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Jul 28, 2020, 07:04 AM
Marc Frank's Avatar
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Discussion

efficiency of reflex vs leading edge droop vs bird tail


i know of the three methods mentioned in the title to create the necessary rotary force to keep the nose of a flying wing down UP
(i count birds to the flying wing category, as the tail isn't really a seperate part of the "plane", it's still somewhat connected to the main wing)

i would guess that the bird tail is the most efficient method, as it can extend rearward, so it has more leverage than the other two methods
for the same reason would i place reflex second in a ranking, as the distance of the leading edge the to the CG line is smaller than the one of the trailing edge

i might be wrong and the most efficient shape is a combination of the three, please correct me if that is the case


combining this with the bell shaped lift distribution and proverse yaw seen on the Prandtl-D, might take the efficiency of that project to another level
Last edited by Marc Frank; Dec 25, 2020 at 06:36 AM.
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Jul 28, 2020, 08:27 AM
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As always, the answer is "it depends".
Been discussed at length in other posts, but here are some points:

1. The choice is highly dependent on the Reynolds number you're flying at. The tail-less flying wing gets much harder to do right at low Reynolds number (say, spans of less than 2 meters).

2. If you need to carry any significant payload volume, the two typical choices are:
A. Thick flying wing (payload inside wing).
or
B. "Traditional" design using a much thinner airfoil with a streamlined cargo pod (fuselage) and stabilizer surfaces at the extremities.

The latter (choice B) will be much less draggy.

Interesting side note: For the B2 bomber (which uses approach A), it was discovered that a math error (caught later in a paper published by Northrop) made them believe their approach was less draggy than the traditional "tube and thin wing" approach. Whether or not this math error was intentional is a matter of heated debate , but the fact is that today it is clear that choice B is more efficient for cases of carrying large-volume payload.

If you're not carrying large-volume payload, then it is possible to design a flying wing with twist (and/or reflex, and/or drooping leading edge) that will be more efficient than a very streamlined, sailplane-like, traditional plane with tail surfaces. In practice, this is very hard to achieve, and I suspect that a typical builder will have better results with a traditional design - the fact that the overwhelming majority of high-performance model sailplane designs are not flying wings is a big hint here

Now, regarding birds: Very little comparison can be drawn from birds for typical models built by typical builders. The bird is using reflex, droop, tail feathers at all times and is varying all of it continually using an onboard computer called "the brain of the bird". It would be very, very difficult to achieve that with the efficiency level that the bird does
Jul 28, 2020, 11:54 AM
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Marc
You only have to fly in a weak thermal with a soaring bird of a similar span to realise just how efficient they are both in using the thermal lift and then flying away at speed using an "air frame" that by model standards would be considered far from ideal.

As nuteman says there is a difference between what is theoretically ideal and what can practically be built, particularly with the control and stability necessary for it to be flown successfully from the ground.
Jul 31, 2020, 05:27 AM
Marc Frank's Avatar
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thanks for your input

i love birds, they are amazing
hard to believe they are a product of random mutations
but that's another topic

i'm looking at this from the slope soaring perspective, so payload is not my thing

i've watched the talk by Al Bowers on "Why Birds don't have vertical tails" and the one by Spencer Lisenby on dynamic soaring
they were very interseting and they both mention the wandering albatros as the ideal example in nature
that would be a flying wing design by my standards

yet both Bowers and Lisenby did not go for the bird tail flying wing design
Bowers took sweep and reflex and twist
and Lisenby built his planes looking more traditional

of course i had to combine everything i saw, and wondered why nobody just built an albatros

especially Spencers planes have a lot of work put into them and are far from the average builders quality
if he is doing so much to optimise, why doesn't he go the step further to flying wings?

it doesn't have to be fully variable like a real bird, but the general shape would surely help with efficiency
Jul 31, 2020, 07:50 PM
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Quote:
of course i had to combine everything i saw, and wondered why nobody just built an albatros

especially Spencers planes have a lot of work put into them and are far from the average builders quality
if he is doing so much to optimise, why doesn't he go the step further to flying wings?

it doesn't have to be fully variable like a real bird, but the general shape would surely help with efficiency
Why don't you contact him and tell him his planes would be more efficient if they looked like birds? What is it about the bird shape, specifically, that you expect would improve the efficiency of a world-class model glider?
Aug 01, 2020, 01:47 PM
Marc Frank's Avatar
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i already know where to find his contact info and i will talk to him

i'm not saying his planes are inefficient, but obviously not optimal in that aspect

watch this and combine it with my initial question
"Prandtl Wing Minimum Drag Update" - Al Bowers (52 min 37 sec)


your question is already answered

if you have anything useful to share regarding the efficiency comparison of droop vs reflex vs bird tail, please do
Aug 01, 2020, 03:08 PM
Registered User
This might shed some light: https://www.google.com/amp/s/api.nat...augott-science

While the article is a very superficial treatment of the topic, it makes it sufficiently clear that biomimicry is far from trivial. When the biggest minds in the business are still grappling with getting albatross-like efficiency with today's means, it becomes easier to understand why other more practical shapes are pursued.

There are some other particularly "heavy reading" sources that go into great detail to explain why bird shapes are not efficient for airplanes that don't have the ability to constantly morph (advantageously) their entire structure the way birds do...
Aug 02, 2020, 11:47 AM
Lee
Lee
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Years ago I was told that a bird has a brain the size of a walnut and we can't compete with that.

We are trying to copy their flight with a few servos while they are using dozens of muscles and hundreds of feathers and their brain that has constant feedback from all the their feathers and nerves that are designed to keep them in the air. Eagles and hawks for example can adapt from soaring to death dives and constantly change their aerodynamics to match the immediate situation.

We can copy one moment in time of their shape and airfoil but as soon as conditions change they have us beat again because they can adapt instantly.

I have found it difficult to figure out how they fly with the CG issues that we have. Many of the birds look tail heavy but it doesn't seem to matter to them.
Aug 02, 2020, 01:06 PM
Marc Frank's Avatar
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everything said here is really interesting
nuteman, please send links to those sources, i'd like to read them

maybe i wasn't clear enough with what i meant by bird tail and what my question was
so i drew a little picture

i wanted to know, if using the little tail shown in the lower two sketches would improve efficiency over the two methods in the upper left

John Woodfield built something similar recently

Stingray build and maiden flight (2 min 54 sec)


i just wanted to combine this with the proverse yaw on the prandtl wing, so there isn't the need for a rudder or v-tail like in the video

i'm not sure if something like that could be built without sweep, i guess it wouldn't correct unwanted yaw movement on its own then


Edit:
i was looking at artificial muscles and came across electroactive polymers
a sort of plastic that expands in one dimension and contracts in the other when a voltage is applied
a flat disk would become thicker and smaller in diameter
they can be stacked with their electrodes to achieve more distance

something like that might enable more variable rc planes to be built
maybe even strong enough to do the flapping motion of birds

the mechanical solutions possible today are horribly noisy
https://www.youtube.com/channel/UC5b...2TxPbzw/videos
i really like the direct servo drive version, i feel like it is the most silent

Servo direct drive Unit2 5th time : First video in 2018 (4 min 39 sec)


but of course, as Lee said, processing power in small packages might be a problem, and power supply
for the builds with many artificial muscles and sensors
nature is way more advanced

it's not easy to mimic that, for sure, but it would be very cool

Al Bowers said in the video above, that the cg changes when using the bell curve span load / lift distribution
that might have to do with the cg of birds, too, maybe not
Last edited by Marc Frank; Aug 02, 2020 at 01:37 PM.
Aug 02, 2020, 02:44 PM
Registered User
Well, you can look up "gust load alleviation system" here: https://www.google.com/url?sa=t&sour...=1596396066276

Note how it functions similar to, and even looks somewhat like - a bird tail. Note that it must be controlled by a flight computer.

This is just one of many articles that address attempts to mimic bird behavior/efficiency. The B2 is one that was only partially successful.

Search Google for "airplane wing biomimicry" to find articles about current ongoing efforts in this area (and some failed attempts in the past).

As your last post indicates: "i was looking at artificial muscles and came across electroactive polymers
a sort of plastic that expands in one dimension and contracts in the other when a voltage is applied" - the approaches that might come close to bird efficiency are way, way, way beyond what even expert builders can achieve with currently commercially available technology.
Aug 02, 2020, 04:31 PM
Marc Frank's Avatar
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yes yes yes
i don't want anything to do with wobbly plastic muscles, too
they are far from impossible, though and i think it will happen someday

look at the little wing i drew
that's what i was thinking about the whole time
Aug 03, 2020, 01:18 AM
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Quote:
Originally Posted by Marc Frank
look at the little wing i drew
that's what i was thinking about the whole time
Yeah, that's why I gave you that link and referred you to the "gust load alleviation system" of the B2 discussed in that article:

Quote:
Originally Posted by nuteman
Well, you can look up "gust load alleviation system" here: https://www.google.com/url?sa=t&sour...=1596396066276

Note how it functions similar to, and even looks somewhat like - a bird tail. Note that it must be controlled by a flight computer.
Did you read that section of the article and see the accompanying picture?
Do you still think something like that (or the one you drew) on a flying wing built by even a skilled builder would yield a benefit worth the effort?
I don't. I think it much more likely that most implementations (even with associated flight controller programming) would end up decreasing efficiency. Elevons only (on the outboard sections of the wings) would almost certainly be less draggy.
My two cents...
Last edited by nuteman; Aug 03, 2020 at 01:37 AM.
Aug 03, 2020, 06:40 AM
Marc Frank's Avatar
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you really have a thing for the b2, don't you?

there are many videos of this model flying which doesn't need the "supercomputers" of the real thing, still some flight controllers but i don't want to get into that
https://carf-models.com/en/products/b-2-spirit

and i actually think the glas serves a different purpose than the tail i drew

i really don't have not much interest in machines designed to kill people and therefore i don't have a lot of the design constraints needed on the b2 to fulfill its purpose, which made the glas necessary

a wing like the one i drew works fine without any flight controllers and i do think just replacing the reflex in the airfoil with the upward angled tail would result in an improved L/D
Aug 03, 2020, 09:41 AM
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Quote:
Originally Posted by Marc Frank
you really have a thing for the b2, don't you?
Funny, practically everything I've said here is critical of the B2 - so if you mean my "thing for the b2" is to criticize it, I guess you're right.
I merely brought the B2 as a "prime" example of an attempt to mimic bird-like shape and bird-like "computerized control" - and to show how hard it is to achieve well.

Quote:
Originally Posted by Marc Frank
a wing like the one i drew works fine without any flight controllers
Without question - but, how well is where we disagree.

Quote:
Originally Posted by Marc Frank
and i do think just replacing the reflex in the airfoil with the upward angled tail would result in an improved L/D
Some food for thought (undoubtedly not all possible considerations, just some):
1. Your approach has the tail very close to the CG (unlike a traditional sailplane), so it would have to be somewhat large to be effective (I believe your drawing is over-optimistic). It might also need to be deflected quite a lot to be effective. Both of these factors could mean quite a bit of drag in many areas of the flight envelope.
2. As you pointed out, the Al Bowers design needs quite forward CG because of its aggressive twist. No reflex or twist in the wing means that the actual/practical required CG is likely even more rearward. This might make the moment arm for the tail you drew even shorter - closely coupled tail means more twitchy (though most wings with high AR and minimal sweep are twitchy to begin with).
3. Al Bowers undoubtedly knew your approach was an option, yet he didn't pursue it (as your first post alluded to) - perhaps this was because some of the issues mentioned in this post and this thread really were issues - or maybe he had other reasons altogether.
Aug 03, 2020, 01:24 PM
Marc Frank's Avatar
Thread OP
i had hoped something like this would be the first comment in the thread

i thought, that the lever of the tail, although smaller than the one of a traditional design, is still bigger than the one of the trailing edge
i had done some experimentation in openvsp before, so i would agree, that it would need to be bigger than expected / hoped

i made this comparison now, with the standard wing in OpenVSP
i added reflex to one by inverting the airfloil and adding camber with the camber location being closer to the trailing edge
for the other i added another wing and just angled it upward

by comparing to results in the vspaero analasys, the values of the reflex and upward angle were tweaked, so that the neutral pitch angle was the same as the angle at which the maximum L/D was

reflex has about 50% L/D
bird tail has about 55% L/D

i hope i didn't mess anything up


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