Ram Air on a Rotary?

[Crazycanuck]

I am planning to build a Cozy MKIV and I have been thinking about and researching the possibility of using a Renesis Rotary for the powerplant. I like that the Renesis is naturally aspirated, because alot of the auto conversion challenges seem to be related to turbo charging. This got me thinking about a ram air system in order to maintain the higher intake presure required at higher altitudes. Has anybody experimented with ram air on the Cozy with a rotary engine, or any other for that matter? Does the extra drag out weigh the power advantage? I was thinking that little scoops under the wings against the fusalage would be the best high presure location. Any guesses on what kind of presure increases could be achieved with such a system?

[Eccentric]

I have a ram air inlet on my rotary install. I notice a slight effect at cruise speed, 160 knots. You don't get a lot of pressure boost from ram air at these speeds.

[Waiter]

Any guesses on what kind of presure increases could be achieved with such a system?

__________________

 

Depending on the design of your air intake system, you should get an increase from two factors:

 

1) Ram air will be under pressure, so you should get a increase in HP.

 

2) On my ram air system, I bypass all filter, This should also get a slight increase.

 

Not a guess, but an actual number. 160kts represents an air pressure of just a little over 1 inch of mercury.

 

Goto my web site and look under DOWNLOADS > PLACARDS and MANUALS.

 

You'll find two items (pdf format) listed under AIRSPEED TABLES. One is from Honeywell and the other is Scanivalve.

 

When you look at these tables, verify that the pressure your looking at is in Mercury or Water -

 

 

Waiter

[argoldman]

Mooney used that system in the 60s on their mode lcalled the super 21 which used an I/O 360 engine. There was a little door in the nose of the Cowell which was cable operated, allowing ram unfiltered air to enter induction system. They claimed a 1 to 2 inch increase in manifold pressure using that system.

 

For reasons known only to the Moonies they discontinued it in their later hot rods.

[Eccentric]

__________________

 

 

Not a guess, but an actual number. 160kts represents an air pressure of just a little over 1 inch of mercury.

 

Waiter

I was too lazy to do the calculation, but that is about what I see on my manifold pressure gauge.

[Richard Schubert]

Well, dynamic pressure is equal to 1/2 * the density of the fluid * the velocity squared.

Lets say 150 mph(thats indicated) 150*5280/3600= 220fps

Density of air = 0.002377 lbs/cuft (approx. )

so (0.002337)(220)(220)= 113lbs/sqft of pressure

divide by 144 for sqin =0.786 lbs/sqin

atmospheric pressure = 29.92 in mercury or 14.7lbs/sqin

2.03 in mercury/1psi

0.786*2.03= 1.6" max theoretical pressure gain

YMMV

"Just finished a fluids course and I couldn't resist":D

[Tyson]

A quote from the mooneypilots.com site article about the Mooney 20E (you can read the full article on the site for free):

 

"I would like to make a comment from a technical standpoint about the ram air system on the E model. Mooney's marketing department made a big deal about the ram air system back in the '60's--a "poor man's turbocharger" and all that stuff. Talked about how it was a stroke of design genius to offer this primary induction system bypass to give added engine power and better airplane performance when flying in clean air.

 

Bull. There is no way a properly designed primary induction air system should have a 1" hg manifold pressure drop across the filter. I think the ram air system on the E model was simply a bandaid [my emphasis] for a poorly designed primary (filtered) system. If Mooney's powerplant engineers had designed the primary induction air system properly in the first place, they wouldn't have had to incorporate this "ram air" idea. It's bad engineering to require the pilot to open an induction air bypass to get the engine performance he deserves in the first place. And unfiltered air is not good for the engine."

 

From a simple-person's point of view, I do not see many super-fast/efficient planes with ram scoops hanging in the breeze. Thus it might be reasonable to conclude that the drag associated with scoops outweighs any benefit.

[Eccentric]

Well, dynamic pressure is equal to 1/2 * the density of the fluid * the velocity squared.

Lets say 150 mph(thats indicated) 150*5280/3600= 220fps

Density of air = 0.002377 lbs/cuft (approx. )

so (0.002337)(220)(220)= 113lbs/sqft of pressure

divide by 144 for sqin =0.786 lbs/sqin

atmospheric pressure = 29.92 in mercury or 14.7lbs/sqin

2.03 in mercury/1psi

0.786*2.03= 1.6" max theoretical pressure gain

 

I think the 0.002337 is density in slugs/cuft.

Didn't you forget the 1/2 factor?

0.5 * 0.002337 * 220^2 = 56.6 lbs / sq ft

At 10,000 feet (where we travel) the density drops to 0.001756 slugs/ft^3

0.5 * 0.001756 * 220^2 = 42.5 lbs / sq. ft.

or 0.295 lbs / sq. in.

or 0.6 inches of mercury.

 

Using 160 knots that I travel at and 10,000 feet you get

64 lbs / sq. ft = 0.9 in. mercury, about what I see on my manifold pressure gauge.

[Richard Schubert]

Hmmm, maybe that's why I got a "B"

[Crazycanuck]

So basically it is probably better to just keep the aircraft nice and slippery and forget about the ram air intakes.

 

Most modern supersport motorcycles come with some sort of ram air system. I guess it must be a marketing gimick.

[Lynn Erickson]

Well, dynamic pressure is equal to 1/2 * the density of the fluid * the velocity squared.

Lets say 150 mph(thats indicated) 150*5280/3600= 220fps

Density of air = 0.002377 lbs/cuft (approx. )

so (0.002337)(220)(220)= 113lbs/sqft of pressure

divide by 144 for sqin =0.786 lbs/sqin

atmospheric pressure = 29.92 in mercury or 14.7lbs/sqin

2.03 in mercury/1psi

0.786*2.03= 1.6" max theoretical pressure gain

YMMV

"Just finished a fluids course and I couldn't resist":D

Thats a lot of numbers. On my IO360, 1.2 is about what I see on the manifold gauge when I switch from the filter to the direct ram air. Numbers aside the difference between the two on a sea level take off is worth every bit of the little drag of the scoop. the difference starts at about 150 mph and makes about a 500 ft./ min increase in climb rate. As to the drag issue, the air has to get to the engine somehow. even if it comes in with the cooling air into the cowling it is still creating drag. if some of the air goes for engine intake then that is air not used for cooling. the total of intake and cooling drag is the same weather it is in one scoop or two. why would you not take advantage of the higher pressure air and pipe it directly to the intake.

[TMann]

"Just finished a fluids course and I couldn't resist":D

What a cointhednthsc.... I jost finithed a cource of flewidz myzelf!

[emteeoh]

So basically it is probably better to just keep the aircraft nice and slippery and forget about the ram air intakes.

 

Most modern supersport motorcycles come with some sort of ram air system. I guess it must be a marketing gimick.

i don't think its that simple. The hayabusa really does generate alot of HP at speed. Many a racer has *complained* about the difficulty of dyno'ing the hayabusa, in my experience.

 

That said, I should also point out that car!=motorcycle!=airplane. They're solving different problems, and in different environments. YMMV.

 

 

... but it really does sound like you should make a slippery plane and forget about the ram air intakes.