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Author Topic: Why use thin flat bottom airfoils??  (Read 871 times)
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wmazz
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« on: May 24, 2018, 01:49:18 AM »

I have been told that "Thin is in" and I understand that they have an advantage
during the climb. Some of the flat bottom airfoils used in wings and stabs look
like they have low camber ~2.5' +/- .5'

When I compare 8% flat bottom airfoils (using xfoil and a upper surface trip) against
9% to 10% thick airfoils with similar camber, and a raised Philips Entry (almost semi-
symmetrical); the thicker air foils with a raised Philips Entry have nearly the same low
drag between 0 to +10 degrees at 30,000rn to 90,000rn. So for climb and glide the
drag is nearly the same between airfoils despite the difference in increased thickness.

This is the part that I am the least certain. What happens in a stall when the drag
bucket narrow or wide? Does the wide drag bucket with significantly less drag from
0' to ~ -8' have an advantage in the trimming process and a forgiving transition due
to the decreased drag?  

Airfoils that increase lift seem (to me) to be unnecessary because of the lightly loaded
wings. Any extra lift is just extra drag.


Thanks

Bill M.

I have one other question. Why use thin flat bottom airfoils on stabilizers? Why not
use 8% to 10% semi-symmetrical airfoils with ~1.5 degree camber because of the
advantage of a wider drag bucket and significantly less (almost flat) moment coefficient.?
BM.
« Last Edit: May 24, 2018, 02:21:44 AM by wmazz » Logged

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« Reply #1 on: May 24, 2018, 12:44:20 PM »

Bill,
Are you using minimum drag as your determining factor for selection?
I am not (intentionally) using camber in any of my stabilizer sections. Mine are (built) flat top and bottom.
Maybe what you are building needs camber there?
How does the cl^1.5/cd polar for these airfoils compare?
Or is that polar part of your consideration during selection?
I am using thin (I think they're thin) airfoils for what I am building.
This is an interesting question, I hope there is some discussion (I have popcorn, cold drink, armchair etc.. Smiley).
Regards.
Tony
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OZPAF
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« Reply #2 on: May 24, 2018, 08:27:50 PM »

Bill I hope the following doesn't sound too "heavy" but it is difficult to be accurate and concise with out sounding a little that way. My back ground is primarily RC Gliders but I'm also a FF fan and a keen student of aerodynamics. I hope the folling answers help.

Quote
When I compare 8% flat bottom airfoils (using xfoil and a upper surface trip) against
9% to 10% thick airfoils with similar camber, and a raised Philips Entry (almost semi-
symmetrical); the thicker air foils with a raised Philips Entry have nearly the same low
drag between 0 to +10 degrees at 30,000rn to 90,000rn. So for climb and glide the
drag is nearly the same between airfoils despite the difference in increased thickness.

It may appear that they are similar but it's the % difference in drag - in the case of minimum drag, that is important. The general rule is that thinner will perform better at low Re no's. The range of CL's for which the drag is low(the drag bucket) and how low the drag is over the " face" of the bucket is a good indication of the overall efficiency of the air foil.

"Phillips Entry' is only a reduction in the camber of the air foil and also slightly moves the high point of the camber line aft.

I did a quick check on XFOIL - using Profili, and I see significant differences between the thick(9.2%) and thin air foils(8%) over these Re No's.

Quote
This is the part that I am the least certain. What happens in a stall when the drag
bucket narrow or wide? Does the wide drag bucket with significantly less drag from
0' to ~ -8' have an advantage in the trimming process and a forgiving transition due
to the decreased drag? 

The best way to assess Stall behaviour is by checking the CL/Alpha graphs. The shape of the lift curve at max CL will be smooth and not a sharp peak for good stall behaviour. Any irregularities in the curve before Max. CL are an indication of problems and possibly an indication that turbulators will help.

The width of the drag bucket is related to the CL/Alpha curve but does not indicate the problem as well.

The range of alpha for which the flow remains attached or at least stable, is what will affect the ease of trimming. The drag is a consequence of the separation but it is the large variations in lift that are the problem.

Quote
Airfoils that increase lift seem (to me) to be unnecessary because of the lightly loaded
wings. Any extra lift is just extra drag.

This is an interesting point. Span wise lift distribution for all wings indicates a drop towards the tip of the wing in the CL relative to the max CL developed. Thus yes the air foils should be elected to match the required duty for that span position. A lot of work goes into the this aspect with RC Gliders ( my background) and other than a few areas(F1B?) I don't think it receives much attention in FF.

Quote
I have one other question. Why use thin flat bottom airfoils on stabilizers? Why not
use 8% to 10% semi-symmetrical airfoils with ~1.5 degree camber because of the
advantage of a wider drag bucket and significantly less (almost flat) moment coefficient.?

The Stab does not or rather should not be carrying too much load as due to it's size it is quite inefficient. Thus as long as the drag is low and it does not suffer separation at it's operating point - the air foil is not all that critical. A thin flat bottom air foil can meet these requirements and be easier to build as well. Symmetrical air foils have ban used and apparently do not offer any significant advantages. Apart from low drag the most important aspect for a stab is to have a stable CL/Alpha curve to ensure good stability response - ie ease of trimming.

Finally as Tony has mentioned you should be considering the power or endurance factor of the air foils you are considering. The climb - speed and time of climb, also needs to be considered and the final selection needs to balance this against the endurance performance, while still being easy  to trim at the required operating points.

There is no one simple answer that will meet all the requirements - the best compromise will be the most effective.

It's also important to understand that even in this simplified approach to air foil analysis that the position of the high points of camber and thickness also play an important part.

John
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wmazz
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« Reply #3 on: May 29, 2018, 03:36:28 AM »

Thanks John and Tony for your answers.

Bill I hope the following doesn't sound too "heavy" but it is difficult to be accurate and concise with out sounding a little that way.
My back ground is primarily RC Gliders but I'm also a FF fan and a keen student of aerodynamics. I hope the folling answers help.

Your answer is just what I was looking for. I always prefer a good tech article compared to a novel.
I flew RC gliders too!

It may appear that they are similar but it's the % difference in drag - in the case of minimum drag, that is important.
The general rule is that thinner will perform better at low Re no's. The range of CL's for which the drag is low(the drag bucket)
and how low the drag is over the " face" of the bucket is a good indication of the overall efficiency of the air foil.

It is the overall efficiency that I was concerned with (and you answered).

I believe the semi-symmetrical airfoil is well suited for stabs on locked AMA Power, but not the wing?
On the other-hand the semi-symmetrical airfoil should be better suited for VIT models.

Back when I was flying gliders, I converted the thin flat-bottom airfoil of my Gentle Lady to a S3021.
Originally the Gentle Lady CG was 40% with a large stab to support the rear CG location. The stab
matches the sq-in recommended for FF stabs in the 59-61 Zaic Year book. The new airfoil only
changed the wings incidence ~1 degree. I believe the original set-up was nearly 0' - 0' on the wing
and stab.

Despite the change that I expected to increase the flight speed and penetration, the plane no longer
floated. Instead it acted tail heavy and handled poorly. Until I moved the CG forward. This change
might compromise trimming a locked AMA Power FF.

Another concern is the narrow drag bucket for thin flat bottom air foils on 1/2a and E-36 models with
relatively short average cord lengths. Trimmed near a stall, these airfoils are operating at very low RE#
and are at a risk when there are changes in air density. Because of the slow speed and small cord length
it will lower the the wings RE# to a point where the thin drag bucket creates a lot of drag as air density
decreases.


Bill M.

Is there an advantage to building a locked AMA FF with Glider airfoils?

 

** Pic #1 is comparing 2 flat bottom wing airfoils.

** Pic #2 is compares a flat bottom and semi symmetrical stab airfoils
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« Last Edit: May 29, 2018, 03:52:03 AM by wmazz » Logged

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wmazz
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« Reply #4 on: May 29, 2018, 03:48:05 AM »

oops!
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« Reply #5 on: May 30, 2018, 09:07:35 PM »

Your graphs highlight the effect of camber and thickness. A "semi symmetrical " airfoil derives it's appearance from a combination of lower camber mainly, and it is more useful to look at from this view point.

A 'flat bottomed ' airfoil is actually a higher camber airfoil with a combination of thickness distribution leading to the flat lower surface.

The depth of the low drag bucket is associated with camber - higher camber airfoils will move the top edge of the bucket up and lower camber moves the bottom edge down as your graphs show. The thickness largely determines the minimum drag - although both the shape of the camber line and thickness distribution affect this as well.

Thus the top and bottom edges of the bucket are a quick guide to performance. A fast straight climbing model needs low drag at low CL - thus a low bottom edge. A slow flying minimum sink model - a high CL with minimum drag - thus a high top edge.

The airfoil needs to be selected for the required duty within the range of the bucket for best performance as the drag increases dramatically outside the limits of the bucket.

However this is only the start you need to find the airfoil that will give the best compromise for climb and glide.

A higher camber airfoil - so called flat bottom - would for example need a spiral climb pattern as it's drag at low CL's is too high and it thus needs to be operating at higher lift and thus will loop at the high speed of the climb. A low camber airfoil will be much easier to trim for a straight fast climb a it can operate efficiently at the low CL's required.

However as mentioned previously the "Power Factor" or duration coefficient needs to be checked as well. Glide duration needs to be balanced against the climb and here the higher camber airfoil in general is more efficient. However for example you can offset the lower "Power factor" of a low camber airfoil by using a lower wing loading - however the higher camber airfoil will sink slower at this same wing loading.

The lower camber airfoils also have another advantage in that their pitching moments are lower and thus this requires lower trim lift from the tail which also reduces drag.

Quote
Back when I was flying gliders, I converted the thin flat-bottom airfoil of my Gentle Lady to a S3021.
Originally the Gentle Lady CG was 40% with a large stab to support the rear CG location. The stab
matches the sq-in recommended for FF stabs in the 59-61 Zaic Year book. The new airfoil only
changed the wings incidence ~1 degree. I believe the original set-up was nearly 0' - 0' on the wing
and stab.
Despite the change that I expected to increase the flight speed and penetration, the plane no longer
floated. Instead it acted tail heavy and handled poorly. Until I moved the CG forward. This change
might compromise trimming a locked AMA Power FF.

This sounds like an example of the difference in pitching moments for the low cambered S3021(around 3%) and the original flat bottom airfoil of around 4.5-5% camber. The CG was originally set for minimum trim drag - best glide, to match the higher pitching moment of thee original flat bottom airfoil, which would have needed a more aft cg compared to the S3021 with it's lower pitching moment. Providing the CG is set for stable good glide - then this should not be a problem with the low cambered airfoil with a locked down FF model.

I've attached a couple of graphs of some other foils at your Re's together with the corresponding lift slopes and power duration graphs.

I would also suggest obtaining a copy of Martin Simmons "Model Aerodynamics" if you haven't got one already. A very good introductory explanation for aeromodellers.

Hope this helps.
John


PS

The extra airfoils are modified to the following

DP - which is 1.91% camber and 7.82% thick and

A BE151FV1 - 1.85% camber and 7.5% thick and

MID 308 - 1.8% camber and 7% thick

Similar in camber and thickness but the differing in camber and thickness highpoints and shape of envelope


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Re: Why use thin flat bottom airfoils??
Re: Why use thin flat bottom airfoils??
Re: Why use thin flat bottom airfoils??
Re: Why use thin flat bottom airfoils??
Re: Why use thin flat bottom airfoils??
Re: Why use thin flat bottom airfoils??
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lincoln
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« Reply #6 on: May 31, 2018, 05:24:05 PM »

I may have missed them, but some other things to keep in mind:

-With a high aspect ratio, the induced drag penalty for high lift foils is less. Looking at it in a somewhat less abstract way, a longer span reduces the induced drag penalty for lift. For a given span, a high lift airfoil means you can use a shorter chord, at least up to a point. Reduced chord means lower Reynolds numbers and probably lower maximum lift and higher drag coefficient, with the latter compensated for by less wing area. And, of course, the reverse applies. The smaller the model, the more important this is.

The rules for the class in question affect the best design choices. For instance, if I understand correctly, in towline gliders, the area of the wing is restricted, and I seem to recall that the stab is counted as part of the area. Plus there's a minimum weight, and not all THAT much need for low drag at high velocity, though I understand that if you get sophisticated enough, you can get a bit of a zoom at the top. These rules seem to have resulted in high aspect ratio models with high lift airfoils and small stabs. As the tailboom gets longer, the stab airfoil can lift more, or else the stab can be smaller. With composites, the weight penalty for a long  tailboom is less.

OTOH, lots of classes don't count the tail area, and may allow it to be as much as 50 percent of the wing area. In that case, it makes sense to use the tail as a lifting surface, though at lower lift coefficients. This will affect the tail airfoil selected. Ideally, you want a foil with low drag at typical in-flight lift coefficients. On a light EZB, the boom is long, the stab is large, and you can actually see the tailboom flex upwards slightly from the lift. (EZB designs are all about having just enough stiffness in the right places. More stiffness means extra weight that isn't necessary. Stiffer wood should result in smaller parts, not stiffer models.)

The best airfoils will depend, not just on the planform, but on the size of the model. Something like a Valkyrie can use a thicker airfoil than an E-20.
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