# Pressurizing ram air scoop

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#### Starman

##### Well-Known Member
I would like to supercharge the engine as much as possible with forward motion, let's assume 250mph or more. What kind of scoop shape would give the biggest pressure boost to the engine with the least airframe drag?

Probably a slot at the wing and/or tail leading edges, right? Or a round scoop with a sharp edge out in the prop slipstream?

... and what sort of pressure gain might we want to consider for something like this?

Edit: sorry, I should have done a search first.

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#### GESchwarz

##### Well-Known Member
Here's a calculator for dynamic pressure...

Dynamic Pressure

Of course the actual pressure in the duct will be less due to the "outlet" into the engine.

You're going to want to minimize ducting weight, so the inlet needs to be near the engine intake and in the prop blast to obtain max velocity. Getting the optimum inlet area to maximize boost, but no bigger, will require empirical test or some knowledgable calculation.

Some planes have up to three inlet paths Filtered, Heated, Direct Ram. I'd worry about injesting a bird with direct ram. No extra horse power is worth losing power.

#### Noah

##### Member
From my experience with cars I know you could see an extra 1-3% from 70-150mph. I'd expect similar results from 100-200 with a small plane.

You will have to tune the carb for more fuel if you're seeing any major gains.

Also, turbulence is a huge problem in engine intakes. Many racing engines see noticeable improvement simply by polishing their aluminum manifolds. I would suggest any intake be clear of the wash. Search autospeed for some good tech articles. Its a DIY performance online magazine.

#### orion

##### R.I.P.
At sea level, 300 feet per second, about 1/2 psi. Even if the induction is 100% efficient, the benefit is pretty minimal.

#### Dan Thomas

##### Well-Known Member
I would like to supercharge the engine as much as possible with forward motion, let's assume 250mph or more. What kind of scoop shape would give the biggest pressure boost to the engine with the least airframe drag?

Probably a slot at the wing and/or tail leading edges, right? Or a round scoop with a sharp edge out in the prop slipstream?

... and what sort of pressure gain might we want to consider for something like this?

Edit: sorry, I should have done a search first.
Very little pressure recovery. Jet engines do it by using a divergent duct. The air inlet is smaller than the first compressor stage fan, and as air enters that smaller area is expands as it moves back. This slows the airflow, and as Bernoulli will tell you, slowing airflow increases its pressure. If we have a funnel with the big end pointed forward we have a convergent duct, the opposite of a divergent duct, and the air speeds up in it and the pressure drops. Backwards to what one would think, but it's true. Both convergent and divergent duct principles are used all the way through a turbine engine; a compressor stage accelerates the air and the stator stage behind it slows the air, increasing its pressure and redirecting it into the next stage. Googling this stuff can reveal much interesting reading.
http://ecampus.nmit.ac.nz/moodle/mod/book/print.php?id=50384

In the early '70s automakers sucked in a large part of the car-buying public by offering "Ram-Air Intake" and that sort of nonsense. At car speeds the pressure recovery was really tiny and the turbulence over the hood ruined it all anyway. Those doodads did nothing but make a lot of money. An example:

See what people are using. The aircraft manufacturers use turbochargers, a few engines have superchargers. If you want to get results for your money those are what you use. And they're not cheap.

#### GESchwarz

##### Well-Known Member
At sea level, 300 feet per second, about 1/2 psi. Even if the induction is 100% efficient, the benefit is pretty minimal.
I got the same numbers. Minimal indeed.

#### Toobuilder

##### Well-Known Member
HBA Supporter
Log Member
A pitot style inlet as close to the prop as possible feeding into a divergent duct will provide good results.

My Hiperbipe picked up more than 2 inches of MP with this air box modification ("stock" was pulling air off the rear engine baffle)

This inlet lip added another .2 inches of MP over the straight 3" tube (2.5" ID). Moving the inlet .5 inches from the prop or 4.5 inches back was worth .3 inches of MP in flight test.

#### Dan Thomas

##### Well-Known Member
A pitot style inlet as close to the prop as possible feeding into a divergent duct will provide good results.

My Hiperbipe picked up more than 2 inches of MP with this air box modification ("stock" was pulling air off the rear engine baffle)

This inlet lip added another .2 inches of MP over the straight 3" tube (2.5" ID). Moving the inlet .5 inches from the prop or 4.5 inches back was worth .3 inches of MP in flight test.

Some engines will run roughly when force-fed by the prop blast. The problem is that the air off the prop isn't a steady flow; it's a pulsating blast that has a spike every time blade goes by. I have read of some guys who got better, smoother performance when they turned the intake away from the prop, and often experience it myself when running up engine on a 172 or 182 with the cowling off so that the prop can drive air directly against the air cleaner. With the cowling on the air flows down and past the cleaner, not directly at it, and the engine runs smoother. On the 182RG the intake is a NACA duct in the side of the cowl to avoid any ram pressure at all.

Pulsating airflow upsets the pressures within the carb, with the float bowl pressure rising and falling slightly behind the pressures in the venturi. Uneven fuel flow from the nozzle results and the cylinders don't like it.

Dan

#### Noah

##### Member
Also, make sure its above the boundary layer!

#### Starman

##### Well-Known Member
In the early '70s automakers sucked in a large part of the car-buying public by offering "Ram-Air Intake" and that sort of nonsense. At car speeds the pressure recovery was really tiny and the turbulence over the hood ruined it all anyway. Those doodads did nothing but make a lot of money.
True but there was one type that worked and those are the ones that took in air at the base of the windshield, at speeds well over 100mph you could get some pretty good pressure there.

Once I decided to see how fast my 1969 Barracuda 340 (Formula S) would go. It happened to be on Long Island and it was misty. As the car went past 90 the windshield wipers lifted off the windshield due to air pressure there, but then past 110 the high pressure at the base of the windshield blew all the rain off in a strip across the bottom of the glass so I was able to hunch down to look under the steering wheel like a little old lady and see the road. It got up to about 140. Indicated speed was 129 but I had larger diameter rear tires and they weren't rated for that speed so they expanded even more so speed was at least 140. Not bad for an old worn out engine with 130K hard miles on it and reving well over redline to boot. No doubt a fresh engine geared right would have gone over 150 easy, stock. It was nice to be able to hear the engine wind out for more than the usual four seconds blast, and the car nose was sticking up in the air a fair amount due to torque reaction on the rear wheels and poor aerodynamics.

Oh yes, and that was without the intake induction at the base of the windshield =) but there was high pressure there anyway.

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#### Toobuilder

##### Well-Known Member
HBA Supporter
Log Member
...Some engines will run roughly when force-fed by the prop blast...
Yes, but a critical part of this system is the divergent duct and large plenum. Bedides, I'm injected, not carburated. A pitot inlet stuck directly on the carb would likely cause problems in some cases, but that said, the Vans RV series often has the FAB (Filter Air Box) bolted directly to the carb, and it is of relatively small volume. My buddy's RV-4 would always deliver more MP than any of us when we flew somewhere.

#### GESchwarz

##### Well-Known Member
Some engines will run roughly when force-fed by the prop blast. The problem is that the air off the prop isn't a steady flow; it's a pulsating blast that has a spike every time blade goes by.Dan
These pulses can be canceled to some degree by having two or more inlets spaced half the angular distance between the prop blades.

#### Toobuilder

##### Well-Known Member
HBA Supporter
Log Member
In the early '70s automakers sucked in a large part of the car-buying public by offering "Ram-Air Intake" and that sort of nonsense. At car speeds the pressure recovery was really tiny and the turbulence over the hood ruined it all anyway. Those doodads did nothing but make a lot of money...
Actually, the only thing the car makers were "guilty" of was creating a misnomer - There were many versions of "ram air" among the car makers, and while the claims of actual pressure recovery was dubious at best, the systems did provide cold air to the engine. This was a welcome relief to the standard ingestion of under hood air typical of the more pedestrian variety of cars of the day, and DID provide noticeable gains. It was also true that the "ram air" packages ultimately included specific camshafts, heads, exhaust and compression ratios (therefore creating "ram air" engines, further confusing the subject), the early packages, such as with the 1965 GTO were simply a dealer installed package with an opened hood scoop and a carburetor pan sealed to the underside of the hood. In this case, the introduction of cold air to the engine resulted in a noticeable gain on the dragstrip. Sure there was plenty of advertizing hyperbole, but most systems DID work to some extent.

In airplanes, a well designed and carefully constructed inlet system may not provide any “supercharging” effect, but it is FAR better than simply hanging a foam filter off the front of the carb like Cessna does. In my experience with the Hiperbipe (which features a pretty “good” system per plans) my modification resulted in more than 2.5 inches of MP at cruise. I don’t know about you, but 2.5 inches of MP is a significant gain to me, and well worth the hundred hours or so of development and fabrication.

I’m convinced that a good inlet system is worth the effort on any serious airplane.

#### Dan Thomas

##### Well-Known Member

In airplanes, a well designed and carefully constructed inlet system may not provide any “supercharging” effect, but it is FAR better than simply hanging a foam filter off the front of the carb like Cessna does. In my experience with the Hiperbipe (which features a pretty “good” system per plans) my modification resulted in more than 2.5 inches of MP at cruise. I don’t know about you, but 2.5 inches of MP is a significant gain to me, and well worth the hundred hours or so of development and fabrication.

I’m convinced that a good inlet system is worth the effort on any serious airplane.
I agree. A good inlet can improve things. There are some really poor setups out there and some of them can cost performance. The typical Cessna single might even be suffering reduced MP because of the flow past the filter reducing the local pressure. The R182 we have here has the side-mounted NACA duct that is pretty much a neutral-pressure setup and would be some improvement. I just don't see gaining much pressure due purely to dynamic pressure since it's so small even at 200 mph, and if one was to create a divergent duct to turn velocity into pressure, the size of it could create more drag that eats up whatever power was gained. Such a system has to be buried in the cowl somehow, and in many there isn't much room. Hanging it outside isn't likely to produce positive results.

Dan

#### Bart

##### Well-Known Member
An OZ car enthusiast magazine studied ram air to augment natural aspiration. They found that it could offset the losses normal to intake flow restrictions such as air filter, ducting, etc. that would otherwise happen, so was worthwhile.

#### Starman

##### Well-Known Member
Instead of thinking of the problems associated with divergent ducts, think instead of a stagnation point pickup, like for a pusher like mine, a chunky base to the vertical tail to create a high pressure zone and taking your air from that. Also thin intake slots on wing leading edge were sometimes used to pick up stagnation point pressure.

Edit: it's no longer a pusher aircraft, just a rear engine.

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#### Toobuilder

##### Well-Known Member
HBA Supporter
Log Member
Inlets and exhaust (and cooling, for that matter) are often at best a compromise, or at worst, poorly done. Many production aircraft suffer from this - almost as if the system was an afterthought. As an example, the Bellanca 14-19-2 Cruisemaster has the carb inlet so close to the bottom of the cowl that the duct from the air filter at the front of the cowl terminates after a torturous path as little more than a 1 inch tall, really wide slot - and then it has to make an immediate 90 degree turn to go up the carb. An otherwise great performing airplane, this thing just falls on its face at altitude. He gives up several inches of MP to me in cruise.

Fortunately, unlike really efficient cooling, good induction system design is not all that difficult for the speeds we fly. It does take some reading and packaging is often a problem but its “free” power.

#### wsimpso1

##### Super Moderator
Staff member
Log Member
I can give you the upper bound on "ram air pressure". Calculate q for the flight condition you are talking about, and that is the pressure available.

q = rho/2*v^2

rho is air density - 0.002378 slug/ft^3
v is velocity - 200 knots is 338 ft/s

output is 136 lb/ft^2, or about 0.95 psi.

So, no matter how good a job you do with it, 1.9" Hg of manifold pressure is the absolute best you can do at 200 KIA. Likely result is 50-70%, 1-1.5" Hg. And remember that the "ram air" has a drag price of 136 lb/ft^2...

Now, you can do much worse, as has been discussed above. But let's hope that we are smarter than that...

Billski