Why do foamies Deltas are so poorly efficient? - RC Groups
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Apr 17, 2018, 04:59 PM
Potato003
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Why do foamies Deltas are so poorly efficient?


Hi my dear friends!

I come here this time to discuss some things that I still cant understand about rc planes/deltas. I was told too many lies and I cant believe anything I read on the internet anymore!

I have a light background in mechanical engineering (despite I was never close to be the smartest guy in any class I attended), and because of that I know a little bit of fluid mechanic and aerodynamics. I also researched A LOT before discussing the subject below. I said these 2 things to let you know I am not coming here to take your time and patience without before investing a lot of my time trying to learn a little more about the things that I still dont get it.

I spent some time building many EZFLYs and BEAGLEs. I probably built at least 30 planes and ended up creating my own plane being a mix of EZFLY/BEAGLE that suits my needs. I always wanted something 1) efficient (I mean, the biggest flight time as possible) and 2) easy to fly.

I need the shape of the plane to be effiient cause here in brazil good batteries, motors, escs... are expensive. So making the design of the plane as efficient as possible, I could get the greatest time flying as possible with my batteris. Thanks to @balsa or carbon I was provided with good batteries, very powerful ESCs and very good motors so I can fly with "not so efficient" plane designs and still get a decent amount of flying time.

This whole story takes me to this: my EZFLY/BEAGLE mix rc plane (picture attached) can fly for 22 minutes in a calm day with a 2200mah good battery. Great. So I was wondering how could I improve this and after reading A TON in the internet and getting advices from inteligent people I was advised to start building deltas/flying wings. The reasons pointed:

1) with delta you dont have a tail so you save lots of weight and drag;
2) with delta you dont have fuselage so you save lots of weight and some drag;
3) with delta the surfaces that generates drag also generates lift, you dont have surfaces that generate only drag and no lift (as happens with planes) assuming you dont use fins or winglets;

Also deltas are easy to build, very crash resistant and you can do many aerobatics with only 3 channels radio.

That looked too good to be true! So I went ahead and spent my last days building the DEAD SIMPLE DELTA. It took me sometime to get it to fly easily (I had to scale it up 2x) and I ended up with a design of my own that flies really good (picture attached). I did some modifications to the delta and I can honestly say that I had a great time flying this plane, even me as a beginner could handle it fine!

I fell in love with this plane, so in love that on the same day I built another with a different motor! I was totally impressed with how easy it was to build and nice it was to fly (very different of the ezfly/beagle mix that I built).

So today I went to field and I promised myself I would fly my both deltas till I get bored or the batteries are dead. There was no wind today and I started flying right at the sunrise! I throwed the delta in the air and started counting the duration of the flight just for fun since I was pretty sure I could get a loooong time flying.

Bad news. After 8 minutes my battery alarm bipped! I COULDNT BELIEVE! I was afraid my battery was not fully charged so I went home, charged it and at the end of the morning I flied again and the battery lasted AGAIN 8 minutes!!! I couldnt get it. So I took my EZFLY/BEAGLE mix plane and putted te same battery on it and took it to the field. I flied with it for a little over 22 minutes!!!!

HOW??? My EZFLY/BEABLE plane has fuselage (which generates drag and zero lift), has a tail that is heavy and also generates lots of drag... and it managed to fly almost THREE TIMES MORE than the delta???

I know I cant compare 2 different planes like that. BUT: as you can see in the pictures both planes have veeeery similar motor (1000kv), exact same ESC, exact same prop size (which @balsa or carbon sent me), exact same battery and veeery similar main wing area. Yeap, the wing of my delta and my ezfly/beagle mix are veeery similar in size (area) and have exactly the same airfoil shape.

After researching a lot on Google I learned the only reason why deltas are not too efficient is because of elevon reflex. It makes sense but I managed to fly my delta with zero reflex, I used the CG a little far from the nose than I should and it worked very well without reflex, my 2 control surfaces are completely flat to the main wing when flight straight/horizontaly.

One argument against delta that nobody brought on the internet and I am guessing that might be responsable for the lack of efficiency is the Reynolds number! On a Delta you will have a bigger "chord" (cause your wing is angled back and the air will flow over a longer distance) and since Reynolds number is directly proportional to lenght, on deltas the drag should be a little bigger cause you are gonna end up with a bigger Reynolds number and you may even have a turbulent flow earlier in the wing.

So is Reynolds number the mistery? This love story ended up too shortly!!! I was amazed with delta, there are tons of videos like this ->
What is the most efficient RC plane? (3 min 9 sec)
where people say delta are the most efficient planes and so on and on.

People use true arguments to tell a misleading story: DELTAS ARE NOT EFFICIENT! This is so true that all the gliders that I saw on the internet have a conventional plane shape, not a delta shape.

My dear friends, would you mind telling me HOW ON EARTH a plane that has no tail and no fuselage behavies a lot worse than a conventional plane with fuselage and tail (considering they both have the same wing size)? Deltas even have a lighter wing load, so how a plane with lighter wing load (delta) can soo poorly behave?

OBS: if you look at my delta you may say the dihedral I added was the curplit of loss of efficiency. It was not, I added the dihedral later trying to get a better plane but it didnt increase my flight time at all.
Last edited by batata003; Apr 17, 2018 at 05:06 PM.
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Apr 17, 2018, 05:28 PM
IMO ( In My Opinion ) →
balsa or carbon's Avatar
Hey Gil !

In My Opinion : for the longest flight times , build a sailplane . A motorized sailplane can climb to altitude with the motor/prop , then glide around with the motor off for a long time ..... depending on conditions . If there are thermals , the thermals can keep the sailplane in the air ..... and also possibly cause the sailplane to gain altitude .

You can also build a DLG ( Discus Launch Glider ) that doesn't even have a motor . RC Groups member boa2110 is ( IMO ) the master of building lightweight planes , here is his most recent DLG :


1.5m foam board glider (4 min 31 sec)
Apr 17, 2018, 05:39 PM
Sopwith Camel's Cousin
Quote:
Originally Posted by balsa or carbon
Hey Gil !

In My Opinion : for the longest flight times , build a sailplane . A motorized sailplane can climb to altitude with the motor/prop , then glide around with the motor off for a long time ..... depending on conditions . If there are thermals , the thermals can keep the sailplane in the air ..... and also possibly cause the sailplane to gain altitude .

You can also build a DLG ( Discus Launch Glider ) that doesn't even have a motor . RC Groups member boa2110 is ( IMO ) the master of building lightweight planes , here is his most recent DLG :

....
I second this.
In "real life", the planes that can stay in the air the longest on little or no fuel are sailplanes, which either have a relatively small motor or no motor at all (gliders).
In one sense, sailplanes are the opposite of many deltas: sailplanes have long skinny wings (long span relative to the chord), while many deltas have short wing spans relative to the chord.
From what I have been told, a lot of a wing's loss of lift comes at the wing tip, so for the same wing area, having less wing tip (less chord, more span) is more efficient in lift than the opposite.

Perhaps at high speeds, having small wings are more efficient: then the wing still produces enough lift, but has less drag. But this assumes that the plane expends enough energy to be able to travel at a high speed.
Apr 17, 2018, 05:56 PM
Registered User
Most crazy efficient plane I have is a wing, but very sailplane like in that it's a sloper with a pretty thin airfoil, but I think it's the wing shape that's magic. But my flyzone calypso big-ass motorized glider comes close.

YG bird, The glide ratio is insane, so much so that it's difficult to land. Too bad they don't make it anymore. Ever since I flew this crazy thing I am a believer in slight forward swept wings and thin airfoils.

They turn smoother, super graceful. "Bird like" is a good way to describe it. I motorized mine and the flight times are insane.

Don't forget efficiency has a ton to do with wing loading (how heavy?) and lift/ airfoil shape. Which is why I don't make wings with 2 full layers of foam for my wings or KF wings anymore, but use a thin upper layer of 1mm depron over a simple foam spar, but I'm on my soapbox.

But I'm in a different boat right now, wondering what's the most stable airframe for going fast (since I have no really fast planes).. it might be a delta who knows?

Too windy to fly R.C ? ...YG Bird review (2 min 45 sec)
Last edited by rotagen; Apr 17, 2018 at 06:06 PM.
Apr 17, 2018, 06:09 PM
Registered User
Engineered Mini Flying Wing (9 min 5 sec)
Apr 17, 2018, 06:25 PM
IMO ( In My Opinion ) →
balsa or carbon's Avatar
This is the slowest flying wing I've ever seen , long span & very low wing loading ..... but I'll bet it can't be flown in any wind . I'm guessing the flights are probably fairly long with those tiny motors/props .


Slowly Indoor Glider (4 min 51 sec)



On his Vimeo channel , the builder has a link to his website with plans , etc .
Apr 17, 2018, 07:04 PM
Build straight - Fly twisty
Whiskers's Avatar
The fact is that all wings suffer from "tip loss."
The high pressure under the wing curls up at the wing tip and reacts with the low pressure air on top of the wing tip, creating a vortex.
The problem with a delta is that the tips form a large large percentage of the wing.
With a high aspect ratio wing, as seen on a typical sailplane, the tip area is a tiny percentage of the total area.
Apr 17, 2018, 07:13 PM
flyin' fool
goldguy's Avatar
This is a delta ........................
Apr 17, 2018, 07:19 PM
flyin' fool
goldguy's Avatar
This is a delta shaped model airplane ........................
Apr 17, 2018, 07:31 PM
flyin' fool
goldguy's Avatar
The delta shape makes a great sloper .......................... http://www.rc-soar.com/pibros/
Apr 17, 2018, 07:34 PM
flyin' fool
goldguy's Avatar
Are we talking about the same thing??
Last edited by goldguy; Apr 17, 2018 at 07:51 PM.
Apr 17, 2018, 08:08 PM
Build straight - Fly twisty
Whiskers's Avatar
Quote:
Originally Posted by goldguy
Are we talking about the same thing??
Ah yes! It seems we may be mistaken when we assume that a guy it talking about deltas when he asks about deltas.
Now where did I put my old crystal ball...
Apr 17, 2018, 08:35 PM
IMO ( In My Opinion ) →
balsa or carbon's Avatar
Delta quiz : which one is not a delta ? ↓

Apr 17, 2018, 08:37 PM
IMO ( In My Opinion ) →
balsa or carbon's Avatar
I know .... I'm just being a wise-ass .....
Apr 17, 2018, 09:47 PM
Potato003
After reading carefully each comment of yours guys I made some notes further below to every kind person that replied (thank you so much for taking your time!). So after all the comments I think I will have to go for a sailplane if I want the most flight duration as possible.

So I promise I will raise one last question in this thread! I searched many sailplanes plans on google and found lots of them. But I dont have access here in brazil cheaply to foam, they are fraking expensive. People always say you buy them on US in the "dolar tree" (which I guess costs 1 dolar) but here in brazil a veeery thin foam costs at least 8 to 10 dolars. I cant afford that mainly if I try to fly and break the plane. So I want to stick with the things that I already have cutted here using some fish box (salmon and tuna) that I got for free in the trash of a restaurant.

Having said that I ask you this: my motor, esc, prop, servos, linkages, battery and a "wing" that I have laying around here wheights about 400g. This is the weight I have to work with, I cant get lower than that, that's already the minimum. My main "wing" is a thick styrofoam which is 90cm X 30cm (rectangular) and also is 2cm thick (no airfoil profile to make this question easier). I ask you this: should I make this wing 90 x 10 cm (instead of 90 x 30) in order to get most flight time? I say that cause with a smaller chord I get smaller drag (Reynolds number is smaller) and wont need too much RPM of my motor BUT I also get smaller lift!

Bear with me: in order to get the biggest flight time I need to have my motor spinning at the slowest speed possible, right? I know BLDC motors have an efficiency curve X rpm (@extremesports already posted a diagram to me in another thread), but in general the lower the RPM the longer my battery will last. Having a wing that has a 500 cm span and 10 cm chord surely will be able to fly with a motor in much lower RPM than if the wing was 100 cm X 10 cm. Surelly a wing with 10cm wingspan and 500 cm chord will be terrible! So assuming my wingspan is fixed at 90 cm (I cant go bigger than that, it wont fit my bedroom and it's gonna be hard taking it inside a public bus transportation to fly) how do I determine the minimum chord so my plane can fly with the lower RPM of the motor and have greater flight time?

See, the only variable that I wanna change is the chord! How does chord relates to the best efficiency given fixed parameters?

I promise I will stop my questions here, no more questions besides this one!



@balsa or carbon thanks again my friend, definatelly I will follow your suggestion and go to a sailplane. That discus launch glider surelly looks very efficient cause event without a motor it sustained lots of time in the air! I will check posts from @boa2110 to make sure I get the most of it! The slowly indoor glider, despite being a delta, has a wing aspect ratio very different than the wings I see on most deltas, maybe that's why this slowly indoor glider gets a better performance and the guy can fly it inside the gym veeery slowly! I went to that guy website and will compare some of his plans with my last delta, thanks for bringing that in! I laughed about your last quiz hahaha but I think, according to @goldguy, all your drawings are deltas, right?

@flying-llama indeed, sailplanes are opposite of delta regarding wing size/aspect ratio, I was going in the wrong direction (building deltas) believing I was gonna get better efficiency! I agree with what you said: it's better you push down gently a little amount of ar (with a big wing span) than pushing hard lots of air (with big chord). That makes sense to me

@rotagen thanks and I am always trying to minimize weight to have better wing loading, exactly as you said! Also I am glad you can get great efficiency with your deltas but unfortunatelly I have no idea why there are so many mixed results: people saying deltas are the best efficiency possible and others saying deltas are not efficient because of reflex. I liked the YG bird, the video impressed me with the wind going up and keeping the deltas in the air!

@ftflyerkevin the video you provided raised me more questions than answers, but it's great! Mainly the 6:50m when the guy talks about the best region to fly (lower drag).

@Whiskers yeap, I also thought about that but I still dont think the vortex generated at the tips of the wing are the main curplit for such a bad flying time I had with my delta (almost 3x lower than flying with a regular plane).

@goldguy I thought deltas are the same as flying wings!!! Sorry for the confusion cause I see this terms being used in so many places as synonymous, but I will take more attention next time!


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