably suit this plane well, since the ailerons are only 20%ish of the wing cord, but I'm going big because I have them in stock.

Analog HS-35s work fine on our spring-string set-up because the servo is constantly holding against the spring, which eliminates the "resting lag". Also we are doing the elevator below the boom so the spring doesn't need to be quite as stiff, between that and and a short servo horn, 12oz-in of torque is plenty to do the job well.
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Old Sep 13, 2012, 05:03 PM
Ride, Surf, Fly, Sleep
Joined Dec 2007
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Left for work late today so started the next part of the build.

I pretty much followed Paul's FF blog. The only real thing I did differently was inject the hard points for the stab under the skin - I still couldn't find my epoxy syringe, so I whacked together a replacement with disposable 'drinking straw' tips (see pic) - simple and worked better than the purpose made job I've lost. Guess I'll keep a bunch of straws around now!

I'm using 60min epoxy which allows me to heat it to get it thin (or rather heat the wing a little before application!). In the 1hr I had I managed to:
1. finish the 2nd fix on the wing join with the GF strip
2. Mark, drill, excavate and inject the hard points for the tail.

I did write a bit more detail about today's efforts whilst at work, but for some reason RCG is blocked on my work tablet but my own website apparently isn't and as I'm in the middle of updating it and happen to have a gallery app in progress...... I did a more detailed temporary build log there instead (that'll teach them!):
Detailed build log for my FireFly DLG

Right I'm off to bed!
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Old Sep 13, 2012, 05:17 PM
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OMG

That straw trick is the most brilliant thing I've seen in weeks!!!

I have a 3-1/2 year old, so no lack of bendy straws around here!!
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Old Sep 13, 2012, 05:33 PM
Ride, Surf, Fly, Sleep
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Originally Posted by Nathan Schmoekel View Post
OMG

That straw trick is the most brilliant thing I've seen in weeks!!!

I have a 3-1/2 year old, so no lack of bendy straws around here!!
Love bendy straws....I've a couple of rug rats about the same age and straws have many, many uses in the world of entertainment for under 5's...and apparently RC

If you do try the trick, don't get the knife too hot, it takes less heat than you think and hold it flat against the straw or you just melt straight through like...well...a knife through a melting straw

Oh and the real genius thing (and where it beats the epoxy syringe hands down)_ is that you can cut the right size of hole in the end for the job, just snip less or more off.
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Old Sep 14, 2012, 07:16 AM
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PLUS- the hole is on the corner edge of the straw so you can see what you're working on around it!!
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Old Sep 14, 2012, 07:46 AM
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USA, OH, Worthington
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How do you get the mixture in? Mine is usually mixed firm enough that it's hard to get in a 3/4" wide syringe.
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