Spruce Goose question

[h34race]

I read in an older Aviation magazine somewhere that I couldn't believe the data they were telling me I can't believe that with eight r43 60s that thing was underpowered four bladed props the only thing that they alluded to was because the engines were not staggered enough forward and back and the Leading Edge of the wing was so enormous it created what they called a bad cavitacion I've heard of this in ships it would apply to airplanes also would it not I'm looking for you airplane Engineers out there to please help me understand this I've seen a lot of YouTube models that are really close to scale I understand that the electric motors will spend those props a lot faster but the wing root deal or Leading Edge just wondering view Aviation Engineers out-there have a clue what I'm talking about or am I just incorrect all the way around we're talking 3600 horsepower times 8 with four bladed props at sea level with the flaps down and it handled like an evil pig as soon as he got it up in the air I just started thinking about this when I saw a scale representation with eight electric motors it took off within 2 feet of power up that's a lot of wing what they were saying was the turbulence from the props that close to that big of a Leading Edge of the wing stalled the wing almost making it ineffective basically all it did was lifted off with the tips for the engines weren't is what they were trying to say I think any knowledge I'd love to have some fun with this just wondering they were saying made sense to me so let's have some fun haha I really curious

[AAFO_WSagar]

Re: Spruce Goose question

I got to meet the remaining crew that flew in 1947 at the 1997 50th reunion held at the Museum, well, it wasn't really a museum yet LOL.. I wish I had asked more questions! I don't remember anyone saying that it was underpowered, I do remember the hydraulics engineer who flew right seat telling me that Hughes remarked that the AC really ballooned when they applied flaps... He also said that he knew Hughes intended to fly when he mentioned flaps during the long pre-flight. Oh heck, here's a link to the brief update that I posted at the time http://aafo.com/goose/50thud.htm

I will say that well into his reclusive period, Hughes would mention to his closest aids that he wondered what she would do if the put engines from a C-130 on her.. I believe he very much loved that airplane.. witness the care he lavished on it until he was too far gone to care any longer!

It lived in a climate controlled hangar and was regularly run up and systems kept flight ready for many years prior to becoming an exhibit.

[Jukka]

Re: Spruce Goose question

I have looked at Spruce Goose very carefully as I have developed similar kinda wood composite as the birch wood wonder HK-1 was made of.

The chief engineer said in Tom Snyder show that it was a year from completion when Hughes flew it...all hydraulics would have had to be replaced etc. It was still in 1978 flyable if a year was used to make it ready.

[Jukka]

Re: Spruce Goose question

I love this video about HK-1 with 0.010 Cox engines; https://www.youtube.com/watch?v=yAFk3oKcGNo

[Jukka]

Re: Spruce Goose question

This 16 ft model has nearly 2 million views; https://www.youtube.com/watch?v=VgJkfPWyH_E

[L.E.D.]

Re: Spruce Goose question

I don't think the engine installation on the Spruce Goose was particularly bad for it. Generally propeller wash increases lift and the extra 'scrubbing' also increases drag, but this is expected and all propeller planes with wing mounted tractor engines deal with it. R-4360s were mounted in all kinds of places on wings. See the B-36 (pusher) and B-50 (tractor.) There was even a design study of a tractor B-36 with the engines looking a lot like the Goose's nacelles.

"Cavitation" occurs when a marine propeller pulls so hard that the pressure drop causes the water to boil. Can't really happen in air. Not with an R-4360 anyway.

Power on the other hand...

Let's do a quick thumbnail performance sketch of the H-4 Hercules, better known as the Spruce Goose.

Empty weight: 250000lbf
Max Gross: 400000lbf
Wing Area: 11430 ft^2
Installed HP: 24000 hp (8 x 3000hp)

If we estimate the lift to drag ratio (L/D) to be 10, that means for every 10 pounds of airplane, there's 1 pound of drag. L/D of 10 is a reasonable guess; nothing special but nothing horrible either.

We know that the goose flew at 135mph on its one flight. It might have been able to fly slower than that, but we'll use the number we know.

Power required is Drag * Speed.

135mph is about 200 feet per second.

at 250000lbf, Drag is 25000 lbf. Power required is 200 * 25000 = 5000000 ft-lbf/sec, or 9090 hp (550 ft-lbf/sec per hp)
at 400000lbf, Drag is 40000 lbf. Power required is 200 * 40000 = 8000000 ft-lbf/sec, or 14545 hp.

The engines make 24000hp, but we have to allow for prop efficiency. Lets say we can get 80% of the installed power out of the props.

Power available = 80% of 24000hp = 19200hp.

Since power available is greater than power required, it'll fly. Of course we usually want the plane to do more than just fly; it needs more power to climb.

The leftover (excess) power gives an idea of climb capability:

At 250000lbf, excess power is 10110hp. Divide that by the weight to get the climb rate: 22 ft/sec or 1320 ft/min
At 400000lbf, excess power is 4655hp. Divide that by the weight to get the climb rate: 6.4 ft/sec or 384 ft/min


There are lots of simplifications baked in, but in rough numbers the Goose would fly. Takeoff and climb would be marginal at gross weight, but then it would have a whole ocean to use for a runway.

[Jukka]

Re: Spruce Goose question

I don't think the engine installation on the Spruce Goose was particularly bad for it. Generally propeller wash increases lift and the extra 'scrubbing' also increases drag, but this is expected and all propeller planes with wing mounted tractor engines deal with it. R-4360s were mounted in all kinds of places on wings. See the B-36 (pusher) and B-50 (tractor.) There was even a design study of a tractor B-36 with the engines looking a lot like the Goose's nacelles.

"Cavitation" occurs when a marine propeller pulls so hard that the pressure drop causes the water to boil. Can't really happen in air. Not with an R-4360 anyway.

Power on the other hand...

Let's do a quick thumbnail performance sketch of the H-4 Hercules, better known as the Spruce Goose.

Empty weight: 250000lbf
Max Gross: 400000lbf
Wing Area: 11430 ft^2
Installed HP: 24000 hp (8 x 3000hp)

If we estimate the lift to drag ratio (L/D) to be 10, that means for every 10 pounds of airplane, there's 1 pound of drag. L/D of 10 is a reasonable guess; nothing special but nothing horrible either.

We know that the goose flew at 135mph on its one flight. It might have been able to fly slower than that, but we'll use the number we know.

Power required is Drag * Speed.

135mph is about 200 feet per second.

at 250000lbf, Drag is 25000 lbf. Power required is 200 * 25000 = 5000000 ft-lbf/sec, or 9090 hp (550 ft-lbf/sec per hp)
at 400000lbf, Drag is 40000 lbf. Power required is 200 * 40000 = 8000000 ft-lbf/sec, or 14545 hp.

The engines make 24000hp, but we have to allow for prop efficiency. Lets say we can get 80% of the installed power out of the props.

Power available = 80% of 24000hp = 19200hp.

Since power available is greater than power required, it'll fly. Of course we usually want the plane to do more than just fly; it needs more power to climb.

The leftover (excess) power gives an idea of climb capability:

At 250000lbf, excess power is 10110hp. Divide that by the weight to get the climb rate: 22 ft/sec or 1320 ft/min
At 400000lbf, excess power is 4655hp. Divide that by the weight to get the climb rate: 6.4 ft/sec or 384 ft/min


There are lots of simplifications baked in, but in rough numbers the Goose would fly. Takeoff and climb would be marginal at gross weight, but then it would have a whole ocean to use for a runway.

Considering that B-52 and Avro Vulcan had L/D of 17 then H-4 had at least 20. There was nothing marginal about it. It was to carry 750 troopers into Europe across the Pond.

[L.E.D.]

Re: Spruce Goose question

Since it's a holiday weekend, I decided to play with this a little more, if you all don't mind some math.

Here are the best "published" numbers I've found from the Hercules "Manual":
Range: 2975 statute miles (No Reserve)
Fuel load: 12,500 gal (75000lbf)
Wo (takeoff): 400,000lbf
Wf (all fuel used): 325,000lbf

So what kind of L/D does the Goose need to have to achieve these numbers?

The Breguet range equation estimates range based on how much fuel is used, and assuming that the airplane operates at maximum L/D for the whole flight, varying speed as needed as fuel burns off.

Here's what the Range Equation looks like

Range(sm) = 375 n/c * L/D * ln (Wo/Wf)

Where:
c = fuel consumption in pounds per hour per horsepower
n = propulsive efficiency. (Includes propeller, cowling drag etc.)
L/D = lift to drag ratio
and the 375 is a conversion factor that gives the answer in statue miles.

For the R4360, fuel consumption c = 0.45 lbf/hr/hp.
We'll be a little more generous than we were before and assume 85% propulsive efficiency: n = 0.85

Solve the range equation for L/D, and put in the takeoff weight (400000 lbf), weight at the end of the flight (325000lbf) and the efficiencies n and c:

L/D = (2975/375/0.85*0.45) / ln(400000/325000)
L/D = 20.2

An L/D of 20.2 is higher than historical cruising data for flying boats indicates; L/D of 15 is the upper end of typical, but of course the Goose is anything but typical.

To get a cut at what kind of L/D the Goose might actually have had, we can do a simplified drag breakdown.

Airplane drag can be separated into Induced Drag, which is the drag that is caused by lift, and Parasite Drag, which is everything else - friction, cooling drag, interference between parts etc.

We can sum all this up with the "drag" equation:

Cd = Cd0 + Cl^2/pi/AR/e
For any Cl, we can compute a Cd, and then divide Cl by Cd to get L/D. We can adjust Cl to get the the max L/D.

The equation has two parts:
Cd0 is the drag coefficient not caused by lift (parasite drag);
and the Cl^2/pi/AR/e is a combination of drag due to lift, and other bits of drag caused when the lift changes.

Non-lifting drag can estimated as a function of the surface area of the whole airplane (the "wetted area"), and then converted to be a a function of the wing area so it can be incorporated with the drag due to lift.

I scaled a three view of the The Goose and estimate that it has a wetted area of about 45510 square feet. Assuming a wetted area drag coefficient of 0.005 per square foot of surface area, the equivalent "drag area" is 228 square feet. Divide that by 11430 square feet of wing, and Cd0 is about 0.02.

(Wetted area side note - There's a fairly clear trend historically relating the wetted area of an airplane to takeoff weight. Heavier airplanes have more wetted area. If you use that trend to predict the wetted area of the Goose you get a value that is only a little bigger than the wetted area of the wings, which is far too low. The Goose is literally off the chart for being large, but light. Compare the 747 gross weight of close to 1,000,000lbf for a similarly sized airplane.)

To estimate induced drag, we need the wing aspect ratio and the "efficiency factor", e.:

The aspect ratio (AR) of the Goose is span^2/area = 320^2/11430 = 9.
"e" is trickier, since it sums up a whole lot of things, but we'll be generous and call it 0.85.

Stick all that into a spreadsheet for a range of Cl and you get a max L/D of about 17. Put that back into the range equation, and we get a max range of 2500 statue miles, which is short of the claimed 2975mi range.

So did we make a mistake somewhere?
- The wetted area is probably right to within 5%
- Engine data is pretty solid.
- We were optimistic about efficiencies, so any errors there are in the Goose's favor.

The biggest assumption is the wetted area drag coefficient. This number is a measure of how "clean" an airplane is - surface roughness, form drag, etc. I picked 0.005 as representative of large planes of the era - B-17, B-29. Modern jets get down in the 0.003 range. A P-51 is 0.004.

If the Goose were as clean as a P-51, the wetted area drag coefficient could come down to 0.004, which would be a 20% improvement and squeak the L/D up to 19.

Is 0.004 reasonable for a smooth, molded flying boat?

I wasn't able to find any wind tunnel data for the Goose, but I did find NACA Report WR-L-683, which details towing tank tests on the hull. It's an interesting read and has nice scale drawings of the hull. (You can download it here: https://ntrs.nasa.gov/archive/nasa/c...9930082228.pdf)

The report contained aerodynamic data, supplied by Hughes for the airplane with the flaps down for use in estimating takeoff performance.

Two interesting points from the report:

The Goose, as it stood in in 1944, was close to being thrust limited for a full gross takeoff.

The aerodynamic data looks to me like it was calculated, not measured. For example, the lift coefficient, flaps down is exactly what you'd get if you used 1940s data (such as Abbot and von Doenhoff) to predict it.

Abbot and von Doenhoff also suggest that Cd0 doubles with a 20deg flap deflection. If we take the report value of 0.0375 and divide it by 2, we get 0.01875.which is a little lower than our estimate of 0.02. (This really applies only to the wing - Cd0 would probably change less for the whole airplane, but again, let's be generous.)

If we revise our estimate to use a wetted drag coefficient of 0.0047, we get a Cd0 of 0.0187, and an L/D max of about 17.8. That's about a 6% change. Even if we throw in another 5%, we only get to 18.7

It's hard not to conclude that Hughes was a little optimistic in their performance estimates, but I definitely give it credit for having an L/D of well over 10.

[RAD2LTR]

Re: Spruce Goose question

The skin of the Goose is pretty smooth. Here are a few shots I got of it when I was there in 2011.




The skin of the bird is very very smooth. There are no rivets just smooth everywhere.

Will

[Jukka]

Re: Spruce Goose question

Since it's a holiday weekend, I decided to play with this a little more, if you all don't mind some math.

Here are the best "published" numbers I've found from the Hercules "Manual":
Range: 2975 statute miles (No Reserve)
Fuel load: 12,500 gal (75000lbf)
Wo (takeoff): 400,000lbf
Wf (all fuel used): 325,000lbf

So what kind of L/D does the Goose need to have to achieve these numbers?

The Breguet range equation estimates range based on how much fuel is used, and assuming that the airplane operates at maximum L/D for the whole flight, varying speed as needed as fuel burns off.

Here's what the Range Equation looks like

Range(sm) = 375 n/c * L/D * ln (Wo/Wf)

Where:
c = fuel consumption in pounds per hour per horsepower
n = propulsive efficiency. (Includes propeller, cowling drag etc.)
L/D = lift to drag ratio
and the 375 is a conversion factor that gives the answer in statue miles.

For the R4360, fuel consumption c = 0.45 lbf/hr/hp.
We'll be a little more generous than we were before and assume 85% propulsive efficiency: n = 0.85

Solve the range equation for L/D, and put in the takeoff weight (400000 lbf), weight at the end of the flight (325000lbf) and the efficiencies n and c:

L/D = (2975/375/0.85*0.45) / ln(400000/325000)
L/D = 20.2

An L/D of 20.2 is higher than historical cruising data for flying boats indicates; L/D of 15 is the upper end of typical, but of course the Goose is anything but typical.

To get a cut at what kind of L/D the Goose might actually have had, we can do a simplified drag breakdown.

Airplane drag can be separated into Induced Drag, which is the drag that is caused by lift, and Parasite Drag, which is everything else - friction, cooling drag, interference between parts etc.

We can sum all this up with the "drag" equation:

Cd = Cd0 + Cl^2/pi/AR/e
For any Cl, we can compute a Cd, and then divide Cl by Cd to get L/D. We can adjust Cl to get the the max L/D.

The equation has two parts:
Cd0 is the drag coefficient not caused by lift (parasite drag);
and the Cl^2/pi/AR/e is a combination of drag due to lift, and other bits of drag caused when the lift changes.

Non-lifting drag can estimated as a function of the surface area of the whole airplane (the "wetted area"), and then converted to be a a function of the wing area so it can be incorporated with the drag due to lift.

I scaled a three view of the The Goose and estimate that it has a wetted area of about 45510 square feet. Assuming a wetted area drag coefficient of 0.005 per square foot of surface area, the equivalent "drag area" is 228 square feet. Divide that by 11430 square feet of wing, and Cd0 is about 0.02.

(Wetted area side note - There's a fairly clear trend historically relating the wetted area of an airplane to takeoff weight. Heavier airplanes have more wetted area. If you use that trend to predict the wetted area of the Goose you get a value that is only a little bigger than the wetted area of the wings, which is far too low. The Goose is literally off the chart for being large, but light. Compare the 747 gross weight of close to 1,000,000lbf for a similarly sized airplane.)

To estimate induced drag, we need the wing aspect ratio and the "efficiency factor", e.:

The aspect ratio (AR) of the Goose is span^2/area = 320^2/11430 = 9.
"e" is trickier, since it sums up a whole lot of things, but we'll be generous and call it 0.85.

Stick all that into a spreadsheet for a range of Cl and you get a max L/D of about 17. Put that back into the range equation, and we get a max range of 2500 statue miles, which is short of the claimed 2975mi range.

So did we make a mistake somewhere?
- The wetted area is probably right to within 5%
- Engine data is pretty solid.
- We were optimistic about efficiencies, so any errors there are in the Goose's favor.

The biggest assumption is the wetted area drag coefficient. This number is a measure of how "clean" an airplane is - surface roughness, form drag, etc. I picked 0.005 as representative of large planes of the era - B-17, B-29. Modern jets get down in the 0.003 range. A P-51 is 0.004.

If the Goose were as clean as a P-51, the wetted area drag coefficient could come down to 0.004, which would be a 20% improvement and squeak the L/D up to 19.

Is 0.004 reasonable for a smooth, molded flying boat?

I wasn't able to find any wind tunnel data for the Goose, but I did find NACA Report WR-L-683, which details towing tank tests on the hull. It's an interesting read and has nice scale drawings of the hull. (You can download it here: https://ntrs.nasa.gov/archive/nasa/c...9930082228.pdf)

The report contained aerodynamic data, supplied by Hughes for the airplane with the flaps down for use in estimating takeoff performance.

Two interesting points from the report:

The Goose, as it stood in in 1944, was close to being thrust limited for a full gross takeoff.

The aerodynamic data looks to me like it was calculated, not measured. For example, the lift coefficient, flaps down is exactly what you'd get if you used 1940s data (such as Abbot and von Doenhoff) to predict it.

Abbot and von Doenhoff also suggest that Cd0 doubles with a 20deg flap deflection. If we take the report value of 0.0375 and divide it by 2, we get 0.01875.which is a little lower than our estimate of 0.02. (This really applies only to the wing - Cd0 would probably change less for the whole airplane, but again, let's be generous.)

If we revise our estimate to use a wetted drag coefficient of 0.0047, we get a Cd0 of 0.0187, and an L/D max of about 17.8. That's about a 6% change. Even if we throw in another 5%, we only get to 18.7

It's hard not to conclude that Hughes was a little optimistic in their performance estimates, but I definitely give it credit for having an L/D of well over 10.

Ok 17.8 sounds reasonable.

[sledge39]

Re: Spruce Goose question

.
originally posted in 2013

...HK-1/H-4 at Long Beach (not surrounded by a bunch of other airplanes)...

31 Oct 1980...the morning after the move to the bank out of the Long Beach waterfront hangar...

...after Feb 1982...inside the Dome next to the Queen Mary...

[AAFO_WSagar]

Re: Spruce Goose question

Most of my exposure to this airplane was while it was in storage, under plastic tarps in Oregon.. It was almost heartbreaking seeing the enormous creation that was once the apple of Howard Hughes' eye! Then disassembled barely safe from the elements... It was truly "Howard would roll over in his grave" stuff. There were a bunch of people working in the trenches to ensure that the airplane found a secure home. And it did!

There's some stuff on the site regarding my involvement..

As far as I'm concerned, the airplane was machine art! The woodwork deep in the barely visible places, as well as EVERYWHERE is beyond belief!

I have a piece of the material that was used in the construction.. it is barely thicker than aluminum skin but is six or seven laminations of wood! Just amazing stuff.

I could go on forever about this airplane!

[supercub]

Re: Spruce Goose question

Most of my exposure to this airplane was while it was in storage, under plastic tarps in Oregon.. It was almost heartbreaking seeing the enormous creation that was once the apple of Howard Hughes' eye! Then disassembled barely safe from the elements... It was truly "Howard would roll over in his grave" stuff. There were a bunch of people working in the trenches to ensure that the airplane found a secure home. And it did!

There's some stuff on the site regarding my involvement..

As far as I'm concerned, the airplane was machine art! The woodwork deep in the barely visible places, as well as EVERYWHERE is beyond belief!

I have a piece of the material that was used in the construction.. it is barely thicker than aluminum skin but is six or seven laminations of wood! Just amazing stuff.

I could go on forever about this airplane!

PLEASE DO !! Love hearing about it.

[L.E.D.]

Re: Spruce Goose question

Y'know.. I have a different Spruce Goose question.

When they took that hangar apart there was (I think) nothing holding it up forward of where the plane's wings were. Anybody got pics of how that hangar was constructed?

[Leo]

Re: Spruce Goose question

Geodesic dome if I remember. With all the interlocking triangles you can remove a lot of the structure and the rest will hold it. Within reason of course. I haven't looked at any photos to see if they place some type of lintel over the opening but I doubt it.