Slow delta wings?

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Norman

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Blasphemer!:roll:

I marked up your drawing last night but didn't have time to write a post. Then when I looked at this thread again this morning I saw that you understood the main gear/thrust moment so decided it would be redundant but now that you have done the same to me it's only fair that I post my mangled version of your drawing. I'll also up the ante with a drawing showing how to figure the effects of tilting a thrust producer away from the line of motion.
thrust_vs_wheel-drag_moment.jpg

thrustvectors.jpg
 

Himat

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While we are at making sketches, look at this one:
ThrustAngles.jpg

On a flying wing, at the bottom in the sketch, there is a engine mounted in a pod.
On top of the pod, for no particular reason other than make this example, there is an arm with a heavy weight. The mass of this weight and length of the arm do relocate the centre of gravity to straight behind the propeller and the thrust line pass straight trough the centre of gravity. The weight and mounting arm is well streamlined and faired in such way that for this example the aerodynamic drag of said devices can be neglected.

Now, when trimmed for stable flight at full power the power is abruptly removed. Does the plane pitch up, down or continue without disturbance to the flight path?

Another case, trimmed for minimum stable level flight with minimum power the power is suddenly increased to maximum. Again, what effect does this have on the vehicle?

As I see it , the location of the centre of gravity and inertia matters, but the placement and direction of the thrust and drag vectors relative each other are as important.
 
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Norman

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As I see it , the location of the centre of gravity and inertia matters, but the placement and direction of the thrust and drag vectors relative each other are as important.
Nobody has said that the drag vector doesn't mater! The thrust vector and the drag vector couple to produce a moment that is constant and easily trimmed out if not too large. The thrust vector and the center of mass produce a moment that it only present during a speed change. Ideally the trust line would pass through both points but that's not possible for most pusher configurations because the prop would be too close to the ground and slicing through the wing wake causes serious vibration problems not to mention a rather large power lose. Puting the prop on the nose decreases stability and moves the pilot aft where he/she can't see anything. Hears a drawing of a propulsion system on a tailles airplane that actually works.
virtual-empennage.jpg
The thrust lost due to tilting the prop is not very large and vibration is minimized by getting most of the prop disk away from the wake
 

dirk_D

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Most deltas seem to be short and appear to have a centre of pressure not far behind the centre of gravity.
In a pusher arrangement, be it high or neutral thrustline, I'm wondering if it may cause some wild dutch roll motion?
Maybe this is why tractors are preffered, I'm thinking F117 with the nickname 'wobblin goblin' for a working example.
I take it a plane that wobbles has a centre of pressure close the c.o.g?

High thrustline is a glaring design challenge, add to that a wobbly design, wow, issues huh?
 

Kingfisher

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Nobody has said that the drag vector doesn't mater! The thrust vector and the drag vector couple to produce a moment that is constant and easily trimmed out if not too large. The thrust vector and the center of mass produce a moment that it only present during a speed change. Ideally the trust line would pass through both points but that's not possible for most pusher configurations because the prop would be too close to the ground and slicing through the wing wake causes serious vibration problems not to mention a rather large power lose. Puting the prop on the nose decreases stability and moves the pilot aft where he/she can't see anything. Hears a drawing of a propulsion system on a tailles airplane that actually works.
View attachment 40660
The thrust lost due to tilting the prop is not very large and vibration is minimized by getting most of the prop disk away from the wake
Yes, this is totally what I meant!

To summarise:
One has to distinguish a dynamic/transient condition from a static in flight condition.

Static flight condition determines trim setting for a given constant speed, e.g. climb out. This trim setting is determined by the couple of thrust vector and drag vector, where a greater distance between the two will require more extreme setting, and the C.G. vertical location has indeed nothing to do with it.

Dynamic condition would be the sudden throttling back during take-off. Assuming the trim was set for the expected static climb condition after take-off, the sudden removal of the thrust-drag couple could pitch the nose up due to
-the elevator trim downforce, larger for greater offset between thrust and drag vectors.
-he sudden prop drag, where the C.G. vertical distance to the prop does matter.

So the high mounted engine would be worse for these two reasons. If prop line is angled on a rear high mounted engine as Norman drew it, the up-trim could then be the same as for a centreline thrust, making it much better. Drag vector would still shift up for prop braking, as there would be no vertical component to that, or would it?

Last thought: As we saw, the high rear mounted engine will always have a penalty in additional downforce, either by trim or by angling the thrust line, so one should only do it if absolutely necessary, like in a seaplane.
If the engine was high mounted, but in front of the CG, one would generate extra lift by angling it up, while also eliminating the couple. So a tractor config should be better in that case?
 

BigBen

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The aoa correction for short span aircraft isn't too complicated:

dCl / dalpha = 2pi *(Ar/(Ar+2))

dCl/dalpha is the slope of the liftcurve in radians. In other words, for an infinite aspect ratio it's 2PI, so for each tenth of a radian (5.3 degrees) you increase your angle of attack the lift coefficient (Cl) goes up by 0.63. For an aspect ratio of 2 that same 5.3 degrees increase in angle of attack yields only a 0.315 higher Cl :)

As such an aspect ratio of 2 gives you only half the dcl/dalpha. That's good for turbulence (only half the gust strength), but for a normal profile that means your angle of attack for stall is in the 30 to 40 degrees angle of attack region. That's a biggie for the landing gear design. Making the aspect ratio a bit higher is the easiest solution.

The above part doesn't include vortex lift, but on your design I doubt it'll play any role ;)
I know this post is old but it is relevant to my question.
Is there an aspect ratio for a clipped delta such as the Verhees at which the beneficial stability/gust response starts to noticeably diminish?
Also at what sweep angle is the loss of generation of vortex lift noticeable?
Possibly restated, at what point does the clipped delta become a tapered plank?
 

Norman

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The slope of the cl/alpha curve becomes shallower as Autoreply described down to AR=2 (corresponding to 52 degrees leading edge sweep) then vortex lift kicks in and the cl/alpha curve jumps to the left a bit but on the same slope. There's a brief discussion of leading edge vortex here
 

Aerofoam

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Greeting all. I stumbled on this thread. This looks like a great forum with some great discussions.
I wish I would have seen this thread a few years ago, I might have saved some discussion.
I am a UAV designer and MFG.
I have built at least 75 to 100 iterations of delta wing designs and other tail-less aircraft. They have all been low Reynolds numbers compared to full scale, but most of the behavior is scalable..
I have been somewhat obsessed with flying wings since the late 60's and started building gliders as a kid.
I am not going to write a dissertation here, because it would just recap most of your conclusions.
If anyone wants specifics, I would be happy to have a phone call, or maybe address them a bit at a time on the forum as I am able.
I would like to add a couple things:
I encountered the thrust line problem on a project in 2003. I was told I would have a smaller 3 bladed prop available for a 130lbs aircraft that looked sort of like a sexy supermodel version of the bloated space shuttle.(I may be able to post a picture) I ended up having to use a 2 bladed prop that was about 12%
longer and that cascaded the problem into higher engine mounting, angled thrust line, taller gear etc.
The result was the plane stayed laminated to the deck with power on. I put a ridiculously tall front landing gear strut on it to increase AOA, so it would fly off the deck, NO JOY!!! On a final run down the tarmac at about 75kts, I chopped the power and the sudden prop disk drag caused the plane to rocket upwards at a 45deg. angle and when I tried to feed power back in, it simply rotated to a nose down attitude at about 100ft. (And below stall speed) I managed to cut power and pancake it in which only required a 12 hour all night session to repair...
This design was never acceptable with the higher thrust line and it wasn't really very high.
If I had this constraint again, I would add a canard on a track that could move fore and aft for CG adjustment. It could move all the way back into "Strake" position once the plane was "on Step".
I would not use a high thrust line on any man carrying aircraft.
Another thing I consider critical to an efficient flying wing design is to NOT have a fuselage.
Anything you do to the wing from 1/3rd. of the distance aft of the total chord length to the LE in the middle of the wing will severely effect the efficiency and drag of the airframe. The Horten brothers referred to this as "Middle Effect" and have a lot of documentation of it. I have empirically explored many aspects of this including adding pylon mounted fuselages above and below the wing. For anything flying below 150kts. you are better off with a thicker wing for payload. I think the Facet-Mobile really confirms this with it's almost offensive airfoil which still performs very well
(I actually think it is brilliant BTW)
I related this story to an A-10 pilot and he told me the A-10 had the similar thrust line issue.
He also said it was normal procedure to pump the nose brake when you wanted to rotate and it would cause the planes nose to bounce upward enough to initiate the rotation.
I took him at his word, but have never confirmed that story...
My bucket list includes at least building an ultralight version of one of my own delta designs.
I will probably build it quick and dirty, then test it from the ground with a couple sacks of concrete for a pilot. If all goes well, I will build the useful version...
It's easy to be brave when you are still on the ground!
 

PureFlight

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Greeting all. I stumbled on this thread. This looks like a great forum with some great discussions.
I wish I would have seen this thread a few years ago, I might have saved some discussion.
I am a UAV designer and MFG.
I have built at least 75 to 100 iterations of delta wing designs and other tail-less aircraft. They have all been low Reynolds numbers compared to full scale, but most of the behavior is scalable..
I have been somewhat obsessed with flying wings since the late 60's and started building gliders as a kid.
I am not going to write a dissertation here, because it would just recap most of your conclusions.
If anyone wants specifics, I would be happy to have a phone call, or maybe address them a bit at a time on the forum as I am able.
I would like to add a couple things:...
Hi everyone, my first post on here. Been lurking for a while but deciding to post since I'm finally getting my butt in gear to get moving on my own homebuilt design. Glad this thread is still alive. Aerofoam....I'm glad you posted here sounds like I could learn a TON form you. I also REALLY want to build a flying wing type aircraft. Seems there are many examples of flying wing type aircraft that go pretty fast relative to their power plant size. That just screams efficiency! I've been flying RC aircraft for many years and have always been impressed with how delta wings handle. I've even built a delta wing biplane that had phenomenal stability and slow flight characteristics. So why don't we see more flying wings in GA?

I'll try not to ramble on too much as I explain what I want to do. As of late I've been experimenting with different aircraft designs in RC models. My original goal was to create an aircraft that didn't take a decade to build or oodles of money....I know probably everyones hopes on the outset of these types of projects. My thinking was that if I could construct a plane out of very few shapes...no complex curves...it might cut down on construction time. And in kit form would also cut down on cost. Imagine a plane where the wing was one shape for most of it's span. So perhaps you could have sections of a wing preformed in carbon fiber that could just be attached together and slid over a spar. The fuselage could be a simple tube that could basically constructed with many of the same piece. For example, four sections bolted together could create a "ring" or short tube and then you could bolt these short tubes together to make a longer one. The basic design I wanted to apply this construction concept too was a high wing twin boom pusher plane. So a whole aircraft could be completed with a small set of simple shapes. The twin booms could simply be long carbon fiber tubes holding up the tail feathers. I've attached a picture of the plane I made. Here is a link to my video testing the plane. https://youtu.be/bq2ii6O_Fk8

So long story short(ish) I went through several other iterations with the wing and parts that made up this plane. On all my variations I either had problems with tip stalls(even after adding wing cuffs) or motor thrust angles. I even tried mounting the motor as a tractor up above the cockpit. It flew OK like that but the plane was getting too weird at that point =).

SO....I'm back to where I really wanted to be in the first place which is finding a good flying wing design that is easy to construct, has a decent top speed.....AND can also land slowly. For now I'll just stick with a tractor configuration even though I REALLY wanted to get the prop out of view while flying. I'm days away from receiving my next test model in the mail. It's an RC plane called the Wingnetic and it's made / sold by HobbyKing It's a flying wing of sorts, apparently handles great as an RC model. Flies fast with a small power system and can also fly pretty slow with out bad tip stall tendencies.

What do you guys think of this design and how it might turn out as a full scale plane?
 

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henryk

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Yes, for a small, lightweight plane like this, I would ignore vortex lift in your planning and calculations. Just go with the conventional formulae. It may be able to develop vortex lift, but you might never see that benefit except in situations where you pull some hard g's or inadvertently raise the nose too high on landing. The delta wing fighters have heavily loaded wings and need that extra AOA to bring their landing speeds down. Your lighter wing loading will allow you to land at a more comfortable AOA (much like the Dyke Delta).
https://www.youtube.com/watch?v=MIMPXr_VgNo

=LE vortex generators=low speed at low AoA...

BTW=KASPERWING can fly well under 20 mph...

https://www.youtube.com/watch?v=Yk2uXO4heY4


https://www.youtube.com/watch?v=WkntHrPOYKs&list=FLK6PTUYw6Ydp6VZ5foD6L3Q

-or BKB1-A =V min=10 m/s !
 
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PureFlight

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Hi everyone, my first post on here. Been lurking for a while but deciding to post ...

What do you guys think of this design and how it might turn out as a full scale plane?
Of course this one has also caught my eye. Hobbyking calls it the Skipper but it's a copy of the "North Star" or "Polaris" ? Not sure who the original designer is. Anyone know if this was ever a full scale plane? I love the rear mounted motor and water handling capabilities. Though I'm guessing with the wing shape it's not the most efficient design for it's wing area.

So between the Skipper(North Star / Polaris) and the Wingnetic what do you guys think is the better design for full scale?
 

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Himat

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Of course this one has also caught my eye. Hobbyking calls it the Skipper but it's a copy of the "North Star" or "Polaris" ? Not sure who the original designer is. Anyone know if this was ever a full scale plane?
The original design is the Northstar by Laddie Mikulasko, designed back in the early 1970'ies if I remember. I do have a pdf of the model airplane magazine article somewhere if I can find it. Balsa USA have made a kit for this RC model since the mid 1970'ies. Much later came the Polaris and the Dragonfly/Skipper. I don't know of a "full size" airplane like this. The layout is not that well suited for a light manned airplane.
 
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