JH25 aerofoil section

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Doc J

New member
Oct 7, 2018
10
1
#1
Hi Guys,
this is the basis of the sections that I have been using for the Schwing, Schwing Corsa, Stormbird, Redshift and the new Firebird.

Designed to be a high response profile that works well with the control surfaces, its a double convex section.

As you can see it looks a bit weird compared to "Conventional" sections, but it works well and will take a nice lot of ballast.

Unlike the "super secret" sections we se so often, this one is open house and available to anyone who wants to use it or play with it.

Cheers,

Doc.

JH.25 polars.png
JH25 general..png
 

purview

New member
Nov 5, 2018
63
0
#2
Dear James,

I need some extra tuition on aerodynamics. I took the chart from Cm (pitching moment) and Alpha (α). The colors should be the same as you released on the 8Th of october. There seen the different Re number spectrum (0.1 to 0.3) we use on modelplanes/unmannedvehicles and quiet not in the range of positive mach numbers. So fast so good.

While disturbing the pitch axis (on the pic within the red circle) at Alpha -2° the detected different behaviours (the spread that starts at 4°α turns into upside-down). Based on different reynold numbers it indicates that different control inputs are required to achieve stabilization.
I am interessted in the drop of near and
JH.25 polars extend.png
below Alpha -5°. Your chart pictures nicely the situation above 13 degrees on the positive side. Is it possible to know more what happens more on the left side of this chart. I certainly do not know why the airfoil changes behaviour (on this chart five times) as indicated by the brown graph and the violet one (or the dark red at Re 0.300). What should a pilot know about this phenomenon or is it just a normal state of flying this objects?

Many thanks for your help. I have no proven education on this type of Things. I if I termed something wrong than please correct.

Cheers
Chris
 
Dec 13, 2017
335
21
Taiwan
#3
HI Chris,
yes the chart does start to do some nice dances around the negative portion of the envelope - but in fact most aerofoils do this.

I think that at higher negative angles the bottom boundary layer is detached or beginning to detach, and that will have some strange effects on the entire lift profile around the transitions - as we see on the graph.

I have never really looked at this in great detail as I have not noticed any bad effects in the flight envelope.

Happily we rarely fly at high negative alpha angles for any length of time.

Cheers,

James
 

purview

New member
Nov 5, 2018
63
0
#4
Hello James,

I need such flying state during pitching manövers and it works very good (the down sections are yet not so fast as the pitching up). In combination to target the spot where I want to be during decending (-80° Alpha ore more, dependent on wind speeds and slopes) the whole aircraft should track precise and it does so. What the above airfoil chart indicates (Cm/Alpha) is the problem of how to get to know more about the effect. On the plane and not only for the airfoil itself. For example while I fly I rarely can use laminar flows. So disturbances are rekon but wihtout what the hell is going on. Maybe pitch response changes by given control input.
But this is an universal problem and related to airfoils.

Your charts are constructed via wind tunnel testing or computer driven simulation? (data collection, given x and y)

I will search for boundary layer and transitions of lift in the web. It will help
Thanks for your words. Sorry to learn about such things in your JH25 thread
Chris
 
Dec 13, 2017
335
21
Taiwan
#5
Hi Chris,
you are right - our models rarely if ever fly in a real laminar flow, at least on the upper part of the aerofoil section. It's all theoretical at best.

This why on any fast slope design its a lot better to worry about keeping the bottom of the wing clean rather than the top, especially when you know that the boundary layer will depart at about 40 to 60% back from the leading edge whatever you do. Unless of course you can go to DS speeds where the laminar flow is actually possible and sustainable - but then you get into other lambda wave problems.

There's no free lunch!

I use computer simulation for design and initial testing because its really fast - but I have found that this can be very inaccurate when compared to actual physical testing. For example a quick glance at a computer analysis of any new aerofoil will indicate if it has a worrying drag bucket at the front on the Cd/Cl chart - in which case I usually send it to the delete file.

Therefore If I have an aerofoil section that I have designed and it looks promising, then I will make the appropriate model and test it in a wind tunnel. The same goes for any aerodynamic "departure" such as the more radical wing planforms I am favouring right now.

On a new plane design it's better to look at the things that might actually make a difference instead of concentrating on one particular aspect like the aerofoil section(s). Once you have a good section thats proven to work its better to keep using it and change other variables on the model so that any difference becomes really apparent. I see many new designs where everything has been changed from the previous version. Problem is - how to tell what's working and what isn't, or wasn't or used to or should be?

Sometimes it's funny. Its a little bit like going crazy on the design of aerodynamics bikes, which people really do - the super aero designs work great - until someone gets on them and messes up all the flows, or they have to ride in an oblique wind condition etc etc.

I tend to try to be very practical and try things one by one.

Cheers,

James