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RV-7 stall angle of attack

D

dmaher

Active Member
Hi, I’m working on an RV7 flight model for Microsoft flight simulator.
I’d like to get the stall speeds accurately set...or better yet the stall AOA.

Convention says it should be 15deg or so, right?
I’m wondering if anyone with a digital AOA gauge has ever made a note of AOA at stall (clean stall).
Also, I have to ask :) ...is it right that the stall AOA is unaffected by weight…unlike stall speed?

Anyone have an estimate of AOA for a clean stall?

Thanks,
Danny
 
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18 degrees

Based on the airfoil of a standard wing or something close to it, a wing like that of a -7 will stall clean at 18 degrees. In mine and my dads -7 we stall at 56mph indicated at 5000' feet with 150pounds in each seat. pretty standard CG. I fly microsoft FSX and have been looking for a -7 or another RV. If you put the model online, I would love to download it for my flight sim!!!
 
Thanks for the replies. This helps a lot.

18 is higher than I expected.
In level flight I guess you really have to work to get the nose up there.

I’ve configured it for 16deg which gives stall speeds that are a bit too high.
I think at 1800lbs it was stalling at 62mph – Vans says 58.
It’s a small difference but it really helps to get this precise.
Errors here will cause problems other places.

For what it’s worth, my maximum coefficient of lift works out to about 1.36

I’ll try 18deg - it will raise CL and bring down the stall speed, hopefully it will hit 58.

Casey, if you want to look at some of the work here’s my website…browse to the forum too.

www.baytower.ca

Thanks,
Danny

Just for some reference, this is a graph of CL vs. AOA for a generic wing - not an RV.
So I'm working on locating AOA(x-axis) at stall and then finding it’s corresponding CL (y-axis).
RV7_tbl_CL_Vs_Alpha_Ref.jpg
 
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I'd be interested in the rest of the model

I assume the model will need inputs for the drag curve(?). I have some data that may come close. The CAFE 6A is a good starting point, but there are some obvious differences. This would be a neat way to make some boring old numbers highly visible. Email me and/or check the info page under my signature. Please and Thanks.
 
dmaher said:
Hi, I?m working on an RV7 flight model for Microsoft flight simulator.
I?d like to get the stall speeds accurately set...or better yet the stall AOA.

Convention says it should be 15deg or so, right?
I?m wondering if anyone with a digital AOA gauge has ever made a note of AOA at stall (clean stall).
Also, I have to ask :) ...is it right that the stall AOA is unaffected by weight?unlike stall speed?

Anyone have an estimate of AOA for a clean stall?

Thanks,
Danny
Click to expand...

Technically speaking, weight has no effect on stall AOA. The wing will stall at a particular AOA independent of weight. Stall AOA is difficult to predict analytically, so our very best source is flight test or wind tunnel testing (of the airplane, not just the airfoil).

Speed - actually reynolds number - has a small effect on stall AOA. At lower Reynolds Numbers, the 23013 stalls around 16 degrees. At higher Re, it stalls closer to 18 degrees. Reynolds number is related to speed, and the viscosity of the air. So, we could only tie stall AOA to weight if we assume a particular flight condition, like level 1g flight.

Incidentally, surface roughness (bugs, layer of dust, etc) will degrade stall performance down to about 12 deg AOA. But this was a wind tunnel test and I don't think the actual airplane is as affected by the usual roughness as the test indicates.

So, for your model, I'd suggest maybe 16 degrees for the stall AOA, if you're OK with that.

Reference: the good old "Theory of Wing Sections", page 498.
 
Bill thanks for the extra information.
16 deg is giving very good numbers.
My CL value is just a bit low…So I’ll adjust that.

Thanks H.Evans .
Actually I am familiar with your work from a few years ago.
So it’s a happy coincidence you responded.
I hadn't visited your site recently.
There’s an incredible amount of excellent detail there.
I will have a good look around.
And I will definitely contact you by email sometime this week.


Cheers,
Danny
 
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That graph you had was of a symmetrical airfoil at a low Re of 500k, whereas the 5' chord at 55 mph is around 2M. Don't forget that when you come up with the stall AOA, that it is of the wing relative to the induced AOA, which will put the body angle even higher. Also don't forget that the AOA is measured from the zero-lift alpha, which for a 23013 is about -1.5 deg. So stall AOA of airfoil plus -1.5 AlphaZL plus induced AOA minus wing incidence angle = body angle. The induced angle of attack is a function of weight, Q, span, and tip efficiency factor, so you need to take all of these factors into account.
 
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gimp2x said:
why not do it for xplane? nobody uses FSX anymore
Click to expand...

Tens of thousands still use FSX. It's even become more popular thanks to new addons such as Orbx. In fact, Microsoft just printed a bunch more copies.

But...........this type of stuff is better argued on a flight sim forum...

L.Adamson --- RV6A
 
I’ve made some good progress…still working at getting the stall speeds just right.
Looking at Van’s stall information, they suggest…

58mph at 1800lbs

Is it reasonable to assume that’s calibrated airspeed?

I’m think these are the numbers I’ll work with…
Stall clean @ 1800lbs 64 mph(CAS)
Stall flaps20 @ 1800lbs 60 mph(CAS)
Stall flaps40 @ 1800lbs 58 mph(CAS)

Do they sound about right?



The last clean stall I tried worked out to…
1.0G stall @1800lbs 64.53mph (CAS) or 62.0mph (IAS) and 16deg AOA

Danny
 
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That's pretty darn close to what I got in re: to airspeeds...but I can't tell you what angle of attack it was. I'd still think 18-degrees.
 
as part of our full flight test program that we are completing over here, here's our data:

our indicated airspeed at 60kts reads between 3-4knots high, giving us a stall speed of 59 KIAS/55.5 KCAS clean @1800lbs

full flaps 53 KIAS / 49.5 KCAS clean @1800lbs

based on this, your numbers are pretty **** close. maybe a tiny 1kt high on the full flaps and dead on on the clean.

even a little power is sufficient to lower the stall speed, so flight idle will play a factor.

unfortunately we have no "hard" angle of attack figure. i can only report that pitch angle during the clean stall was 20?-22?!
but as elippse has suggested, the aoa of course will be lower.
with full flaps the nose felt significantly lower by the way.

rgds, bernie
 
dmaher said:
I?ve made some good progress?still working at getting the stall speeds just right.
Looking at Van?s stall information, they suggest?

58mph at 1800lbs

Is it reasonable to assume that?s calibrated airspeed?

I?m think these are the numbers I?ll work with?
Stall clean @ 1800lbs 64 mph(CAS)
Stall flaps20 @ 1800lbs 60 mph(CAS)
Stall flaps40 @ 1800lbs 58 mph(CAS)

Do they sound about right?



The last clean stall I tried worked out to?
1.0G stall @1800lbs 64.53mph (CAS) or 62.0mph (IAS) and 16deg AOA

Danny
Click to expand...
Given that it is very difficult to determine the errors in the airspeed system at the stall without fitting a flight test airspeed system, I'd assume that Van provides stall IAS. How did you determine the CAS at the stall for your aircraft?
 
kevin,

danny is talking about his flightsim model (which i suppose is aerodynamically in a "perfect world")... he's working on coming up with a valid .air file.

on our airplane, we did airspeed calibration throughout the full range of airspeeds using the 3way gps method.

so, taking the airspeed error at 60kts and the indicated stall speed of 59kias, gives a kcas of 3-4kts lower. of course this isn't perfect, but as good as it gets within reasonable means. instrument error by the way is less than 0.5kts (advanced efis) as verified during pitot/static check.

rgds, bernie
 
Yeah, I’ve had mixed feelings on how to interpret Van’s number.
I imagine folks looking at the Van’s catalog are interested in IAS…I’d think CAS is kind of abstract.
But looking through a lot of online material it seemed to me it might be CAS.
Bernie’s numbers for instance correspond better if Van’s number is CAS.

I figure for a builder/pilot it’s not really significant one way or the other :)
And likely Van’s would say it’s only an estimate your results are going to differ.
The margin of error in measuring alone could account for discrepancies too.

How did you determine the CAS at the stall for your aircraft?
Click to expand...

FSX can produce numbers for any parameter, but CAS isn’t one of them :)
I don’t know, maybe CAS doesn’t actually play a role in the sim’s physics.

I use a third party tool that will calculate CAS for FSX…based on density and angle.
It’s hard to know if it’s to be trusted.
The error is suggested to be around 2.5mph.

I’ve also made a script for collecting stall data.
If I fly a series of stalls the script will average the speed of any stalls that meet 1G level stall criteria.
Only about 1 in 4 stalls is good enough to consider…but the results are very solid and repeatable.

Btw I don’t remember seeing any photos of the RV-10 or RV-12 fitted with specialized test instruments.
 
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CAS is IAS corrected for instrument and installation error. So, in your sim world I'd suggest the two are essentially the same.

In the real world, at high AOA, there will almost certainly be errors in IAS due to the nature of the pitot-static system.

The FSX tool to calculate CAS is probably a WAG, or slightly better, based on some idea of how a standard pitot might be affected by local flow angles. Just a guess...
 
Bill, Turns out you're right :)
The tool calculates a suggested error…that includes the angle of relative flow.
But the simulator does not account for the angle of relative flow.
The tool's information is provided just as a guide for scripting a more accurate gauge yourself - if you cared to.

There actually is a provision for ‘calibrating’ the airspeed in the sim by means of a global scale and offset.
The SDK refers to the default speed as CAS and the adjusted speed as IAS…as you might expect.
But it’s very limited, and not of any use in this case.
So, IAS = CAS ;)
 
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dmaher said:
Yeah, I’ve had mixed feelings on how to interpret Van’s number.
I imagine folks looking at the Van’s catalog are interested in IAS…I’d think CAS is kind of abstract..
Click to expand...
I still remember when I was shopping for an airplane to build. I had zero interest in the stall IAS. I was interested in the retained energy in an off-airport engine out landing so I wanted to know the CAS. Whether or not the indicator said 35 or 75 did not and does not matter to me.
 
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