# Briggs vanguard conversions

### Help Support HomeBuiltAirplanes.com:

#### blane.c

##### Well-Known Member
HBA Supporter
So, how much direct drive thrust should we expect from the 810cc engine?

Disclaimer: I’m not a propeller expert. The “method” below may or may not be valid.

As the point of all this tinkering with the engine is to make thrust for an airplane, it’s reasonable to ask how much thrust we can expect. For the purposes here, I’ll assume we want to know how much thrust we’ll get at 2000’ MSL from an engine rated at 30 HP at SL, and driving a fixed-pitch wooden two-blade prop at 3400 RPM. I know there’s reason to believe that the engine will produce more HP than that at higher RPM, but this may be a conservative appraisal of what the engine can make on a continuous basis.

Methodology: The table contains estimates for several propeller lengths and design speeds. In each case I used Jan Carlsson’s propeller design program to give the expected efficiency of the propeller at its design speed. I was not able to make the program give me reliable projections for a propellers at airspeeds other than the design speed, so I estimated “off design speed” propeller efficiency using a graph from Raymer’s Aircraft Design: A Conceptual Approach (Ed 1, Fig 13.10). That lookup table, made from the published graph, is in the spreadsheet attached.

In general, relative to its design airspeed, a propeller loses efficiency fairly slowly at airspeeds below its design speed, but loses efficiency rapidly at airspeeds above the design speed (at airspeeds above its design speed the propeller blades are at low AoAs). For example, if a particular prop has a design speed of 100 MPH, it might be 75% efficient at that airspeed. At 30 MPH below its design speed, 70 MPH, its efficiency is reduced to 0.82 of its 100 MPH design efficiency, so its new efficiency is 0.75 x 0.82 = 62%. Now, if we go the same 30 MPH above its design speed, to 130 MPH, the prop is just 50% as efficient as it was at its design speed (so, 0.75 x 0.50 = 37% propeller efficiency).

After finding/estimating the efficiency for the prop at various airspeeds, I found the thrust by applying the following equation:
Thrust (in lbs) = Engine power (HP) x propeller efficiency x 375 (a constant) / airspeed (MPH)
The tyrannical term in that equation is the divsion by the airspeed at the end. Because of that, thrust declines with increasing airspeed even for propellers designed for higher speeds.

I’ve attached an excel spreadsheet in case you’d like to sort things differently (don’t blame me if it falls apart!) or catch my spreadsheet errors. The PDF version may be useful if you just want to look at the numbers. The process I used has a lot of manual steps, which means it is both slow and prone to error.

Observations from the estimates:
- Though these 42-28" props are short compared to "regular" GA aircraft props, because of the modest HP we are using, they have very reasonable propeller disk loadings (lower than a C-152, etc). Even at modest climb speeds, at these RPMs there appears to be little gain from going with long propellers. If these numbers are right, it looks like most people choosing these engines and running direct drive to climb and cruise a small sportplane at 60-120 MPH will probably choose a 44"to 46" propeller.
- As noted above, prop efficiency (and, especially, thrust) numbers fall off relatively rapidly above a propeller's design speed, but the efficiency (and esp the thrust) hold up somewhat better at airspeeds below the propeller's design speed. For folks looking to balance a desire for higher cruise speed and good climb rate, this would argue for choosing a propeller optimized for speeds closer to the cruise speed than the climb speed. OTOH, from a safety standpoint, acceptable climb rate almost always trumps cruise speed. So, more compromises . . .

If anyone has prop design/thrust information for this engine (or other small engines) in direct drive that we could use for comparison, that would be great. I believe MiniSport uses a Helix 48” composite prop on their SE-33 engine, but I’m not sure of the pitch or of the thrust they are getting in flight. M. Colomban reportedly went through quite a development effort to design the Arplast propeller used on the MC-30, but I’m not sure of the specs on it, performance, etc. The data here is “open loop”--based on Jan’s propeller calculator and some extrapolation for off-design speeds. It would be great to get some real world numbers.

Notes:
– The prop planform is the “Jan Carlsson Standard”
– The prop pitch optimization I chose in Jan’s program is “standard”, so, it is coarser than a pure climb prop but finer than a pure cruise prop .
– The tables show the blade width at 75% span because the propeller blades designed by Jan’s program get quite narrow (and thin) at longer prop lengths and these relatively small HP levels. Some people may choose to give up a little bit of efficiency to get a propeller that is more robust.
- The estimates are for a 30HP (at sea level) engine taken to 2000’ MSL (so, producing 27.9HP) at 3400 RPM. The propellers are wooden, 2 blade and (obviously) fixed pitch.
- Jan’s calculator does not provide thrust values below 40 MPH, and traditional methods don’t do well at those low speeds. So, for takeoff roll, etc we’ll have to use other means to get thrust estimates. Even a completely stalled blade produces thrust (though inefficiently).
- For many small airplanes, rate of climb is very sensitive to the available thrust. Even a few pounds difference in thrust levels can significantly change the climb rate. My data has some obvious glitches (e.g. I can't explain why the props designed for 70 MPH are shown as being more efficient at 60 MPH than the props designed for 60 MPH. The method I used probably also overstates the 60 MPH capabilities of the higher-pitched props at 42" diameter--surely all of the 42" diameter props have efficiencies of less than 35% at 60 MPH). The estimates here (and elsewhere) may be useful for planning, but getting real numbers in flight will be critical.
I am curious if the airspeeds are indicated or true? This could effect design choices quite a bit.

#### blane.c

##### Well-Known Member
HBA Supporter
What are the parameters of a flexible propeller, 3600RPM flattest pitch @ what RPM is coarsest pitch and what would the pitches be? How do you design a flexible propeller that has predictable pitch variation?

Also putting the "P tip" on a propeller is supposed to effect its radius about 2" so a 42" propeller with the "P tip" would have the performance of a 44" propeller. Does anybody understand how to incorporate a "p tip" onto a blade that works?

#### Vigilant1

##### Well-Known Member
I am curious if the airspeeds are indicated or true? This could effect design choices quite a bit.
Short answer: It is a little ambiguous, but I think the thrust numbers will correlate most closely to TAS.

Long answer: Per the (too long) text in my earlier post, I started by getting an "on design speed" calculation of propeller efficiency from Jan's calculator. This required that I put in an airspeed for which the prop would be optimised, but Jan's program doesn't specify if this input is supposed to be TAS or IAS. The output thrust graph his program provides is labelled "TAS," so (to me) that's an indication that TAS is expected in the entry form.

After I got the prop efficiency, I used a formula and a graph of off-design prop efficiency corrections to get the off-design expected thrust. The design->actual conversion is definitely a function of TAS.

As a note: The relationship of on-design propeller efficiencies to off-design propeller efficiency is a function of "advance ratio" ("J").
J = aircraft velocity / (RPM * prop diameter). As "J" changes from the "J" that the prop was designed for, efficiency declines (this is just the compromise we make with a fixed-pitch prop). Notice, though, that if our velocity changes by 10% (say, from 100 KTAS to 110 KTAS) but we also increase our RPM by the same amount (say, from 3200 to 3520), then "J" remains unchanged, and our prop is still the "ideal" prop for this new airspeed.

The prop efficiency conversions that I did in that previous post accounted >only< for a change in aircraft velocity. In this respect, they may be overly conservative since when we are flying faster it's likely we'll also be at higher RPM (and the converse when going slower). So, we may get more thrust at off-design airspeeds than I estimated.

This is all a bit like using a micrometer to measure a marshmallow. We can get more refined when we know max torque numbers at various RPMs.

Last edited:

#### Vigilant1

##### Well-Known Member
What are the parameters of a flexible propeller, 3600RPM flattest pitch @ what RPM is coarsest pitch and what would the pitches be? How do you design a flexible propeller that has predictable pitch variation?

Also putting the "P tip" on a propeller is supposed to effect its radius about 2" so a 42" propeller with the "P tip" would have the performance of a 44" propeller. Does anybody understand how to incorporate a "p tip" onto a blade that works?
While I can't help much in answering your question, I think Lonnie Prince (owner of Prince Aircraft Company, manufacturer of the P-Tip props) can provide useful assistance when we have enough baseline info for him to work from. I suspect the "coning" and pitch change functions of his props is fairly subtle. The PAC web site claims that the pitch varies approx 4" from takeoff to cruise, but I would think this might also be different for different diameter props. For perspective, according to Jan's program the ideal pitch for a 42-47" diameter prop at 3600 RPM and 60 MPH (climb?) is 19", and at 100 MPH (cruise?) it is 31". So, a 4" change by the P-Tip will be a help, but nobody should expect that it'll be perfect.
Lonnie Prince has experience with small props (his company makes props for small UAVs, too). I know that he's been diligent in helping folks who were installing the Aerovee Turbo package and needed a lot of adjustments/pitch changes. I'm not advertising for him, but he does have a good reputation in the Sonex community.

When M. Colomban was working on the Luciolle I know he spent a lot of time working to get the prop right, and it seems likely that MiniSport (the SD-1 folks) have done development work on whatever prop they are fitting to their B&S derived engines, too.

Last edited:

#### blane.c

##### Well-Known Member
HBA Supporter
While I can't help much in answering your question, I think Lonnie Prince (owner of Prince Aircraft Company, manufacturer of the P-Tip props) can provide useful assistance when we have enough baseline info for him to work from. I suspect the "coning" and pitch change functions of his props is fairly subtle. The PAC web site claims that the pitch varies approx 4" from takeoff to cruise, but I would think this might also be different for different diameter props. For perspective, according to Jan's program the ideal pitch for a 42-47" diameter prop at 3600 RPM and 60 MPH (climb?) is 19", and at 100 MPH (cruise?) it is 31". So, a 4" change by the P-Tip will be a help, but nobody should expect that it'll be perfect.
Lonnie Prince has experience with small props (his company makes props for small UAVs, too). I know that he's been diligent in helping folks who were installing the Aerovee Turbo package and needed a lot of adjustments/pitch changes. I'm not advertising for him, but he does have a good reputation in the Sonex community.

When M. Colomban was working on the Luciolle I know he spent a lot of time working to get the prop right, and it seems likely that MiniSport (the SD-1 folks) have done development work on whatever prop they are fitting to their B&S derived engines, too.
When I think about a flex prop I think max hp and max RPM = max flex (finest pitch) and when you back off the power you eventually get coarsest pitch at whatever power and RPM that is. It seems logical that a short thick prop will flex less than a long thin one as well. So for these engines we may get less than 4 inches of pitch change but anything will be better than none considering the RPM band.

For these engines to work well the propeller is going to be as critical as the cooling.

#### pictsidhe

##### Well-Known Member
I have pondered a twisting prop, but have moved it to my after-it's-flying list...

#### Vigilant1

##### Well-Known Member
When I think about a flex prop I think max hp and max RPM = max flex (finest pitch) and when you back off the power you eventually get coarsest pitch at whatever power and RPM that is.
There may be different types of flex props. From the description on the PAC site (on their FAQ page). In part:
How does the Prince P-TIP propeller change pitch? The method the Prince propeller changes pitch is due to the amount of pull the propeller has. When a propeller is at high rpm and slow airspeed the forward face of the blade has the greatest pull. When the propeller reaches the designed airspeed the propeller face has zero pull. To take advantage of this condition we taper or sweep the leading edge of the Prince propeller profile. When the propeller pulls or cones forward in this high rpm slow airspeed mode the outer portion of the blades pull forward. As the blade pulls forward it will untwist the propeller and offer about 4" less pitch. The pitch will start to move back to the higher pitch carved setting as the airspeed increases.
According to this explanation (as I read it), the unflexed "relaxed" state of these props is coarser pitch and seen at higher airspeeds. They "flex" to a >finer< pitch when the blades are loaded up at lower airspeed (in response to the pull of the prop, the blade tips arc forward and this "untwists" some of the pitch that was carved into the blades. I see how that could work, but I can't say from experience that it actually does).
For these engines to work well the propeller is going to be as critical as the cooling.
Yes, the prop will be important. For uses similar to the SD-1 or Luciolle (relatively clean airframe, 65 kt climb, 90 kt cruise), the props already in use by these planes and fitted to these engines would be the best place to start--or finish--the quest. But for folks doing something different (e.g. "next generation DA-11 " cruising at 130+ MPH, or a draggy slow plane, or a multi-engine design that has hard-to-accomodate requirements for good thrust at both low airspeed (SE climb) and high airspeed (normal cruise), etc)--they may need to do something different.

Last edited:

#### Vigilant1

##### Well-Known Member
On a semi-related tangent: I've been watching the financial news concerning Briggs and Stratton since I got interested in these engines. It would seem pretty important that the company remain in business and healthy to keep these engines in production. Anyway, the company's stock (ticker: BGG) has been in quite a slump and now trades at $6.25 per share, less than half of its 52 week high of about$15. The company is closing a mower factory in Kentucky and taking some other steps to cut costs.
I was pleased to see this article that was bullish on the company's future, indicating that some of the trouble was due to one-time events (closure of Sears stores, unusually poor weather, tariff issues, etc). I still have concerns.
Note: I don't own any of this stock and have no interest in promoting it. I'm just interested in the overall health of the company, which may or may not be reflected in the price of this particular equity.

#### BJC

##### Well-Known Member
HBA Supporter
That is interesting.

Random data point: When my daughter’s family moved a couple of years ago, they left their gasoline powered riding mower in Florida and purchased a battery powered  riding mower for their hilly yard in Georgia.

BJC

Last edited:

#### BBerson

##### Well-Known Member
HBA Supporter
Is that an electric riding mower? I haven't seen any, and I assume they are not inexpensive.

#### BJC

##### Well-Known Member
HBA Supporter
Is that an electric riding mower? I haven't seen any, and I assume they are not inexpensive.
Yes, a riding battery powered lawn mower for a yard with steep hills. I edited my post above to clarify.

BJC

#### Vigilant1

##### Well-Known Member
I was tempted by a cool Ryobi electric riding mower, you can talk yourself into it if you estimate a high enough cost for maintenance of a gasoline powered riding mower (esp annual maintenance done by someone else of the stuff under the deck-- belts and spindles, etc). The electric mowers have direct drive motors for the blades, they'll probably need less attention. Still, the out-the-door price for an electric rider is double the cost of a gas unit.
I suspect the cordless push and self-propelled walk behind mowers will be the first place where electric takes a significant bite out of the gas engine market. That's happening already if the display area at Home Depot is any indication of relative sales.
The 810cc vertical shaft engine that we've discussed a lot in this thread is primarily used in beefy zero turn radius mowers most often sold to commercial users. I don't think there's much risk of electric power making inroads there (though there have been a few attempts to sell electric ZTRs to homeowners). B&S should be making them for a long time.

Last edited:

#### Hot Wings

##### Well-Known Member
HBA Supporter
Log Member
The 810cc vertical shaft engine that we've discussed a lot in this thread is primarily used in beefy zero turn radius mowers most often sold to commercial users. I don't think there's much risk of electric power making inroads there
This is the reason the 810, or equal, will be around for a long time. Much like our airplanes battery energy density just isn't there for commercial use. I mow my personal yard with a corded electric and have for well over a decade. Even though I have the tools and knowledge to maintain a gas mower the electric de-complicates my life.
I have a friend in the commercial lawn service and he has gone to all battery trimmers and blowers because his employees can't destroy them as fast as the gas. He has lots of batteries. I have both gas and battery string trimmers. I use the battery at home but need the gas for heavy stuff. Getting hard to find a good 4 stroke trimmer for a reasonable price.
I also have a gas rider that I can't even imagine how I'd replace with battery for the same reason my friend is still using gas mowers commercially. Battery might be fine for a small yard but for large yards, or commercial use, we still need the energy density of liquid fuel.

With the trend going to outsourcing and distributed use - like Uber, Lyft, and AirBnB - I expect battery powered equipment demand to be reversed and the percentage demand for gas going back up..............except for those progressive and enlightened cities where battery is mandated.

Vertical shaft mower engines will be with us far longer than the Corvair was, and probably the VW as well.

#### blane.c

##### Well-Known Member
HBA Supporter

Couldn't find fel-pro asbestos but found Percy graphite with steel core. a couple size's

https://www.summitracing.com/parts/php-67-070/overview/
https://www.summitracing.com/parts/php-67006?rrec=true

Think I may try copper sheet and Gasgacinch
http://gasgacinch.com/home/technical/

Last edited:

#### pictsidhe

##### Well-Known Member
I've made a few 2 stroke cylinder head gaskets from soft copper and aluminium. The can be re-annealed for reuse. I would lilke to make up something to make embossed steel gaskets. They usually use expensive dies to stamp the ring in, but some kind of jigged roller thing might work for small quantities.

#### blane.c

##### Well-Known Member
HBA Supporter
I am just wondering if Copper Sheet and Gasgacinch would be adequate for pressures developed at 9 to 1 compression ratio and naturally aspirated?

#### Armilite

##### Banned
=====================================================

Many Old Engines used Copper Gaskets. There was a guy on eBay that made Custom Copper Gasket that is really reasonable. You can send him a CAD file and he will give you a quote. You can also use one of them Cri Cri Machines that cut Paper Gaskets, Leather, Vynil, and Copper, etc.

Also, if you Port your own Engines, you can make and extra Thick Template Gasket so you can use to Trace on the Case. Also, to use longer rods.

Last edited:

#### Armilite

##### Banned
I am just wondering if Copper Sheet and Gasgacinch would be adequate for pressures developed at 9 to 1 compression ratio and naturally aspirated?
===========================================

Racers use Copper Gaskets and their using up to 16:1cr! Also, Aluminum was used on some Engines. You want to make sure you Anneal Copper before you Assemble the Engine, it can Harden over time. Rotax Base gasket, Head gasket and Exhaust Gasket.