View Full Version : Why CG affects stall speed?


Commander
2nd Jul 2001, 05:17
I am ashamed for posting this; I know I've learned it sometime, but forgotten. Why is the stall speed different from the most rearward CG to most fwd? Please post something quickly - I've got some explaining to do tomorrow!



Cardinal
2nd Jul 2001, 05:29
I knew this admittedly simple concept perfectly well but I coudln't put it into coherent English when asked last week, sometimes we all have lapses. Here's a better try:

Aft CG = Less elevator downforce required = Less lift (AOA) required from wing for a given weight.

john_tullamarine
2nd Jul 2001, 08:52
In addition, if the elevator power is a bit light on, then it may be a case of minimum flight speed .. that mushy sort of thing which sometimes passes for a stall ...

BEagle
2nd Jul 2001, 11:09
The nose down pitching moment caused by a forward centre of mass must be opposed by rearward control column movement generating tailplane downforce and an increased wing angle of attack. Hence at the same IAS, an aeroplane with a forward centre of mass will be operating at a higher angle of attack than one with a rearward centre of mass; it will, as a consequence, reach crtical angle of attack at a higher IAS than the aeroplane with the rearward centre of mass.

Commander
2nd Jul 2001, 13:15
Yes Cardinal, that was excactly what happened. A student of mine asked and I was trying to explain about the total lift and the tailforce, until suddenly I told him that I didn't want to dig myself deeper by explaining in such a hard way, I would answer him next time...

THANKS EVERYONE!!!


god I love this web ;)

compressor stall
3rd Jul 2001, 16:43
OK.....technical statements for the masses 102.

"So mr pilot why have we climbed so high?"
"We have climbed to 10000 feet because the airplane performs better at altitude"
"OK, but the Airspeed indicator is showing 10 kts less than cruise at a lower altitude?"
"That is because the air is thinner"
"but how can a propeller be more efficient through thin air?"

Any suggestions on a succinct simple explanation to explain the above to a lay person over the noise of lycoming IO-540s 2 metres away?

------------------
Those who restrain desire do so because theirs is weak enough to be restrained.
William Blake

Checkboard
3rd Jul 2001, 19:07
Hi Stallie!

I was thinking about posting a brilliant answer that would have all of the users "Oooh'ing" and "Ahh'ing" about my incredibly sound grasp on the subject - but in reallity it would all be from this site (http://www.db.erau.edu/research/cruise/)!

If that is all too tech. for people I can give a one paragraph summary :)

BEagle
4th Jul 2001, 01:42
1. We're at full throttle height.
2. Big deal. The TAS is higher. The fuel flow is less.
3. Propellers don't give a rat's about IAS; they only know about rpm and TAS - the relative airflow over the blades means that they are at their optimum l/d ratio equivalent at this speed, height and rpm combination.

Tinstaafl
4th Jul 2001, 04:05
Hi Stallie

Tell 'em that at low altitude the engine has to be artificially restricted ie throttled, to produce the amount of power that is correct for the airframe's most efficient speed. This causes a dramatic loss in efficiency.

Going higher reduces the throttling, restoring the engine's potential efficiency.

At the same time, the airspeed indicator is built to be accurate only in particular atmosphere conditions. The result is that it is not very accurate at altitude & under-reads.

They'll probably then ask why one isn't made for altitude. Point out that it would fill the a/c to have an ASI for every single variance in altitude & temp. Then show them the adjustable TAS scale on either the ASI or your whiz wheel.


Of course, if you were Slasher you could probably just tell them to "F*** off or I'll lose my concentration & then we'll crash and die." :) :)

Slasher
4th Jul 2001, 11:16
Depends on whos doing the asking Tins! ;)

Tinstaafl
4th Jul 2001, 15:50
Good point Slash.! Didn't think of the more pleasant aspects. How about "If you can't show me your t*ts, f*** off or I'll lose my concentration...." etc etc? :)

PS: Whatever happened to that thread about inane questions that SLF ask? I was in stitches with Slasher's answers.

[This message has been edited by Tinstaafl (edited 04 July 2001).]

compressor stall
4th Jul 2001, 17:13
thanks all.

If it's some annoying bloke I don't have any time for, then i will use BEagle's answer.

If it's some PYT then I'll take Tinny's suggestion.... :)

------------------
Those who restrain desire do so because theirs is weak enough to be restrained.
William Blake