IAS for sidewards and rearward hover

[mderdem]

Hello,

It is your simulationist again. This time my question is about how successful the pitot-tube based IAS measurement for rearward and sidewards flight is ?

Since the inlet of the pitot-tube points to forward, it is surprising for me if sidewards speeds can be measured correctly as good as forward speeds. Can the IAS reliably measured when helicopter is hovered rearward or sidewards ?

Thanks!

MDE

[IntheTin]

I think you answered your own question! The pitot tube measures the IAS going forward and its not really acurate until around 20 or 30 knots forward airspeed. :}

[mderdem]

That was a fast response :)

Thank you InTheTin :ok:

[helopat]

Hello,
It is your simulationist again. This time my question is about how successful the pitot-tube based IAS measurement for rearward and sidewards flight is ?
Since the inlet of the pitot-tube points to forward, it is surprising for me if sidewards speeds can be measured correctly as good as forward speeds. Can the IAS reliably measured when helicopter is hovered rearward or sidewards ?
Thanks!
MDE

I think that the most important differentiation to make here is airspeed (measured by the pitot-static system) and ground speed (measured by eyeball, or in a more complex aircraft, by a doppler radar system)...the doppler groundspeed is usually indicated on a digital display.

The pitot system is quite unreliable at low speeds and not reliable at all at measureing sideward or rearward flight. Forward flight below 25-30 kias is not reliably indicated.

Hope this helps.

HP

[NickLappos]

mderdem,

The pitot really doesn't wk below about 20 knots, so most helos (with side and rear limits in that neighborhood) will not benefit even if we pointed extra pitots in various directions.

A number of systems have been developed to provide the answer, and some oare very good. Here is an overall discussion, the autho might not have been aware that Comanche had a blade-mounted system that was excellent:

http://www.dsto.defence.gov.au/publications/2550/DSTO-TN-0495.pdf

[mderdem]

Thank you helopat & NickLappos :ok: It's all obvious now.

[CareBear]

Pitot tubes are notoriously innacurate in anything other than forward flight, and they also vary from aircraft to aircraft. As an example - when demonstrating a low or zero speed autorotation in a jetranger, you can reduce airspeed to zero indicated - the aircraft will now be descending almost vertically (if there is any headwind the track over the ground will actually be backwards). As therate of descent increases, the pitot tube experiences disturbed airflow as a result, and the indicated airspeed will then actually start to increase throught 40kts or so. In actual fact the real airspeed is still zero or possibly even a negative value ie backwards !!
Similarly it is not impossible to be sat on the ground (or in the hover) with a 30+ kt crosswind / tailwind and the indicated airspeed will be zero.
I would guess that the only time you get anything like an accurate reading is in forward flight, straight and level and in undisturbed (ie non turbulent) air.
Cheers
Jay

[bolkow]

Thank you helopat & NickLappos :ok: It's all obvious now.If they made the pitots from flexible tubing and mounted them near the window, I could bend tham backwars to check my speed when going rearwards? LOL

[Graviman]

A number of systems have been developed to provide the answer, and some oare very good. Here is an overall discussion, the autho might not have been aware that Comanche had a blade-mounted system that was excellent:
http://www.dsto.defence.gov.au/publications/2550/DSTO-TN-0495.pdf

Interesting article, Nick. It mentions various rotor mounted systems in section 3.2. Specifically hub mounted OADS and MI-28 Havoc having blade tip pitots, along with Sikorsky algorith for using sinusoidal airspeed variation.

Was this type of system accurate enough for the FBW control system to generate dihedral restoring moments, or did the system use accelerometers for small period movements? The accs would be needed to null out any dynamic oscillations.

Mart