Fairing design approach

[jbennett]

I need to design a fairing to deflect air flow and water/ice from an open area on the nose of an aircraft. We did a modification which required cutting a large hole (8.5x13.5) in the avionics bay nose door, and creating a type of a chute from inside the area. Concern has been shown towards this area when the aircraft is flying in icing conditions, the customer is concerned about this cutout acting as an ice scoop. The probability of there being an issue is low, but definitely there. They would like me to design some sort of air dam or fairing that would deflect the air flow and ice enough to keep ice from building up on the aft face of the chute. However, I've never approached a fairing design on the nose of an aircraft before, and I'm unsure where to start as far as effects on air flow, performance in those regards. To make matters more difficult, there is a vent just aft of the area in question which pulls air out of the A-bay through use of creating a lower pressure outside of the aircraft. The fairing must be large enough to deflect any ice buildup, yet be small enough to have minimal affect on the overall aircraft performance and on the vent performance. Any guidance would be greatly appreciated.

James Bennetts
Aerospace Engineer

 

[rb1957]

consider looking at a Dash-8 nacelle ... they've got a deflector and a by-pass duct for ice/snow/slush

what about bird strike ?

 

[jbennett]

It's funny you should mention a Dash-8, since the aircraft we modified was a dash-7. However, we can't use any sort of by-pass ducting for this, we need some sort of fairing. I don't think bird strike is a major concern in this case, my major concern is making something that is effective enough to do the job, especially at large angles of attack during climb to cruise altitude, without loss of vent effectiveness which would cause the avionics bay to overheat.

James Bennetts
Aerospace Engineer

 

[rb1957]

would it be easier to add an anti-ice system (eleectrical, or possibly hot air) ?

 

[jbennett]

Not really, I'm dealing with an area that is only 13.5 inches long by 3 inches wide, and on a door that is opened before and after each flight. Therefore, I don't think an anti-ice or deice system is feasible.

James Bennetts
Aerospace Engineer

 

[Sparweb]

You might use a dam in front of the duct opening that the pilot can deploy when in icing/cloud/precipitation.

The dam could open with the hinge on the leading edge, or push straight out like a brake. Accumulation of ice on the dam might impede retraction, in either case.

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A solution used in helicopters is a reverse-flow fairing, where the engine intake is completely covered. Engine air comes in through two ducts open to the aft, travels forward in the fairing, and then enters the normal engine duct.

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Steven Fahey, CET

 

[jbennett]

Good points, but this is not a ducting. It's an installation which requires an opening to function. I'm afraid I can't go into much more detail than that. However, being in the nose (which is a kevlar/nomex composite), and just aft of the nose radome, there's not much area for a fairing structure. Also, due to the type of installation this is, the original composite door had to be scrapped and replaced with an aluminum door. All said and done, this door weighs on the order of 100+ lbs with equipment installed, and the face which could produce ice buildup is a significant part of the loadbearing structure, hence it must remain continuous. I personally don't believe ice build up is an issue at this location, but without any sort of smoke tunnel testing or simulation, I can't accurately gage how the boundary layer will react to this cut out. If we end up with nothing more than a thin brine of ice over the length of the plate, roughly an inch wide and up to half an inch thick, there is no cause for concern. However, if we end up with a 3 - 5 inch thick coating along the length of the plate, it could be disasterous.

I know this is hard to visualize, I wish there was a way to attach a photo, but I don't think there is. Ah well, any thoughts you have are appreciated, including those already given.

James Bennetts
Aerospace Engineer

 

[FieldTeam]

Are you trying to make a fairing to protect the entire door or just the opening? Could you modify the exit tube/opening area so that instead of having a fairing in front of the hole, you taper the aft side in a tear drop style until the possability of ice accumulation or ingestion would not be a factor inside the device? Sort of like a reverse ram scoop or indentation on the door. You might have to reposition the item inside the mod door.
Otherwise, you could place a 3" wide air dam straight down far enough to protect the hole and fair it as best you can.
I don't think either of these would effect your avionics cooling discharge if it solely works of differential pressure to discharge air.

 

[jbennett]

They want the fairing to protect the cutout, not the door. Modifying the exit tube is not feasible though, the flat aft face is the major load bearing member in the installation, and the exit vent is only 3 inches aft of the chute, not a lot of room on this door. And the vent can't be moved any further aft because it would interfere with the closeout bulkhead that separates the avionics bay from the nose gear bay.
The 3 inch fairing/air dam is basically what they are looking for, but I'm a little concerned about the flight performance effects that would have, not to mention that would require another flight test which they are hoping to avoid. The reason I'm concerned with how that would effect the air flow out of the vent is because the lower pressure is generated by the air flow across the vent cut out (this vent is very similar to the reverse ram scoop you mentioned), and I'm not sure what kind of effect turbulent flow would have on that system. I have only a rudimentary knowledge of aerodynamic flow (two undergrad courses) and no wind/smoke tunnel to test it.

James Bennetts
Aerospace Engineer

 

[rb1957]

g'day again,

for obvious reasons i think you can't be too forthcoming about this installation. actually you can link web pix onto posts (but don't ask me how, but i've seen it on other posts) ... but then i think you very limited in what you could post without getting into trouble (nudge, nudge)

anyways, it sounds like there isn't a large demand for airflow (in or out) ... could you add a deflector, open at two ends (like a "top hat" channel) ? this would provide some airflow (but then i think your question is how much ?) but would also remove any ram effects that would be there with an open inlet, but maybe this isn't important ?

 

[jbennett]

Yeah there's not much I can go into as far as details. It gets very confusing because we're dealing with two cutouts, one for the installation, and one which is a vent for the avionics bay. The cutout for the installation is a 8.5 x 13.5 inch hole, roughly, and 3 inches behind that are two 6x6 inch holes roughly, which are for the vent. Cram all of this on a door which is double contoured and only 25x30 (rough dimensions, as the front is tapered), and there's not much room left over. The deflector would actually have to be located on the nose structure, forward of the door by at least 5 inches. This is where the effectiveness comes into play. No airflow is possible at the installation cutout, it is a sealed installation. However, due to the shape (which is a necessary evil for the installation) it could have ice build up. The vent draws air out of the avionics bay via pressure differential. I know I've probably stated most of this previously, I'm just trying to give a better picture of what's happening here and the lack of space we have to work with.

Regardless, you all have offered up some good information and ideas, and I appreciate it greatly. I think I'm pretty much dead in the water unless I can get some simulation programs or some detailed airflow analysis to go by, and that's not likely in this case.

Thanks again for the ideas and by all means offer any others you have, it's greatly appreciated.

James Bennetts
Aerospace Engineer

 

[Sparweb]

How about a flight manual supplement that prohibits flight into known icing conditions? I've had projects face these kinds of challenges, and circumvented them in some cases by using flight limitations.

Like RB1957, I'm guessing this isn't the run-of-the-mill project, so have you checked that the aircraft isn't already in the restricted category?


Steven Fahey, CET

 

[jbennett]

It is indeed not in the restricted category, and no I'm afraid we cannot limit their flight conditions in that manner. The customer would almost prefer to see a total redesign of the forward installation than limit the flight conditions, especially with some of the locations these aircraft need to fly. We've gone about as far with it as we can at our facility, unless more data becomes available. Thanks again all for the ideas.

James Bennetts
Aerospace Engineer

 

[rb1957]

that's a way to respond to (sidestep) the certification aspect, but i thought the original post was more like (due to the extra-curricular nature of the operator) ... if it's icing when we're flying, then we'll be flying in icing conditions, whatcha gonna do about it !

 

[jbennett]

That's correct rb1957. They fly in an area where they have experienced, or could experience, icing conditions. It's the mission profile of the aircraft, nothing to be done to avoid it.

James Bennetts
Aerospace Engineer

 

[karlbamforth]

I know someone mentioned it before but it sounds to me like a small heated mat similar to the type fitted to piston engine sumps in cold environments. It would prevent the icing and it could be fitted on the inside, to avoid airflow disruption.
This would be a simple installation connecting it into the normal anti-ice systems. Pitot/static etc.

Karl

 

[jbennett]

That was one of the ideas I had had, but it seemed like a major issue to get something put in there. Do you have any recommendations for a place to locate a heated mat like you are mentioning Karl?

Thanks much for the input.

James Bennetts
Aerospace Engineer