# Direct Drive Honda/B&S limits

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#### LHH

##### Active Member
Thanks all.
Will start with Billski's advice and model in Solidworks and then model against propellers to see what happens.
Honda price is almost identical to Kohler and B&S for same HP. All are around $2k. #### Armilite ##### Well-Known Member I have been reading the Briggs conversion messages. I understand moment of inertia issues, twisting and bending to some extent, but what is the best way to arrive at a factual answer? For example, X industrial engine can be used as a direct drive if the propeller weight is less than x and or RPM is less than X. If propeller weight is greater than X or RPM exceeds X, then a reduction drive must be used to handle the load. Can this be calculated accurately using data from the engine manufacturers? I am sure I am missing a fact or two, but you get the idea. Thanks in advance ============================================== The fellow using the Honda GX200 Clone in Direct Drive on a Lazer was turning them at 4400rpm. Small 7/8" PTO Shaft (The Rotax 185UL when upgraded to 7/8" PTO used 5000rpm), the Honda/Clone GX390+ Singles uses a bigger 1.0" PTO Shaft. A Preditor 670 V Twin uses a 1.0" PTO. The Biggest I have seen used on V Twin is 1-1/8". The Stock Flywheels are good to 5500rpm Max, and 5000rpm is Max I would turn them for Plane use. Most of these Industrial Engines need to Save Weight, so People use the Billet Aluminum Flywheels which are good to 10,000+rpm which Saves a lot of weight over the Stock Cast Iron Flywheel. Use the Billet Aluminum Rod which is stronger than the Stock Rod. They usually take out the Balance Shaft to save 4-6 lbs of Weight and just have the Crank Balanced. The only thing you need to worry about is using the right Size Prop and Pitching it for the Engine's Max Hp. Like if you're turning it 5000rpm, you Pitch it -100rpm for 4900rpm. I personally would not use Direct Drive on these, I would use a Good Belt Drive like from ACE. I haven't seen anyone Adapt a Gear Drive to one of these Honda/Clones yet. Kawasaki FH680D-FS08S 675cc replacement engine with a horizontal PTO shaft measuring 1-1/8 in. x 3-15/16 in. 5000rpm in Direct-Drive is going to regulate you to a Max 48" x 10 (2) Blade Prop making 355.83 lbs Static Thrust. Needs 45.080 hp. ----------------------------------------------------------------------- 5000rpm with 1.8 ACE Belt-Drive is going to give you 2,777.7rpm, Pitched for 2,677.7rpm, an Option to run a 72" x 10 (2) Blade Prop making 516.64 lbs Static Thrust. Needs 35.053 hp. 5000rpm with 1.8 ACE Belt-Drive is going to give you 2,777.7rpm, Pitched for 2,677.7rpm, an Option to run a 68" x 12 (2) Blade Prop making 411.05 lbs Static Thrust. Needs 33.467 hp. So as you can see, using a Belt Drive allows you to make more Thrust running a Bigger Prop. You can play with the Numbers for whatever Engine you are thinking of using. An ACE Belt Drive for V Twins I consider Best for these Honda Clones shipped to the USA is$649 & $699. Freight to Mainland USA - - US$109
So $758 &$808.

For Singles: US $569 Freight to Mainland USA - - -US$99

#### pictsidhe

##### Well-Known Member
Thank you for the offer, just emailed Ace to try and get one made to keep the engine centered due the low shaft position.
I won't be selling redrives for several years, if ever.
I'm a little dubious about the Ace units. Without spending \$1000 on one to examine and test, I can't say if they are likely to hold up or not. John is not willing to give out enough info to do a proper paper assessment.

#### Victor Bravo

##### Well-Known Member
HBA Supporter
As has been mentioned already, a big spinner in front will greatly reduce the bad shape of the engine for a motorglider, and also give you a nice low-drag shape. Since a motorglider is likely going to have a low drag fuselage with a reclined pilot, that spinner will be helping quite a bit with the whole airplane.

Direct drive will save a lot of weight, money, and fiddling. A motorglider should have low enough drag to be able to use this.

#### Vigilant1

##### Well-Known Member
Trying to find an EFI four stroke that will not require major reworking with a small frontal area.
Honda seems to have the smallest
Here's a sketch that Hot Wings made showing the relative frontal area of an 810cc B&S (28HP stock, more to be had) in direct drive mode. That's a 6' 2" pilot and a 10" x 10" spinner. There's more (incl some sketches with redrives) at this post.

from the side:

Although these engines aren't the "conventional" airplane engine shape, as the drawings show, they are quite small.
If prop clearance from the ground is a significant concern in your application, TiPi's work on flipping the engine over will give you a few more inches. It also puts the jug "humps" underneath which may clean up the cowl aerodynamics a bit. The direct drive approach saves the weight, cost, and some of the risk associated with a PSRU. For a motorglider, the direct drive approach makes a lot of sense (IMO). For a draggy trike, a PSRU would be best.

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#### aeromomentum

##### Well-Known Member
As a manufacturer of engines with PSRU's and of the PSRU itself I may be a little biased but there are some compelling reasons for turning the prop slower. Keep in mind that a faster aircraft has a lower L/D just like a faster turning prop. If you keep the prop the same length a direct drive prop turning 2700 rpm will require about 10% more power for the same thrust than a 2240 rpm prop on an engine with a PSRU. A two gear PSRU will have a loss of less than 1% so you net about a 9% reduction in required power for the same thrust with the same diameter prop.

Of course a larger diameter prop will also provide more thrust for the same power and a PSRU will allow this with less loss due to the increased prop velocity.

Direct drive also has it's own risks. There can always be a resonance between the prop and the engine and this can break props, cranks, etc.

#### LHH

##### Active Member
Spinner diameter is already 17 inches.
Had not considered flipping the engine.
Lots of good points to consider.

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#### Hot Wings

##### Grumpy Cynic
HBA Supporter
Log Member
Has not considered flipping the engine.
Unless you really 'need' an inverted engine I wouldn't. There are other industrial engines besides the B+S 810 that could be adapted. I decided on the 810 mostly because it is easier to invert than the horizontal shaft engines. If I was to use an upright engine I'd have to seriously look at some of the other options. Probably a lot less work to adapt.