Stall speeds/ Rough calcs

[piperboy84]

I was advised many moons ago that for every 80 pounds under gross, book VS0 is reduced by 1MPH, is this a good rule of thumb?

Also would this apply to best glide?

[Firewalled]

Today's weight/gross weight, find the square root of that, multiply that by the published Vspeed, and you have the new Vspeed.

For example at 750kg, Vs=45
For Vs at 680kg divide 680kg by 750kg and find the square root of that, which would yield 0.952. (You can think of this as a percentage too). Multiplying that by 45 we get a new Vs of 42.8 knots.

As you can see the difference is often very minimal, and not worth the effort

[Genghis the Engineer]

Most of what firewalled said.

In his calculation, with Vref = 1.3Vs, that takes the final approach speed from 59 to 56 knots. Enough I'd like to get it right and not have that extra energy to get rid of during the last part of the landing, especially in a taildragger.

Best glide follows different rules and unless you have good reason to, just use a single figure.

G

[peterh337]

There is a lot of this stuff on the net.

Google Professor Rodgers - example.

[jxk]

It's your half roe V squared S innit?

[piperboy84]

As you can see the difference is often very minimal, and not worth the effort

Not sure about that

My VS0 at gross is 47MPH at 2400lbs when i fly light the empty weight is 1440 lbs, I am 200 lbs and 20 gallons of fuel at 120lbs for a total of 1760 lbs, which is 640lbs under gross. If indeed the 1 MPH per 80lbs is correct my stall speed should be 8 MPH slower, which is significant especially when 3 pointing a taildragger

[24Carrot]

If you are finding big differences, then you should probably go the whole hog and convert from IAS to CAS first (as per POH table), then do Firewalled's square root thingy, and then finally convert back to IAS, (as per POH table).

CAS always seems to get progressively bigger than IAS at low airspeeds.

[FlyingStone]

Like it was said before me: the important thing is that you use CAS in calculations (well, the mathematical correct way is to use EAS, but CAS = EAS in spam cans):

Vs = Vs0 * sqrt(m/m0)

Again, as said before, IAS tends to become lower than CAS with increasing angle of attack. For example, quick glance at an ASI of C172P would reveal stall speed of 33 KIAS. One might think Cessna made significant improvement to aerodynamic characteristics of C172 to allow normal cruise speed combined with very slow stall speed. But a bit more detailed analysis of the performance chapter of POH gives you the real stall speed - 46 KCAS - which is normal for this kind of aircraft.

[B2N2]

Um - it is perhaps worth pointing out that most light aircraft ASIs really don't have great accuracy at higher angles of attack.

Trying to nail that last couple of mph below the normal stall speed "because you're light" may not necessarily be the best idea in the world.

I would second that. Unless you have the indicated/calibrated speed chart from the POH taped to your cockpit window it is an academic but useless exercise. At those speeds most airspeed indictors are off by as much as 5-7 knots/mph due to the angle of the pitot tube/mast to the relative wind.

At those speeds it is much more about what you as the pilot "feel" through the controls and the airframe then any instrument.
Your "feel' is what allows you to fly in that area nibbling on the stall.

[mad_jock]

Just make a note of your landing attitude which always works and the speed will look after itself.