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New airfoil for Bird of Time?

Old 08-15-2009, 09:20 PM
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skeeter_ca
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Default New airfoil for Bird of Time?

I've been scanning over alot of the threads concerning the Bird of Time sailplane. I saw several mentions that a new airfoil has been used. Was this airfoil used on the second improved release of the kit or is this a aftermarket design? What improvements did this do?

skeeter
Old 08-16-2009, 12:22 AM
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kwmtrubrit
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Default RE: New airfoil for Bird of Time?

Go to Skybench aerotech (skybench.com should get you there). Check out the "Bird" series. My understanding is this is a better version of the BOT. It's not a BOT but looks similar.

Keith
Old 08-16-2009, 09:28 AM
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soarrich
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Default RE: New airfoil for Bird of Time?

The original was a Dave Thornburg airfoil, most people thought it was an Eppler 193, but it was Dave's own. It's my understanding the Marks Models version used the same airfoil as Dave's. The SkyBench used a S3021 if I remember correctly which was a tweaked E-205.

The advantage to the new airfoil was that it had lower drag, which equates into more range for searching. Dave's airfoil also had a definite step, if you flew it a little too slow it fell out of the sky like a stone, but if you sped it up just a little the plane would fly so much better, I would think it had something to do with a lamiar separation bubble at slow speeds.
Old 08-22-2009, 12:22 AM
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Currawong
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Default RE: New airfoil for Bird of Time?

I'm researching prior to building a BoT - based on the Dynaflite kit to start - with original or developed wing.

I have some ideas about minimal wing strengthening, spoilers and intend to build as a 3-piece [instead of 2].

Re new airfoils: came across the Mark Drela designed 'Bird of Time Laser Set' from Mountain Models, who claim they don't have any detailed information on this 'short kit'. Seems odd they are selling it then. Cannot find detailed description, drawings or even reports on a BoT completed with this set. Is anyone the wiser?

There are a few links on R/C groups - eg http://www.rcgroups.com/forums/showthread.php?t=261775 - but they are old [2004], out of date, incomplete and lead to dead ends.

I wonder if the Sky Bench series of birds is related to these goings on?
Old 08-27-2009, 04:41 PM
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Default RE: New airfoil for Bird of Time?

A few years back I started a discussion on a Bird of Time-New Millenium back in, apparently, 2002.... Where DOES the time go? It centered around an updated airfoil and a slightly longer tail and nose moment length.

At some point this either ended up inspiring others or was more or less taken at face value and someone asked Mountain Models to make a Bird of the Future short kit.

It's been a while but as I recall the discussion was aimed at using the Drela AG34 airfoil and this may or may not be what is used in the short kit. You may want to email them

http://www.mountainmodels.com/produc...70d7fabe041101

Obviously this is wing parts only. You'll still need the RCM plans or the Dyna-Flite kit to work at completing the rest of the model. And frankly having seen what an early Dyna-Flite kit was like I'd STRONGLY suggest building all the rest of the model from scratch. If it was me doing the build I'd increase the tail moment by 1.5 inches and the nose by 1.5 inches. Or if you think you can keep the tail construction light enough only increase the nose by 3/4 to 1 inch. The longer tail will provide a stronger rudder leverage to allow turn entry with less rudder throw. It'll also avoid the "level and leave" issues if you slow down too much while in a thermal. Something that I found was quite prevalent on my own BoT.

At no time were these improvements incorporated into any of the Dyna-Flite kits or the currently available ARF version as far as I know.
Old 08-28-2009, 08:21 AM
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tkilwein
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Default RE: New airfoil for Bird of Time?

I built a couple stretched BOT's and used carbon stripping on the spars to strengthen up a bit.
Both are made from the current kit sold on tower hobbies.

Flaps on one and ailerons and flaps on the other. Full flaps seams to be the best verses crow/butterfly.
I recommend flaps as it slows down to a fast walking speed for landing or even flying.
Make sure you can do 90 degree down flaps as it produced the slowest flight speed on mine.
Used the ribs and some planks from the kit and the rest is scratch.
One is a two piece wing and the other is a three piece wing.
If I had a longer trailer I would have made as a one piece wing.
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Old 09-01-2009, 08:01 AM
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Currawong
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Default RE: New airfoil for Bird of Time?

Thanks for the info fellas. Very helpful.

BM - I did email Mountain Models - the response claimed to know nothing beyond what is published on their website - scant info indeed.

I have secured the Dynaflite kit as a starter. I had been toying with increasing nose and tail mpoments . . . interesting.

. . . and yes. Where does the time go?
Old 10-04-2011, 01:54 AM
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Default RE: New airfoil for Bird of Time?

The Bird of Time aerofoil is a Clark Y thinned to 9%. A reprint of the original article by Dave Thornburg c/w original plan is at the Plans Section of www.colinusher.infoNote the details re the leading edge shape.

Colin Usher
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Old 10-04-2011, 06:10 PM
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Default RE: New airfoil for Bird of Time?

Currawong, I believe people were also building the fuse much narrower than stock. The smaller ,stronger new servos allowed a sleeker build.

I have an old kit version built by a very productive builder in my area. I wish it had spoilers, it has been converted to electric, and I love the freedom that allows.. I think there is a thread somewhere on RCU with my setup, just something thrown together cheap.. not much testing or comparing of numbers
Old 10-05-2011, 12:15 AM
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Default RE: New airfoil for Bird of Time?

If you inspect the ORIGINAL Dave Thornburg drawing you will see details of a much narrower fuselage. (smaller frontal area) However if you are building as a slope soarer narrower means weaker and slopers do not usually have the luxury of a flat landing area. Other suggested Airfoils are S 3021, Eppler 193 Eppler 205. What is the point. If you are building a vintage glider build as per the original plan. Would you build an exact copy of a Type 35 Bugatti & fit a Rover engine. Note in Dave Thornburgs 1979 RCM article the leading edge shape.
Colin Usher @ www.colinusher.info
Old 10-05-2011, 03:20 PM
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Default RE: New airfoil for Bird of Time?

Dave basically built the original around the servos he was using at the time and then custom fitted the pushrods to let the forward servo's linkage pass over the top and linkage of the rear servo. So basically it sounded to me like they were built into the model for good. The article in RCM mentioned that but showed a wider "one size fits all" width. His note in the article suggested that the builder had the option of modifying the width to follow his original concept or not as they wished.
Old 10-05-2011, 10:52 PM
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Default RE: New airfoil for Bird of Time?

There another Bird of Time airfoil post floating around on RCU that I got a kick out of reading

If you read Dave's RCM article you know that the BOT started with the design philosophy of "These current gliders have too much lift which equates to drag"
So with this idea Dave designed an airfoil that had less lift and less drag.
Now he took this airfoil and build a plane around it.
He was proven correct by winning the Nationals with this design.

So now comes a bunch of guys that want to modernize the BOT and the very first thing they want to do is to change the airfoil to a high lift one. LOL

I'm kind of calling the kettle black here because I'm planning the bash the BOT big time. Mostly I like the BOT's wing planform and will change the tail,fuse, and wing position. Not to mention the size.


Old 10-06-2011, 01:49 AM
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Default RE: New airfoil for Bird of Time?

I am not sure if I agree with Dave's philosophy, although the results he achieved cannot be disputed. As he say's Lift = Drag but lift in level flight always = weight. (exactly) You cannot have too much lift or the model would go up like an elevator. The EXCESS lift is traded into speed, the modern sailplane gets from LIFT A to LIFT B (thro' sink) as fast as possible, burning off all that excess lift as speed. A model whose lift just about equals its weight will always stagger along on the verge of a stall. We have all seen flyers trying to stay up staggering along just on the stall, a bit of down elevator & the model shoots off & the lift piles in. With the Cof G in the correct place the model should fly with the tailplane at more or less zero incidence, flying with loads of UP or DOWN elevator will produce drag. The answer is to use an aerofoil that produces its lift over the estimated speed range & produces min drag. Unless you are using CNC milled wings to the exact design profile you are wasting your time anyway with ribs & a sagged covering. The QUANTUM leap in modern sailplane performance has come about due to the computer generated aerofoil design being transalated 100% into the actual wing & not a 50% near copy.
Old 10-06-2011, 06:21 PM
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Default RE: New airfoil for Bird of Time?

Keep in mind that while Dave knew he wanted to go faster and cover more sky he didn't have a lot of good data to go by back then. It WAS only 1979 after all. All the models of the day were designed largely with gut feelings and past experience. Selig's work was still many years in the future. Much of the influence back then for thin and slippery would have come from such things as F1B glow power free flight models and other sources.
Old 10-06-2011, 10:40 PM
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Default RE: New airfoil for Bird of Time?

I shocked going by RCM's plans that Dave designed such a sleek and beautiful plane, then from there only to make the same box three times.

The HLG Sunbird looks cool though. LOL javelin launched glider.


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