Characteristics of Deltas vs Wings

[Slow Riser]

I have been designing small flying wings this summer. With the help of all you here, I have had, at least in my mind, much success.

I am thinking of designing a delta similar to a Mirage.

Question I have is although both wings and deltas, at least at the models I have seen online, use elevons. What flying characteristics to deltas have over wings?

I imagine both have reflex assuming you use a non-reflexed airfoil. The only advantage I see is a delta, is having a shorter wing-span but longer root chord. This will result in much better roll characteristics.

Is there any other redeeming characterics of a delta over a wing?

Thanks!

[RMason]

Other than being able to sustain a higher angle of atack with a delta, a wing has all the advantages. With full scale a delta gives you a larger wing root depth for putting gear, fuel and equipment.

rob

[Sparky Paul]

About the only visible characteristic a delta has that would be superior to a wing would be roll rate.

[Dereck]

In both cases, you use elevons because you have moved the elevators forward to the TE of the wing. The ailerons were there to start with, hence the two functions of pitch and roll get done by the same surfaces. Wing or delta are about the same here.

Flying differences. A flying wing, or at least the six or so I've designed, flies not too far removed from a regular planform in that they roll similar, will stall in much the same sense but can be a little heavier in pitch.

The delta will roll real fast - shorter span to length ratio. Tends to be similar in pitch to a short span, low AR flying wing until you approach the stall. Whereas a FW will stall if balanced correctly (Simitars are nose heavy!), a delta will just keep on going nose high until it runs out of power to drag it along,whereupon it will sink like a brick - but still under control in roll.

You'd need an aerodynamicist to tell you just why, but a delta does not have a classic stall like a regular shaped flying machine.

However - to enjoy this wierd corner of the flight envelope, you need absolutely masses of power - not what small electrics are reknowned for. My last glow delta could end up so nose high that even full power could not sustain level flight, and I regularly beat out OT models at "slow flight races" across the field - they couldn't stay in the air at speeds my delta could handle at easily.

My last electric delta could not get near to this nose high, slow flight regime - just not enough power available from its S400. It could go like a bat out of you-know-where and roll like crazy, but didn't have the power to slow fly nose high at all.

Another point is that a delta will rapidly go from low drag (very fast) when in low angle of attack flight - to real high drag and with speed bleeding off very quickly when the nose is raised. Means, amongst others, that one fast, tight turn and your screaming delta is wallowing along barely able to hold its height.

The classic and easiest baby delta was Garrison's "Positron" - now 'remaindered' with the last few for sale at New Creations. An object lesson in how to do it cheaply and simply - flew great as well.

Regards

Dereck

[Dave Hederich]

Isn't it possible to correct some of the delta's bad habits by extending the nose of the aircraft and adding canards up front?

[Slow Riser]

Just wondering if there is an optimal dimension of a delta?

From the models I have seen, the ratio of the wingspan to length of the chord (root) is about 2:1. This would yield a leading edge sweep of 45 degrees?

[MtnGoat]

I've had great luck with several models at the following delta ratio:

span 17
root 12
tip 5

I don't know what the sweep for this works out to be, but it's a lot less severe than 45. While it doesn't roll as quickly as a 45 sweep it also has a better aspect ratio and doesn't suffer from problems in slow tight turns, given a decent foil anyway.

I've scaled this planform to 48 and 72 inch spans and it's worked quite well. I use the MH45 foil after trying several with various EH series foils and I will never go back.

While the EH's worked fine, the MH is head and shoulders better, lower drag at identical reynold numbers, and will pull higher CL's for tighter slower turns too. The difference was immediately noticable when I first tried an MH45 vs the previous incarnation with the same planform but an EH foil, the polars showed this should be the case but flying them you can see and feel an obvious improvement.

The MH also has a great speed range, dial a click or two of up in for light air and it will slow down and stay up incredibly well, my wing is usually the last thing in the air if the slope lift gets light. And if it picks up, unlike my EH 72" bird, my MH 72" delta has *never* needed ballast, all I do is a couple clicks of down trim and it hauls.

One other thing I have noticed about deltas (given my standard planform!) is that they seem to hold a turn better than swept wing planforms, maybe that is another factor however. In flying various Zagis and a Boomerang I notice that in thermal turns the plane must be managed a bit to stay in the turn, they tend to flatten out, whereas my deltas just stay put at a fixed bank once you set them there. As long as it's not so tight it approaches stall!

Which bring up another thing, the stall characteristics of the MH45 are also better than the EH's IMO.

[Slow Riser]

Thanks for your dimensions.

I building around an IPS DD drive with a 3 x 2 prop. I have just created a jig to cut foams for a delta wing that has a 10" root chord and a 14.5" wingspan and 2.5" tip chords. This will yield an area of about 100 sq inches. I'm aiming at an AUW of 5 oz which is about a 7.2 oz/sq ft wing loading.

I looked at the MH45 but the trailing edge is too thin to hang elevons off of...at least thats what I think...am I wrong?

I'm working with a highly modified clark y for my prototype. The foam cores I have cut seem to glide nicely.

[MtnGoat]

it's a darned thin trailing edge, even at large scale, I agree with that. I'm using balsa elevons but have considered a light glass job to stiffen it up.

it's the foil used on the "wedgie", that rocket fast s400 swept wing kit, so it ought to work at small sizes like you are discussing.

good luck, when will it be flying?

[Dave Hederich]

Slow Riser, I hope that you will share some photos of your small foam delta when it is done. It sounds intriguing. Will you be using the standard IPS motor or the 4.8V EDF-50 motor? The 4.8V motor could obviously save some weight by using fewer cells. Some have mentioned running the 4.8V motor direct drive with the 3x2 prop and a single lithium cell.

[Slow Riser]

I have one but it draws too many amps on 2 Qualcom's. I haven't tried a single cell but I figure that would be a big draw on a single cell as well but it can't hurt to test this scenario out. All you would need is 1 cell at 0.9 oz for a Qualcom or 0.75 oz for an UL LiPoly! If you have a link to persons testing the EDF motor with a single QC cell, would like to see as I haven't been able to find it.

Asperkins sold me some of his "surplus" Ultralife Li-Poly cells as I think he is making the move over to the Kokams. About 1.5oz for a 2 cell pack which I will use. The following link is a page I created for some friends and other "beta" testers:

http://www.royalkensington.org/~wing/home.htm

All my designs thus far are based on the IPS DD with 3x2 and lithium batteries. The Micro E-Wing is about 132 sq in and flies extremely well but to going smaller presents a challenge as the airfoil thickness is getting too thin and its too hard to embed the r/c gear. I thought going to a delta would solve some of the structural limitations of a small wing and that has proven the biggest winner so far.

The biggest challenge to cutting a foam core is getting the technique down to pulling the hotwire 10" at the root versus 2.5" at the tips in a consistent manner...getting pretty close last night though.

Once I get the cores cut, it will take an hour to put in the gear. One of the advantages so far is the thicker section at the root. Plenty of room to embed all the equipment. As soon as I get the cg worked out, there is enough room to embed the battery as well. Another plus is all I need is strapping tape to hold the delta together. In my E-Wings, I was embedding CF rods to keep it stiff.

I'll post some photos after I'm finished.

[Dsegal]

I built a .25 IC powered delta this year and the biggest problem was the overly long takeoff run. I have heard from an aerospace engineer that this is a characteristic of full-scale deltas and canards.

Dave Segal

[dtroyer]

Isn't it possible to correct some of the delta's bad habits by extending the nose of the aircraft and adding canards up front?

The "bad habits" are the set of compromises you get with the delta wing planform. The delta will always have very high drag at high angle of attack, whether it be a flying wing, or have canard or tail planes for pitch control. Examples of all three layouts have been used in full scale high performance aircraft. They land relatively slowly with high angle of attack, but care must be excercised to not get behind the power curve.

[Matt M]

The reason deltas don't stall at high AOA is that they make lift differently at those high AOA's.

Once you get past the 'normal' region for AOA (up to about 18 degrees) you have a vortex that is shed from the leading edge of the wing, which creates low pressure above the wing and the high pressure below is created by the dynamic pressure of the oncoming 'free stream' air.

It makes more sense if I could show you a picture or cartoon diagram, but I have not found one that is suitable.

The deal with canards is that (at least for 1:1 airplanes) is that they are designed to stall before the main wing. This prevents stalling the main wing (pretty much) entirely since you can't get to the AOA required to stall the main wing because the stalled canards prevent further pitch up.

[Slow Riser]

I have started a new thread in the Foams section with my delta. We'll see how she flies this weekend.

http://www.rcgroups.com/forums/showthread.php?s=&threadid=63631

I found that I did not need any, or very little, reflex. Is this a characteristic of a delta?

[Sparky Paul]

Little to no reflex.. on such a small plane, any reflex will be effective.. but also, that indicates the c.g. is possibly a bit aft.
The airfoil looks OK in your pix.
I have to say your whole plane is smaller than most of the horizontals on my planes! :D
.