Belly Pan slowing me down

[Otto]

How has your belly pan mod affected cabin noise levels, esp. at highway speeds?

[wildfire8]

Originally Posted by winkosmosis

I'd say this illustrates why the concept of a rear grill block makes no sense. By letting the air flow through, you decrease drag.

Going threw the engine bay the air has to flow around all the stuff in there. So that makes turbulent air. If you stop the air from going it then it will reduce drag.

[wildfire8]

Originally Posted by Otto

How has your belly pan mod affected cabin noise levels, esp. at highway speeds?

Its the same. I have always had tire noise on the highway cuz they use a different kind of road. When I am on streets I don't have tire noise. Even if it did affect cabin noise I didn't notice.

[winkosmosis]

Originally Posted by wildfire8

Going threw the engine bay the air has to flow around all the stuff in there. So that makes turbulent air. If you stop the air from going it then it will reduce drag.

That's what it was like when it was completely blocked off with no exit. No exit means no air going in. The grill is just acting like a parachute with air going in a certain distance and spilling back out


frank lee and I were arguing about this months ago.


The grill opening is small, the outlet is small. That means air slows down quite a bit going through the BIG engine compartment. Let's say the open cross section of the engine bay is 4x that of the inlet. That means the air would slow down to 1/4, and have 1/16 the drag as if it were full speed.

That would explain why the outlet vent on that plane reduces drag so much... Because of the small cowl opening, engine bay drag is low, and it acts as a tube to let air through.

[Otto]

Originally Posted by winkosmosis

That's what it was like when it was completely blocked off with no exit. No exit means no air going in. The grill is just acting like a parachute with air going in a certain distance and spilling back out


frank lee and I were arguing about this months ago.


The grill opening is small, the outlet is small. That means air slows down quite a bit going through the BIG engine compartment. Let's say the open cross section of the engine bay is 4x that of the inlet. That means the air would slow down to 1/4, and have 1/16 the drag as if it were full speed.

That would explain why the outlet vent on that plane reduces drag so much... Because of the small cowl opening, engine bay drag is low, and it acts as a tube to let air through.

Internal ducting is a black art, but an important one for minimum drag. NACA did a lot of research on this during WWII. Once the cooling air gets in, it needs to slow down and increase pressure to reduce its transit drag, but importantly to have time for its higher pressure air to absorb engine heat. Once having done so, it needs to re-enter the slipstream in as efficient a pathway as possible.

The inlet should be at stagnation point on the nose, where pressure is greatest for ram-air effectiveness, then exit at the point of max. speed and therefore lowest pressure along the sides or bottom. Nature has perfected this, as seen with fast-swimming fishes.

[budgenator]

Originally Posted by winkosmosis

I don't understand how the heat could result in thrust from the exit port. The heating and expansion of air wouldn't push air out the back any more than it would push it out the front. The way I see it, it should have no net effect on flow.

It looks to my non-aeronautical engineering mind the the shape of the P51D radiator housing is a classic air-foil and would form a lifting surface and that lift would oppose the lift generated by the wings. It's position slightly behind the wings would put its "lift" behind the aircrafts presumed center of gravity, pitching the nose upward. The enlarged exit duct looks like it would be a spoiler to that opposing lift and therefore reduce drag. In a seat of the pants measurement, reduced drag is increased thrust.

[winkosmosis]

Originally Posted by budgenator

It looks to my non-aeronautical engineering mind the the shape of the P51D radiator housing is a classic air-foil and would form a lifting surface and that lift would oppose the lift generated by the wings. It's position slightly behind the wings would put its "lift" behind the aircrafts presumed center of gravity, pitching the nose upward. The enlarged exit duct looks like it would be a spoiler to that opposing lift and therefore reduce drag. In a seat of the pants measurement, reduced drag is increased thrust.

The Bernoulli effect is insignificant in the flight of a plane. I've read that the effect on the wings is something like 1% of total lift, and that's with BIG wings, rather than the little radiator cowl. I'm sticking with my theory that the bigger rear facing outlet vent allows the cooling system to act as a passthrough.

Edit: Usually the tail is used to push the rear down, and lift the front. So if there was an effect like that from the cowl, it would mean less of that force has to come from the tail. Adding the outlet thing, which would act like a spoiler, would then require more input from the tail.

[sepp]

just a question, you say that the front blocks made your temps go up. i suppose thats from reading your scangauge? but im interested to know if you also notice that your standard car temp needle or whatever is higher than usual? i would be interested to know that, as i have blocked my entire front and my temp needle stays at the verry same position as where it used to be before. but i don't have a scangauge to doublecheck.

[evolutionmovement]

I read somewhere that a properly designed NACA cowl over a radial engine could supposedly generate forward thrust. If that is true, I wish I could think of a way to harness that effect on a road car (and correctly model it).

[lunarhighway]

i've never heared of a naca cowling generating thrust. it did however cut down drag greatly when it was introduced, because it allowed for both better cooling flow and reduced drag. previously it was believed that the best way to cool a radial engine was to have all the cylinders exposed to the airlow... most cowling that did exist before that where more wide and there to protect the pilot in his open cockpit from oil splatters (especially in rotary engines wich where radial engines where the entire engine turned with the prop for better cooling!)

because drag was reduced so much planes saw an increase in top speed, and because improve cooling meant leaner mixtures more HP was on tap from the same ammount of fuel.

However the p51 uses an inline engine, with water cooling and the radiator is indeed mounted in a spacial radiator housing, that was developed by North American, although the idea had been tried unsuccessful by Curtis. the shape of the radiator housing is such that the hot air that exits does actually provides thrust, even so much that it will cancel out the drag induced by the radiator. This was called the "Meredith Effect".

All of this happens at rather high speeds compared to cars, and what's more a plane is always moving, even when flying "slow", these planes still go faster than the top speed limit for cars... a car needs proper cooling when it's crawling along at a snails phase, to a completely sealed cooling duct might call for a fan to assist airflow, canceling the benefits.

that said there is possibly quite some room for improvements in the cooling departement