Whirl Wind 70" versus 75"

[herman pahls]

I am shopping for a new prop more out of curiosity than need.
Model 4 Kitfox (700#'s) , 912 RotaxULS (100hp), 29" ABW's.
The Warp Drive 70" taper tip with leading edge protection performs well both short field and cruise (100-115 mph above 10,000msl).
It is starting to show the abuse from a steady diet of sand and gravel bars.
I would not mind going to a prop with less mass moment of inertia (MMofI).

Whirl Wind (WW) suggested their 75" fits my flying style but I will lose some cruise.
The WW 75" will have more MMofI than the 70" but less than the current Warp.
McDeans say they sell mostly 70" WW for the 912's (100hp) and 75" to the 914's.

Questions.
1. Any experience out there with the WW 75" with 912ULS?
2. WW says their blades fit the Warp Drive hub 80% of the time.
Has anyone tried WW blades on a Warp hub?
3. Any experience switching from a WW 70" to a WW 75"?

I am considering a big bore kit in the future.

I do not want to make a mistake thinking bigger is better.
Apparently most of the Air Master users are using WW 70" blades with 100HP.

Thanks
Herman

[Dave S]

Herman,

Not an answer to your question; but, your question inspired a question I have (also running the same warp drive prop/engine combo you have).

With your thinking that the Warp is close to needing help from wear and tear, I am curious how many hours of service you have been able to get out of it so far?

Mine has 310 hours on it and is a ways away from needing attention...but we don't have a lot of gravel strips out here either.

Another option is Warp apparently can take a well worn prop and redo it to new condition including new nickle edges....not sure what the price is but it is less than a new prop and if the hub is fine there is no need to replace that.

[WWhunter]

I have both but have used them on a RANS S7. My engine is a 105hp Zippered ( originally an 80 HP) engine. I think either one would serve well. Only reason I changed was due to wanting to have the 'latest and greatest'. Plus, the 70" was developing severe cracks in the outer finish that I wasn't sure was safe. Whirlwind said it was only cosmetic... I say BS!

The 75" pulls a little harder but I never really put enough time on the 70" to give you an honest comparison.

[herman pahls]

Dave S
Of the 600 hours on this Warp, I have only flown 150 of them.
I was not aware that Warp Drives could be reconditioned.
Even with a few small chips and dings in the leading edge protection it still performs great and smooth.

Whunter
Thanks for letting me know your experience between the Whirl wind 70" and 75".
I ordered the 75" thinking it would be appropriate when or if I go with the 115 hp big bore kit.
I really wanted to try the Prince 78 or 80" but was concerned about not enough ground clearance even though I am on 29" ABW's and standard Model 4 Grove gear.
I am not ready to spend $2500 for extended Grove or Cub Style gear so I spent $1900 on a prop when the existing prop works very well and I really want to try a Prince.
Why did I do that???

Wish list.
115 Hp big bore kit $5000.
Extended gear $2500.
Prince Prop $2300

[herman pahls]

I recently installed a 75" Whirl Wind (WW) prop that was replacing a 70" Warp Drive on a model 4 Kitfox with 100 hp Rotax 912.
The Warp gave me excellent all around performance.
I do most of my flying near sea level but was looking for better short take off and climb when I go to the Backcountry of Idaho or Nevada (Nevada has some strips at 9000').
I did a pull test before removing the Warp to get a baseline.
I am using a 550 pound scale that looks like a large fish scale that I purchased on Amazon.
The Warp pulled 400 pounds at 5360 RPM.
The Whirl wind pulled 480 pounds at 5300 RPM at 15.7 degrees pitch.
Who knows how accurate my scale is but the seat of the pants method told me the WW definitely pulled harder than the Warp.
With the Warp I could easily hold a full power run up with the brakes, with the WW the brakes can no longer hold the additional thrust.
I was told by WW that their 75" prop was well with in the Mass Moment of Inertia (MMI) that is recommended by Rotax.
I was also told that the Warp I took off was borderline or exceeded the Rotax recommended MMI.
I have not done the MMI tests so I have to go by what I have been told.
I was disappointed that the complete 75" WW including their 10" spinner weighed 11 pounds- 4 ounces which is 2 ounces more than the Warp.
The only explanation as to why the 70" Warp has a higher MMI than a 75"WW when it weighs 2 ounces less is that the Warp blades must be heavier at the tips than the WW.
My 100 HP rotax does not have the soft start that the newer 912 ignition modules have so it takes a cold start up procedure to prevent kick back.
The WW blades seem to start easier so that may be an indication of less MMI than the Warp.
I knew that I would lose cruise speed and I have probably lost about 10 mph at the current 15.7 degree pitch angle.
With the Warp I often get 110 to 115 mph above 10,000' with 29" ABW's burning 5.5 GPH with out a HACman.
I have noticed on the fuel flow, that at low altitudes the 75" burns more fuel.
This is a beautiful prop to look at but I am not sure it would be the best all around performance prop.
I would think the 70" WW would be best all around in the WW lineup.
For my local gravel bar bashing I am pleased with the prop but may need to go back to the Warp for long distance flights.
I am sure this would be an excellent prop for floats.
Herman

[av8rps]

Over the years I have found the 68 to 70 inch 3 blade props to provide the best overall performance on the 912 series engines on airframes like the Kitfox. I know the longer blades like the 75 inch do better in the hard STOL environments, but they suffer immensely in cruise due to having too much drag. As an example, I have a 70 inch 2 blade Sensenich carbon fibre prop on my 912S powered Highlander, and it is a very impressive performer. It will climb solo at 1800 fpm and yet cruise at 108 mph at 5400 rpm. And that is with large diameter 8.50 tires and an open tube gear. For a Highlander that is not only a great cruise but also a great climb.

For comparison I tried a 72 inch 3 blade Warp with tapered blades with nickel leading edges and lost more than 20% in climb and 10% in cruise. I have a Sensenich 3 blade 69" (aka 70") I will be trying one day as I suspect that will be the best overall prop on my Highlander. Of course I'm not pushing my Highlander on mountain tops like you are. But even with my current 70 inch 2 blade prop I can outperform pretty much any Supercub. So I'm happy.

I think the 912 gear reduction providing such a low prop speed changes a lot of what we thought about propellor and engine efficiencies. That's the only logical explanation I can come up with.

Try to find someone in your area that will lend you a shorter blade to try to see if what I'm saying applies to your plane like it does on mine. That 75 inch WW is a VERY NICE prop, but I'd be surprised if a 70 or 72 inch prop wouldn't be more to your liking.