Minimum Drag Nose Cowling for Electric Planes

[danmoser]

Trying to minimize the drag from the cowling on a tractor propeller plane is highly dependent upon engine cooling requirements.
However, cooling requirements are greatly reduced for electric propulsion, allowing potential drag reduction by resculpting the nose cowling.

Modern pusher propeller aircraft designs --such as the Synergy-- plan to use maximum laminar flow in the nose to reduce drag, and utilize suction and entrained flow on the aft body just upstream of the prop.
But what if you want or need the prop in front?

It seems safe to assume that a front-mounted propeller will result in turbulent flow along the nose cowling and this flow would be much higher in velocity than a pusher would experience.
Can this characteristic be exploited to gain performance?

The Front Electric Sustainer is mounted on sailplane noses in a way that blends in with the standard laminar nose shape of the original sailplane.. obviously designed for minimum power-off drag with folding curved blades, and little pockets to receive them.
FES utilizes a small cooling air inlet in the center of the spinner.


The Yuneec e430, which is more of an airplane than a sailplane, has a slightly more conventional looking cowling and spinner .. but with smaller side cooling air inlets, yet still retaining a sailplane "look".


I was thinking there may have been some studies done to determine optimum nose cowling on front-mounted liquid-cooled engines, with radiators located away from the nose.. but haven't found anything like that yet.
The P-51 is such a craft, and it has a giant spinner almost the full width of the fuselage.. perhaps there's a benefit to this? .. granted, that's pretty far from being a "homebuilt airplane." :gig:

 

[autoreply]

But what if you want or need the prop in front?

It seems safe to assume that a front-mounted propeller will result in turbulent flow along the nose cowling and this flow would be much higher in velocity than a pusher would experience.
Can this characteristic be exploited to gain performance?

Folded you don't loose as much as one would think. The canopy edges are already turbulent (wedges), so a folded blade only leads to a marginal increase in turbulence. About 8% more fuselage profile drag it seems.

Not entirely sure what the rest of your question is. For the minimal cooling requirement of an electric motor (about 100 times less for the same SHP), a simple hole in the nose is plenty and unless one really screws up, cooling drag is going to be infinitesimal anyway?

 

[Himat]

Answering the last question first. During WW2 designers experimented with different spinner shapes on different aircraft. If this was extended to a systematic research I don’t know. It does look like aircraft with liquid cooled engine often got large spinners, airplanes with air cooled radial engines more often got smaller spinners.

The cowling shape I would guess was mostly decided by the shape of the engine. A note here, when DeHaviland designed the Mosquito follow on, the Hornet, they had Rolls Royce design a new Merlin mark with all ancillaries at the rear to reduce frontal area.

 

[danmoser]

Folded you don't loose as much as one would think. The canopy edges are already turbulent (wedges), so a folded blade only leads to a marginal increase in turbulence. About 8% more fuselage profile drag it seems.

Not entirely sure what the rest of your question is. For the minimal cooling requirement of an electric motor (about 100 times less for the same SHP), a simple hole in the nose is plenty and unless one really screws up, cooling drag is going to be infinitesimal anyway?

Thanks for the reply.
I hadn't realized flow was turbulent already at the canopy.

My main question was intended to be applicable to an airplane more than a motorglider .. I should have been clearer about that.
I was researching minimum drag nose (cowling & spinner) shape when engine is running and thrust is being made, rather than the drag of a folding blade power unit in stowed position, power-off mode .. finding published work on the subject seems difficult.

Propwash induces high velocity, fully turbulent flow along the front cowling, and I was wondering if there is a better shape than a conventional sailplane nose for this condition .. or maybe a conventional sailplane nose shape is as good as it gets, prop or no prop, no matter the propwash velocity/turbulence.
But then the question is; how does one choose the size of the spinner?
Perhaps it pays to make the spinner as large as possible in diameter?

Stemme motorgliders illustrate both the large spinner and mounting a retractable prop as far aft as possible in a slot behind a non-spinning nose .. ostensibly, these too are made primarily for power-off soaring.

 

[henryk]

Trying to minimize the drag

Modern pusher propeller aircraft designs --such as the Synergy-- plan to use maximum laminar flow in the nose to reduce drag, and utilize suction and entrained flow on the aft body just upstream of the prop.

=Goldschmids aerodynamic theory\investigations\ are very promissing,
but I dont know many practical constructions?

=Yours opinion?

=my future attempt \50 kW, 0.7 m impeller diametr\.

 

[danmoser]

=Goldschmids aerodynamic theory\investigations\ are very promissing,
but I dont know many practical constructions?

=Yours opinion?

View attachment 33989 =my future attempt \50 kW, 0.7 m impeller diametr\.

I don't know, Henryk .. it certainly looks interesting, but I have no idea how well that would work.


Back to the topic, I've read somewhere that a spinner --AKA propeller hub-- should be ~1/4 of the prop diameter.
It seems almost all installations are smaller than that.
But here's an interesting example of one that is approx. 25%, the Davis DA-11.
It looks unusual, but there's no arguing that the DA-11 is a VERY efficient airplane, though perhaps not very practical.

Leeon Davis designed an annular inlet for cooling air around the large hub, which is reminiscent of the old NACA cowling for radials.
But with electric propulsion, the slot can be eliminated and the cowling contour could be blended in smoothly with the spinner.

 

[autoreply]

Thanks for the reply.
I hadn't realized flow was turbulent already at the canopy.

My main question was intended to be applicable to an airplane more than a motorglider .. I should have been clearer about that.
I was researching minimum drag nose (cowling & spinner) shape when engine is running and thrust is being made, rather than the drag of a folding blade power unit in stowed position, power-off mode .. finding published work on the subject seems difficult.

Laminar pusher, like Henryk posted, like Strojnik, Carmichael. 300 mph on 70 hp seems about the upper limit of what's feasible.

Propwash induces high velocity, fully turbulent flow along the front cowling

Except for sailplane, that flow is turbulent anyway. (Seams, cowling etc). Even on sailplanes, the sides are turbulent.
Do the math for the iduced velocity. For a typical fast airplane, we're talking about a Vi of 10% of the TAS. 21% more fuselage drag (probably a bit more), considerable, but not as bad as one would think.

and I was wondering if there is a better shape than a conventional sailplane nose for this condition .. or maybe a conventional sailplane nose shape is as good as it gets, prop or no prop, no matter the propwash velocity/turbulence.

IMHO it doesn't matter much. Bigger prop, lower Vi and thus lower extra drag. Biggest prop and you run into diminishing returns.

But then the question is; how does one choose the size of the spinner?
Perhaps it pays to make the spinner as large as possible in diameter?

The smaller the better. A spinner is just a fairing to optimize airflow. Either the engine or the cockpit defines the min XC of the fuselage. Shape, once turbulent isn't critical, minimal wetted area and no detached flow is all that matters.

Stemme motorgliders illustrate both the large spinner and mounting a retractable prop as far aft as possible in a slot behind a non-spinning nose .. ostensibly, these too are made primarily for power-off soaring.

Only important for the highest possible blade size.

Calculate the prop efficiency of the Stemme. You wouldn't believe how inefficient it is, mid 40's percent.

 

[JamesG]

Another tack with the electric tractor is that your spinner does not have to be conical shaped, it could be a "hollow" inlet drawing the high pressure air thru the hub to cool the direct-drive motor.

 

[danmoser]

Except for sailplane, that flow is turbulent anyway. (Seams, cowling etc). Even on sailplanes, the sides are turbulent.
Do the math for the iduced velocity. For a typical fast airplane, we're talking about a Vi of 10% of the TAS. 21% more fuselage drag (probably a bit more), considerable, but not as bad as one would think.

IMHO it doesn't matter much. Bigger prop, lower Vi and thus lower extra drag. Biggest prop and you run into diminishing returns.

The smaller the better. A spinner is just a fairing to optimize airflow. Either the engine or the cockpit defines the min XC of the fuselage. Shape, once turbulent isn't critical, minimal wetted area and no detached flow is all that matters.

Only important for the highest possible blade size.

Calculate the prop efficiency of the Stemme. You wouldn't believe how inefficient it is, mid 40's percent.

Thank you.
Excellent guidelines with regard to Vi of prop in relation to turbulent fuselage drag.
And I've never heard the spinner size put in those terms, but it seems reasonable.
It would be interesting to see how fast the AR-5 would go with electric propulsion.
It was estimated that 8.5% of its drag was cooling drag, but I don't think that included cowling drag .. the fuselage was ~1/3 of the drag, IIRC

Prop efficiency under 50% of the Stemme is much lower than I would have expected.
But I guess low propulsion efficiency is considered acceptable to get self-launch and high performance power-off soaring.

The FES unit looks great .. pretty good propulsion efficiency, I'd guess.
But it's $30K .. even if you manage to convince them to sell you one! :shock:

 

[danmoser]

Another tack with the electric tractor is that your spinner does not have to be conical shaped, it could be a "hollow" inlet drawing the high pressure air thru the hub to cool the direct-drive motor.

Yes, I like that approach .. like in the FES, Front Electric Sustainer.
View attachment 34010

The Greenwing/Yuneec eSpyder uses an very small spinner and small fan-enhanced cooling air inlet for their electric motor .. but this is a separate propulsion pod, away from the fuselage, like so..
View attachment 34009

 

[Himat]

...Back to the topic, I've read somewhere that a spinner --AKA propeller hub-- should be ~1/4 of the prop diameter.
It seems almost all installations are smaller than that...

Could this come from test’s where poor propeller blade design at the blade root was simply masked by a large spinner?
I would track down the references and see what is really behind this statement.

I tried a few internet searches on the subject, but I didn’t get that many interesting hits. Here are two old references I found:

“Development of Cowling for Long-nose Air-cooled Engine in the NACA Full-scale Wind Tunnel”
“Tests of Five Full-Scale Propellers in the Presence of a Radial and a Liquid-Cooled Engine Nacelle, Including Tests of Two Spinners”
NASA Technical Reports Server (NTRS) - Development of Cowling for Long-nose Air-cooled Engine in the NACA Full-scale Wind Tunnel
NASA Technical Reports Server (NTRS) - Tests of Five Full-Scale Propellers in the Presence of a Radial and a Liquid-Cooled Engine Nacelle, Including Tests of Two Spinners

Newer studies probably exist as turbo prop regional passenger aircraft are common.

 

[danmoser]

Could this come from test’s where poor propeller blade design at the blade root was simply masked by a large spinner?
I would track down the references and see what is really behind this statement.

I tried a few internet searches on the subject, but I didn’t get that many interesting hits. Here are two old references I found:

“Development of Cowling for Long-nose Air-cooled Engine in the NACA Full-scale Wind Tunnel”
“Tests of Five Full-Scale Propellers in the Presence of a Radial and a Liquid-Cooled Engine Nacelle, Including Tests of Two Spinners”
NASA Technical Reports Server (NTRS) - Development of Cowling for Long-nose Air-cooled Engine in the NACA Full-scale Wind Tunnel
NASA Technical Reports Server (NTRS) - Tests of Five Full-Scale Propellers in the Presence of a Radial and a Liquid-Cooled Engine Nacelle, Including Tests of Two Spinners

Newer studies probably exist as turbo prop regional passenger aircraft are common.

Thanks, Himat .. good find! .. useful data!!
Isn't it remarkable that testing done 84 years ago is still quite relevant today?
They had never even conceived of CFD or any other type of software at the time.