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ski pedestal height

Randy

FOUNDER
S.Dakota
Although is was 95 degrees here yesterday :) .....I have a question about ski installations on Cubs.
In an article I found on the net....it reads...

64. LANDING GEAR MOMENT REACTIONS.

In order to avoid excessive moment reactions on the landing gear and attachment structure, the ski pedestal height must not exceed 130 percent of the axle centerline height with the wheel and tire installed.


My question is....if you have bushwheels...and since that would increase the height of the centerline of the axle from the ground....would you be able to increase the axle centerline 30% above that with higher ski pedestals?

I'm thinking specifically about losing propeller clearance and AOA when changing from bushwheels to straight skis.
It seems like you would lose a lot of clearance when going to a straight ski like a Federal.....maybe some other kinds of wheel replacement skis have higher pedestal height?.

I'm sure other folks have thought about this before...and I was just curious in an academic sense how they worked around it.

This is an informative link about ski installations........
http://avstop.com/maint/alterations/ch5.html
Randy
 
Hi Randy,

I've fooled around w/ all different types of skis, and different gear lenghts. That said, I have found what works good for my application in real world situations. However I can't quote exact prop clearences for different props and/or skis instalations or STC requirments, ck w/ your mechanic..

The Landis 2000's will do the job, however they are fairly small for powder conditions and do not offer maximum floatation. They have a fairly short tower height wich mean you're not sitting at an optimum AOA on stock lenght gear (poor short field take-off performance), also I have broke more Landis 2000's than any other ski on the market. I have used these w/ stock gear & 3" extended and have never had a prop clearence problem (always use an 82" wheel), however I'm certain you'll need 3" extended gear to be legal.

The Landis 2,500's seem to work well for a wide variety of snow conditions. These skis have a taller tower hieght which puts the plane at a better AOA on stock gear, plenty of prop clearence, and a durable ski used in harsh conditions. This ski is a little long which mean you'll need a little extra room for turning, however they are an all around good ski.

Areo bought out Federal many years ago. The Areo skis made today are a cheap copy of the old orginal Federal skis. Areo makes a variety of different sizes, these skis are mainly applicable to deep powder conditions as they have wide bottoms and are fairly short for easier turning. However this skis is not made for icy or hard frozen snow conditons, they are light & flimsy and won't last 2 season under harsh conditions or heavy use.
They also make a fony-baloney cable-spring semi-retractable ski that never really gained popularity, however they will work under certain conditions. I'tried them once, more of a pain in the azz than anything.

There are a few of the old Atlee Dodge skis still around, these are heavy and ugly....probably tough as nails though!

The old [original] Federal 2,500 or 3,000 have a good tower height, which puts the plane at a great AOA on stock gear. There are realatively lightweight and built tough. They seem to work good in both deep powder and icy-hard pack snow conditions. When in very deep powder, w/ a heavily loaded plane, you'll need to learn the trick of getting them "on step" for better take-off performance. Because they are short they turn easy, but in very deep powder they are similar to floats. These skis are tough....you'll break the gear off and do damage to the lower longs before tearing up one of these skis. These skis are getting hard to come by, when you see a good set you better buy em' on the spot!

I know of at least 3 different outfits working on "new" skis for Cubs. I suppose because of the fairly small market it is hard for them to justify putting to much into R&D. However I think we'll see some more ski options within the next few years. Some of the newer carbon fiber-plastic technology is making its way into ski design.
Personally I run the Federal 2,500's and think they are an all-around good ski for my application. It seems like you can walk up to 5 different Cubs and see 5 different type of skis, choose whats right for you.

good Flying...>Byron Lamb
 
Luke/Byron:

I have what appears to be an anomolous pair of Federal A-1500 skis. I just bought them from another Champ guy. They have bottoms that are nearly 12 inches wide. The other Federals I've seen are skinny little things, maybe 6 or 7 inches wide.

Mine have 1/8 inch UHMW bottoms and skegs as well. I'm looking forward to installing them and having fun in the snow.

This particular set was manufactured in 1946, long before Aero bought the company, I imagine. Aero Ski is just 30 miles from me, in Brooten, Minnesota, a town of about 600 people (on a good day).
 
Hi Jon,

What you're decribing is NOT un-common. Keep in mind that the tunnels & towers of the Federal "1,500", "2,000", and "2,500" are nearly identical, or are actually the same. The only difference being the width of the bottom skin.

Biggest 1st difference between the Aero and Federal is the tunnel lenght & width. The Federals have a wider/stronger tunnel, plus its longer to support the ski better. Secondly the Federal towers are massive, and the third primary difference is the dictinct convex bottom on original Federals.

Taking into consideration we're talking about 50-60+ year old airplane plarts, it is very plausable that someone has changed the bottom skin of your "1,500's" at some piont in thier life. It sounds like you now have a nice set of "Federal 2,500's" , regardless of what the data tag says, great skis!

I also have some w/ a mfg. date of 1946 or 1947, belive me...I run them hard! Average about 300-400+hrs per year. No other ski on the market can hold up to harsh use and perform as well in deep powder & hard pack snow/ice with a heavy plane. I sure wish they weren't getting so hard to find these days!
Wanna sell yours? :D

Good flying..>Byron
 
Luke/Byron and Jon B.

Interesting information about the skis....the Federals seem to be pretty tough...my Dad hunted with the same set for many years....a narrow bottom set that he said would "cut through" the knife edge hard snowbanks common out here where the wind has to blow the snow almost immediately into hard drifts.

I haven't done much ski flying myself...but hope to change that as soon as practicable :)

Winter flying works out better for me anyway...more time to enjoy it.

As far as the height of the axles when changing from Bush tires to skis....It just seems like most wheel replacement skis I have seen would lower your propeller clearance and AOA...unless you had some high pedestals....

Looks like the front plane in this picture must have some higher pedestal skis and maybe an extended gear also?

http://sparky.supercub.org/photopost/showphoto.php?photo=1277

maybe it just appears to have a higher AOA than the airplane parked behind it?
Beautiful picture by the way :)

Anyway....You would hate to lose the height/clearance if you were used to flying big tires.

Thanks again...Randy
 
Randy:

My brother flies a Husky which currently sports an 82-inch MT propeller. On floats and 26" tires, he's OK. When he goes back to his wheel-skis this winter, it won't technically be legal, as the minimum tire size is 8.50 x 6 for this prop. He has to revert to 7.00 tires for the skis to fit - they're Fluidyne (WipAire) hydraulic retracts.

My A-1500s had likely been rebuilt several times in their 60+ life. Someone decided they needed more floatation and added the 2500-type bottoms. And, NO, they're not for sale. I just got them three weeks ago. They're in nice shape, have Champ rigging and a set of really nice dolly wheels. They were relatively cheap, too - $900 complete!

Of course, I have a 7FC (formerly a Tri-Champ) so they're not on the TC as approved components. I have to get a field approval to use them. The nose wheel went away 7 or 8 years ago, so '57B is essentially a 7EC (except in name, as the data plate still reads 7FC). The approval *should* be easy. I already asked the MSP FSDO guys.

Jon B.
 
As Luke points out, there are a lot of options when it comes to skis. One of the best investments you can make on a Cub when going to skis is to purchase a set of 3 inch extended gear legs. That really helps with pedestal height, angle of incidence on surface, and of course, performance and prop longevity.

If your old Federal skis are original, the bottoms will have a "tunnel" in them, on the bottom. In other words, the ski bottom will have not be totally flat. Right under the tunnel on top of the ski, there will also be a convex area, as Luke noted, in the bottom, in place of a center skeg. If the bottoms are flat, they've been replaced.

For most of the world, the Aero 3000 skis or the Landes 2500's work just fine. You have to beat on them pretty hard, in pretty tough conditions to break or bend those. I agree with Lukes comments, though: The Landes take a bit more room to turn, and the Aeros turn a bit better, but don't work as well on hard pack and rough snow.

Where you run into damage on these things is in a place like Luke is, with lots of hard pack, bare spots, melting and freezing, etc.

In the Interior of Alaska, most of these skis will work fine.

And remember, if you put that much energy into the ski, and the ski holds up, that means it is transferring ALL that energy and forces into your airframe. Which is more valuable, and harder to replace: your airframe, or your skis?

The Federals are tough alright, but they are also horribly heavy beasts.

I'd take a set in a heartbeat, though,

MTV
 
so, as I understand, the pedestal height of Federal 2000 and 2500's is the same?

Also, have to replace the bottom of my skis this fall. any suggestions? can I go wider than the ski? any tips for the bottoms regarding runners? I have 3 inch wide runners now. thanks
 
Ski Pedestal height

My 2500's are higher than my 2000's,about 10" axle centerline to bottom vs about 8 1/4". 2500's substantially beefier than 2000's.
 
Standard gear with the ThrustLine mod and 850x6's there was plenty of clearance with the Borer prop. I think it could also work with stndard gear and wheel skis. I should have written down the measurement as measured from the center of the axle height to prop height in the level position. :oops: Thought I'd never forget. At 50 my memory is excellent---just short. I will get that measurement again the next chance I have. If anyone needs it sooner----give a call. Excellent prop clearance on floats also. Have a buddy Kris in PWS spotting salmon with considerable ocean ops heavy. His comment was that he had negligable prop erosion. Those that operate in the salt know how abrasive that can be. Nothing like he used to get. Don't mean to be a commercial but I just thought some of you would like the info.

Thanks

Mark
 
I've been doing a lot of thinking and asking around about this subject. Here are my conclusions, thus far:

Prop clearance - What is legal and what you do can be two different things. I have to say that the threat of "ground" contact on snow does not really worry me all that much, since the ground will be snow. If I thought I would be on a lot of ice, that would be different.

Extended gear - I'm using standard length gear with 31" bushwheels in the summer and 3" extended for skis in the winter. The reason for the 3" extended is not to make me legal or to get me more prop clearance. Those things are side benefits. I wanted 3" extended for ski flying to give me a more consistent angle of attack in each season and so ease the swap over each season (swap gear, not tires/brakes for skis).

Skis - I'm using Aero 2000's (1971 model if that matters). They are light at 25 lbs each. A buddies Landis 2500's weigh 50 lbs a piece. I like the weight of my skis. I'm not bashing hardpack or ice, so I think the durability of the Aero 2000's will be fine.

Ski Bottoms - I'm going with 1/4" UHMW, extended 2" past the edge of the ski, all-around. I'm thinking this will give me near Aero 3000 floatation. The cost will be about $150 for the new bottoms and hardware.
 
Erik,

Don't fool yourself that snow won't destroy a propeller just as quick as rocks. There is also a somewhat seditious effect one can get into with powder snow, where you run your prop (generally happens with Borer props) through a lot of loose snow. At some point, you begin to notice that the performance really isn't what it used to was.

I've found more than one of these very thin blade props where the blade pitch has actually been flattened out, presumeably from running them through snow.

"Legal" prop clearance is a good thing, and not something I'd try to skirt around, for any reason. There are a lot of really good practical reasons to justify maintaining "legal" prop clearance.

Frankly, it just makes sense to provide as much protection to that very expensive tool out front, and with the current range of legal gear legs available, it makes no sense to skimp on engine and prop protection.

Oh, and by the way, you have read Lycoming's newest recommendations on "prop strikes", I assume? There unto itself (even though its not mandatory for part 91, try to get a mechanic to put his signature on one that hasn't complied) is a great reason to maintain a "legal" prop clearance.

Also keep in mind that your skis will not still weigh the same after adding 20 pounds or so of plastic to the bottoms. By the time you're done, the little skis will weigh just about as much as a set of 3000's, I'd bet. And, you can't compare Aero 2000 skis to Landes 2500 skis, they are a totally different beast, in many ways, not the least of which is durability.

Lots of folks hang the plastic over the edges of the skis to make bigger bottoms. I don't like the idea much (even though my skis are too small), since I think you'll start seeing cracks in the plastic sooner by doing so, since you're applying a lot of stress with no support along the edges. The stuff fails soon enough, and its costly and time consuming to replace. Figure out your real savings on weight (apples to apples) and your actual cost savings over ten years, and I'll bet it would be nearly as cheap to just buy a set of 3000's. But, what you're proposing will work.

MTV
 
I agree with Mike.......about the prop in the snow.

On the other hand, I like the overhanging plastic idea. It works. Yes, it weighs, but the extra float is something you'd be very unhappy without, in certain conditions.

...Just another compromise to make.
 
Mike,

Loose snow counts as a prop strike? I have read Continental's policy on prop strikes, but not Lycomings. Continental says that any strike that reduces engine RPM requires a mandatory tear-down inspection. I assume Lycomings policy is no different. Can virgin snow do such a thing - reduce RPM? I'm talking about the prop going into new snow a few inches for a second. (I'm sure you'll bash me for this paragraph!)

In general operation, loose snow will erode a prop? If so, what is the best way to avoid "kicking up snow". What is a good technique. I assume coming in smoothly with power and getting the plane moving before higher power is applied, such as for dirt strips, is a sound technique. I haven't put a single nick in my new Borer yet, using this technique in gravel/dirt.

Regarding the legality issue - I agree that if you need to use extended gear to protect the prop, then get the extended gear. Yes, this is very expensive equipment and the cost of new/extended gear is relatively inexpensive. But if you are convinced you are not going to damage anything by running standard length gear, yet have technically a "legal" issue, well that's up to you on what to do. I'm concerned about reality, not legality on this matter. They many times are the same thing, but not always.

I was not comparing the Aero 2000's to the Landis 2500's in terms of quality or durability. I was comparing Aero's with extended edges to possibly being similar in footprint to the Landis 2500's. But I am not sure how close they are. I haven't done the math. I am strictly talking about a footprint/snow pressure comparison.

As I posted in an earlier thread, the only reason I am considering going with Aero 2000's in the first place is because I own them (from a previous plane purchase). I would not buy them new. If I were buying new, it would be the Aero 3000's (although I think they are ugly) or the Landis (Airglass) 2500's. I can put a few hundred into my Aero 2000's or I can fork out $2500 to $3000 for new skis. At this point, I'd rather have a nice new belly pod next summer (so I'll use the money saved for that)! If I don't like my skis after a winter, I'll sell them and roll the money into a bigger/better set next season.

I have no direct experience with the UHMW at this point. But I have to believe that 1/4" of the stuff has got to be tuff. And the bottoms of the Aero 2000's I have are sloped upwards. So I thing the middle of the ski bottom/skags will take the brunt of the beating and the outer edges will come in nicely in the softer snow when you need more floatation. Yes, the 1/4" UHMW will add weight. I'll post back once I've determined how much weight. I hope it's not 20 lbs. But maybe.

Thanks, Erik
 
Erik, the prop/snow issue that's most acute is the way "a couple inches of snow" bends the prop. It's not so much about erosion, in my opinion.

When the tips go into the snow, they load up and bend forward. Note that it's at the end of the blade, where there is much greater leverage than near the root. In my experience, you come out of this scenario with a prop that has bend forward, not just a "flattening of the pitch".

I'm a believer in longer gear being better. I wasn't ask, but there it is anyway. :D

DAVE
 
Dave,

You were asked, in a sense. When I post, I figure I'm really talking to everyone, even if I put an "to" name at the beginning.

So an operating prop that makes it only a couple of inches into soft snow can do that kind of damage huh? That surprises me. I'll have more reverence for the subject then.

Thanks, Erik
 
Dave did a much better job of explaining specifically what I was describing. The point is the prop gets bent forward at each tip, with such a small radius bend that sometimes it's difficult to even see (though I've seen a couple that were pretty dramatic) and that has the EFFECT of flattening out the pitch of the prop.

Its easier to do than you might think, particularly on a very long thin prop like the Borer props.

I routinely run the Husky prop in deep snow with relative impunity, since the blades are so much thicker on that prop, and it's not as long. The Husky has great ground clearance, as well, about the same as a Cub with 3 inch extended gear.

There are many, many traps to be found in deep snow. If you haven't done so already, get some good flight instruction on skis before you get too wild and crazy.

Oh, yeah, take yer camping gear, and warm clothes,

MTV
 
Airglas skis

The Airglas L2500A straight skis are legal on standard (heavy duty) landing gear with an 82" Borer prop. The L2000A skis are not legal with standard gear, you have to use 3" extended gear. The new Airglas L2500A skis weigh 33 lbs out of the factory, because our bottoms are bonded on with 1/8" thick UHMW. The L2000A skis are 27 lbs brand new out of the factory. Many people buy the old black bottom L2500A skis and use 1/4" thick UHMW and then attach it with screws. The skis then end up weighing 50-60 lbs. The new LH4000 thermo plastic ski weighs 39 lbs, which is amazing in that the ski is easily twice the size of the L2500A straight ski. The cub LH2600(or LH2700 that is still being decided on) is going to be really light. And if it works like the LH4000, straight skis will be a thing of the past, because the wheel ski is working just like the straight ski, with a completely plastic bottom (including the runners).
 
Another factor to consider for prop clearence these days is the addition of the thrustline mod. Does anyone know exactly how much extra clearence it gives? Besides the extra clearence, the new angle fights the nosing-over tendency better.
 
ground loop said:
Another factor to consider for prop clearence these days is the addition of the thrustline mod. Does anyone know exactly how much extra clearence it gives? Besides the extra clearence, the new angle fights the nosing-over tendency better.

Figuring 4 degree change with 28" from firewall to prop, about 2" of extra clearance. Educated guess on the 28" from firewall to prop path based on cowl supports.

Tim
 
Re look at this thread? It has always sort have been interesting that everyone seams to accept that the difference between small tires 8:50s
With say a 10" axle height and Bushwheels 30/31 with a 15" height is a huge advantage because of the extra AOA is a no brainer,
But when it comes to ski flying which can get you easily into some of the toughest conditions you will ever face, are right back down to 8-10" avg heights? Flying out of two feet of snow off a 10" axle height is like your "hung yourself position" as the minute you lift the tail up , to get it out of being a brake , your in a far to flat AOA, for the wing to help . I am guessing your distance to get in the air on a 10" axle height compared to a 15" height would easily be 50% plus further. Yet to this day no manufacture addresses the this. A cub with 6" extended gear works perfect on normal skiis (Landis, Aero, Federal) and a Scout or Citabria with Scout gear ditto as well. But a cub on standard gear on say Aero 2000 with an 8.5" axle height (we wont even bring up if you unbolted the Borer prop) is a long way from a "ball of fire" in two feet of fresh snow! Cessna seamed to be better at addressing this with longer gear legs?. Of course this all shows up on a digital
Level when you put it on the bottom of the the wing , a 185 a Scout and a Cub on 6" gear now all start to look similar in the AOA relation, just food for thought. Here is a great example of two Citabrias one with oleo gear and one with Scout gear look at the huge difference in AOA............... that GCBC worked great on the Scout gear and had tremendous travel to the legs.
IMG_0937.jpg
 

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