Grob 115 stall speed

[no name]

Can anyone explain why the stall speed for the Grob 115 aircraft actually increases with the selection of flap in the 60 degree position when full power is selected?

Thanks.

[Genghis the Engineer]

I don't know the type, but from experience on other similar designs I'd guess that it probably doesn't. What is most likely happening is that the selection of flaps is either reducing the pressure at the static port through direct flow affects, or increasing the pressure at the pitot by causing the aircraft to fly more nose-down. The result is that the INDICATED speed goes up for a given ACTUAL speed.

Take a look in the POH and see if there are different airspeed correction graphs for different flap settings.

G

[no name]

Thanks for the info...

Your suggestion is possible but I can't determine if it is correct from the flight manual.

At max power and at a particular weight the stall speed for a particular flap setting is;

Flap 0 = 43 KIAS
Flap 15 = 40 KIAS
Flap 60 = 44 KIAS

The Airspeed calibration table for the particular flap settings only goes as low as 53 KIAS.

However from the table I can say that;

At a IAS of 60 kts the CAS for flap 15 degrees is 58 kts and for flap 60 degrees 56 kts

[djpil]

The Pazmany PL-2 is one airplane where the P.E.'s do strange things. The pitot-static is on top of the fin. I can't remember the details as it was many years ago - perhaps some-one can refresh me - something like the stall warning goes on at a lower IAS than the stall IAS.

For the Grob- tell us the power off stall speeds and the location of the pitot and statics, give us more to go on.

[Intruder]

It is also possible that at 60 degrees, the airflow separates from the flap section earlier. I suspect the setting is more for drag (allowing steeper descents at relatively low airspeed) than for lift purposes...

[Final 3 Greens]

Can anyone explain why the stall speed for the Grob 115 aircraft actually increases with the selection of flap in the 60 degree position when full power is selected?

Yes. A test pilot flew the thing and noted the IAS at which it did certain things.

[Guido]

I know that the military version (sorry, civil registered flown by the military) has the same disparity in indicated stalling speeds.
However it does have an alternate static source tapped from the cockpit, unaffected by external flow over the static vents. With alternate selected, the indicated stall speeds with flap and power are lower than those with normal selected, but I'm not sure if they are lower than the clean, power off speeds.
This may help.

[no name]

There are two static ports on either side of the fuselage just ahead of the empennage.

The stall speeds at idle are:

Flap 0 - 54 KIAS
Flap 15 - 52 KIAS
Flap 60 - 51 KIAS

So the airflow at full power could be distrupting the static vent reading to give a higher IAS than the true CAS.

[djpil]

Let's try a change of tack.

What's the stick position at the stall for all configurations?

I'd hazard a guess that minimum speed at full power with flaps 60 is limited by elevator deflection. Can you bring it down to YMMB one weekend for me to try it?