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stall speed vs minimum speed

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baddles created the topic: stall speed vs minimum speed

Hi all

A dumb question: is "minimum possible airspeed" the same as "stall speed (power on)" ?

The CASA syllabus for CPL Aerodynamics includes:
"2.5.1 Using power required and power available graphs, identify the following: (a) stall speed (power on) ..."

Bob's text explains that the 'power available' and 'power required' curves determine the minimum and maximum possible airspeeds. These are the speeds where the two power curves intersect.

I can't see how else to read off the required stall speed from the power curves unless it is the same as the minimum possible airspeed.

Naively I imagine they are the same thing, but ....!

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bobtait replied the topic: stall speed vs minimum speed

You would have to assume the question is talking about level flight. Providing the aircraft is maintaining level flight, the minimum possible speed that could be maintained would be the stalling speed or the speed at which the controls become ineffective. The power, flap and weight configuration would determine exactly what that speed would be.

If it is anything but level flight, all bets are off..
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  • John.Heddles
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John.Heddles replied the topic: stall speed vs minimum speed

If I may add a comment or two to Bob's, above.

You might need to cut Bob's text a little slack. Were he to put in all the caveats and such like, the book would be 1000 pages long before he got anywhere near finished. To some extent, we have/choose to cut the detail a bit (lot ?) for the target audience.

The intersections of the curves represent the sensible min/max speeds at which the aircraft can be flown, typically, in level flight. That is to say, at the low speed end, all the available power is expended overcoming drag power (in this case, mostly lift-dependent, or induced, drag associated with the high CL ) while at the high-speed end, surprise, surprise, all the available power is expended overcoming drag power (in this case, mostly lift-independent, or parasitic, drag associated with our inane desire to go a bit faster than we might really need - certainly a good way to increase the fuel bill for very little sector time gain). The low speed intersection doesn't, necessarily, involve stall; that will depend on where the intersection is in relation to the stall speed in level flight.. Sure, you always can stall with high power but, often, you will need to slow down somewhat which involves descending - much the same as stalling with a lesser or idle power setting. I would be horrified to think that there are instructors out there running stall sequences with maximum power settings - not a good idea.

Consider the risks, though, with really high power stalls.

At the low speed end, should you have enough power to be back at the stall speed in level flight and you do actually stall, you might be in a world of hurt with the recovery - generally, it's not a really bright idea to swan about doing high power stalls as there are few situations where a civil pilot needs to exploit that part of the envelope. Similar considerations apply if you don't have enough power and stall during a high power descent. Either way, propeller forces can do funny things as there are forces at play other than thrust with high alpha, high power, operations. We need to have some exposure to moderate power stalls to provide some training for mishandled go-rounds and the like but that is quite different to a maximum power stall situation.

At the high speed end things aren't that bad - go faster and you have to start descending as there is nothing left in the power larder to bring to the table.

So, when Bob's text suggests that the "curves determine the minimum and maximum possible airspeeds" you need to add, mentally, something along the lines of "for level flight". The low speed end might, or generally might not, coincide with the relevant stall speed. Either way, be prepared for a roller coaster ride if you do stall at very high power settings.

Using only the power required and available curves doesn't, necessarily, tell us anything about the specific stall speed - except that the power required should end at the low speed end somewhere aligned with the stall speed. I suggest that the syllabus is talking a little more generically and considering the situation where, with the addition of power (to whatever extent), the curves will provide an indication of the resulting low speed equilibrium cruise speed but should only be thought of in a manner similar to the sort of approach we do to a certification stall, just with a different (ie a bit more) power.

And do keep in mind that the POH certification stall speed (at low/idle power) will see quite measurable reductions as power is increased for the approach to the stall. In addition, the PEC becomes a bit problematic and the ASI might get quite imaginative - think power on stalls in the smaller Cessnas where the IAS, apparently, goes towards zero.

Engineering specialist in aircraft performance and weight control.

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baddles replied the topic: stall speed vs minimum speed

Thanks for your response John.

My question is not a criticism of Bob's text - on the contrary.

I'm simply reading through the CASA syllabus for CPL aerodynamics (as advised in Bob's text) and trying to anticipate possible exam questions. Since CASA seem to be fond of semantic niceties, I thought I should check whether the term 'stall speed (power on)' mentioned in the CASA syllabus could be interpreted - for the purpose of an exam question - as being the same as the minimum speed for level flight. If so, then I know how to answer the question to CASA's satisfaction.

It's clear that this relates to level flight because the 'power required' curve is 'power required for level flight, assuming a particular weight and configuration' (and I do mentally insert that definition).

Your commentary about the realities of stalling and other accidents is very interesting, thank you.
But my question is simply about the use of terminology to satisfy the examiner.

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baddles replied the topic: stall speed vs minimum speed

Thank you Bob!

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