Reno air race aircrafts evaporative cooling

[mvdmitri]

How Water-Injection Cooling Enables Race Planes - Tested.com

Evaporative Cooling 1

Evaporative Cooling 2


During 8 minute race at 500 mph aircrafts consume about a gallon of avgas every 8.5 seconds and a gallon of cooling water every 5 seconds. It seems that a better solution could be cooling radiators with the compressed air. Higher air pressure will magnify Meredith effect even if some power will be consumed by cooling fan.

 

[pbehn]

How Water-Injection Cooling Enables Race Planes - Tested.com

Evaporative Cooling 1

Evaporative Cooling 2


During 8 minute race at 500 mph aircrafts consume about a gallon of avgas every 8.5 seconds and a gallon of cooling water every 5 seconds. It seems that a better solution could be cooling radiators with the compressed air. Higher air pressure will magnify Meredith effect even if some power will be consumed by cooling fan.

Which compressed air?

 

[mvdmitri]

Evaporating cooling system requires additional water tank with almost two times more volume than gas tank. As a result, aircraft range at maximum speed will be 3 times less.


Compressed air flow through radiator is already present at high flight speed. Adding fan in front of radiator can boost this pressure even higher.
Air with higher pressure leaves radiator nozzle at velocity faster than speed of flight.

 

[wuzak]

Evaporative cooling removes more heat.

 

[Dash119]

Evaporating cooling system requires additional water tank with almost two times more volume than gas tank. As a result, aircraft range at maximum speed will be 3 times less.


Compressed air flow through radiator is already present at high flight speed. Adding fan in front of radiator can boost this pressure even higher.
Air with higher pressure leaves radiator nozzle at velocity faster than speed of flight.

M mvdmitri ,

First, I am not an engineer or physicist but the heat transfer from a radiator to air is far less efficient than the heat transfer from a radiator to liquid.

Second, even if a fan were practical it has to draw energy from somewhere. This will either sap power from the engine itself, or be battery powered. Unlike a coolant tank or open loop boil off system, the weight of the battery and motor/fan would remain constant throughout the race rather than decreasing. Even the improved thrust from the Merideth effect, which has limits, cannot offset the energy required to power the fan, or the additional lift and attendant drag required by the weight of system itself. There is no free lunch.

Third and most importantly, as one of the articles points out, all of the evaporative systems used at Reno were either designed by Pete Law or based on his designs. Pete Law worked at Lockheed Skunkworks for decades on some of the most advanced air vehicles ever built. While I can appreciate your out of the box thinking, I believe if your idea had merit Pete Law would have seen that. Ask any member, of any Unlimited Racing team since 1964 and I'm sure they would all concur.

 

[pbehn]

Evaporating cooling system requires additional water tank with almost two times more volume than gas tank. As a result, aircraft range at maximum speed will be 3 times less.


Compressed air flow through radiator is already present at high flight speed. Adding fan in front of radiator can boost this pressure even higher.
Air with higher pressure leaves radiator nozzle at velocity faster than speed of flight.

The range of a Reno racer is geared to the Reno races, if the races were longer or on a different course they would look much different. The Schneider trophy racers went to museums because that's basically all they could do, calling them sea planes is a stretch of the imagination.

There was a very elegant explanation of the Meredith effect on the P-51 posted here. The speed and pressure of the incoming air is actually reduced by the shape of the system in front of the radiator, the radiator is designed in such a way as to not resist airflow and the shape of the ducting after the radiator makes use of the heated air's speed and pressure to produce some thrust effect. However your figures for fuel consumption are much higher than I recall a standard Merlin being which is about 150 gallons/hour on full power, so it is producing much more heat and therefore needs much more cooling.. Reno air races are held close to the ground but Reno is above sea level, this doesn't help a radiator cooling set up at all. Also compressing air to any significant amount heats it up.

 

[gumbyk]

From memory, water absorbs something like 25 times more heat than air. So it's far more efficient.

 

[pbehn]

From memory, water absorbs something like 25 times more heat than air. So it's far more efficient.

Beat me to it I was just looking up the latent heat capacity and heat of vaporisation. The difference between the heat needed by water heating it up by a 1C and that required to turn it to steam is also huge .
Heat capacity - Wikipedia
Enthalpy of vaporization - Wikipedia

 

[gumbyk]

mine was just based on cold-water survival. But yeah, water is far more efficient at carrying heat than water.

 

[mvdmitri]

This may explain why by 1945 aircraft piston engines where great, but already totally defeated by first imperfect turbojets. As aircraft goes beyond approximately Mach 0.65 the required power goes up exponentially. Also propeller efficiency drops down quickly. Piston engine rejects to cooling water/or air about 25% of the fuel energy. This means that an aircraft radiator not much larger than automotive one has to remove 2000-3000kW of thermal energy to a very large volume of cooling air with small temperature rise.

 

[gumbyk]

This may explain why by 1945 aircraft piston engines where great, but already totally defeated by first imperfect turbojets. As aircraft goes beyond approximately Mach 0.65 the required power goes up exponentially. Also propeller efficiency drops down quickly. Piston engine rejects to cooling water/or air about 25% of the fuel energy. This means that an aircraft radiator not much larger than automotive one has to remove 2000-3000kW of thermal energy to a very large volume of cooling air with small temperature rise.

Yes, shedding the heat generated was one of the limiting factors in piston engine development. One of the reasons Reno racers are able to make a lot more power than the same engine in WW2 is because they only need to fly for minutes not hours, so the heat build-up isn't so great.

 

[Dash119]

Yes, shedding the heat generated was one of the limiting factors in piston engine development. One of the reasons Reno racers are able to make a lot more power than the same engine in WW2 is because they only need to fly for minutes not hours, so the heat build-up isn't so great.

Given that they are producing significantly more power than envisioned in the original designs, I think it is the ability to dissipate the heat for a short period of time rather than the lack heat build-up.

 

[gumbyk]

Given that they are producing significantly more power than envisioned in the original designs, I think it is the ability to dissipate the heat for a short period of time rather than the lack heat build-up.

a bit of both.
To takeit to an extreme: drag racers don't have any cooling systems, because they only run for seconds at a time.

I suspect we're both talking about the same thing, but different terminology.

 

[Dash119]

a bit of both.
To takeit to an extreme: drag racers don't have any cooling systems, because they only run for seconds at a time.

A Top Fuel drag race is 4 to 5 seconds at full power. The Reno races are about nine minutes for the top racers in the Gold Class Unlimited race. Producing 3,000 or 4,000 horsepower for that time period is going to result in quite a bit of heat build-up...

 

[gumbyk]

A Top Fuel drag race is 4 to 5 seconds at full power. The Reno races are about nine minutes for the top racers in the Gold Class Unlimited race. Producing 3,000 or 4,000 horsepower for that time period is going to result in quite a bit of heat build-up...

yes, but the engines can cope for that short a period without dissipating all of the heat produced.
e.g. if they shed 90% of the heat produced, they may only reach critical temps at the end of the race.

I've had G.A. aircraft engines quit due to components failing due to heat soak, and it's often after a longer period than you'd think.

As I said, I think we're both talking about the same thing, but from different perspectives

 

[jmcalli2]

yes, but the engines can cope for that short a period without dissipating all of the heat produced.
e.g. if they shed 90% of the heat produced, they may only reach critical temps at the end of the race.

I've had G.A. aircraft engines quit due to components failing due to heat soak, and it's often after a longer period than you'd think.

As I said, I think we're both talking about the same thing, but from different perspectives

These engines are very expensive; the racers can't afford to burn them up totally.

 

[pbehn]

These engines are very expensive; the racers can't afford to burn them up totally.

The airframe and pilot are quite important too, I would feel like that if I was a pilot anyway.

 

[Dash119]

yes, but the engines can cope for that short a period without dissipating all of the heat produced.
e.g. if they shed 90% of the heat produced, they may only reach critical temps at the end of the race.

I've had G.A. aircraft engines quit due to components failing due to heat soak, and it's often after a longer period than you'd think.

As I said, I think we're both talking about the same thing, but from different perspectives

In the case of the most modified Merlin powered racers, they are producing more that twice the horsepower of a stock motor. The scoop and the frontal area of the coolant and oil radiators is the same or less than the stock airframe. The altitude at Stead field where the races are flown is over 5000' above sea level. I agreed that they don't need to shed all of the heat produced in the start-up, take off, form up for start, full race and landing. However they do need to shed the bulk of the heat generated during the race itself. Hence the use of spray bars on the radiators, and the attendant weight penalty of those systems. Unlike a top fuel dragster, you don't shut down the motor at the end of the 4 second run, you need to keep flying and to land the airplane. Albeit at much lower power settings.

 

[pbehn]

In the case of the most modified Merlin powered racers, they are producing more that twice the horsepower of a stock motor. The scoop and the frontal area of the coolant and oil radiators is the same or less than the stock airframe. The altitude at Stead field where the races are flown is over 5000' above sea level. I agreed that they don't need to shed all of the heat produced in the start-up, take off, form up for start, full race and landing. However they do need to shed the bulk of the heat generated during the race itself. Hence the use of spray bars on the radiators, and the attendant weight penalty of those systems. Unlike a top fuel dragster, you don't shut down the motor at the end of the 4 second run, you need to keep flying and to land the airplane. Albeit at much lower power settings.

There is a difference between an enhanced radiator system and a total loss boil off cooling system the main one being a draggy air intake. Weight isn't a huge penalty in speed records as compared to drag.

 

[Dash119]

There is a difference between an enhanced radiator system and a total loss boil off cooling system the main one being a draggy air intake. Weight isn't a huge penalty in speed records as compared to drag.

I agree, but none of the most modified Merlin/Griffon powered racers are currently using a boil off system for the engine coolant, and they retain a belly scoop.

Steven Hinton's world record run in 2017 was in Voodoo, which has a radiator and modified belly scoop.