Very low aspect ratio planes?

[Sockmonkey]

Most tractor-mounted single engines and props are in pretty much the same place unless you are doing something like a Weedhopper or a Flaglor Scooter. In that sense the engine and prop are equally in the way in terms of visibility, so I don't think that's really the visibility issue. That UFO design could easily have a clear panel on as the access door.

Twin engines means you can put a clear panel between them so the nose isn't going to block any of the forward view. Prop length means at least a meter between the engines.

 

[cluttonfred]

I get it, I was just pointing out that the problem you are trying to solve has nothing to do with the delta wing or LAR, it's common to most tractor-engine singles.

 

[Vigilant1]

With this oft-mentioned idea of a clear panel in the floor, where does the instrument panel go? Maybe to a cluster on either side (viewed with iguana eye scanning technique).

Looking through glass/plexiglass at very low grazing angles makes seeing things pretty tough.

Maybe it works fine, but it sounds like it would be problematic unless it was something like a glazed nose or helicopter bubble.

 

[rotax618]

With a many LAR Aircraft there is a large internal volume, and it would be possible to have a glazed panel either side of the cockpit floor that would allow downward and forward vision, the FMX4 had something similar.

 

[Voidhawk9]

It would not be difficuult these day to have a camera on the nose feeding a screen on the panel that would look almost like a 'see-through' nose.

 

[pictsidhe]

It would not be difficuult these day to have a camera on the nose feeding a screen on the panel that would look almost like a 'see-through' nose.

yeah, but windows dont suddenly break on you. Oh, wait a minute.

 

[berridos]

Installing large windows in the floor of a monocoque composite structure has a high weight penalty. Besides in that area you have the landing gear retraction cage, the radiator outlet scoops, your legs, the front spar.....
A screen on your panel with gyroscopic cameras in front and a redudant remote vision googles is the way to go. Maybe someday I take a trip to holland to see those floor windows in real life and convince myself otherwise

 

[Arfang]

At low CL the vortices are skinny and weak but grow as CL increases. At high CL the tip vortices affect the flow about 1 to 1.5 times the tip chord inboard of the tip and delay separation but at low CL the vortices are very skinny and probably don't have much effect farther inboard than a few inches.

Thank you for the clarification, it make sense now.

I read the post you linked:

It doesn't help that we aren't all speaking the same language either. The notation from different labs can look a bit different and notations from different countries have changed over time. CP is probably the worst example of this drift over time. Before about 1935 everybody plotted the position of the center of pressure (c.p.) which leads to infinities because the pitching moment does not drop to zero when the AoA is such that the wing isn't producing lift. So by the mid 1940s all the labs had switched over to using the theoretical AC at c/4 and a moment coefficient (Cm) so the engineer has real numbers that he can do math with. Now we also have a new notation, Cp, that stands for "coefficient of pressure". Cp is the pressure distribution on the airfoil surfaces not the old c.p. but it's a similar looking notation so gets confused with the older notation.

Sorry this is kind of short and disjointed. I may wright something better after the holiday

What you wrote left me confused, what is the difference between center of pressure and coefficient of pressure? There's two different Cp? Many sources I read define cp as center of pressure, for instance in Airplane performance, stability and control by Perkins and Hage: ''center of pressure, cp, is the distance from the leading edge to a point on the chord through which the resultant of all the pressure forces on the airfoil section is assumed to act.''

About the aerodynamic center, from the same book:

''Hydrodynamic theory shows that for a particular position on an airfoil section the corresponding moment coefficient is a constant, independent from lift coefficient. This point is defined as the aerodynamic center ac. The aerodynamic center usually lies very close to the chord and from 22 to 26 per cent of the chord from the leading edge.''

Now, from what I learned:

CG is where the weight is applied, and neutral point being the most aft location at which the CG can be placed in order for the airplane to remain stable
CP is where the resulting lift and drag forces are applied, this point moves forward as alpha increase
AC is the 'fulcrum' around which mg and L acts

Then, I put together what I found on the relationship between CG, AC, CP and Cm (correct me if I'm wrong):
at CG-> Cm = f(alpha)
at CP-> Cm = 0
at AC-> Cm = constant

Now, is Cm around the AC caused by torque caused by mg and lift force around the AC or is it a separate moment created by the action of air on the wing and 'inherent' to the airfoil shape? I tried to represent both scenarios with a statics diagram but it doesn't seem to work.

If the position of AC determines where we put our CG to stay inside a given SM and that the usual 'put the CG ahead of the quarter MAC' can lead to dangerous situations, it seems to me that knowing the exact position of the AC is vital.

Thank you for bringing up that subject.

If I'm too off-topic I see no objection to the above been moved elsewhere.

 

[Arfang]

Very low aspect ratio planforms are usually very stable in pitch and can’t be compared with planks or high aspect ratio swept wings, the 2D airfoil data is a poor predictor of the general stability and behaviour of LAR, particularly at low speed. As to the CG, I found the only way is to fly a model, it is nearly always further forward than you estimate. Circular planforms seem to be at about 25% root chord irrespective of the airfoil.
I use the root chord as MAC is meaningless on most LAR planforms.

This is interesting, could you elaborate on that? If 2D airfoil data and calculations based on MAC don't work well, what tools do you use to predict or analyze the behavior of your models? Or is it something you 'see' when doing test-flights?

 

[rotax618]

I’m no theorist, I build, fly and observe. My method is fraught with problems, there a many variables which effect the results, small differences in rigging and shape have a greater effect on a model, scale effect, surface roughness etc.. To really get very accurate results requires an accurate shape in a wind tunnel. Never the less I do get some pretty interesting results and a lot of entertainment.
As Norman has said the 2D predictions of airfoil performance pretty much apply to all wings irrespective of shape But as Zimmerman showed this does not apply universally at high alpha where certain shapes at very low aspect ratios continue to produce lift long after higher aspect planforms have stalled, there is a drag penalty to produce the lift, but if the craft is landing, drag is an acceptable trade off for a very low landing speed, stall and spin resistance

 

[rotax618]

As to vision, polycarbonate sheet is considerably stronger than fabric and doesn’t usually shatter unless it has been subjected to some solvents, it isn’t even very heavy in 2mm thickness, is used in windscreens which are subject to far greater wind loads.
It’s amazing what the nay sayers can dredge up as a reason not to consider as simple solution.

 

[Sockmonkey]

I get it, I was just pointing out that the problem you are trying to solve has nothing to do with the delta wing or LAR, it's common to most tractor-engine singles.

Right, but in narrow-bodied planes it's usually easy to see the ground by looking down on either side of the engine, which you can't do in a facetmobile or verhees delta. Small twin engines does solve two other issues. Those being asymmetrical prop torque and engine cost.

 

[cluttonfred]

I hear you, Sockmonkey, but I am going to have to throw down a flag on that last point. There are very few situations in which one powerplant of X horsepower power more expensive than two similar engines/propellers/installations of X/2 horsepower. Usually, when two small engines are used it's either because of the perceived benefit of a twin over a single or an attempt to reuse engines or components already on hand.

 

[pictsidhe]

As to vision, polycarbonate sheet is considerably stronger than fabric and doesn’t usually shatter unless it has been subjected to some solvents, it isn’t even very heavy in 2mm thickness, is used in windscreens which are subject to far greater wind loads.
It’s amazing what the nay sayers can dredge up as a reason not to consider as simple solution.

Polycarbonate is very bad for stress cracking. That can be avoided with careful design and construction.

 

[Vigilant1]

I hear you, Sockmonkey, but I am going to have to throw down a flag on that last point. There are very few situations in which one powerplant of X horsepower power more expensive than two similar engines/propellers/installations of X/2 horsepower.

I think things get a little crazy on the low HP end of the scale. Two new 30 hp industrial engines fitted for flight and with props can probably be had for less than one new 60 HP VW based aero engine. And two new 75-80 HP VW aero engines can be had for less than one new O-320 engine. But whether these are comparable in other ways (esp resale value) would be another issue.
Most important, in general a twin that can't remain safely airborne on one engine is much less safe than a single. This drives a lot of compromises in the design of the twin (prop pitch, wingspan, max takeoff weight, etc).

 

[pictsidhe]

Harbor freight 212s are a bit over $100 a pop. Might be ok for 10hp. A pair of those on something like a Lazair, VJ or woodhopper could be the cheapest way fly. Also a very cheap engine to experiment with.

 

[WonderousMountain]

The second cheapest way to fly being three of them.

 

[Norman]

I read the post you linked:

There were several links. This post was way down one of those pages so you may have missed it. It has an illustration of where the pitching moment comes from.

What you wrote left me confused, what is the difference between center of pressure and coefficient of pressure?

c.p. (see dot pee dot) is a point that moves around a lot depending on AoA and camber. It's the sum of all the vertical pressure force on the airfoil ie lift. The distance between the c. p. and the AC is a lever arm and the lift is the force. Force times distance produces a moment. Cm is that moment





Cp (big C little p (no dots)) is the Coefficient of pressure. The OpPoints window in XFLR5 shows both of them when the <pressure> box is checked but the c.p. is not mentioned because it's obsolete. The reason it was dropped from the polar charts and replaced with Cm is that at zero lift there's still a pitching moment implying a moment arm of infinite length ie a moment arm of finite length can not produce a torque without a force acting on it. So the pitching moment is a couple produced by the pressure forces acting on the top and bottom of the airfoil, not a torque produced by a single lever.

There's two different Cp? Many sources I read define cp as center of pressure, for instance in Airplane performance, stability and control by Perkins and Hage: ''center of pressure, cp, is the distance from the leading edge to a point on the chord through which the resultant of all the pressure forces on the airfoil section is assumed to act.''

Hopefully the forgoing illustrations have clarified the difference between Cp and c.p.
Cm and c.p. are just different ways to show the same thing but c.p. leads to infinities at low AoA which causes problems for engineers trying to do math, Cm is a real number. Cp is the pressure distribution around the airfoil and is not related to the moment although moment is derived from it.

I hope I didn't screw that up too badly but there are people on this forum who may jump in and correct me. They have in the past but for some reason don't speak up until after I put my foot in my mouth.

 

[Arfang]

Thank you for your explanation. It makes me realise how much I still have to learn.

So, if I understand correctly, instead of summing Y forces and using the resultant force to create a moment around the AC, we keep both Y+ and Y- components, and use that instead, now we always end up with a moment even with zero lift. And since both magnitude and chordwise position are moving depending on alpha, the moment at ac stays the same.

I just don't understand how forces (or pressure coefficient) located between the leading edge and the trailling edge can give a resulting force outside the airfoil. Or if coefficient of pressure is a dimensionless number, how do we use it to produce a moment since it's not a force? Also I suppose there are forces acting in the x axis (drag for instance, or X-axis lift component), shouldn't those play a role in producing a moment on the airfoil?

Regarding the ac position, I finally found a paper explaining a method to find it in a manner I can understand:



Would you say it looks right? If so, how do I know where on the x axis was Cm measurement taken, for instance on the graph below:


The Cm value being all over the place means it's not Cm measured at the ac but at some other point, correct?

Thank you again for your time.

 

[nestofdragons]

Just my idea about pilot position and overal view in very low aspect ratio.
Why not place a thicker root chord and get as much as possible the pilot inside the wing? Just put his eyeline over the wing-surface so he can see above the wing. Due to the pilot being mostly inside the wing, you only need to make the front transparent. Not really the underside.
About the spar. I came up with this (probably crazy) idea to create a struss spar in which a pilot can be seated in between. With a reinforcement between front and rear of rear besides the pilot, torsion might be no problem. Remember ... i am not a engineer. I am just guessing.
Here a draft (from the past) of a glider i drew in 3D using that idea.