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Author Topic: Air speed data from engines  (Read 3044 times)

Offline syhprum

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Air speed data from engines
« on: 02/02/2015 10:57:31 »
Several aircraft crashes have been attributed at least partially to failure of air speed indicators.
Their must be a strong correlation between the airspeed and the pressure in the cowling behind the fan and the rotational speed of the fan, the pressure increase should be little affect by icing due to the high speed of the fan.
I take I am not the first to think of this so what is wrong with the idea ?.
There are normally at least two engines working so a check could be made between their indications and that of the regular air speed indicator 


 

Offline alancalverd

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Re: Air speed data from engines
« Reply #1 on: 03/02/2015 00:05:54 »
Problem is that as the aircraft accelerates or decelerates, the cowling pressure will not match the true airspeed pressure as the fan won't be in equilibrium.

I've toyed for some years with the idea of an ultrasonic doppler system for measuring TAS but never got round to it. Perhaps now is the time!   
 

Offline David Cooper

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Re: Air speed data from engines
« Reply #2 on: 03/02/2015 17:41:57 »
On a boat in the old days, they'd chuck a wooden thing over the side on a rope with a series of knots in it and count how many knots went out in a set time. They'd then give it a yank to make the wooden thing collapse into a flat shape that was easier to pull back in.

I'm not suggesting that would work for planes, but something similar could - you just want to miss out the rope part and use something disposable like a small radar reflector made out of tin foil. It would be so light that it would soon be stopped by the air, and all you need to do is track it to get the plane's airspeed relative to it. These things could be released from a wing and tracked by a little radar system along the side of the plane.

Not great for the environment, unfortunately, but you'd only need to use them in icy conditions.

[Edit: It wouldn't work to well if it gets caught up in the air blasted out of the engines, so maybe these things need to be shot out of the top of the plane near the front and tracked by a little radar system along the top of the plane.]
« Last Edit: 03/02/2015 17:45:19 by David Cooper »
 

Offline CliffordK

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Re: Air speed data from engines
« Reply #3 on: 03/02/2015 20:29:02 »
There are a number of indicators that might be useful to determining a plane's flight status.  However, it may get to a point where it would take a computer to integrate all the data, and either use it for an autopilot, or for a create an integrated picture of the filght status for the pilots to view without having to determine which sensors accurate and which are faulty, or making conversions in their head.

For an engine based airspeed, perhaps one could use engine RPM, instantaneous fuel consumption, and air pressure/altitude.  Strain gauges might also augment a HP determination.

Lift from various sensors on a wing or fuselage would also help.

One could also judge lift from pressure (hydraulic?) on the flaps, although this would be altered by added weight of ice (could that be estimated?)

As mentioned earlier, GPS can also be used to give an absolute speed.  Not relative to wind, but nonetheless important.

Is it possible to better make ice-free sensors? 

Anyway, humans too often fixate on a single sensor whereas a computer can be good for integrating information from multiple sources, and with the right programming, determining which sources are valid or not.
 

Offline alancalverd

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Re: Air speed data from engines
« Reply #4 on: 04/02/2015 00:06:24 »
The essential parameters in determining whether a plane flies, are airspeed and attitude. If the airspeed indicator dies (usually due to ice on the pitot or static head) you can in fact fly entirely by attitude (which determines airspeed in a glide), and in a well-equipped airliner you might expect to find two independent attitude gyros, one "steam" gauge driven by a vacuum pump and one electronic with a dedicated battery backup. This should be sufficient to land or ditch an intact aircraft from any adverse event such as loss of engine power or stall, provided that you have been trained to fly on attitude alone, and are in current practice. 

It is for this reason that "the authorities" are reviewing training and currency requirements, and are suggesting that airline pilots should from time to time fly a sailplane or simple single engine machine.
 

Offline Colin2B

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Re: Air speed data from engines
« Reply #5 on: 04/02/2015 11:12:51 »
I've toyed for some years with the idea of an ultrasonic doppler system for measuring TAS but never got round to it. Perhaps now is the time!   

until recently, masthead wind sensors on sailing yachts have relied on cup anemometer and vane sensors. These are very sensitive to SRE (Starling Roosting Effect) so there has been a lot of interest in ultrasonic systems similar to http://gillinstruments.com/products/anemometer/windsonic.htm which you are probably aware of. Are commercial links allowed? If not apologies, search for Gill Instruments.
Not sure if sensors like these are suitable for aircraft speeds. As they can work in 3D, would they show effects like wind shear, ?if compared to an inertial guidance system? Extra thought - on sailboats the guidance sensors include roll, pitch and yaw as in planes, do planes also include vertical fall/rise rate sensors - these aren't often of use in a boat!
« Last Edit: 04/02/2015 13:14:40 by Colin2B »
 

Offline alancalverd

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Re: Air speed data from engines
« Reply #6 on: 04/02/2015 16:59:37 »
do planes also include vertical fall/rise rate sensors - these aren't often of use in a boat!

Yes, but the vertical speed indicator is often the first instrument to fail - ice or water in the static vent line will produce all sorts of interesting effects, none of which is related to the actual movement of the aircraft (been there!)

I'll look into Gill Instruments. Doppler airspeed should work better at higher speeds.
 

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Re: Air speed data from engines
« Reply #6 on: 04/02/2015 16:59:37 »

 

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