Bigger spinners !?

[BJC]

Retroflyer said: "I may have exaggerated the meaning of the spinner here...spinner is part of the completely aerodynamic fuselage"

Exactly, and all portions should be coordinated with the others. John Thorp told me that he had to "coordinate" 5 pressure distribution curves from the spinner to the firewall on the T-18.

mjb

And then almost all builders ignored what Thorp did and installed a different cowling.


BJC

 

[Dan Thomas]

Exactly, and all portions should be coordinated with the others. John Thorp told me that he had to "coordinate" 5 pressure distribution curves from the spinner to the firewall on the T-18.

Which points out how much is involved in designing an efficient airplane.

 

[H.Evan'sRV7A]

Here's the flame.

Prop makers have known that for a long time. That's why the TCDS for certified airplanes specify which props may be used, and those props are very often designed to satisfy the airframe manufacturer's engineers. They might want more or less pitch in the center area for more or less cooling flow. They might even want more blade area in the center. If you sight down some propellers, especially along the trailing edges, you can see some fairly abrupt changes.

The other problem with the prop: it's travelling upward on one side and down on the other, so the two sides see different airflows, even in level flight. Pretty hard to design a prop to handle both sides well. If the airflow is moving outward and downward around the cowl, for instance, the upgoing blade has a higher AoA than the downgoing blade. At the bottom, the airflow is moving sharply downward, while at the top it's almost straight.

Wind tunnels with smoke streams have been used for a long time to see this. Computers can probably do it much more easily and cheaply now. But the problem of uneven divergent flow is still there unless you're flying a round engine.

Dan's reply is mostly excellent and factual. But let me get into it a little for the benefit of anyone who is not already rolling his eyes.

Much of the "unexpected" changes in physical pitch from tip toward root (common) is in direct response to their perception that the air in those regions is slowed by the cowl behind them sort of pushing the air back, forward. That is the at-least-partial error I was talking about. I haven't been able to find anyone making props for our little EA planes that alters the airfoil specifically for the elongated effect at essentially stream speed. In many cases when they make the change for the slowdown they think is there, they get an improvement. OK, but that is not proof, it is a related effect. I do know for certain that the original Andy Bauer program that was used to design the Whirlwind series of RV props does not handle the angle of the stream but just treats it as slowdown. I don't know what Whirlwind did after they used Andy Bauer's program, but I do know what that program does and does not do

Smoke in wind tunnels is pretty convincing stuff. That is why I urge anyone to Google "bagger" (as in motorcycles) to view the videos of the smoke streams around the streamlined fairings. I can't see any slowdown. All I see is diverting air. Maybe it's just me but I thing this needs better measurement. I have looked at a lot of old and recent videos on the 'net for this reason and I'm just seeing it differently than others do. So to what extent are we seeing what we expect and not necessarily what really exists?
example: https://www.youtube.com/watch?v=1hEV92gBsoM&ab_channel=HattonBrown

 

[Retroflyer_S]

There is one small detail about big spinner that you forget ( possibly ).

See the spinner meets the stationary air..which pushes the prop axis towards the engine..at the same time as the prop axis is pulled by the propeller forward..instead of meeting the fuselage...see my point here ?

 

[Marc Bourget]

There is one small detail about big spinner that you forget ( possibly ).

See the spinner meets the stationary air..which pushes the prop axis towards the engine..at the same time as the prop axis is pulled by the propeller forward..instead of meeting the fuselage...see my point here ?

No, I don't see your point. From what I understand of your point, I believe the thrust of the propeller is much, much greater than the drag of the spinner. So much so to be insignificant.

mjb

 

[WonderousMountain]

I think there's a big difference between loss of thrust and increase in drag.

A propeller that goes from 90% efficient to 88% efficient was 20% more drag; but maybe only 1 mph difference in total performance.

The air goes through the prop at an angle parallel to the fuselage, and more streamline as you get further away.

It's a lot more like an "open" system than a fixed stream tube. The energy it takes to be diverted is acted on the fuse, not so much the prop, which doesn't care a lot if the air is at 15 degrees off normal.

LuPi

 

[Topaz]

No, I don't see your point. From what I understand of your point, I believe the thrust of the propeller is much, much greater than the drag of the spinner. So much so to be insignificant.

mjb

Not only does the thrust have to be significantly larger than the drag on the spinner, it has to be significantly larger than the drag of the entire airplane, else the airplane couldn't ever climb, nor go any faster.

 

[Dan Thomas]

There is one small detail about big spinner that you forget ( possibly ).

See the spinner meets the stationary air..which pushes the prop axis towards the engine..at the same time as the prop axis is pulled by the propeller forward..instead of meeting the fuselage...see my point here ?

That's an intuitive idea that may or may not be true. The air accelerates as it passes along the spinner, same as the leading edge of a wing, and the pressure falls and will generate lift perpendicular to the surface that will be largely countered by drag on the spinner. The net effect on the spinner could be rather low drag. There will also be pressure in the cowl behind the spinner trying to force it forward, as long as the gap between spinner and cowl is small.

 

[mcrae0104]

I haven't been able to find anyone making props for our little EA planes that alters the airfoil specifically for the elongated effect at essentially stream speed.

I believe Jan does. He has spoken before about needing to know about the airframe and cowling shape before designing a proper propeller. Surprised he hasn't jumped in.

 

[Retroflyer_S]

That's an intuitive idea that may or may not be true. The air accelerates as it passes along the spinner, same as the leading edge of a wing, and the pressure falls and will generate lift perpendicular to the surface that will be largely countered by drag on the spinner. The net effect on the spinner could be rather low drag. There will also be pressure in the cowl behind the spinner trying to force it forward, as long as the gap between spinner and cowl is small.

View attachment 44384

I may try to say that the friction created in the propeller shaft affecting the ball bearings may be lesser in big spinner cases and ensuring more lifespan for the engine...or then again it is insignificant.

 

[Dan Thomas]

I may try to say that the friction created in the propeller shaft affecting the ball bearings may be lesser in big spinner cases and ensuring more lifespan for the engine...or then again it is insignicant.

Crankshafts in engines don't use ball bearings. Some ultralight two-strokes might. Most engines are just plain bearings, and the thrust bearings are also simple plain bearings. Oil is the only thing that really supports the whole thing and takes all radial and axial forces while running.

My old Gipsy Major engine had plain bearings and a really thin axial tapered roller bearing for propeller thrust forces. never seen that in other engines, though perhaps some of the really old Menascos or Rangers may have been built that way.

In any case, the thrust bearings last far longer than the rest of the engine.

 

[bmcj]

There is one small detail about big spinner that you forget ( possibly ).

See the spinner meets the stationary air..which pushes the prop axis towards the engine..at the same time as the prop axis is pulled by the propeller forward..instead of meeting the fuselage...see my point here ?

If I understand you correctly, you are saying that the extra drag of the spinner helps counteract the pull of the prop so that the net pulling forces on the crankshaft are reduced, thereby reducing the stress on the crankshaft?

I suppose that is possible. To carry it a bit further, we could make a very big, draggy spinner that completely counteracts the pull of the prop so that there would be no forces imparted on the crankshaft.... and no force left to pull the plane forward either. :gig:

 

[Dana]

Not only does the thrust have to be significantly larger than the drag on the spinner, it has to be significantly larger than the drag of the entire airplane, else the airplane couldn't ever climb, nor go any faster.

No, the thrust has to be equal to the total drag in unaccelerated flight, regardless of whether the plane is climbing or not.

Dana

 

[WonderousMountain]

The torque caused from thrust is proportional to the thrust provided along the length of the propeller.

At 1/3 span you have 2 twice the drag created at 2/3 span, but when multiplied by it's distance from axis of rotation the contribution is equal.

While that doesn't necessarily imply that there is nothing to be benefited from a larger spinner; it does go to show why the majik formula has eluded us.

Preferably, we would have use very low drag airfoils with short chord tips at best overall L/D. Practically, we are choosing between things like climb, cruise and racing, along with diameter, number of blades and weight.

Anything that can do 90% is very good.

LuPi

 

[Starjumper7]

Not only does the thrust have to be significantly larger than the drag on the spinner, it has to be significantly larger than the drag of the entire airplane, else the airplane couldn't ever climb, nor go any faster.

No, the thrust has to be equal to the total drag in unaccelerated flight, regardless of whether the plane is climbing or not.

Allow me, unequaled in Physics except by Tesla and Einstein, to step in here for a sec. Thrust equals drag in unaccelerated level flight only.

For climbing: thrust = drag + change in potential energy per unit of time.

 

[bmcj]

For climbing: thrust = drag + change in potential energy per unit of time.

Let's not confuse force with energy or power.

In unaccelerated CLIMBING flight, Thrust = Drag + arcsin of the climbing angle

 

[Topaz]

No, the thrust has to be equal to the total drag in unaccelerated flight, regardless of whether the plane is climbing or not.

A climb is not unaccelerated flight. Which is why it takes more power to climb compared to level flight, just as it does to accelerate longitudinally, or sustain a constant-rate turn over time.

 

[Starjumper7]

Yes, I glossed over some of the details so the non physics types would comprende.

to be specific. Force X time = energy ... energy X time = power (psst: change in potential energy means changing altitude )

In plain english = you gotta work to go uphill. End of story. So add that to the drag.

 

[Topaz]

Let's not confuse force with energy or power.

In unaccelerated CLIMBING flight...

There is no such thing. While the airspeed may not be increasing, the total energy of the aircraft most certainly is, translated into a vertical velocity. This is no different than turning flight, where the increase in energy is expressed as a change in horizontal direction. In both cases, there is an increase in "g". If you are maintaining a constant positive vertical velocity against gravity, you are accelerating. If constant-rate-of-climb flight were non-accelerated, you could do it with the same power as level constant-airspeed flight. If you don't add extra energy, your vertical velocity will not be constant. It's the same as a turn, except that the acceleration is horizontal in the latter case. You can explicitly think of a turn as a "horizontal climb". The fundamental physics is exactly the same.

Look at it this way: In straight and level flight, you have added enough energy to the airplane (through the engine and prop) to counter a 32 ft/second^2 acceleration - that of Earth's gravity. In a constant-vertical-velocity climb, you have added more energy to create that climb. However, if you could magically increase the acceleration due to Earth's gravity, you could bring that climb back down to zero. Math being what it is, what you have done, even in a constant-rate climb, is accelerate away from the Earth at a rate that is higher than the rate imposed by gravity.

The only possible "non-accelerated" flight is straight and level at a constant airspeed.

 

[Dana]

A climb is not unaccelerated flight. Which is why it takes more power to climb compared to level flight, just as it does to accelerate longitudinally, or sustain a constant-rate turn over time.

Sure it is, if you're climbing at a constant airspeed and constant climb rate. But yes, it takes more thrust (power) to offset the fact that the gravity vector is no longer perpendicular to the direction of flight. But that's not the same thing as acceleration, which is defined as a change in velocity.

Dana