cooling fan ideas

I'm looking for some input/ideas for cooling on a 2 seat helicopter of around 100hp. Every helicopter i've seen (piston) uses a fan similar to what an aircooled car would use.

The drawback to this that i see is that the fan is really only needed for hover and low speed flight, in cruise it should be possible to use airflow in the same way a fixed wing does.

Just a quick search seems to indicate around 7-10% of engine power (based on VW engines) goes to driving the fan, so eliminating that drag during cruise would give me either more power or less fuel consumption.

So, the ideas i can come up with to do that are: electric fan(s) or a fan clutch. Both these ideas give more things to break down.

The other idea i had was to somehow use the fan to make thrust and offset it's drag on the engine. This would eliminate the need to turn off the fan, but presents other challenges.

So to sum it up, anybody got and other ideas or input on how to come up with the best system?

One very successful idea that was investigated in detail some years back was the use of ejectors on helicopters. This provided cooling while allowing the engine to maintian its power by not siphoning anything off with something like a fan. In order for this to work the engine has to be enclosed as in a conventional airplane. The exhaust is then used within a properly designed ejector to entrain and "pump out" the air from the plenum below the engine.

If this ejector idea is incorporated into a properly shaped cowling...not only will it provide cooling it will provide a small amount of thrust as well...

Current F-1 teams say they generate up to 150lbs of thrust from their cooling and exhaust ducting... I'm not sure I buy that...but its in the ballpark...

neat stuff...

Dave

Originally Posted by orion

One very successful idea that was investigated in detail some years back was the use of ejectors on helicopters. This provided cooling while allowing the engine to maintian its power by not siphoning anything off with something like a fan. In order for this to work the engine has to be enclosed as in a conventional airplane. The exhaust is then used within a properly designed ejector to entrain and "pump out" the air from the plenum below the engine.

Darn, you beat me to it Orion!

I was going to suggest that very idea, though I wasn't aware that it had been done before... just seemed like it might be something worth trying.

Bruce

Actually Dave, I think the thrust number may be pretty close. Designing a proper configuration can be a pretty involved process and the number of variables allow you to optimize for whatever condition you want to emphasize, thrust being one of the variables. Unfortunately though, if you optimize for thrust, you do reduce the cooling effectiveness somewhat. But therein lie the trades.

Originally Posted by orion

Actually Dave, I think the thrust number may be pretty close. Designing a proper configuration can be a pretty involved process and the number of variables allow you to optimize for whatever condition you want to emphasize, thrust being one of the variables. Unfortunately though, if you optimize for thrust, you do reduce the cooling effectiveness somewhat. But therein lie the trades.

Thats why I love engineering...it actually makes sense...LOL

You decide what is "perfect" to you...or for your application...and make it so!

Oh! If that were the only variable...

Dave

so would an ejector system move enough air to cool the engine during hover on a hot day? i would think if it could keep it cool for at least 3-5 minutes on a 45*c day would be sufficient.

any links on how to design such a system?

Georden, I don't know much about it, but if you look up thrust augmentation as used on pulse jets, I would think it's kind of similar in theory. Not sure because of the pulse effect, maybe there's a difference between that and pure venturi in a ejector? - but the exhaust is pulsed - I guess you want tuned exhaust.

I tried some quick searching on google without any success. most of what i found related to jet engines. I understand the concept of it, but looking for some info on how to design/optimize such a system and maybe how much air flow could be expected.

i would think if it could keep it cool for at least 3-5 minutes on a 45*c day would be sufficient.

I would think that you would want more than 3-5 minutes in whatever temperature regime you see yourself operating in. In a helicopter, all take-offs and landings "could" be done in that amount of time but there is plenty of reason to want to hover for longer than that. At gross weight waiting for take-off clearance is not really a time you want to be worried about cooking your engine. What about hover practice? Hovering is almost the most fun you can have in a helicopter so being able to do if for more than 5 minutes is a nice feature.

But, please take what I say with a grain of salt because I have no time in a piston powered helicopter. Maybe living with those types of limitations are the norm. My helicopter has 30 min, 5 min, and 12 second Turbine Gas Temperature limits. But these time limits can be reset by simply lowering the collective for about 3 seconds and then I can go right back into the limit. Not sure how this would work on a recip.

Here is a link to a thread I started in 2004 that may have some relevant information.

Jake, I'm curious, what sort of performance envelope does each of the limits correspond to? I guess that a straight-up climb would be the hardest on an engine?

The limits themselves don't have a direct link to a particular flight mode. You are right though, hovering straight up and continuing past about 1.5 times the rotor diameter is going to produce the highest demands on the engine. At that altitude there is no ground effect to help make the rotor more efficient and it's just plain brute force holding all 5200 lbs aloft. The way we check to see if we are going to have power to perform our most demanding maneuvers is to climb up to an Out of Ground Effect (OGE) hover and do a slow left pedal turn. On cooler days our limiting factor will be Torque available. On Hot days it's usually the TGT that will limit us before the Torque will. The left pedal turn actually places more of a demand on the engine because the tail rotor is at a higher pitch setting. This time of year there is just no way I can even dream of getting to an OGE hover when I first take off, even though we are just about sea level here. When I pull back out of the re-arm/re-fuel point on hot days I almost always make a right pedal turn. In my very under-powered helicopter, every degree and every % Torque makes a difference. We live at Max Gross weight taking every rocket and every ounce of gas we can get away with.

Originally Posted by Jman

At gross weight waiting for take-off clearance is not really a time you want to be worried about cooking your engine. What about hover practice? Hovering is almost the most fun you can have in a helicopter so being able to do if for more than 5 minutes is a nice feature.

my logic was, at 45* c you aren't gonna be practicing you're hovering while you cook away in a giant glass bubble, and worse case, there is the option of setting down to wait for clearance. it would be nice to have no limit on how long i can hover at gross, but if i can give up the fan and still manage 5 minutes on the hottest days i think i could get by.

read through your post from 2004, i'm just wondering if it has been done succesfull on a helicopter since most of what is talked about is increasing flow, not generating it. did get the idea of using the exhaust augmenter to maybe provide most of the cooling needed and incorporate and electric fan to help out when needed.

my logic was, at 45* c you aren't gonna be practicing you're hovering while you cook away in a giant glass bubble, and worse case, there is the option of setting down to wait for clearance.

Yeah...I flew at those temps last summer and am about to do it again starting in a couple of weeks for the next 3 months. It's already up to 40 degrees here on a regular basis. It takes me about ten minutes to go from lifting up to a hover to getting take-off clearance from tower. Again, I'm not sure how the recip engines work in a helicopter. If dropping down cools it enough to give you more time to continue hovering for another 5 minutes then it could be workable.