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Old Mar 20, 2012, 11:30 AM
Lift is cheap - Drag sucks
Tom Harper's Avatar
Socorro, NM
Joined Jul 2004
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Hanson,

Assuming that your Aerobat flies at about 200 Watts per pound, a scaled 17 gr model will require 4 W input for similar performance (assuming similar motor efficiency). You need to prop the motor so that you draw at least 1.1 Amps on the bench.

Your bench set up looks good, but a torque measurement would be meaningful rather than static thrust.

Tom
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Old Mar 20, 2012, 11:53 AM
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richard hanson's Avatar
United States, UT, Salt Lake City
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Quote:
Originally Posted by Steve Anderson View Post
When you write "rounding the edges" do you mean a wing section with a radius on the leading edge and the same size radius trailing edge and constant thickness from leading to trailing edge?

Do you have any idea what the elevation above sea level was at your location?
Yes and 4350 ft.
Those little 1/2 a models were just about as good without the engines running
the plastic ones -would barely fly without whipping them --those I saw .
I just got rid of an old collection of Tee Dees - Baby Bees etc.
I kept my Wasp
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Old Mar 20, 2012, 11:57 AM
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Quote:
Originally Posted by Tom Harper View Post
Hanson,

Assuming that your Aerobat flies at about 200 Watts per pound, a scaled 17 gr model will require 4 W input for similar performance (assuming similar motor efficiency). You need to prop the motor so that you draw at least 1.1 Amps on the bench.

Your bench set up looks good, but a torque measurement would be meaningful rather than static thrust.

Tom
On that test rig I have tried varied prop on same motor/batteries/esc and the " thrust to power absorbed" is always about the same- no matter what prop is used .
In actual practice the rig is accurate for the results I want. low speed thrust
Speed on indoor aerobats is not required - any speeds over 5 meters per second are excessive.
I guess I may setup a wattmeter with connectors for the really little stuff but . I now have a feel for combos which work- and the motors which are available set the stage for weights which are practical
Do you fly this stuff?
It's quite interesting
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Old Mar 20, 2012, 12:46 PM
Lift is cheap - Drag sucks
Tom Harper's Avatar
Socorro, NM
Joined Jul 2004
3,661 Posts
No, but it sounds interesting.

Static thrust measurements are not distinguishing. What you really need is Watts input to the motor and motor torque. With that you can calculate motor efficiency. I suspect that very small motors take an efficiency hit.

Perhaps, I will buy one and do some measurements.

Tom
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Old Mar 20, 2012, 12:52 PM
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Old Mar 20, 2012, 01:29 PM
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United States, UT, Salt Lake City
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Quote:
Originally Posted by Tom Harper View Post
No, but it sounds interesting.

Static thrust measurements are not distinguishing. What you really need is Watts input to the motor and motor torque. With that you can calculate motor efficiency. I suspect that very small motors take an efficiency hit.

Perhaps, I will buy one and do some measurements.

Tom
The beauty of the test rig -you can skip the torque/efficiency stuff
you look at watts and oz thrust - then just compare .
believe it or not
the prop really does not matter for this testing
the result is simply watt to thrust -good enough.
eficiencies run about 90% at best now - the little super cheap brushed motors I use for one ounce models are less.
calculating prop efficiences is a waste of time-
actual results are the only meaningful method
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Old Mar 20, 2012, 03:44 PM
Lift is cheap - Drag sucks
Tom Harper's Avatar
Socorro, NM
Joined Jul 2004
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Hanson,

You have observed the problem. Static thrust is largely meaningless as a bench test technique.

I agree with the actual results approach, however you need to use the proper results. Watts, torque and efficiency could provide you with insight.



Tom
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Old Mar 20, 2012, 03:59 PM
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pressure distributions around airfoils

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Originally Posted by dave1993 View Post
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.
* You might be on to something, or you might not be on to something, but consider--

* Lots of readers on this forum with a wide range of modelling interests-- I suppose you are meaning slow-flying RC or freeflight indoor planes or park flyers?

* Would be interested to hear more about what you are basing your conclusions on. Tried spoilers on the top of the wing and they didn't work? Or is it purely a gut feeling?

* After all, even a flat plate could, hypothetically, generate a larger low-pressure surface on top than a high-pressure surface on the bottom. I don't know if this is true or not, or if the answer is strongly dependent on Reynolds number, I'm just saying it could be true.

Surely someone can dig up a plot for the pressure distribution around a flat-plate airfoil at various Reynolds numbers?

Hang on, here we go http://www.windsofkansas.com/atoz2.html

The author is writing about dragonfly wings. In developing the theory, he includes figs 5 and 7-- pressure distribution around flat plate, and pressure distribution around flat plate bent into a cambered airfoil shape (zero thickness)-- in both cases the low-pressure area above the wing appears to make a greater contribution to lift than the high-pressure area below the wing.

An even better picture is given in Figure 1 http://www.windsofkansas.com/Fig1AtoZ3.jpg of the following link by the same author:

http://www.windsofkansas.com/atoz3.html

Last week I posted this link:
http://wind.nrel.gov/public/library/3387.pdf

See pp 19-21
Especially airfoil 3 in diagram 14-- pressure plot of top and bottom surfaces--
Airfoil optimized for low-speed low-Re -- airfoil is thin (but also cambered) (because airfoil is thin, bottom surface ends up being undercambered.)

The supporting text (pp. 20-21) says that "Airfoil 3 produces lift from high pressure on the lower surface."

However I can't see that the pressure plot supports this conclusion. Looks to me like if we draw a horizontal line at "1" on the pressure plot, and assume that this line represents the free-stream velocity or pressure, then there is roughly equal or maybe even more force generated by low pressure (high velocity) on the top surface, than there is force generated by high pressure (low velocity) on the bottom surface. Just comparing the area "trapped" above the horizontal line and below the top-surface pressure curve , versus the area "trapped" below the horizontal line and above the bottom-surface pressure curve-- these two areas look about the same to me on the graph. What am I missing?

* On another note, here is some interesting reading re an airfoil that uses flat plates rather than curved surfaces for most of the outline of the airfoil (but not particularly relevant to slow flight, also not spectacularly different from a more standard airfoil, just kind of interesting reading http://www.eaa1000.av.org/technicl/onedesaf/1desaf.htm

Steve
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Old Mar 20, 2012, 04:46 PM
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richard hanson's Avatar
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Quote:
Originally Posted by Tom Harper View Post
Hanson,

You have observed the problem. Static thrust is largely meaningless as a bench test technique.

I agree with the actual results approach, however you need to use the proper results. Watts, torque and efficiency could provide you with insight.



Tom
I understand your approach.
However for what I am after -which is thrust in excess of the model weight - the test is valid
Simple comparison data is all I need -
The argument that static thrust is is meaningless is familar
I could use calibrated prop and check rpm against a scale and compensate for altitude temp etc..
I simply don't need more info than the power in vs thrust (the load )
I like to keep things simple
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Old Mar 20, 2012, 04:47 PM
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New York
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Quote:
Originally Posted by aeronaut999 View Post
Lots of readers on this forum with a wide range of modelling interests-- I suppose you are meaning slow-flying RC or freeflight indoor planes or park flyers?
exactly (maybe take out the word "freeflight" because thats really a niche).

naturally we tend to think everyone flies the same type wings we do but in fact there is a most common one. hint: it aint pylon racing, edf, or dlg. for example just now i went to the most active rcgroups forum (scratch foamies) and looked at the largest thread on page one (blu baby). 18,000 posts with most others only a few dozen or hundred. guess what type wing i found?



this exemplifies wing type found on most rc models, be it scratchbuilt, horizon parkflyer, bnf, um, indoor rtf, silverlit toy, lrf, ect etc.. there is a reason we gravitate to the undercambered single surface and it aint necessarily economy. some of those models cost hundreds of dollars.

so we can go on and on about dragonflies, 747s, hummingbirds and rogallo hang gliders but my comments generally refer to the kind of wings most rc hobbyists are concerned with.

Quote:
Originally Posted by aeronaut999 View Post
Last week I posted this link:

http://wind.nrel.gov/public/library/3387.pdf

See pp 19-21
Especially airfoil 3 in diagram 14-- pressure plot of top and bottom surfaces--
Airfoil optimized for low-speed low-Re -- airfoil is thin (but also cambered) (because airfoil is thin, bottom surface ends up being undercambered.)

The supporting text (pp. 20-21) says that "Airfoil 3 produces lift from high pressure on the lower surface."
you pretty much answered your own question. there are spoilers on my mx and they are effective drag inducing devices but im still convinced its my ol bud newton at work underneath that provides most of the lift even on those HUGE wings. even more so on our tiny little rc models.

as ive said from the beginning theres obviously low pressure forces at work on the top. but so far i see little evidence this results in lions share of lift. edge effects, flow efficiency, vortex separation, blabitty bloobitty blah blah blah.

notice, again, im no longer referring to those top forces as bernoulli because bernoulli is an over-used word and since bernoulli may not be the only principle in effect here i will refrain from saying bernoulli too often. note that another reason to avoid the term bernoulli is because the old school equal transit guys used bernoulli a lot and we dont want to be associated with them. you wont hear me saying bernoulli, at least not very often.

ps i also agree with rh that thrust/watt is most useful info when trying to develop efficient power sytems. not so much for trying to look smart on forums or endlessly arguing about aerodynamic theories to ward off the lonliness.
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Old Mar 20, 2012, 04:56 PM
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richard hanson's Avatar
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A lot of info can be gathered by using a simple model -the VAPOR- andthen spending a few hours in a gym ,flying the model at all speeds and workable angles of attack.
Any plate which produces a pressure difference top to bottom is a wing it can be flat or curved or painstakingly shaped to an exact shape as dictated by load/speed n size
ALL of the othe r info , Bernoulli ,Coanda Newton etc., describe some of what is going on and is necessary if one likes to calculate stuff.
Bottom line- lift is simple a result of pressure differential at work
trying to decide wether the top does more than bottom seems to me to be a waste of time
Monday I was doing trimmed ,hands off flight and the model was ever so slightly climbing.
I added a touch of down trim - the climb rate increased
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Old Mar 20, 2012, 04:59 PM
Lift is cheap - Drag sucks
Tom Harper's Avatar
Socorro, NM
Joined Jul 2004
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Interesting, what data are you gathering.

Tom
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Old Mar 20, 2012, 05:04 PM
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Curving relative wind, maneuvering point, pitch stability

Quote:
Originally Posted by richard hanson View Post
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
I want to comment on this but without hijacking the thread onto another topic -- briefly, for lightweight slow-flying planes, there is one effect at play that allows for very aft CG locations that has nothing to do with the pitching moment, lift coefficient, etc of the wing and tail airfoils as measured in a wind tunnel. Remember that the relative wind is curved in turning flight. At slow flight speeds, the radius of any curvature in the flight path decreases, so that it is not all that large compared to the linear dimensions of the aircraft. This causes different parts of the aircraft to feel different relative wind directions. This creates a pitch "damping" effect, which is stabilizing. This allows stable flight even if the CG is far aft of the "neutral point" as calculated based on purely static considerations.

For more, (or to discuss/ comment), please see the new thread "Curving relative wind, maneuvering point, pitch stability"
http://www.rcgroups.com/forums/showthread.php?t=1617808

Steve
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Old Mar 20, 2012, 05:09 PM
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New York
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hey tom, it just occurred to me that "drag is cheap" and lotta guys here think "lift sucks".
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Old Mar 20, 2012, 05:25 PM
Lift is cheap - Drag sucks
Tom Harper's Avatar
Socorro, NM
Joined Jul 2004
3,661 Posts
Dave,

It seems that way.

From a design standpoint, lift is a given - fixed by weight. Drag is a budgetary item.

Tom
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