Below are the Emails with Lee discussing prop design and fabrication. I
used cut and paste and edited out most of the the chit chat.
If you can put this on the server, the group can pick it up from here.
I went through 6 different props, some borrowed, all 2 blade. The best I
had was a Hendrickson that I modified several times with thin carbon tips.
There was a wide difference between them all. Last year I got pissed off
trying to find the right prop since I changed to a gear PSRU with reverse
rotation. Best I could find with a less than 3 month delivery was a custom
made 3 blade from Performance at $1750 US plus shipping and crating. Hell
with that. Neccessity being the mother of invention prompted me to go one
step further and build my own.
So I did the aerodynamic design with Don Bates' Propopt and did the stress
calcs in mathcad. Made a 3 blade with select grade pine core with glass
spars and outer skin. It was easy as hell to do and took about 3 weeks of
small tasks. It cost $70 for enough wood for 2, plus some glass and epoxy,
say $50. I made a second one, a 2 blade and flew it for about 1 hour total.
It was better than any other 2 blade I had tried but not even close to the 3
blade so the 3 blade went back on.
If you can build a Cozy you can build a prop better than you can buy. I can
help you with the design and send you the Propopt application. I can
explain how to make the spars which loop around a centre insert giving a
designed safety factor on mine of 5 at 3200 RPM with a weight of 9 lbs.
Phil is going to do the same for his SVX Cozy but he likes to build stuff
like I do.
I don't broadcast this kind of thing because few undestand, they think its
all magic which of course it isn't, but you seem to be a good candidate with
a need. There are a bunch of pictures of my 100 HP VE on
johhny-enterprise.com if you want to see the prop, high helix, high aspect
ratio, swept blade, semi-constant speed, $50. Can send the design files if
you can read mathcad, the Prop OP file is just asci text. My fee is zero.
Let me know if you are interested in further discussion, otherwise I will
just go back in my hole and be quiet.
>I looked over the pictures of your custom prop on Johnny's www page. It
>looks very nice. I may be a candidate to make one. Do you think I could
>use the design data you've generated for your plane or would I have to
>generate new airfoils? I ask this because I believe our HP and cruise speeds
>may be similar.
Prop aerodynamic design is far beyond my little mind as there are many
variables which all interact, and the math is very complex. But I have a
nice piece of software called Prop optimizer which contains all of the
algorithms. It performs hundreds of iterative solutions until it converges
to the best solution within user defined constraints and then provides the
output including the blade shape and templates for 9 of 10 blade stations
It has proved to be very accurate. I use this exclusively for the
aerodynamic portion of the prop such as dia, pitch area etc. It does not do
the structural calcs so I do that in Mathcad but a spread sheet would be
just as good.
>Also would you describe the steps and tools you used?
This is a brief description of how its done but if you decide to make one I
will write a much more detailed description and fax you some sketches.
I start by making a dwg of the hub and one blade showing the blade planform
and draw out 3 of the 9 airfoil sections from the ordinates provided by the
application. I then cut out the blade laminates with a jig saw from soft
select grade pine and then laminate each blade blank individually using
recorcinol or epoxy adhesive. (This is like making a wing core before
hotwiring) The zero pitch line is marked on the leading edge of the laminate
with pencil then the dots connected with a french curve (I use a cozy wing
template for this). I make saw cuts on the flat side up to the line then
rough cut out the wood in between with a chisle. A spoke shave gets it
closer and reasonable smooth and accurate but not yet finished and true.
Then I do the top curved side the same way approximating the airfoil shape
at the tip, root #2 and number 5 about 1/2 way, the rest are easy to eyeball
into a natural shape, but still roughed in accuracy. Then the other 2
blades are done likewise. This gives 3 blades roughed out ready for joining.
The blade roots are cut at + and - 30 degrees to form a butt joint with the
blades at 120 degrees apart. Don't shiver here as the wood carries no load,
its just a high density core like a wing core, so the joint is to establish
form only. The blades are fitted together on a flat steel plate on the bench
and weighted with a lump of lead to hold then in place. When the blades are
set to 120 degrees with the tips on the same level plane then the joint is
floxed together, weighted and allowed to cure overnight. This gives a wood
prop core ready for final shaping.
The flat side of each blade is then finally shaped with 80 grit sanding belt
and block to get the blade angle correct at each of the 9 stations. This is
the 2nd most critical operation but not hard to do. I use a bubble
protractor sold by Warp drive to set these angles exactly. Then the top is
curved to the correct airfoil using outside calipers at 3 stations. The
blades are then checked along their length to get them all identical in
thickness and shape. This step takes one full evening but is not difficult.
The airfoil is not critical but should be symetrical between blades. I can
expalin this in more detail when the time comes.
Then the centre of the hub is bored for a .75 inch diam aluminum insert full
depth front to back. The ends of this hole is further relived with a 1.125
inch spade bit dowm 3/8 inch to form a moat around the insert to take the
spar material. The bolt hole locations are also relieved 1.0 diam by 3/8
deep both sides for flox pad for re-inforcement and crush protection. Spar
grooves are cut on both sides of the blades with a small chisle tangentially
from this insert down the blades to about 40% .25 inch wide and deep at the
root tapering in depth to almost nothing at 40 % where they join together.
Strands (tows) of Uni spar tape glass are then laid up in these grooves
looping around the insert and back down the blade. Done one tow at a time
alternating each blade in sequence thereby overlapping each other in the
centre moat until the troughs are full plus a little protruding. When cured
they are sanded down flush with the blade surface. These spars provide 1/2
the strength required of the blades and to keep the hub together.
Carbon tips are optional but make for a thiner and stiffer tip with higher
efficiency, mine all have thin carbon tips. The wood tips are relieved on
the bottom to allow for 3 plies of uni carbon about 5 inch long. When cured
they are trimmed to the blade shape then the other side is done the same and
cured and sanded to a nice sharp leading edge. Then the bolt hole pads are
filled with flox and Uni glass is laid up full span right across the entire
hub, alternating each blade, then a second ply out to 3 inch from tip, then
a 3rd halfway, and a 4rth out about 5 inches from hub, then 2 ply Bid on the
hub area and just onto the blade shanks. Cure and trim it all down just
like a wing is done with a tapered overlap on the LE. The TE is relieved of
wood about .2 inch in for a flox bead full span, then skin the top same as
bottom. This provides 8 plies of uni total at the hub area for the other
half of the blade tensile strength plus the Bid pad which I didn't count in
Now you got the basics of a prop with excellent properties. Sand the glass
overlaps to a smooth transition while balancing it on a bearing spindle
until it will stay put with no heavy blade. Then fill, prime and paint
keeping it balanced. Balance is the most important property and needs to be
checked at each step once the core is roughed out. Drill the bolt holes
using a crush plate as a drill guide. Erosion tips are added using soft
aluminum duct tape, better than anything else I have tried and easily
replacable. You can leave the spinner behind, it only adds looks, weight
and makes bolt torque checks a bitch plus upsets the balance.
> Anything with compound curves seems a little daunting to me, maybe I'm
>missing something. Also, when ever I asked IVO or Performance Props about a
>3 blade, the seem to indicate that it would only be helpful with higher HP.
> You've tried both and found it to work better than a 2 blade on your 100HP
>engine. Why do you think that is?
The theory says a 2 blade is better in efficiency. But everyone I have
spoken to who has tried a 3 blade says the opposite. That has been my
experience also. It climbs better cruises better and much smoother. Once
you go 3 blade you will never go back, don't know anyone who has gone back
to 2 blades. But I don't know why this is so, just a few theories only.
Let me know if you want to try this prop making thing and I will help you
through your first one, as I know it won't be your last once you have it at
your finger tips. Its not as bad as it sounds, just takes a bit of patience
and carefull work, no different than building your Cozy. I can run all the
numbers for you and provide the design file.
>Would you send me a starter package and I'll see if I can make a go of it?
> I'd be more than happy to reimburse you for expenses incurred in mailing me
>the information. You can also send me an example design file to see if I
>can figure out how to use it. I assume there will be either .dxf or
>coordinate files to make the templates, etc. What other information is in
>the design file?
I have attached the design O/P file for my 3 blade to show what it provides.
Its a DOS text file so read it with whatever you have to handle that,
Notepad, or any WP application should work OK. The first part is the Input
file repeated, and without the user manual the variable names and
constaraint numbers, it may not make much sense, but it will give you the
Give me a day or so and I can prepare a package for you. If you can send me
you work fax # I cand send some construction sketches and the mathcad
structural file. Also send what data you can on your LE, current prop,
speeds and power setings and flat out speed any thing else you have. I can
input this into prop opt to calibrate it for drag calculation. If you have
the torque curve for your engine that is needed also but I can probably get
this prety close from data I have.
I've had a chance to look over the data you sent me and I was able to
decipher most of it. A lot of the abreviations needed no translations, but
there were a number that I did not understand. I'll formulate a list of
questions after I've had a change to digest it. You asked for my fax number.
It is xxxxxxx Since it is a shared fax, please make sure to put a
cover page with my name on it so it doesn't get lost.
As far as data, I'd be happy with the LongEZ book figures for the plane. I
can fax you those pages out of the manual if you like. My numbers are as
posted in the original message. Let's see if I can remember them... At
7500' DA, 132kts@2650rpm, 147kts@2950 (full throttle). Empty weight is
982lbs, gross is 1425. The current prop is 62x66. HP is 108.
>I'm a little curious how you made the templates from the data. Did you input
>it by hand into a CAD package? Perhaps this will become more apparent with
The airfoil ordinates in the output file was hand ploted on graph paper
using every second data point then drawn in with a french curve. I only
draw 3 of them so I can see the shape but I plot the angles of the bottom (
pitch line) for all stations so I can establish the reference line on the
wood blanks for carving. I use outside calipers on the blanks to get the
final airfoil shape with coarse sandpaper and block. You probably improt
those ordinates into a CAD application and make it draw the templates then
cut them from mylar if you wanted to get fancy, but I don't use CAD at all
so only a guess.
Have run a prop design for you as attached. It is optimised for cruise with
some margine left for full power with the nose down a bit without overspeed.
I could re-run it for 2900 RPM max if you wish as this will result in a bit
less pitch and better climb but will tend to overspeed if you push it like
on a low and over. I would use this one though. My 3 blade is optimized
for top speed and tends to overrev if I put the nose down even 1 degree. I
will make another this winter with just a bit more blade area but same pitch
so I can beat the field up at full throttle.
Will try and get some fax sketches to you in coming week. Also working on
getting the stress calcs into a Lotus SS but cant get it to do integrals
like mathcad so may have to plug in a few constants for blade CG location
etc to make it much simpler.
> Hi Nigel,
> I got your fax. Thanks for the sketch. It will be quite helpful. I
> was wondering if you use the coordinates to size the wood blank or the
> finished prop. Since the glass adds size, it would affect the prop
> numbers if you carved the wood to shape and began to add it after the
> fact, although probably not by very much. It would be a little more
> difficult to size the wood if you had to take into account the build
> up from the glass. Please tell me which approach you take.
I have done it both ways. I would recommend subtracting the ply thickness
(.010 per ply) from the airfoil ordinates top and bottom before plotting
them. But these uni plies are sanded at the transition to a gentle taper
over about 3 to 4 inch in span. Yes I know common practice is not to sand
the structure but the plies were made longer than needed to allow for this.
The result is you will need to do a little interpolation if you really want
to plot each airfoil station to exact dimensions. This is not necessary as
you will find when you shape your first blade and the airfoli is really not
that critical, just the pitch. Stations 2 and 3 are too thick to worry
about so plot 4 , 7 and 10 and shape the blank to these accurately. You
will see by the patterns in the laminate glue lines if you are off in
between. A little sanding with 40 or 60 grit on a hard block span wise will
quickly get things into proportion.
The tip station is a special case if you go with carbon. The wood is shaped
to match the ordinates accurately as there will be no uni right at the tip
when it gets tapered there later. Relieve the wood to allow for 3 plies
carbon uni spar tape (the stuff Wicks sells) and lay it up and cure, then
trim to blade LE and TE. Then relieve top side which will result in sanding
away a lot of wood back from the TE and some at the LE exposing the bottom
carbon in a taper shape. Then layup the 3 plies carbon which gives a good
bond at the joint leaving a small wood core in the centre right at the tip.
When you get to skin the blades the first ply of uni glass goes over the
carbon right to the tip but after cure can be tapered back a bit until
carbon is exposed right at the tip. This will preserve the thin sharp
section needed at the tip for good performance.
You may want to try one blade just for fun to get the feel. Get some good
select pine (no knots) or other soft straight grained wood such as mahogany
or virola pine, the lighter the better. Make 5 or 6 clamps from pieces of a
two by four, drill holes and get some threaded rod, washers and nuts. Cut
out your blanks in top profile (helps here if you draw one blade first to
get the size). The first 2 are full span and the rest get shorter as you
move to the root. Think mine took 5 laminates at .75 inch thick. Use a
good quality carpenters glue or recorcinol or epoxy and stack them up nice
and even then tighten the clamps slowly. They will try and slide around a
bit initially so use patience to keep them aligned vertically until they
grab, then tighten up the clamps nice and firm. You could drill a .25 hole
near the root and use a wood dowel to keep them in alignment.
Plane or belt sand the LE of the blank square and smooth. Mark your blade
stations with pencil on the bottom and extend up on the LE.
You have a choice of making a straight TE, LE, or curve both. The easiest
and best performing is a straight TE like Warnke and Performance (and Me)
use. Plot up vertically on each station where the bottom of the airfoil
(zero pitch line) is as measured from your diagram and make a pencil mark on
the LE of the blank. Now join the dots with a Cozy main wing root airfoil
template fitting the curve up to station 3. Now cut with a hacksaw across
the bottom of the blank up to this line and just short of the TE. Remove
the wood in between cuts with a chisle and mallet, but watch the grain and
try not to go too far. Then use a spoke shave, followed by 40 grit on a
block. Then do the top same way only carefull with the saw. Cut down to the
TE and LE at an angle to approximate the airfoil but don't go too far. Do
the chisle and plane thing again but check the progress with your 3
templates. Finish it with 40 or 80 grit and block. Do your final adjusting
for pitch angle and airfoil thickness the next day when you are fresh, like
it was fine tuning a part that you just bought.
The blade root transitions can be done quickly with a belt sander using the
nose to get the curves to blend. Leave the final shape at the roots until
the blades are all together as then it becomes much easier to see and measure.
Oh I forgot the wood is easier to find in 5 inch widths. This is what I
used and it leaves a small gap where the blades join at the hub which is 6
inch diam. This is not a problem, the small gap is filled with flox just
prior to skinning the blades. Remember the wood caries no load it is just
core material. It will be the best prop you ever flew if you keep it thin
at the tip and accurate. Its not that hard really.
Could you read the spread sheet I sent. You can see that the safety factor
has a wide margine for overspeed.
> Your construction technique is very elegant. Did you think this up on
> your own or do you know of there others out there doing it?
> I was looking through the back of Sport Aviation and noticed a few
> classified ads for instructions for making your own props. Have you
> ever sent away for any of this information? If so, have you found any
> good references?
No I did this on my own, so far its the only one flying built this way (
that I know of). My 3 bld has 96 hrs now with no sign of stress or
problems, still looks like new.
In reading your spreadsheet, it appears
> that each blade weighs 3 lbs. Does this neglect the hub weight? If
> so, what is the weight of a finished 3 blade prop?
Mine weighs 9 lbs which is in the input section of the SS. So I made it
divide by the number of blades to get individual blade weights.
What type of
> spoke shaves/drawing knifes would you recommend for carving a prop?
Mine is a flat faced Stanley about $25 up here. You will need a fine wet
stone also to keep it nice and sharp. When you get to the carving stage
clamp the blade root to your work table with a C clamp and allow the blade
to extend over the table edge at about 45 degrees or so. Get yourself a
chair and park your butt at the tip of the blade. Use 2 hands on the spoke
shave and draw it towards you. When you are getting close adjust it to take
very fine cuts then you can keep things accurate. Keep it sharp for best
results. But save the final shaping until the blades are assembled together
then you can get them all matched more easily. This bit needs some further
> Also, I was wondering how you determine the planform of the blade and
> how you go about gluing the laminates together. Do you make them all
> the same size or do you try to match them up with the eventual 3D
> shape of the blade? The table at the end of the airfoil data seems to
> indicate block widths and heights. However, I am not sure I
> understand what they are trying to say with all of these measurements.
Yea thats a good question. I basically ignored that table. I cut the first
blade top planform laminate full span and width from the board using a power
hand jig saw. First though I laid out the profile on the board in pencil,
just started with the blade radial line parallel to the board edge and
plotted a bunch of points on the LE and drew in the TE then connected the
dots with a curve. I checked it against the dwg I made, then it became my
master template for all of the other laminations. From then on just plunk
it on the next board and draw around it with pencil. The inner ones are
shorter only but same section. They are glued into a square stack with _NO_
stagger. I suppose you arguably waste a bit of wood this way but it sure
makes life easier. If you drill a .25 inch dowel hole on the radial line in
about 1 inch from the end it really helps keep them aligned when glueing.
Make the dowel a tad shorter than the stack height then just slip them on
and clamp but keep the tip aligned and square as you wind in the clamp pressure.
> I am also having a little difficulty interpreting the data coordinates
> at the leading edge radius. There is a point at X=0 where YU and YL
> that are equal to the radius of the leading edge. However, if you
> plot those points and fit a circle to it, it drops down below the
> bottom of the airfoil which is flat. (I am assuming that YL of the
> bottom of the airfoil is 0) What is the program trying to show here?
Never ran into that but the bottom is flat up to X= a little bit, or just
enough to give a slight under curve to form the LE radius. At the tip I thin
out the airfoil about 25 % more than that shown. I have not gone into that
yet but wood props have thick tips for strength otherwise they will easily
split. This plays hell with efficiency, so by using a carbon laminated tip
you can make it much thinner more like an aluminum prop. I know you say
what about the airfoil accuracy?? As I mentioned before the airfoil is not
particularly critical. All the marketing BS you read about fancy prop
airfoils is just that, BS IMO. It is posible to get a very small
improvement with weird looking transonic airfoils _in the lab_ but as soon
as you hit the first bug, it becomes just another turbulent airfoil. You can
literally draw your own airfoil around the sole of your shoe and it will
work. What is critical is the angle of attack, (pitch). This has to be
matched exactly between all blades to get a really smooth running prop. And
of course balance is everything. If you take outside calipers and measure a
commercial wood prop, you will find some variation in thickness and airfoil
shape. This is from the final sanding process to get it into balance and is
common accepted practice. When you get to the final balance stage after
skinning and transition sanding, if you have a pesistent light blade, you
can add a ply or two of 3/4 Oz. glass as I had to do on my 2 blade. This
cured it and only added a couple thou in thickness. You have some leeway
with paint also as an extra coat on one side of a blade or just at the tip
will make a change.
> What do you use as the overall thickness of your hub? I believe you
> mentioned that the diameter is 6".
The 6 inch diameter matches most hubs but check yours and make it to match.
The thinkness depends on how far in you want to try and keep an airfoil with
the correct pitch. My Performance 2 blade is 4 inch thick but not much
airfoil until about station 3. My 3 blade is 5 inches and much better. My
2 blade is 6 inches which is probably overkill. If you go with 6 laminates
at the hub you will get 4.5 inches plus the glass thickness and that should
be fine. You could go with 5 laminates you will get have a thinner root
section with little change.
I just realized that the hub raises a possible problem that you will have to
consider. Most hubs have a centering boss which protrudes about 3/4 inch or
so. You will not be able to counter bore your prop to clear this boss due to
the alum insert which supports the spars. I see a few options to get around
this. You could make an alum spacer to take up this boss depth. You could
machine off the boss, I don't use one the bolts center it fine. Or you
could change the spar pattern on the forward hub face to loop around alum
inserts where the bolts hole are drilled. Or you could counter bore the
wood blank and use a piece of .25 inch wall thickness alum pipe set into the
blank on the forward face and loop the spars around it. Have to confirm
that they clear the bolt holes by carefull positioning. I think I would go
with a spacer, you could even epoxy it right to the prop when you bolt it on
the first time.
> Is your trailing edge straight in both TE indentation and TE
> elevation? I noticed that these number vary only a little between
> each station and it makes is simple if you could keep them all in a
> straight line.
Yes I make the TE dead straight and use it as a reference for all other
measurements. This results in a unique side profile with the blades
appearing to angle back slightly, looks racy. But it has proved to be a
> In making the saw cuts before chiseling the wood, about how far apart
> do you need to space them?
Radius over 10 spacing, or just mark and cut at each blade station. The
wood will split out quite easily with a mallet and chisel, just watch the
grain. If it wants to split into the blade then come at it from the other
direction. If by chance you do mess up and cut into what should be solid
blade core, as long as it is not too severe, you can fill it with flox after
final shaping just before skinning as the wood caries no load.
> I have attempted to arrange the blades in several ways. The first was
> to rotate the blades aft so that the hub radial intersected the
> leading edge of the outermost blade radial. This gives the straight
> trailing edge a slight backwards angle with respect to the hub's
> How did you do it?
I swept mine back so the radial line intersects the blade tip LE, pretty
much as shown in the sketch and as you mention above. This puts the centre
of lift behind the radial line causing the blade to twist slightly at high
power, high thrust as in the climb. This gives a semi constant speed effect
to some degree. The more you sweep it back the more pronounced the effect.
Imagine an airplane with wings swept back at 45 deg. Then you pick it up by
the wing tips which loads up the wing. The tips will flex up and reduce the
incidence at the tips. The trade off is the twisting stress introduced with
the blade GC behind the radial line as you mentioned. I used pure uni cloth
span wise only to allow it to twist a little. If you put a BID layer on at
+ - 45 it would become very stiff in torsion and loose this effect without
gaining much more strength, just more weight and another couple thousand
pounds of centrifugal load.
The sweep has another benefit for fast turning props (mine turns 3200 due to
my engine and GB arrangement) as it delays shock formation at the tips
allowing a higher tip mach number. This improves efficiency slightly. This
is the same reason the swept wing fighters ie sabre jet, were faster than
the straight wing models.
You could make it with the thickest part of the airfoil right on the radial
line and then plot the LE and TE from there. This should work well and have
better stress distribution but may cost a little in climb performance. But
the 2 props I made are swept back and work great so don't really know if you
would loose any performance with zero sweep. I would suggest you make it as
above as I know that works well and your RPM and stresses will be lower than
> The CAD system I use has a lot of 2D capability, and pretty good 3D,
> but I'm afraid that it cannot do sculptured surfaces. I can get a lot
> of the properties from the 2D section like area, moment of intertia,
> CG, etc. In looking it over the the largest section, it appears that
> station 2 is about 4 in^2 in area. Machinery's Handbook specifies
> most woods for maximum working stresses between 650-1500 psi. I don't
> know what safety factor is included in this table. Hmmmm, in looking
> at your spreadsheet, it's pretty easy to get the centrifugal load up
> around 9500 lbs. This seems well beyond what this prop would be
> capable of handling if it were made entirely of wood.
Yes you are very correct here. A pine prop with butt joints at the blade
roots would come apart long before you got to full power. But the pine, or
whatever wood you use, does not carry any load in this design, it is just
core material. You could probably use high density clark foam also. The
stress is all carried in the glass spars and skin once the prop loads up at
operating RPM. The load is almost purely radial centrifugal like a
helicopter rotor which has hinged blades that would droop down and hit the
ground when stopped if it were not for the droop stops. So the wood strength
was not included at all in the calculations as it doesn't matter. A pure
wood prop is another story however, thats why they use laminated rock maple
and make them thick.
I got a fax from an ME friend of mine on structural properties of glass,
carbon and kevlar. Its a lot of good information which I will fax to you on
Mon. What came out of this is the tensile strength of glass seems to be
much greater than what I allowed for in the stress calcs at 87,000 PSI. And
carbon is right up there at 450,000 PSI and lighter also. Might make my
next one from carbon over pine. But I am concerned about the epoxy matrix
bond as the weakest link so will keep it very conservative as I did in the
SS I sent with lots of safety margine.
Hope this explains it.
> I ordered a spoke shave and some other tools yesterday so I am moving
> forward with this project. I also got your fax on the fiber material.
> It looks really useful. Thanks!
Great, its fun to make but your cozy will suffer. Mine is still just a
bunch of big parts waiting for the spar to go in then engine, etc, etc.
Having way too much fun with the VE.
> I'd like to fax you some of the output of my CAD work to see if it
> looks OK to you. The aspect ratio of the prop seems a little spindly
> compared to what I am used to and especially when I compare it to the
> general shape of your sketch.
Certainly thats the correct number, would love to see your CAD dwgs. Got to
look into this CAD stuff myself. Seems like a great way to pass files
around via Email and make suggestions and doodles etc.
> Another question I had was that my LongEZ has a 62" dia prop on it
> now. The data you sent me had a 64" prop. Is it OK to increase the
> diameter of the prop? That is, do you know if a LongEZ has enough
> ground clearance that a 64" prop will not produce any problems. I
> suppose I can trim it back a little if it is.
The 64 inch should not be a problem. We put a 64 on my friend JC's LE this
summer and he had lots of clearance. Try lifting the nose up way high as in
a very high flare attitude with your current prop vertical and see how much
clearance there is. The only time it could be a problem is if you drop it
in hard with a high flare angle then you could get a prop strike but
probably not. If 64 looks too big let me know and I can re-run the numbers
and change the dia constraint to 63 or 62, the SW will adjust the rest. But
it found 64 as best within that constraint so should give best results. When
you are getting ready to cut wood let me know and I will re-visit the design
just to be sure its the optimum. But as you say its best to be a little
over as its much easier to trim it than add some.
Yes they do look spindly, but remember there are 3 blades now to share the
load. Perhaps that is why the 3 blade is better, the blades are thinner,
have a higher aspect ratio and lower tip vortex even though there are 3 vice
2. I looked at a bunch of 3 blades on EZs and Puffer's MK IV at Osh and
they looked spindly to me too but with BIG triangle shaped hubs. I think
this design is better, certainly a lot cheaper, and lots of pride as few
people make their own props especially 3 blade. You will get lots of
questions about it when you fly in somewhere.