The hovercraft is surprisingly dirty after just a few minutes flying around a "clear" parking lot.
Dirt collected under the thrust engine
More collected on the side
Another view of the dirt at the rear
There's even dirt in the center of the fan. Clearly there's a lack of airflow there...
After cleaning off as much dirt as possible I set to work to improve the hover performance.
I took two approaches. First I started by clearing out more of the grill wires. It made a large difference for the thrust fan. And since the lift fan is actually well protected, I cleared out 3 of 4 on the bottom, and 2 of 3 on the top except for the outer ring where I did 3 in a row where I expect the highest speed.
In addition there was a gap between my metal diverter plate and the board that is attached to the skirt attachment leaving about a 3/4" gap across the whole width. And with the wires removed I could now relatively stretch some Duct tape across the gap. I think this might be my first correct use of Duct tape ever!!
Sealing the diverter with duct tape front view
Duct tape rear view
And reassembled
The propeller is much mor eexposed. i plan to build up a higher housing around the lift engine to keep me and other things farther away from the lift engine to avoid any problems with foreign object entries. I'm even thinking of a shroud/hood that will scoop forward air and also dampen the noise hopefully.
As I identified in the last post there's quite a bit of pressure buildup in the thrust fan. So I've been reducing the drag induced by the protective grill.
The 1cm width is narrower than necessary. So with help from Wim, I've removed every other wire from the front. And 2/3 from the rear of the thrust fan housing.
With these gaps I still cannot reach the propeller even if I try with my hands since it has 3+" of clearance at the front and 6+" inches of clearance at the back.
View from the front of the reduced lift fan grills
Back left view
Back right view
In this process we removed a lot of clippings. The ones I collected weighed 2.4 lbs. Not bad for decreasing the weight at the back.
Most of the clippings removed
Clippings bundled in cling wrap to be weighed. (2.4 lbs)
With so very unscientific testing of standing downwind of the fan, it felt noteably stronger. And throttling up and down did not result in the front grill flexing like it used to do.
We held back on the lift fan since it's both closer to the pilot as well as lower volume due to having back pressure. With the improved fan mounting the craft is floating much higher, but the CG is still notably aft. To take a passenger I'll probably need to add weight at the front with the current configuration.
Since I was having trouble with the lift fan propeller and I've tested it more than the thrust fan I decided to do the same operation for the thrust fan just in case.
Dissassembling it there were no major signs of wear from spinning like the lift fan which had shavings everwhere and clear galling and grooves. There is a little bit of wear.
The thrust fan shaft after
The collet freshly removed
Since I used the end cap in the middle of the lift engine hub I needed to make another one.
Here's the end result, not pretty, but effective enough.
The hub assembled with longer bolts extra washers and locknuts.
The final assembly from slightly behind.
The hub before assembly with the keys installed.
The keys and bolt in a fit check.
And here's the evidence that the fan stayed on. For at least 5 minutes....
While running this test I was watching closely for wear and tear and any malfunction. And I noticed that the grill protecting the fanwas flexing quite a bit when I throttled up. So I setup the camera in a different vantage point. Sorry it's a little bouncy since the tripod is now on the hovercraft. But you can see the grill flex when I throttle up best by watching how much of the engine shaft you can see.
TODO lift
I also did some continued testing of the lift fan. I found two more points of failure. Since i've dissassembled and reassembled the lift fan so many times, two of the screws holding the grill down wiggled out of place. it was kind of exciting to watch one of them start bouncing across the grill and then disappear between the wires never to be seen again. One more reason I'm glad that I have the metal housing. And there's the thick piece of wood between me and the lift fan too.
I filmed the lift engine tests but unfortunately the gopro had operator error and so I have two unexciting pictures for each test. One before the test, and one after the test. During this test I was excited to realized that the tilting of the hovercraft does effectively allow me to move it side to side by releasing air out from the side of the skirt. By leaning I could choose to loosen or tension the docking lines on either side of the vehicle.
The other interesting result of the lift engine test was the below grill failure. As far as I can tell this wire was vibrated free from its housing. It does not appear to have had a traumatic experience from cutting and just sprung itself loose from a weakness in the weld under the high vibration environment of the lift engine.
The one piece of wire that has proven to be not well attached. You can also see the hold where the sheet metal screw has self vacated.
The whole loose strand, and both screw holes which were vacated.
I am going to be looking for ways to decrease the vibrations of the lift engine. Mounting it on the two horizontal bars is generally strong enough but leads to a lot of resonances which I think are making everything harder to keep in place. And also makes things louder and less comfortable on the craft.
After all my trouble with the collet slipping I have upgraded the fan mounting technique.
I ordered some shaft keys since both the shaft and collet are keyed. And I also ordered some 3/8-24 bolts to thread into the end of the engine shaft. And I modified the outside mounting plate with a center hole to allow it to bolt to the end of the engine shaft.
The blot ane key fit checking
The key fit in nicely. Though surprisingly the woodruff key I ordered was the wrong size. I didn't expect that it and the may key would be different width. The main keyway was 3/16" but the woodruff key slots on the back were clearly 1/8". But the collet was also 3/16" so that works better.
The 1-1/4" was the right length to fit in.
Key fit check
The face plate modified with a center hole for the shaft end blot.
Fully Assembled hub, with the new plate visible between the hub and the propeller assemble.
An external view of the assembly
I identified that the grill housing is going to be vibrating against the engine mount, but listening to the running sounds I think it's a minimal amount of the noise of the system.
And finally the moment we've been waiting for. The test!!
It worked quite well. I hovered much higher than I can really remember except for a brief moment in one of the recent tests as I got the hub super tight. Which suggests to me that it was never really not slipping previously.
This is great! I'm looking forward to more tests soon!
I took it all apart, and found out why I got such a different sound.
Damage to the lock nuts
With the slightly shorter collet mounting meant that the locknuts holding the fan to the housing impacted the screen housing before the mounting bolts impacted the bottom of the engine.
The nuts before fully removed.
Doing some research I've found that the B&S engine shaft appears to have 2 woodruff keys slots as well as a standard keyway. And the end of the shaft is tapped with 3/8-24 and is about 1.3" deep.
I will use the end cap plate, to hold the assembly to the end.
End cap plate, which will need to be drilled out in the middle.
And I've ordered keys to go into the keyways and stainless bolts to secure the plate to the bottom of the shaft. The plate will keep it from riding up, and the bolt will keep it from dropping down. And the keys will keep it from spinning.
With the new collet installed I then moved onto testing the engine. Since I was on my own you'll see that I rigged up some eyes in the walls of the garage and secured the hovercraft from moving significantly laterally.
Here's my first flight.
Everything went relatively well but there's some ominous rattling near the end. It's easier to notice now that I listen to the recordings. But what do you do after a successful test? Try again...
Yeah, not so good. It was clearly not doing things properly since it didn't even start hovering. And by then it had been loosened so of course it fell off again. But it's nicely contained in the housing.
So I took it all apart again. Measured that it hadn't worn more than a few thousands off the bushing, and put it back together again. This time I put more than the recommended 95 inch pounds, but not excessively. But then I ran a very short test:
It hovered well, but I took it apart anyway and there was an extra quarter turn I was able to get out of the tightening bolts. So I tested again.
This time I throttled up a bit and everything seemed ok. But I took it all apart and was able to tighten each bolt a quarter turn again. So one more check.
Things sounded better, and the bolts did not have freedom when I took it apart. So I decided to try again.
Everything sounded good and it was hovering very high. I've had some balance issues so I tried adding some weight at the front to feel the balance. It was riding super high and super stable when I throttled up. However as I throttled down to feel it settle, the propeller started hitting the underside of the engine as it had slide up the shaft during the testing. That's what made the horrible noise at the end.
So I'm a little frustrated now...
I think that there's some work to be done. Leveraging the keyway seems like a good idea. And also using the end of the shaft, and the end bolt would make sense. This is the mounting technique for most lawn mower blades.
So after waiting several weeks for Univeral Hovercraft to send me a replacement collet I was excited to get it mounted up again and get hovering again.
Some new parts
When dissassembling I found a lot of metal filings.
The removed propeller with worn collet and filing visible in the old housing.. Also note the shorn off bolt.
And based on those metal filings I thought I'd check the size of the collet new vs old.
New collet lower edge measurement
Old collet lower edge measurement
There's an exciting ~15 thousands of an inch extra clearance which
New collet top side measurement
Old collet top side measurement
And the wear was almost as much on the other side of the collet too.
For reference I also measured the shaft. And it's pretty close to the expected size still.
Shaft size near the end
Shaft size where there appear to be the most wear
So I moved onto reassembling the lift fan. Taking note of the bushing instructions.
Collet bushing instructions
I did notice some differences in dimentions of the bushing mounting blocks, but the critical dimension where the bushing rides appeared to be the same.
New and old bushing housings side by side
New and old bushing housings side by side.
I put everything back together. And I installed the collet to the shaft before installing the propeller so I could see and access it better. Everything looks good.
Reassembled collet end view
Reassembled collet side view
The bolts were all secured with lock-tite.
A close up trying to show the lock tite
I added the propeller on last before reinstalling.
