The lighter the better in weak lift ?

[yclui]

I am not too sure. . . . .

It was rather foggy ( so¡K NO thermal for sure ) off the slope today and the wind was really weak so I flew my Highlight HLG ( I believe it's the equivalent of Omega in the US. info at http://www.euro-sailplanes.co.uk/uk/html/sport-hlg/highlight.htm ) thinking that my model is probably the only one that can stay aloft. Much to my amazement, my friend¡¦s all-mold mini NYX ( http://www.euro-sailplanes.co.uk/uk/html/sport-hlg/mini-nyx.htm ) did equally well, may be even slightly better.

Both models are 1.5m in span and have similar wing area but my friend¡¦s moldy is twice as heavy as my HLG. The fact that both models can stay aloft equally well makes me wonder whether weight is such a dominating factor in determining how floaty a glider is under weak-lift condition.

I know the information given here is too rough and incomplete for any systematic analysis but if someone can suggest what MIGHT have counteracted the weight penalty of my friend¡¦s moldy, I will appreciate it.

I have came across the theory that up to a certain point, increasing the weight helps to increase the speed and hence the ¡§Reynold¡¦s number¡¨ which is supposed to make the airfoil more efficient. To be honest, I have not done sufficient reading on aerodynamics to understand that theory well but I did try it out today by putting 10% more weight into my HLG but there was not any perceivable improvement. Having said that, the test was very rough so it really didn¡¦t prove or disprove anything. If anyone has any ¡§thumbs-on¡¨ experience in this regard, I would like to hear about it.

Thanks,

Y C Lui

[Ollie]

The airspeed and sinking speed increase as the square root of the wing loading. In the case of a ten percent increase in wing loading, the increase in airspeed and sinking speed will only be 4.88%. As you have guessed, it will be very hard to tell the difference by casual observation.

In a thermal soaring environment where the low altitude lift is weak and very small in diameter, increased wing loading has a bigger effect because it not only increase the sinking speed in straight line flight but increases the radius of circle for a given angle of bank. To stay in the strongest part of the lift zone a heavier loaded model must bank steeper for the same size circle, further increasing the sinking speed.

In a slope soaring environment there is a fairly sensitive test that allows you to compare two models. If both models are flown in competition with each other, at the same time, to see which can gain the greatest altitude in weak lift, the model with the lowest sinking speed will win the competition.

The soaring column in the current issue of Model Aviation magazine discusses the difference in performance between airfoils like the SD7037, RG15 and MH32 which were designed to work well at reynolds numbers in the range of 100,000 to 200,000 (corresponding to heavy 3-meter sailplanes) and thinner lower camber airfoils like the AG16 which were designed to work well at reynolds numbers below 100,000 (corresponding to 2-meter and HLG). In the example, at a reynolds number of 80,000, the AG16 has about a 30% lower profile drag than the MH32!

[davidleitch]

Ollie

Thank you for that clear and concise explanation. What are the main determinants of a Reynolds number?

[Ollie]

Reynolds number is equal to density times velocity times length divided by viscosity. In a standard atmosphere, reynolds number is equql to 6363 times velocity in feet per second times length in feet. When applied to the airfoil of a wing, the length is the wing chord.

Reynolds numbers are useful in determining similar patterns of flow. When the reynolds numbers are the same and the shapes are similar the flow pattern will be the same even though the lengths and velocities may be different. When lift, drag and pitching moment coefficients of an airfoil are measured in a wind tunnel at various reynolds numbers and angles of attack, the results can then be applied to aircraft with a variety of chords and velocities using that airfoil and operating in that range of reynolds numbers.

[ChrisP]

Originally posted by Ollie
...........the results can then be applied to aircraft with a variety of chords and velocities using that airfoil and operating in that range of reynolds numbers. .......but not forgetting, of course, that induced drag is a function of aspect ratio which in turn is related to wing chord.

So you are playing a game between increasing wing chord to increase the Reynolds number (and thereby aerofoil efficiency) vs decreasing it to increase aspect ratio (and thereby lower induced drag).

I think there may be a case that if you want a small model to float well in light lift, a lower aspect ratio could help. Certainly there used to be an old rule of thumb in the free flight model glider world that a model with a chord of less than 6 inches ''didn't fly worth a darn''.

Just my two Eurocents.

[Ollie]

Generally speaking, the thinner and lower cambered the airfoil, the lower the reynolds number at which the trade balances between profile drag and induced drag for a given wing area. The design of the airfoil contour and and its control of laminar separation bubble component of profile drag is also an influence on where the trade balances. The trade also depends on whether you are optimizing for minimum sinking speed or maximum lift to drag ratio.

[ChrisP]

Originally posted by Ollie
The trade also depends on whether you are optimizing for minimum sinking speed or maximum lift to drag ratio. It was a very long time ago that I got my Batchelor of Science Degree in Aeronautical Engineering, but my brain still keeps whispering 'Breguet Range Formula' to me.

Yes, I hear what you're saying (great stuff by the way) but I'm a pre-Woodstock aerodynamicist. Actually my degree was in Aeronautical & Astronautical Engineering pre Moon Landing !!

The world belongs to the Young. Peace brother !

Cheers !

ChrisP

PS - Sparky Paul. Where are you when we need you ?

[Ollie]

ChrisP,

How old are you? I'm 72 years, 3 months and counting. I don't even have a degree on aeronautical or mechanical engineering. Never even took a course in fluid dynamics. Most of what I have learned is from casual reading and people like Dr's. M. Drela and Michael Selig also, Martin Simons who are kind enough to teach us about model aerodynamics. You just have to be interested and dig for it.

It helps to be interested in sailplane design where drag reduction is the name of the game. The power guys have it a lot easier where their main requirements are met with a bigger engine and more control authority.

[ChrisP]

COOOOOOOOL !

I guess most clubs have an 'aerodynamic expert' or two without any formal training at all. They have the strangest ideas. I had one guy try and convince me the way to increase spin rate was to increase the size of the fin & rudder. When I told him it was exactly the opposite he told me I had no idea and should listen to the experts !
So it's tremendously encouraging to hear that there are people like you who are getting enjoyment and knowledge out of digging in to this fascinating subject. Impressive !

I was 56 last month. I went to the same school that Frank Whittle (father of the jet engine) went to and did my degree at Southampton University 1965-68. Gave up aircraft design for car design (34 years with General Motors).

Cheers

ChrisP

[davidleitch]

I put two smallish servos (Hitec 55s) on the top of the elevator of a V tail electric glider (Silent Dream 2.2 metre). I did this because I dislike installing the long linkages and gluing tubes for said linkage inside the fuselage.

Question is how much induced drag am I thus creating? I did provide the balsa mounting block.

[Ollie]

If you mounted the servos outside the skin of the tail surfaces, you interfered seriously with the flow over that part of the tail and effectively reduced the stabilization effect of the tail area. The main increase in drag will be parasitic drag. Parasitic drag is an important part of the drag budget at high speeds and less important at slow speeds.

Induced drag is mainly associated with the tip vortices and lift distribution of the wing and to a much smaller extent with the tail. Induced drag dominates at airspeed below that of the best L/D.

[davidleitch]

Here's a photo of offending V tail. Servos on top. To date the servo efficiency and simplicity have been worth a lot to me. In theory I can also remove the V tail for transport easily.

However I am now worried about the impact the parastic drag will have on performance.

How do I estimate the increase parasitic drag and its impact on L/D ratio at cruising speed, say 40 KPH?

One could remount the servos in the body of the fuselage but at the tail. That would combine the efficiency of short linkages with lower parasitic drag. However its a material amount of work and I'm reluctant to fiddle with a system that from day to day usability is working well unless there are sufficient flying benefits?

[BMatthews]

As much as that makes me cringe I have to admit that it's probably not as bad as we are making it out to be.

Consider that the servos are mounted down in the apex of the surfaces and near the fuselage. There would be a reasonable amount of turbulent flow along that area anyway so it's not like the servos are hanging out in clear air. At a guess I'd say this arrangement is probably only about 1/2 to 2/3 as bad as we have been making it out to be.

Now if you could bring yourself to cut those root areas of the surfaces out and sink the servos down into there that could help a lot. But then I suppose the joiner wires are in there.

One day you may find yourself flying in the same air as another model of the same type but without the servos hanging out. If this happens talk the other pilot into a drag race and see who gets back first or who looses the most altitude to maintain the same speed.

And as the others said. If most of your flying is floating around then it's not a big deal. If you like to put the nose down and speed around searching large areas for lift then it's a bigger deal.