I maidened an plane for a gentleman the other day (his own design) and it was a beast. Although the elevon throws were overly sensitive, in both roll and pitch, I think the primary problem was a rearward CG as opposed to too much throw. At cruise speed, which was about 1/4 throttle with an MVVS up front swinging a 12x6 at about 18,000 rpm, the plane was sensitive but relatively controllable. and I was able to get in several circuits and trim the plane without too much difficulty. However, when slowing the plane for landing the pitch control became significantly more sensitive to the point that pulling back a click on the elevator would put the nose at about 60 degrees and likewise pushing a click would put the nose down about 60 degrees. As a result, the only way I could attempt to manage the glide slope for landing was to throttle back to try to get the nose down which then would result in a nasty tip stall to the left. About 20 attempts later, I did finally get it down without damage but I think it was more luck than skill. Before this thing goes back up, I want to make sure the CG is really correct. I had checked it on the ground and it seemed ball park to me, at least for a normal plane, but cursory research indicates that a tail-less plane may require a significantly more forward CG than a conventional design. Currently, the plane balances at the front most tan stripe on the wing tip. Is there a CG calculator out there that handles this type of planform or is there a good rule of thumb that I can apply.




Thanks, Tony

Your problemcould bethe Centre of pressure in relation to the centre of gravity. The aircraft may well have the CoP and CoG too closely coupled.

In the shown design, CoP should be at the point in the wing where the chord is at max thickness since it looks fairly symmetrical from leading edge to trailing edge.</p>

Another thing to remember is that a wing with such a large chord is going to be VERY sensitive to angle of attack, it will stall faster and it sound to me like thats probably whats going on, its stalling.

This aircraft is going to requirea CoG further forward and to be flown down to the landing to prevent a stall. You have a massive amount of parasitic drag at low speeds

Also check the engine thrust line. the pic does not show it very well. If you could take some pics of a side view, top view and front view that would be fantastic just to get an idea of wing shape, chord symmetry and dihedral/anhedral.</p>

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Interesting design btw.</p>

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i agree
i had this issue with a "flying flag" which is a square profile wing with elevons

cg with accurate but when 75%TH it stalls, either to the LT or to the RT

see my thread maybe it can help you improve your troubleshooting: http://www.rcuniverse.com/forum/m_88...tm.htm#8850568


nice plane though

Flying wings have their centre of pressure at about 25 percent of the chord, similar to the wings of airplanes having horizontal tails. Tailless airplanes must have their center of gravity no farther aft than 25 percent of mean chord before becoming dynamically unstable and nearly impossible to fly. I have found, during wrestling with several tailless designs, that CG about 22 percent of mean chord may the the practical maximum aft location for friendly handling. Tailless airplanes will usually be more sensitive in pitch, although this broad-chord example should exhibit reasonably friendly handling with the right CG.

You are a good pilot, Tony; you have saved the beast!

Consider that the area of the elevons is big and that small deflections may be just enough.

Regarding the left wing stalling, verify that it is not just lateral unbalance, which shows up at low speeds, or wing built-in twist.
It could also be a tendency of the design to go into a spin, helped by the engine’s torque, which could be remediated increasing the vertical fin area.
Stalls shouldn't happen when the nose is pointed down.
Delta wings and low AR wings are hard to stall.

Now, for the CG location, you need to:
1) Determine the MAC of each area that form one half wing
2) Divide each MAC length into four equal sections, and mark the first one closer to the wing’s LE.
3) Use the formula in the attached schematic to calculate the length of the MAC of the full wing, which should be located in the central line.
4) Repeat #2 for that MAC; that is where the neutral point (NP) of the full wing is.
5) Mark the CG range between the 10% and 20% closer to the forward point of that MAC.

Check these old posts as a reference for determining the MAC of each area graphically:

http://www.rcuniverse.com/forum/fb.asp?m=9268143

http://www.rcuniverse.com/forum/fb.asp?m=9265664

woo wee

darn, you are good

very nice answer , i think i will save your user name on a note on my PC desktop for times of truble

god job helping the guy!!!

Thanks, Gadix;.................just trying to give that nice plane a second chance.

actually delta's and low aspect ratio wings stall easily. Hence the use of canards with these types of wings.

Looking at the graphic layout so nicely done by LNEWQBAN it's apparent that the CG was at or even a hair behind the NP. Shifting it ahead by a goodly amount will likely turn the tiger into a far more agreeable play thing.

Thanks to all, especially LNEWQBAN, for some great information. Will run some calculations and give it another shot. Stay tuned.

Tony

tony
just keep them pics and videos comming

good luck

I may be wrong, but I believe that delta wings can stand higher AOA before stalling.
........I have to research on that.

You are welcome, Tony.[sm=thumbs_up.gif]

Delta's are not very good at high alpha because of the long chord length. airflow breaks away quickly and its' difficutl to rstroe hence the design has never been in favour with any designer looking to produce an agile aircraft that is aerodynamically unstable.

they are great for speed due to lower induced drag. thats why they were so popular on the MIrage III, 2000, and other variants.
In all applications where high aplha flight is required, the Delta is accompanied by a closely coupled Canard wing to keep the airflow over the top of the wing attached.

In RC models because the reynolds number is different the flow separation is less critical for normal flight but when we get down the slow speed flight it becomes critical again.

Remember something called a Diamond Dust? Thats a high speed delta, also does not fly well at low speeds or high alpha in the landing pattern because it stalls too quickly.

I've seen this characteristic on delta's low aspect ratio wings, flying STOP signs, lawn mowers, flags. Great at speed, poor at low speed high alpha due to stall

Actually a sharply angled delta (more than 50 degree leading edge sweep and where the root chord is equal or more than the span) can be VERY good at high alpha. This is because the air flows strongly around the leading edge in a vortex that fills in the top surface and strongly aids in reducing the effect of the wing stall. This vortex flow is so strong that delta wings with it can fly under full control at crazy angles of attack. For example the old Delta Dart and Dagger, the Hustler and the Concorde all use this very high alpha vortex flow and associated high drag to slow down for landings instead of using traditional flaps. And models with a high engough leading edge sweep angle can also take advantage of this.

The speed deltas such as the Diamond Dust did not do well at this because the leading edge sweep angle was not high enough and the tip chord was too big. In effect they were more of a sharply tapered and swept flying wing than a delta wing.

AH YES very true but that effect does not scale down to rc planes at low speed very well. Rc Plane delta wings behave differently to full size delta's.Full scale has leading edge extensions and or flaps to control that vortex formation and position and intensity with airspeed.  Then there is also the intricate wing shaping. Concorde particularly used a highly complex 3d shape wing to achieve its flight performance throughout the envelope, another example was the XB-70 Valkyrie.

Dart and Dagger were agile by fluke but not at the kind of low speeds at which dogfights traditionally come down too. They  also tended to land fast with a long roll out. Even Concorde had a very long roll out for it's size and it certainly landed a lot faster than any other similar sized aircraft employing a more traditional swept wing w/tailplane layout. Most of the older Delta wing aircraft could stay in the fight as long as the speed remained high. Hence they were good interceptors but not good air superiority fighters. Delta's in an air superiority role that includes dogfights do require aerodynamic instability, active controls for vortex formation and canards to keep flow coupled to perform at the same level as swept wing. Most designers of the 4th and 5th generation fighters use a highly swept root chord leading edge and then ease the sweep.


RC planes are not that sophisticated although I do know of a few UAV's...

Good post.

Really TimBle? I have been flying a Pete Russel '362' delta for a number of years and I can state with absolute certainty that it can, and does, fly both slower, and faster, than an equivalent 'normal' layout. The AOA attainable at slow speed cannot be attained with a normal plank wing. The drag in this condition is high, and you can't accelerate from low speeds as quickly, but as the AOA decreases the high speed attainable is higher than you would think. This delta has a 60deg angle, and as BMatthews states, is very difficult to stall. I do have to admit, that if one is trying high 'G' aerobatics that need a high AOA then the delta drag is higher than a normal layout and this may be one of the reasons that the fancy add-ons are used in full size. In model sizes, the delta is difficult to beat at the usual stuff. If you haven't tried it...
Evan, WB #12.

Timble, you should try a sharply raked delta. I admit that I have yet to get around to it but all the articles I've read about model deltas that have the more sharply swept wings indicate that the small models follow the full size faithfully. Keep in mind that this ONLY works with strong leading edge sweep angles. The Diamond Dust and others of similar layout do not have the angle needed for the vortex rolling action over the leading edge.

Concorde did indeed use a fancy wing shape but it was for efficiency in the super sonic end of the speed range. For landing drag they relied on the self formation of the vorticies that came with the high angles of attack.