Power Off - 1-G vs. "Controllable" stall

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undean

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I've been looking at available Part 103 aircraft, AC 103-7 and the appendices, Other FAA documents, this and other forums, and am attempting to reconcile what appears in AC 103-7 (appendix 2 Stall Speed graph) to be a reference to 1-G "stall" and a number of designs which appear to have been designed to critical AoA stall.

Using the oft referenced stall calculation [ V = √( 2 m g / ( ρ S Clmax ) ) ] and even using the over-predictive 2-D Cl_max at appropriate Re I've noticed some airplanes list a much lower Vs0 than what that stall calculation would predict. However, making a simplified/idealized version of the plane in xflr5 appears to show controllable flight below the formula predicted velocity (e.g. it will close on a solution with W > L).

For one popular UL (to my knowledge) the above formula indicates the need for a CLmax of ~2.15 at Re = ~1,100,000 without the use of flaps, slats, or other high-lift devices on what is roughly a Clark Y to meet the 24 knot stall which is above its listed Vs0.

All of this being said, I am having difficulty with what appears to be finding a velocity (x) with, maybe, sink rank (y), just prior to the point where it will presumably nose down as opposed to just entering deep-stall as the latter, if "controllable", would mean it could be argued its not in "stall".

Perhaps I am misunderstanding something or it's one of those winked at things but at a minimum I cannot reconcile a stall calculation which over predicts stall onset despite using CLmax. I've found some interesting discussions on this topic here and elsewhere but at this time I would rather not officially inquire about this but if I have missed something obvious to someone else I would like to hear that as well.
 

Vigilant1

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I'm not sure I understand your question. Are you asking about the regulation or the aerodynamics?
From an aerodynamic standpoint, there's no such thing as a "stall speed." Stall occurs only when the wing exceeds the critical AoA. It can happen at any airspeed, and it is possible to fly well below the published "stall speed" without experiencing a stall (e.g. when flying a "bunt" maneuver).
 

TFF

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1 G stall= maintaining same altitude until the airplane stalls. As the plane slows toward the stall speed, the pilot has to increase the AOA to maintain altitude. At some point they can’t and the plane stalls. That speed is the stall speed.

An airplane can stall at any speed with the right conditions, like high Gs.

Calculating what you want is one thing. The test flight is another.
 

undean

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If the equation V = √( 2 m g / ( ρ S Clmax ) ) is accurate for Vs0 how do planes like the Legal Eagle have a Vs0 well below the equations prediction (from my understanding, estimates, and sim)?
If the equation is not accurate, what is?
Has anyone run the numbers of that or similar planes and found differently?
If so, perhaps they are able and willing to explain how they arrived at their different findings?

My current thoughts are:
The equation implies W = L but with design features such as washout a plane can be controllable where W > L
With enough elevator authority a plane can delay nose pitch forward long enough to appear to have a lower Vs0 than might otherwise be reasonable
Thoughts, comments, explanation, or contradictions?

There are quite a few others buried in there but those are my primary focus and reason for posting.
 

mcrae0104

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If the equation V = √( 2 m g / ( ρ S Clmax ) ) is accurate for Vs0 how do planes like the Legal Eagle have a Vs0 well below the equations prediction (from my understanding, estimates, and sim)?
L=qSCl is reliable. Remember that time published Clmax values for a two-dimensional airfoil are different than a three-dimensional wing. One of BJC's first threads was on the difference between published stall speeds and real stall speeds--suggested reading.
 

BBerson

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If an ultralight owner complies with the AC103-7 graphs, there is no requirement to test to the FAR 103 stall speed.
AC103-7 provides alternate approved criteria if desired ;).
 

undean

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L=qSCl is reliable. Remember that time published Clmax values for a two-dimensional airfoil are different than a three-dimensional wing. One of BJC's first threads was on the difference between published stall speeds and real stall speeds--suggested reading.
Yes, unless my recollection is off the 3-D wing Cl will be less than the 2-D idealization which uses assumptions and simplifications based upon an infinitely long wing. I found a post to that effect and thread that discussed the intricacies of which vehicles had to follow which parts of 103, how a helicopter could be considered stalled, and the like. If neither of those sound like what you mention I would appreciate a link if its not lost to time.

If an ultralight owner complies with the AC103-7 graphs, there is no requirement to test to the FAR 103 stall speed.
AC103-7 provides alternate approved criteria if desired ;).
Very true. The wings I've designed based upon the 103-7 graph & the above equation drops my estimated peak L/D from ~40 to ~30 with a high kurtosis and heavier weight compared to the ones that have solution closure with W > L in xflr5. I've only been able to get an idealized LE UL to do the latter...
 

BJC

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L=qSCl is reliable. Remember that time published Clmax values for a two-dimensional airfoil are different than a three-dimensional wing. One of BJC's first threads was on the difference between published stall speeds and real stall speeds--suggested reading.
Here is a copy of a presentation for an EAA chapter meeting about claimed stall speeds.


BJC
 

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Dana

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The graphs and calculations in AC103-7, which the FAA accepts, are rather conservative. In most cases the AC calculated stall and maximum speeds will be lower than the actual speeds. The manufacturer may be quoting the lower, but legally acceptable, speed.
 

pictsidhe

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Physics is physics. It does not bend.

Flying straight and level while stalled with the power off isn't always the easiest thing to do. While testing stall speeds, there's a very good chance that the aircraft is not pulling 1G. Another big factor is that the ASI can be very inaccurate at high angles of attack.
My plan to get an accurate stall speed is if I can hold it in stable stalled gliding condition, have GPS log my exact motion, then compensate for the glide slope numerically.


Cos(glide_angle) = effective g.
1G stall = glide stall/(sqrt(cos(glide_angle)))

Actual CLmax = CLglide*cos(glide_angle)


Perhaps some testers don't do that? With the high drag typical of a full stall, the slope is pretty steep if you hold it, and does very nice things to the quoted stall speed if not compensated for. Stir in an inaccurate ASI and you get stall speeds that have a Clmax of double of what is actually achievable...

An added complication of doing a gliding stall test is that you may well be able to fly past the stall and below Clmax if your aircraft has good manners. Once compensated for the glide slope, your apparent stall speed will be worse than it actually is.

If your numbers are believable, unlike the LE ones, you won't have problems with the FAA. A few knots out is no big deal, considering how hard it is to verify. The actual LE stall speed is probably low 30s mph. ~10% over the required 103 stall speed, rather than 10% under. Near enough that the FAA have not bothered to say anything. I am planning to meet all the 103 requirements. That means compared to the LE, I am using 15% more wing area and effective flaps but still think it will be much nearer the 28mph 103 spec than the LEs quoted 25mph stall speed... If you design something with a sane CLmax, you should be fine with the FAA.
 
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BJC

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My plan to get an accurate stall speed is if I can hold it in stable stalled gliding condition,
If the airplane is descending at a steady (unaccelerating) rate, it may be close to a stall, but it isn’t stalled.


BJC
 

BBerson

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The rule is "unpowered stall speed". So that requires descent. I don't think the FAA wanted to bother with defining "unpowered stall speed" so AC-103-7 was issued instead.
 

Dana

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If the airplane is descending at a steady (unaccelerating) rate, it may be close to a stall, but it isn’t stalled.
BJC
Not necessarily. Even at a high (stalled) AOA, there will be a speed where lift equals weight and the airplane isn't accelerating. Whether any particular aircraft has the control authority to hold it in that attitude is another matter.
 

pictsidhe

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The rule is "unpowered stall speed". So that requires descent. I don't think the FAA wanted to bother with defining "unpowered stall speed" so AC-103-7 was issued instead.
Descent, or deceleration. I belieev that a deceleration rate is suggested somewhere.
Using deceleration mkaes it a dynamic test, which isn't the easiest the measure speed in, especially if you are also trying to fly a stalling aircraft.
 

BBerson

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Descent, or deceleration
Why not do a "stall turn" then? (Hammerhead)The speed will be zero.
Why do you care when AC103-7 is available to use?
Or why not follow what EAA recommends (and the popular LE) and simply disregard the speed rules somewhat. (like 90% of Americans on the highways). The FAA doesn't want to enforce the speed rules. Only the 5 gallon and one seat rule.
 

Kyle Boatright

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I've been looking at available Part 103 aircraft, AC 103-7 and the appendices, Other FAA documents, this and other forums, and am attempting to reconcile what appears in AC 103-7 (appendix 2 Stall Speed graph) to be a reference to 1-G "stall" and a number of designs which appear to have been designed to critical AoA stall.

Using the oft referenced stall calculation [ V = √( 2 m g / ( ρ S Clmax ) ) ] and even using the over-predictive 2-D Cl_max at appropriate Re I've noticed some airplanes list a much lower Vs0 than what that stall calculation would predict. However, making a simplified/idealized version of the plane in xflr5 appears to show controllable flight below the formula predicted velocity (e.g. it will close on a solution with W > L).

For one popular UL (to my knowledge) the above formula indicates the need for a CLmax of ~2.15 at Re = ~1,100,000 without the use of flaps, slats, or other high-lift devices on what is roughly a Clark Y to meet the 24 knot stall which is above its listed Vs0.
Don't make the assumption that the published stall speed for an ultralight reflects the *true* stall speed. Depending on how the person obtained the data around stall speed, it could be corrupted by the use of power, by using IAS as opposed to CAS, or by pure marketing baloney. Odds are, not many ultralights get thorough testing in that regime to shake out all of the potential sources for error.
 

pwood66889

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"Don't make the assumption that the published stall speed for an ultralight reflects the *true* stall speed."
Nor for any other type of fixed-wing aircraft, Kyle.
Regarding "reconcile," that may not be possible.
Yet, it could be a valid exercise. I wish undean all the luck, but like a math teacher will say "Show your work."
Yet again, s/he may be trying to reconcile market hype...
And thanks, BJC, for that presentation!
 

Dana

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Descent, or deceleration. I belieev that a deceleration rate is suggested somewhere.
Using deceleration mkaes it a dynamic test, which isn't the easiest the measure speed in, especially if you are also trying to fly a stalling aircraft.
Stall speed is a constant. It's the speed an aircraft flies in unaccelerated gliding flight right at Clmax. You're descending, but your rate of descent is constant. You can fly that speed all day and maintain a constant descent rate, no acceleration, but you can't go any slower without accelerating downward.
 

BBerson

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But if you put it in a deep stall like a Kasperwing, the forward speed could be nil, with greater vertical rate. So is the legal stall speed a measure of horizontal rate or vertical rate or combination of both?
 
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