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Apr 17, 2018, 11:11 PM
IMO ( In My Opinion ) →
balsa or carbon's Avatar
Gil , do you still have this motor and battery ( in photo below ) ? If so , they would be a good choice to use on a very lightweight sailplane .
Here is a 2 meter wingspan motor glider ( built & flown by boa2110 ) that uses the same motor :

2m UMX motor glider (2 min 53 sec)



The key ( as I'm sure you know ) is to make the plane as lightweight as possible .


If you make a DIY hot-wire cutter , you can slice your fish-foam into useable sheets .
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Apr 18, 2018, 01:29 AM
Kimbers Keeper.
BHOFM's Avatar
I have a 1m 165g wing that handles wind like a pro. flies on about 60w.
With a 2s 1300 flight times are over twenty minutes. Flat DTF, no air
foil, no spar, filament tape on the bottom. Myself and several others have
tried to break it. Ain't gon'a happen.

1000X165wing (1 min 21 sec)
Apr 18, 2018, 07:33 AM
Registered User
Extreme Sports's Avatar
Gil, I'm no expert on aerodynamics, but like you, I am an engineer (sometimes) with almost an obsession to understanding how things work. I will try to answer your questions, but hopefully some real experts on aerodynamics (there are only 4 or 5 on RCG and you can tell who they are by the quality of their posts) will chime in and correct where my semi-layman explanations deviate too much from the truth.

First, I'm not sure where you got the idea that deltas are efficient. Delta (i.e. triangle shaped) wings are used on supersonic jets because they have properties that are useful at transonic speeds (i.e. close to, and above the speed of sound). They are well known to be terribly inefficient at low speeds....and probably even worse at the very low Reynolds numbers that model planes fly at. Can you think of a single successful commercial light aircraft with a delta shaped wing? Now we do use them a lot for model planes 'cause they have some other nice properties (look cool, don't tip stall too easily, strong, great high alpha performance etc) and our over-powered brushless motors can compensate for the poor aerodynamic efficiency at model plane speeds....as long as we are happy with 5-10 min flight times - which most of us are. The reason for their inefficiency was suggested a few posts back - low aspect ratio and relatively large tip vortices. Remember some time back I told you about the two types of drag? Well, deltas have a LOT of induced drag, and ID is highest at low speeds...which is where most model planes are flown (even 'fast' model planes fly slower than full sized planes, so all except record breakers are 'slow' ). So get the idea out of your head that a delta will be efficient and give you long flight times.

Second, don't confuse a delta wing with a flying wing. You can have delta shaped wings that are quasi flying wings (I'll explain this later) e.g. the correx 'Mugi' and deltas that are not flying wings e.g. the US F16 jet has a delta wing, but also a tail. Then some in between such as the French Mirage (delta wing with fuselage, but no tail), and the Swedish Gripen or Eurofighter (delta wing with fuselage and canard). Note, these are all supersonic fighters....not subsonic passenger jets (where fuel efficiency is critical). Of course the Concord was a delta wing too, but it was a bit of a design dead end for commercial air travel.

Very few true flying wings exist - google 'Horten wings' to see what a real flying wing looks like. No fuselage and no vertical stabilizer. True flying wings can be very efficient if designed to be such - i.e. very long, high aspect ratio wings. But true flying wings are very hard to design and can be hard to fly. Yaw stability comes from the twist in the wing and other mechanisms rather than from a vertical stabilizer. Take a look at the B2 bomber as an example of an update of the Horten concept. They need computers to fly them. There is a sub forum on RCG on Horton style wings - take a read there to understand more.

So most model flying wings, like your design, are 'quasi' flying wings - a wing and vertical stabilizer, but no horizontal stabilizer and no fuselage. You saw how it flew without the vertical stablizer. Or rather did not fly . Note too that flying wings do not have to be swept - there are many model 'plank' designs that have zero sweepback, or even some forward sweep.

OK, so that deals with deltas and flying wings.

So what if you want to make a very efficient plane for long flying duration: Well, as everyone has pointed out, the easiest is to make a glider style plane with a long thin wing (high aspect ratio = low induced drag, so can fly very efficiently at low speeds). You don't need to purchase an expensive one - there are lots of plans, but the trick is to make sure your wing structure is up to the task for the long wing span - so proper spar design is important. As others have pointed out, the airfoil starts to matter now too - no KFm or big blunt edges. So your construction techniques need to improve a lot, but you can still use cheap materials.

You could also design a high aspect ratio quasi flying wing, especially if you goal is to fly far and faster - take a look at something like the Goblin FPV wing. Very efficient airframe, but totally unsuitable for your current pilot skill level, or the places you fly.

So, definitely a gilder style plane is your best bet. But you get gliders and you get gliders - and the more you push the efficiency frontier, the more you will have to trade off docile flying behaviour and cost. So start your journey with a beginner style glider shape - if you can't find a foam design, look up classic balsa designs such as the Gentle Lady, Wanderer, and Drifter II. These are what I learned on, but I bet you could make something very close from your foam using a hot wire cutter and come up with some sort of strong spar system - I'm more than happy to help when you get there.

Lastly there is the issue of power train efficiency. I'm glad you remember my earlier post where I showed you that brushless motors are most efficient at lower power levels. There are a few additional factors to consider: 1) using a motor at low power means that you arguably carry the weight of a larger motor than is strictly necessary, and 2) propellers sadly, are more efficient at the higher end of their rpm range and 3) most people are obsessed with motor and prop charts at FULL throttle. When you are flying for long duration, you will normally be flying at part or quite low throttle - so you might match a different prop to the motor than you normally would. How did I work this out? I'm busy optimising a correx wing I built to break the 100km mark. And my measurements suggest that the conventional wisdom of a large, slow turning, low pitch prop does not always give the best overall efficiency, measured in Wh/km. The underlying principle remains valid, but you need to adjust it to the rpm you will cruise at, not the max throttle rpm where most folks do their thrust and tip speed measurements. And of course the drag curve of the airframe. So it quickly gets complex and very empirical.

So what does this all mean for you? Well, first, ignore the last paragraph - the nuances of props and motors are a level of complexity that you don't need yet - leave that optimization until you have a plane that is the right design and flies well.

So what plane? My suggestion is to leave the flying wings for now - they are going to be hard to make work in the flying sites that you have. Instead, follow the good advice you have got on this thread, and build a version of a proven, simple, beginner suitable glider design (I say 'version of' because I know you are incapable of building a plane EXACTLY according to plans ...sometimes because you don't have the same materials, other times just because you like to be different ).

PS: If you go the flying wing route, like BHOFM's wing, then you really have to build LIGHT. Note his 1m wing weighs just 165g. You can see how it literally floats about.
Last edited by Extreme Sports; Apr 21, 2018 at 05:53 AM. Reason: "Horten" not "Horton" :)
Apr 18, 2018, 11:29 AM
Scratch building addict
rotagen's Avatar
Very interesting thread satisfying my geeky tendencies.

Good that people clarified what should be called a "delta".

I think the reason people (like me) started posting alternate wing shapes, in my case even a forward sweep, is we all sorta agree that a true delta is not the most efficient (if "efficient" equals less drag and more lift giving longer flight times), and not to forget wing loading, a heavy plane needs more power all the time.

I suspect one of the big reasons deltas are not efficient in that definition is because they have pretty lousy stability in the roll direction due to the lower wingspan/bigger chord and spend a lot of their time with one wing lower than the other.

Very good point on wing tip vortices increasing drag, a vertical component on both tips helps this .. in theory... and in my experience with a panther jet (it has cylinders on the tips), that thing flies way better than it should with those stubby wings. Also I suspect slight forward sweep helps this, and definitely helps keep the wings level, because it forces air to move over the wing towards the fuselage.

As people pointed out, there's no substitute for a lightweight motor and battery. Personally my most used battery by far is a 3S 500maH. I have used heavier batteries on same motor and the flight times actually went DOWN.

As an aside, more power (watts) are needed to go the same distance as the KV of your motor goes up, mainly due to smaller props and smaller area props being less efficient. But with sailplanes people also worry about the drag of the prop itself.

I'm just geeking around as we say.

edit: Sorry I just read the OP reply. YG bird is definitely NOT a delta, it is a wing however, but quite the opposite, just to clarify.
Last edited by rotagen; Apr 18, 2018 at 11:53 AM.
Apr 18, 2018, 11:42 AM
Potato003
@balsa or carbon Sure, I have that motor you gave me and the 2 batteries! I even waterproofed the ESC (I removed the transparent heat shrink of it, painted it with nail polish 3 times and used a white durex around it) so if I fall in the water it will not be damaged! I managed to get a good rubber band to lock the prop in place from a pipe store that uses o-rings - photo attached! Also that 2 batteries you sent me I am also using them with my MT2204 motor which is 3S but works fine with the 2S battery you sent me and it makes my planes lighter! I liked the plane in the video, it uses the same motor you sent me with a bigger prop, I will try with the prop you sent me and if needed I will use one of the other big props you sent me! The only thing I dont like in that glider is that it uses motor on the front and I usually fly over people and I am afraid a motor in front might cause serious injuries if my plane falls from the sky and hit someone! That's why all the planes I build are pushers. But I will certainly build that plane and will only fly it away from people and give it a try! One question: that glider has also aileron, do you think I can make it fly with only rudder/elevator since there is a lot of dihedral in that plane? In this other video below the guy putted even more dihedral!
90" motor glider (3 min 1 sec)


@BHOFM liked your delta, you placed the prop in a slit and I still didnt see that! It looked very stable!!!

@Extreme Sports Thanks a lot for your reply! Lets go in parts: 1) it makes a lot of sense what you said about commercial airlines not having a delta shape, they have indeed in all cases a conventional airplane shape (in some cases with wings swept back by a lot but still not a delta). 2) I thought delta are efficient because of the so many lies that "professionals" tell on the internet. That youtube video I posted on #1 is from a guy that got almost 200k views telling lies, he uses truth arguments to tell a fake story. But now that you all made things very clear, I know deltas are not efficient! I think deltas are useful for transonic flights reducing drag and for low speeds it is not efficient - this thing that you said makes a lot of sense to me. Also thanks for pointing the difference of delta and flying wings and providing examples, now I got it! I was not aware that the swept back was not completely needed for flying wings!

@Extreme Sports yout tips about making an efficient plane are great, indeed I am gonna have to improve my building skills to make an efficient glider. I liked the Goblin, looks like a delta mixed with a plane, but as you said I should stick to a glider! I will take a close look at the gliders that you learned on but I think the biggest chalenge will be doing a strong spar with such a wide wing. Also the other chalenge will be looking for an efficient prop at low RPM since you pointed out that despite motors being more efficient at lower RPM, props are more efficient at higher RPM! But as you recommended I will leave this last chalenge to take care later, first I will focus on the wing!

@Extreme Sports I will follow your advice and choose a well proven efficient glider/sailplane and will start building it trying to get as close as possible to the instrcution hahaha Sometimes I cant find the right materials to make the planes exactly as they should be and sometimes I like to be different (as you precisely said hhahaa) but at this point I will take a break from flying wings and focus on gliders and try to keep them as light as possible! I will take a look at the Horton wings later today as you recommended.

@rotagen I find it hard to believe this stuff "vortices in the wing tips" and induced drag. I just started believing on it when I saw a video of a plane taking off and the smoke started spinning right at the tips of the wing! Maybe that's why people, as you also suggested, recommend using vertical fins at the tips. I will also look for some friends here in my city which could give me to try a smaller 3S battery, the smallest one I have is 90g 1000mah. Getting a smaller battery would certainly help A LOT! Thanks for your considerations, it just solidify what I am learning
Apr 18, 2018, 11:59 AM
Scratch building addict
rotagen's Avatar
Wow, fast reply. Just one more thing : wingtip vortices are definitely a concern but I wouldn't worry too much about them especially if you are designing a smaller chord wing...or a forward swept.... in my experience chord does matter but total wing area matters even more at the low speeds we fly, and at low speeds a bigger chord provides more lift per area.... so there's a tradeoff there.

Something around the YG bird's aspect ratio I found to be about perfect.

YG bird is not a delta, quite the opposite? as I wrote in last post, it is a "flying wing" however.

Try a slight forward sweep if you can, you'll love it.
Apr 18, 2018, 12:29 PM
IMO ( In My Opinion ) →
balsa or carbon's Avatar
Quote:
Originally Posted by batata003
One question: that glider has also aileron, do you think I can make it fly with only rudder/elevator since there is a lot of dihedral in that plane? In this other video below the guy putted even more dihedral! https://www.youtube.com/watch?v=ozCuxg8PJIE
The only thing I dont like in that glider is that it uses motor on the front and I usually fly over people and I am afraid a motor in front might cause serious injuries if my plane falls from the sky and hit someone!
Yes , a glider with enough dihedral/polyhedral will make turns with rudder & elevator . The very first RC plane I ever flew was a Gentle Lady glider with a 2M polyhedral wing , it had only rudder & elevator .


Here is someone glide testing a Gentle Lady glider :


RC Glider test flight #2 (0 min 49 sec)





You can also get a motorless glider to altitude with a bungee high start , like in this video :


Hi-Start Launching (2 min 12 sec)





A motor glider with the motor on the nose can be safer if you use a folding prop with a soft spinner . Or you can have the motor on a pod that's NOT on the nose .
Apr 18, 2018, 12:56 PM
Registered User
Extreme Sports's Avatar
Quote:
Originally Posted by batata003
2) I thought delta are efficient because of the so many lies that "professionals" tell on the internet. That youtube video I posted on #1 is from a guy that got almost 200k views telling lies, he uses truth arguments to tell a fake story.
First of all, that guy never once mentioned the word 'Delta' so be careful attributing stuff to him that he never said. Second, he is clearly making a case for why he likes flying wings. What he is saying is not wrong, but, as so often happens with psuedo-science, he leaves out some important factors. In particular, he forgets to mention that a flying wing airfoil needs to be shaped to give longitudinal stability in place of the tail, and such an airfoil is not the most efficient. He also correctly points out that fuselages cause drag, but does not mention that this is FORM drag, which is low at low speeds, and can be minimized with proper design. That is why most gliders do not fly at very high speeds and have very sleek fuselages - they are designed to minimise the drag penalty of the fuselage. He also forgets to mention that, although all the wings he shows have a pusher prop, placing the prop in the wake of the wing is not a very efficient thing to do. Probably cancels out all his other efficiency features combined.

But he is not 'lying' entirely. Designed properly, a flying wing can be a pretty efficient plane, especially for higher speeds. But none of the wings he shows in his video look particularly efficient to me.

Also, bear in mind that there are at least 2 measures of efficiency in aircraft, and in most full sized aircraft, these lead to two different 'efficient' speeds: 1) Speed for maximum range and 2) speed for maximum flight duration. They are seldom the same, and in your case, I suspect that you want to optimise for the second.

But, to be honest, at this stage most of this is academic. Do as BorC suggests - build a simple glider with a pylon mounted motor if a tractor prop worries you. But bear in mind that there is a trade off - a glider is less likely to be as sporty to fly as a plane that chows batteries. But then again, a glider will provide you with a new set of challenges, such as catching and using thermals etc.

So go read up about basic RC glider designs and come back with your next set of questions!
Apr 18, 2018, 01:17 PM
Scratch building addict
rotagen's Avatar
Quote:
Originally Posted by Extreme Sports
. In particular, he forgets to mention that a flying wing airfoil needs to be shaped to give longitudinal stability in place of the tail, and such an airfoil is not the most efficient.
I know it's subjective, but I find in real practice the "shaping" (with scratch-builders usually just adding some reflex with elevons) doesn't lower lift enough to notice.

And here's a weird thing I did on my little kestrel flying wing, I put a small amount of epp on end of v-tail pointing up slightly to replace the reflex, and it did indeed work as in the amount of reflex I have if any would take a microscope to see.

Here's something real non-scientific, but typical "sailplane" airframes with tail and thin long wings, are just not as fun to fly for me. Everything is sorta "slow motion" when you're up there.

Everybody's a damn expert. (my quote for today, referring to myself of course).
Apr 18, 2018, 01:51 PM
IMO ( In My Opinion ) →
balsa or carbon's Avatar
Quote:
Originally Posted by rotagen

Here's something real non-scientific, but typical "sailplane" airframes with tail and thin long wings, are just not as fun to fly for me. Everything is sorta "slow motion" when you're up there.
batata003 wants long duration flight , so a sailplane is probably the best choice for that .


But if you want exciting glider flight , how about a LeFish sloping on the side of a mountain in 60MPH wind :


Acrobatic flying with Le Fish, NE Bowl of Mt Elden, AZ (3 min 42 sec)
Apr 18, 2018, 02:55 PM
Registered User
Extreme Sports's Avatar
Quote:
Originally Posted by rotagen
I know it's subjective, but I find in real practice the "shaping" (with scratch-builders usually just adding some reflex with elevons) doesn't lower lift enough to notice.
That's because you're operating in a different paradigm. Your typical 'scratchbuilt foamie' does not have anything close to an efficient airfoil (nor does it need one ), so you are quite correct that adding some reflex won't really make any noticeable difference. That is kinda ironic in the video - he goes on about how to make a plane very efficient but none of the wings he builds are anywhere close to efficient.

The top level competition sailplane guys operate in a paradigm where airfoils do matter, and they would see the difference.

But you make a very important point that Batata003 should get his head around: While the theory is all very interesting, at the level of an early scratch builder, only the first order of magnitude aerodynamics are relevant - things like aspect ratio, wing loading , basic airfoil shape, choosing the right motor/prop combo and a few others. Leave optimising airfoils and worrying about how efficient the motor and prop are for much later in your RC career....if ever. Until you have the materials and ability to build super efficient air-frames, you just need to be in the ballpark. You simply won't notice any difference from second and third order optimizations.

By all means understand the theory.....but then just ignore it and go have some fun flying whatever style of plane takes your fancy
Last edited by Extreme Sports; Apr 18, 2018 at 03:01 PM.
Apr 18, 2018, 03:22 PM
Scratch building addict
rotagen's Avatar
good points.

One thing that struck me though from my scratch-builder experience, is a smooth thin airfoil makes a big difference- much floatier, gentler, more stable in pitch, they don't lose their lift as soon as you point the nose down like the KF airfoils or a simple flat plate do. So for that reason and for keeping light I would advise even a beginner to do my 1mm depron upper surface thing, in the beginning I even used colored packing tape as upper surface, ugly but floated forever. Smooth upper surface is what matters more than thin although thin makes less drag and longer glide. Slight undercamber makes for a great slowflier, but I'm usually too lazy so I usually go with flat bottom and a little sanding of lead and trail edge.

I have a homemade sloper modeled after "le fish", made the body with ethafoam (kayaker stuff - tougher than epp), and an epp wing core pw51 airfoil I think, or some similar sloper foil.

It is indeed a lot of fun. I need to get out and slope but the wind never hits my slope just right...
Last edited by rotagen; Apr 18, 2018 at 03:30 PM.
Apr 18, 2018, 03:30 PM
IMO ( In My Opinion ) →
balsa or carbon's Avatar
Quote:
Originally Posted by rotagen
good points.

I have a homemade sloper modeled after "le fish", made the body with ethafoam (kayaker stuff tougher than epp), and epp wing core.

It is indeed a lot of fun. I need to get out and slope but the wind never hits my slope just right...
When I lived in San Diego ( most of my life ) , I would slope at the Torrey Pines cliffs right on the ocean . On a good day , the first thing my plane would do when I launched it was : take the express elevator straight up to the 100th floor ! Nothing like that here in the Portland area ....
Apr 18, 2018, 06:08 PM
Kimbers Keeper.
BHOFM's Avatar
Quote:
@BHOFM liked your delta, you placed the prop in a slit and I still didnt see that! It looked very stable!!!
Main reason for the mid engine setup was to get the CoG without adding any ballast. Yes,
it is very stable and very easy to fly, it's ability to handle wind is amazing considering it weighs
165g ready to fly.



I do a lot of wings and a lot of deltas of my own design.



This was the first prototype, now flying #3, also flying the forward section and working on the
separation mechanism.

Last edited by BHOFM; Apr 18, 2018 at 06:21 PM.
Apr 18, 2018, 10:17 PM
flyin' fool
goldguy's Avatar
Quote:
Originally Posted by Extreme Sports
.................. 'the theory' ..... but then just ignore it and go have some fun flying whatever style of plane takes your fancy
I learned the theory, and the facts, as I progressed through the hobby over the decades (), mostly to correct my mistakes. That way I got in lots of flying, not much reading and was able to see it in action. All we had was model magazines published once a month and most of us were all on the same learning curve. Today, you guys are way to spoiled.

I usually recommend a proven simple conventional design to start with. If it's built as designed, then the student can spend more time learning the skills of flying, rather than fiddling and fussing to even get it flying in the first place.

Generally ..................... flying wings, deltas, swept swings and planks require a bit of patience to fine tune and can be frustrating to get it right.

I've watched lots of people teach themselves to fly with many different designs with little input from others. So, it also depends on the individuals too. I have FB that have been in the hobby as long or longer than me and still haven't mastered the skills.


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