SMALL - espritmodel.com SMALL - Telemetry SMALL - Radio
Reply
Thread Tools
Old Mar 19, 2012, 12:08 PM
RC beginner
New York
Joined Oct 2008
6,054 Posts
Quote:
Originally Posted by DPATE View Post
Are you saying that most airfoils (including "blocky" ones and flat plates) have similar characteristics at low Re, or are you saying that these characteristics aren't important?
yes.

however note that i do differential between phrases like "less important" and "arent important".

spending many hours in an mx and a c150 of similar size i got the feeling airfoil played a significant role in the difference. then observing behavior of many types of foils on a 4' foamie it looked like it was less important. finally experimenting with lrf and sub-gram attempts it seemed profile had little to do with flying characteristics. like hanson implied weight was an overwhelming factor down there.

who is this richard hanson anyway and why is he trying to spoil my fun? richard klein of kf fame (get well soon dick) impresses me as being very knowledgeable and experienced too. the fellow who got me started here and taught me most of what i know was also named richard. seems like all the really smart guys are dicks. coincidence.... or SOMETHING ELSE?????????
dave1993 is offline Find More Posts by dave1993
Reply With Quote
Sign up now
to remove ads between posts
Old Mar 19, 2012, 12:57 PM
Grad student in aeronautics
United States, GA, Atlanta
Joined Oct 2010
466 Posts
So this smaller planes have lower weight, size, top speed, etc. And you conclude that it's due to Reynolds number?
DPATE is offline Find More Posts by DPATE
Reply With Quote
Old Mar 19, 2012, 01:08 PM
Ascended Master
Sparky Paul's Avatar
Palmdale, CA
Joined Oct 2000
13,499 Posts
The improved performance of small planes, birds, insects isn't a result of Reynolds number.. which is just a measurement.
It's a result of the size of the object and the density of the air, which doesn't scale up or down.... it just is.
Doing strange things to air... I recall observing the C-5 when it made a million pound lift record flight from Edwards.. I was at Willow Springs Raceway (20 miles away) when it flew over.... Low! You could hear the air molecules protesting holding that thing up!
The foamie flitters swim in air, just like small birds and insects.
I've seen an "expert" hang glider criticizing ducks because they don't soar like he does... They have two things not in their favor for soaring.. unswelled heads, and high wing loadings! Lots of body weight, tiny wings.
A swan or a heron has the ability to soar. Massive wing area relative to the weight. Ducks glide only when on the landing (watering?) approach.
They can't do anything else.
There's points where some engineering facts just stop being relevant.
Sparky Paul is offline Find More Posts by Sparky Paul
Reply With Quote
Old Mar 19, 2012, 01:12 PM
Registered User
richard hanson's Avatar
United States, UT, Salt Lake City
Joined Oct 2007
7,357 Posts
Quote:
Originally Posted by Steve Anderson View Post
Richard,
To use a blanket statement for slow flying models based on pattern planes and small foam aerobatic planes is absurd. The planes you are talking about need inverted performance (symmetrical airfoils) and use high drag to help control speed on the down leg of maneuvers, correct?

If the airfoil shape made no difference other than structural there would be no need for glider classes that do not allow camber change.

My first model airplanes were control line. A friend and I went to a hobby store and bought the same kit, .049 engine, prop, etc. We would fly in the same circle and my friend’s plane was sooooo slow. As you can imagine he was not very happy so we took his plane to the hobby store for help. After they stopped laughing… they pointed out that my friend skipped the “sand the balsa wing to an airfoil.” Yep, it had a square leading and trailing edge wing, just like a Depron plane.

His plane probably would have been far superior to mine for control line stunt though, wouldn’t it?
actually I qualified my statement to the small aerobats- NOT gliders
PS the squared edges on those 1/2A's made no difference - Iremember that issue
.
I was flying Ucontrol when the basic trainer was a Brave with a backport O&R 23
richard hanson is online now Find More Posts by richard hanson
Last edited by richard hanson; Mar 19, 2012 at 03:22 PM.
Reply With Quote
Old Mar 19, 2012, 01:15 PM
Registered User
richard hanson's Avatar
United States, UT, Salt Lake City
Joined Oct 2007
7,357 Posts
Quote:
Originally Posted by Sparky Paul View Post
The improved performance of small planes, birds, insects isn't a result of Reynolds number.. which is just a measurement.
It's a result of the size of the object and the density of the air, which doesn't scale up or down.... it just is.
Doing strange things to air... I recall observing the C-5 when it made a million pound lift record flight from Edwards.. I was at Willow Springs Raceway (20 miles away) when it flew over.... Low! You could hear the air molecules protesting holding that thing up!
The foamie flitters swim in air, just like small birds and insects.
I've seen an "expert" hang glider criticizing ducks because they don't soar like he does... They have two things not in their favor for soaring.. unswelled heads, and high wing loadings! Lots of body weight, tiny wings.
A swan or a heron has the ability to soar. Massive wing area relative to the weight. Ducks glide only when on the landing (watering?) approach.
They can't do anything else.

There's points where some engineering facts just stop being relevant.
Yeh-- little is it's own thing
richard hanson is online now Find More Posts by richard hanson
Reply With Quote
Old Mar 19, 2012, 01:30 PM
RC beginner
New York
Joined Oct 2008
6,054 Posts
Quote:
Originally Posted by DPATE View Post
So this smaller planes have lower weight, size, top speed, etc. And you conclude that it's due to Reynolds number?
well no, like sparky says re is a measurement. my point (first made very early in this thread) is that airfoils are not as important as most think for the small slow planes most rc hobbyists fly. the main funcion would be cosmetics or high performance. i can appreciate the elegance of some vintage and scale builds even though i havent gotten involved myself. yet.

more on topic is another theory of mine that top of wing forces are also less important with types of wings most of us are concerned with. notice i didnt say bernoulli this time because there may be factors involved other than bernoulli. so im going to try avoiding the use of bernoulli as much as possible even though bernoulli is a really cool sounding word.
dave1993 is offline Find More Posts by dave1993
Reply With Quote
Old Mar 19, 2012, 01:35 PM
Lift is cheap - Drag sucks
Tom Harper's Avatar
Socorro, NM
Joined Jul 2004
3,664 Posts
Wing loading is proportional to linear dimensions for models of the same planform.

So:

6 ft span, 1 ft chord, 12oz/sq ft = 72 oz AUW

For similar performance at 1/6th size:

1 ft span, 2" chord = 12/6 oz per sq ft = .33 oz AUW

Tom
Tom Harper is offline Find More Posts by Tom Harper
Reply With Quote
Old Mar 19, 2012, 01:53 PM
Registered User
richard hanson's Avatar
United States, UT, Salt Lake City
Joined Oct 2007
7,357 Posts
here is a little info - which may be of --little interest
for good indoor aerobat flyers - you need a plane which will fly easily at under about 4 yards per second in level cruise
IF the model is aprox 100sq in total area --the wing loading will need to be around ONE ounce per sq ft.
We put in about 3 hours , three times a week ,flying indoor stuff and finding what is really happening has been part of this ..
A VAPOR which weighs 17 grams all up will fit in this criteria.
Now try and make an aerobat which is about same size and will fly at about same speeds
You will find , the ONLY thing which makes a difference is wing loading- aerobats can't use reflexed or undercambered wings - you need a symmetrical shape or flate plate (good as any)
In doing this , I have found that 5 grams makes a very noticable difference.
an interesting point is that I can shift the CG to ANY point along the chord -and still fly the model. The AOA and speed of the model changes a lot doing this - but you can do it.
Tiny aerobatic planes are a great way to learn about what really matters in these sizes and weights
richard hanson is online now Find More Posts by richard hanson
Reply With Quote
Old Mar 19, 2012, 02:16 PM
Registered User
Joined Sep 2007
221 Posts
...
Steve Anderson is offline Find More Posts by Steve Anderson
Last edited by Steve Anderson; Sep 30, 2014 at 07:35 PM.
Reply With Quote
Old Mar 19, 2012, 03:42 PM
Lift is cheap - Drag sucks
Tom Harper's Avatar
Socorro, NM
Joined Jul 2004
3,664 Posts
Hanson,

A 17 gr Aerobat scales to 12.5" span with everything else in linear proportion. Velocity scales as the sq rt of linear ratio, so the flying speed should be 60% of the larger model.

What's the wing area of a full size Aerobat?

Tom
Tom Harper is offline Find More Posts by Tom Harper
Reply With Quote
Old Mar 19, 2012, 05:45 PM
Registered User
richard hanson's Avatar
United States, UT, Salt Lake City
Joined Oct 2007
7,357 Posts
The problem here is that actual practice shows the scaling isn't exact .
and areas of various full scale aerobatic stuff is something I really have not paid any attention to it as I am only dealing with the model . If you can get your 17 grm model to be aerobatic and I am guessing the wing area is something like 50/60 sq inchs you are doing well .
That's a tough neighborhood to be in -if you want hover and roll /snap performance.
I found that you really struggle once you drop under about 100 squares
richard hanson is online now Find More Posts by richard hanson
Reply With Quote
Old Mar 19, 2012, 05:48 PM
Registered User
richard hanson's Avatar
United States, UT, Salt Lake City
Joined Oct 2007
7,357 Posts
Quote:
Originally Posted by Steve Anderson View Post
If you say so. Looks to me you are talking about sailplanes in a couple of your posts.




Really? I didn't know you were there. So his plane flew as fast as mine after sanding an airfoil, but the real reason it was slow is what exactly?




NOT gliders what???
You are just having fun--
also I really screwed up my editing.
whay your model was slower is a mystery to me but rounding the edges is an old trick which in actual practice on these little flat wing 1/2 A control line jobs never paid off -
Maybe in todays enlightened world it works --
richard hanson is online now Find More Posts by richard hanson
Reply With Quote
Old Mar 19, 2012, 06:53 PM
Lift is cheap - Drag sucks
Tom Harper's Avatar
Socorro, NM
Joined Jul 2004
3,664 Posts
Hanson,

Aerobatics is a power loading issue. What is your watts input to the motor on the full size Aerobat?

Tom
Tom Harper is offline Find More Posts by Tom Harper
Reply With Quote
Old Mar 19, 2012, 07:32 PM
Registered User
richard hanson's Avatar
United States, UT, Salt Lake City
Joined Oct 2007
7,357 Posts
Power loading -- on the 29 gram model , I am using a Spektrum 6400 rx with integrated ESC and this on a 160 ma single cell Lipo and the motor is one they sell in their little EXTRA .
so-at full tilt it is producing about 40 grams thrust at best and the power consumpion has to be at likely something under one amp- I have not run this one on my design thrust/power consumption stand and Horizon does not give any figures on it.
On larger stuff I can measure all this using a Whattmeter and a gram scale -which gives power consumed in watts and thrust in oz and grams
The nice thing about electrics is you can get usable power consumption numbers -unlike IC stuff which is loaded with variables.
My son's new FAI model will pull upwards of 2600 watts! My friend's big YAK pulls over 10kw.
richard hanson is online now Find More Posts by richard hanson
Reply With Quote
Old Mar 20, 2012, 10:55 AM
Registered User
Joined Sep 2007
221 Posts
...
Steve Anderson is offline Find More Posts by Steve Anderson
Last edited by Steve Anderson; Sep 30, 2014 at 07:36 PM.
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion From whom can we rent a pool to run boats? LouScarlino Dock Talk 12 Nov 11, 2011 07:41 AM
Discussion loading down a T-28 Trojan, or Taking Pictures From the Air nemoskull Scratchbuilt Indoor and Micro Models 3 Sep 19, 2011 03:22 AM
Discussion Can we build something like this from scratch? mrexcel Foamies (Scratchbuilt) 6 Jul 07, 2011 02:55 PM
Discussion Can we move on from choo choo trains now? plowboy1966 Life, The Universe, and Politics 26 Mar 31, 2011 09:34 PM
Help! Can we get 1S power from 2S LiPo? Merog Batteries and Chargers 11 Mar 08, 2011 02:48 PM