enginehistory.org

avasko · Joined: 13 Jul 2003 Posts: 37 Location: Ft Collins, CO USA

Engine Cooling Fans
I have wondered why no (at least in my search) American radial engines used cooling fans. I here of course exclude the pusher installations such as the Northrop XB-35 flying wing and their XP-56 and the Convair B36 and XC-99.
Never seen anything about how effective the B-36 cooling was.
Both the Brits and Germans made extensive use of nose=case fitted cooling fans. Bristol certainly did as did the FW-190.
Often wondered if things might have been better for the B-29 if they had been fitted with fans. Knew an ex-B-29 FE who described the taxi out laden with bombs and fuel and watching the CHT go up, start TO roll with cowl flaps open and closing them on the roll hoping they got closed before liftoff. Those big cowl flap ears spoiled enough lift (+ drag) that you couldn't get off with them open yet looked at CHT at its max.

kmccutcheon · Posted: Tue Sep 21, 2021 06:20 Post subject:

I can recall only one U.S. tractor installation that used cooling fans --- the Republic XF-12 Rainbow. In that case the tight nacelles and extremely high-altitude operation suggested the fans.
_________________
Kimble D. McCutcheon

avasko · Joined: 13 Jul 2003 Posts: 37 Location: Ft Collins, CO USA

I forgot the Republic Rainbow. One other item, the drag from cowl flaps is pretty high as noted in my first post. Those big ears sticking out didn\t help aircraft performance.
Makes me wonder why sliding cowl air exits that didn't stick out in the airstream were rarely adopted. Rainbow had them. They also allowed full cooling airflow even on takeoff while conventional cowl flaps had to be closed when needed most.
And still no reeason why US manufacturers never adopted cooling fans. I remember seeing, I think it was a Rotol publication that offered cooling fans and I think I remember they said they only used 55 hp to drive.
From a mechanic's point of view, the Lockheed 749/1049/1649 had the best cowling to allow access to the engine. Single rod disconnect and up went the cowl with cowl flap attached. Full easy access. Douglas was not so nice.

avasko · Joined: 13 Jul 2003 Posts: 37 Location: Ft Collins, CO USA

The Republic XP-72 also was fan cooled so that makes two US aircraft with them. It apparently did have cowl flaps though
Digging a bit deeper into the XF-12 cooling, it had a two-stage fan up in front. The aircraft was intended to use R-4360-31 with dual drive shafts for contra-props. The Hughes XF-11 used them and proved the stage of contra-props technology. The -31's were converted to the -37 model with a single prop drive shaft. I am so far unable to find if the two-stage cooling fan installation was done by P&W or by Republic or some other. I haven't seen anything by P&W to indicate they had forward fan drives fitted to the their engines.
Anyone seen any pictures of that fan installation?
Quote from article below.

https//military.wikia.org/wiki/Republic_XF-12_Rainbow

tfey · Joined: 13 Jul 2003 Posts: 104 Location: Arlington Hts., IL

The two speed fans on the Rainbow were driven off the propeller hub, but I've never found any photographs of diagrams of the mechanism. I believe the fans were made of a plastic material. There are flight notes that state when at altitude, low speed on the fans did not provide sufficient cooling. However, switching to high speed provided sufficient cooling, but sucked too much power from the engines, thus no increase in airspeed. The R-3350 powered JRM Martin Mars had hub/spinner mounted cooling fans behind the Curtiss Electric propellers.

avasko · Joined: 13 Jul 2003 Posts: 37 Location: Ft Collins, CO USA

Thanks much Tom for that info on the cooling fans on the Republic Rainbow. The Brits and Germans made their cooling fan as part of the engine. Frustrating no photos or tech drawings of the installation. One question though: You know how many can not get "two-speed supercharger" and "two stage" supercharger completely confused.
Was the Rainbow's fan a "two-speed single stage" or a "two-speed two-stage" fan.? Ditto for the XP-72.
Would love to see pics of the installation, of how they drove it off the prop hub, and diagrams of the speed shifting mechanism. How was it lubricated.? Who built it?
Shame no one asked these questions back in late 1950's. Bigger shame no one thought to preserve a complete engine QEC with cowling, fan, prop. Truly unique engine/cowling installation from a USA manufacturer.
Of course we can also curse the Air Base commanders who destroyed the XB-15 and XB-19 to "clean up their bases" in spite of orders from General Arnold. So much history lost.

tfey · Joined: 13 Jul 2003 Posts: 104 Location: Arlington Hts., IL

Tony, the XF-12 twp speed cooling fan set up was built by Curtiss. Low speed fan was propeller speed and high speed was 2.283 prop shaft. The shifting was electronic via the FE station, and the fan blades were indeed plastic. If you send me your email address I'll send you the Phase II flight Test document that Kim dug up which has the best, but only partial, description of the XF-12 cooling fan system. I believe the XP-72 cooling fan (like the JRM-2 Mars units) was basically a spinner component that spun at prop speed. The XP-72 did have a fixed contra vane behind the rotating fan. Tom

rwahlgren · Joined: 15 Aug 2003 Posts: 324

No need for a cooling fan, especially on something like a C-97.
The propellers have cuffs to help aid with cooling air flow during ground operation, once the aircraft reaches speed, there is sufficient air flow for cooling. A fan represents more mechanical complexity, more parts to manufacture. Then the least desirable things would be the added weight, on a 4 engine plane that would be X4 the weight of course, and lets not forget, the added drive system, as well as the fan it self is just something else that could fail in flight. Maybe the engineers just decided not to fix something that wasn't broke?
The B29 just needed what they did to FiFi. The exhaust collector in the rear not in front.

tfey · Joined: 13 Jul 2003 Posts: 104 Location: Arlington Hts., IL

It was only the contra-prop version of the XP-72 (#2 ship) that had the cooling fan. The first prototype XP-72 used a single rotation Curtiss Electric prop with blade cuffs to do the cooling work. Aeroproducts was queried by Republic to build up props and fans for the Rainbow, but Aeroproducts declined. The Blackburn Beverly transport used cooling fans, but with up to 2,850 horsepower per sleeve valve engine and a cruise speed of 173 mph, I'd guess the limited airspeed of the Beverly, like the cooling-fanned Bristol Freighter (164 mph cruise) necessitated the fans.

avasko · Joined: 13 Jul 2003 Posts: 37 Location: Ft Collins, CO USA

I certainly am no fan of additional complexity. I was quite intrigued when I ran across a series of videos explaining why the FW-190 A series were so fast. Its BMW engine used a cooling fan. This is why the cowling air inlet of the FW was so small. I was also intrigued when the presenter pointed out there were no external scoops associated with the the engine.
The carb air intake was under the cowl benefitting from a small boost in pressure under the cowl. The oil cooler air intake was also under the cowl. Is it any wonder the FW went so fast?
I have gazed into the big nose cowl opening on Lockheed and Douglas transports with their large oil cooler scoop under the engine not to mention the big carb air scoop. I will admit Lockheed did a better job of fairing in the carb intake on the Connies than Douglas on their DC-4/6/7. Still, looking at the Rainbow's small cooling air intake and can only wonder how much the big piston transports could have benefitted from such a design.
I do know the Airborne Early Warning Lockheed RC-121 and WV-2 had their cooling problems due to their relatively low speed on station. Many had the fixed afterbody aft of the prop spinner removed in order to get a little more air in.
I also still speculate if the B-29 could have used a fan. The redesigned 3350 used in Fifi wasn't available in any quantity if at all in 1944/45. The miserable BA model engine was a dog and its forward facing front row exhausts and the big hot manifold certainly did not help cooling. Maybe a fan would have helped there.

tfey · Joined: 13 Jul 2003 Posts: 104 Location: Arlington Hts., IL

Tony, yep, B-29's had their problems for sure. Did you get those Rainbow documents on your aol account OK? Tom

avasko · Joined: 13 Jul 2003 Posts: 37 Location: Ft Collins, CO USA

Yes, Curtiss props early on had cuffs. The Curtiss equipped Lockheed L-749/749A had cuffs. L-1049 with Curtiss had no cuffs. L-049, 1049, 1649 HS props had no cuffs.
Boeing B.377 Strats with Curtiss had cuffs. HS hollow steel appear to have small cuffs but when they converted to solid dural had no cuffs. American Overseas B.377 Stratocruisers Curtiss too. Those were converted to HS props when Pan Am bought AOA. United Air B.377 also had Curtiss but converted to HS when BOAC bought them. Apparently there was considerable weight saving in the change but the HS hollow steel (and DH props on Brits) were quite snakebit.
Only DC-4/6/7 I ever saw with Curtiss was a conversion at Lockheed NY of a USAF SAMFLEET VC-118 (DC-6). One aircraft was converted from HS Hydro's to Curtiss. I believe it got cuffs but it was a long time ago. However, early DC-6 at American had Curtiss with thin chord blades, no cuffs. Later all converted to wide bladed HS.
Early Convair 240 at American had wide blade square tip Curtiss, I presume hollow steel, then all went to HS wide chord blades. No cuffs.
Foreign operators flying Lockheed L-749 and L1049 frequently ordered Curtiss Electrics as did Seaboard and Western in USA. However never saw one on a Douglas civil in my 33 years at KJFK (KIDL).
Apologize for thread drift.
Returning to cooling fans, can't find it now but have seen some Rotol brochures claiming only 30-hp in one and 60-hp in another as power consumed by cooling fans. Sounds optimistic to me. I assume the Curtiss two-speed single stage fan on the Rainbow must have consumed a lot more hp especially in high.

tfey · Joined: 13 Jul 2003 Posts: 104 Location: Arlington Hts., IL

I just found an ad for early Convair 240's with the square-tipped CE prop blades. Pretty cool. As for the cooling fans, it would be interesting to know how much power they consumed. The cooling fans on the Rainbow ran at 2.283 propeller speed in high, and I don't know how power consumption scales with cooling fan speed, but if low (propeller) speed took 60 horsepower, then the fans in high could be taking nearly 140 horsepower. It might be hard to really know due to airspeed, temperature, density altitude, and how much power the engine itself is making under those conditions.

avasko · Joined: 13 Jul 2003 Posts: 37 Location: Ft Collins, CO USA

Not to beat a dead horse but, if the Rotol numbers are correct, the installation of a cooling fan costs little weight or engine power. I grant this is a sales brochure. Some Bristol engines came with fans installed. I still think the big air nose inlet could have been reduced lowering drag. Note how small the CV-440 inlet is as the over-the-wing exhaust helped aspirate the air through the engine.

The use of cooling fans by US aircraft (traction) engines was not limited to the Republic XF-12 Rainbow and XF-72. It turns out that one model of the Martin PBM Mariner had them installed. I believe it is the PBM-3D version with uprated R-2600-22 of 1900-hp. (Replaced R-2600-12 of 1700-hp. I did not record specific reference at time I saw it.

From AEHS online
Rotol Ancillary Equipment List of Spinners, Fans and Feathering Pumps
Compiled by Kimble D McCutcheon - Published 1 Mar 2012

Rotol Cooling Fan CF/1A Weight 39 lbs., absorbs about 20-hp of engine output
Source: Page C5.

Rotol Cooling Fan CF/2A Weight 38 lbs, absorbs about 20-hp of engine output shorter blades than CF/1A Page C-6. Difference between the two examples is they are fitted behind different Rotol propellers and spinner with differing sizes.

tfey · Joined: 13 Jul 2003 Posts: 104 Location: Arlington Hts., IL

Hi Tony, thanks. I know the two surviving Martin Mars JRM (R-3350) fire bombers have fans on the spinner backplates of their Curtiss Electric props. These are of course heavy transports and Wiki says they cruise at 190 mph, so relatively low speed like the Beverly and Bristol Freighter.

https://www.aopa.org/-/media/images/aopa-main/news-and-media/2016/july/airventure/martinmars/0801_martin_mars_24.jpg?h=177&&w=315&la=en&hash=3ED99AC314A96140230E52A55A83AAA179360405

The lone surviving Martin PBM-5A residing at Pima does not have cooling fans, but I don't know too much about the PBM's.

The Fw-190A / BMW 801 fan was geared 1.72 to the crankshaft but I don't recall ever seeing information regarding how much horsepower the fan consumed. Great stuff, thanks as always for the info.