Cooling Exhaust Fouling Pusher Prop

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RJW

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How bad would leading edge radiators foul the rear prop on a push/pull tandem arrangement? Would downwash over the wing keep the hot, low-density air out of the prop? Or would the rear prop draw this messy air back up through the arc of the propellor? If the messy air does go through the prop what kind of undesirable effects would be expected?

The attached drawing is meant to illustrate the question but not, of course, the actual airflow. For pushers and tandems the practice seems to have been to exhaust the hot air at the base of the prop. I could do this on this design but there are packaging problems that would be improved if leading edge radiators turned out to be workable. Ideas?

Thanks,
Rob

fouled prop.jpg
 

Vigilant1

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How bad would leading edge radiators foul the rear prop on a push/pull tandem arrangement?
I don't have any figures, but I wouldn't think there'd be much effect. And, remember that of the waste heat generated in a typical IC engine, there's more in the engine exhaust than what is lost to engine cooling. So, if this important at all, the placement of the exhaust pipes is more important than the effluent from the radiators.
 

Jan Carlsson

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The induced air at the prop plane is probably 6-7% of forward speed, at this powers and speeds we talk about here.
check the Hawker Tempest Mk.1 version (and Mk. III) of the LE radiators tested in some very fast version with Sabre IV engine. (war production made for slower Mk.V rad)

The Hawker Tempest Page
 

RJW

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Thanks for the replies. Are there any books or papers (NACA) that deal with this topic specifically? Pictures of X-planes (XP-55, Shinden, Black Bullet, etc.) and the Do-335 all show what appears to be careful control of cooling exhaust placed very close to the base of the prop.

Rob
 

Jan Carlsson

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I have not seen any specific on that what I remember, if you relate to prop efficiency, but looking to your model the cooling air hits a small part of the prop, and probably even less with down wash from wing in mind, also remember that at Reno at those speeds they hardly never "turn" straight a head, but have a more or less constant G load.

There are many papers on cooling on the NACA site.

The thoughts that lie to ground for all cooling of airplanes is, to place air inlet in a high air-speed, high pressure area, with clean virgin air. below the rad if possible, and have the outlet in low pressure, or disturbed air, higher then the radiator. specially in the climb attitude on most all normal airplane, but also in high speed with these race plane. and let the air out parallell to the airstream, at about the same speed.
 

RJW

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Thanks Jan. I’m probably worrying about this too much. I see that the Saab J21 used radiators buried in the wing roots with the exhausts dumping directly into the prop. The J21 was a successful design AFAIK. I also see that the XP-54 cooling exhaust was in close proximity to the prop. I’ll look through the NACA papers.

Thanks again,
Rob
 

Jan Carlsson

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The J21 was a miss with two booms :) (realy funny in Swedish, where miss and bom can mean same thing = No hit!)
It had some problem with cooling, specially on ground. SAAB J21 in detail but it had one of the first if not the first catapult seats.
 

revkev6

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Thanks for the replies. Are there any books or papers (NACA) that deal with this topic specifically? Pictures of X-planes (XP-55, Shinden, Black Bullet, etc.) and the Do-335 all show what appears to be careful control of cooling exhaust placed very close to the base of the prop.

Rob
looking at the DO-335 the cooling system doesn't appear to be much different than any other system of that era. the front engine used the annular system like the FW190. and the rear had an under slung scoop like a P-51. I don't know if it was because of a conscious effort to keep the hot air close to the fuselage or if it was just the most efficient cooling system they could come up with
 

Radicaldude1234

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looking at the DO-335 the cooling system doesn't appear to be much different than any other system of that era. the front engine used the annular system like the FW190. and the rear had an under slung scoop like a P-51. I don't know if it was because of a conscious effort to keep the hot air close to the fuselage or if it was just the most efficient cooling system they could come up with
For the Do-335, at least, I think the intent was to use the rear propeller in conjunction with the outlet flaps to provide some sort of suction cooling whilst on the ground or at low speed. The rear engine of the Do-335 captured by the Americans (the one currently in the Smithsonian) was actually trashed when the rear flaps were wired shut and it rapidly overheated. In German hands, the plane did remarkably well in staying cool on the ground; most rear engine fires were traced back to a flaw in the engine which resulted in spark-plugs being ejected and resulting in a fire. At least two Do-335s (a prototype and one captured by the British) went down this way. That, and the ejection seat never worked, except once when a pilot tried to use it, gave up, and the seat was set off by the impact of the emergency landing.

I don't have the sources to back it up, but the scoop itself looks to be a conceptual imitation of the scoop found on the Mustang.
 

Starduster II

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You might want to look for data on the Piaggio P.180 or the Beech Starship. Both dump the entire exhaust flow of their PT-6s onto the props. Effulent from a cooling radiator would seem to me to much less of an issue.
 

wsimpso1

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I would not be so worried over the cooling outflow going into the prop. The two big problems with your scheme are that the downwash off the wing makes the prop inefficient, and the shallow hieght/small volume available for radiators make your cooling system draggy.

I doubt that you can really make a low drag cooling system within the wing volume and you are messing with the airfoils big time by routing air internally. A system like the Mustang has a radiator several times taller than the inlet and really reduces air velocity through the radiator, then squeezes back down to a narrow outlet, taking the air close to its original speed on the outlet. Very low drag, and not particularly dirty flow either.

The down wash off the wing and further wash off the tail will have the AOA of the prop changing continuously as the prop blades go around. Noisy, and lowered efficiency. This effect is given credit for both the Beech Starship and the Piaggio Avanti being less speedy either than originally forecast or than you would predict based upon hp, frontal area, and wing area.

Now if you want to set up low drag high speed twin, put both engines in clean air, build only as much airplane as you can hide behind the engines, use long tail arms to get trim drag down, and put the engines and fuselages only far enough apart to keep them from interfering with each other and then put the wing and tail area under the center of gravity. Yeah, I just described the Boomerang. At EAA 2011, Burt told us that the 400 hp Boomerang had to fly at something like 18% power to fly the same speed as the 320 hp Defiant, and the Defiant was no slouch.

Billski
 

TFF

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The exhaust on the Piaggio kills the props. I think they have to throw the blades away at 6500 hrs. That may seem a lot to a light plane flier, but that is about 5-10 years with that class of plane.
 

Aircar

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The Douglas XB 42 ("Mixmaster") was probably the most thoroughly tested and documented pusher --it was written up in Aviation mag Feb 1947 "Design Development of the Douglas XB 42 " Pg 37- it also had the exact same wing internal cooling system as you show . Bruce Carmichael gives references and some data showing negligible drag increase over the basic airfoil with optimized internal ducts like this (you could expect thrust at really high speeds like the Meredith duct ) The Fairey Firefly was another with wing mounted leading edge cooling inakes ( used to have one undergoing restoration that I shared hangar space with ) the DH Mosquito and Hornet and many others did the same (but not pushers and benefitted from the propwash for ground cooling --you would have both effects )

Propeller swirl is likely to be an effect on the push pull set up and the props should rotate in opposite directions as seen from the front (same handedness) --on the XB 42 a quick re read reminds me of the effect of the cooling system doors' wake on the prop vibration (they stated that the wing and tail wake were of no concern but that the extended landing gear,wing flaps down, bomb bay open and the cooling flaps open were )
 
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