# Tri-Mower Design

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#### blane.c

##### Well-Known Member
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I am designing (loosely worded) and hopefully building a "two place tandem, taildragger, Tri-Mower experimental airplane". The design even if it doesn't look like it will be heavily influenced (to put it lightly) by the Evans VP series of aircraft and construction techniques. It won't have a VW engine (100ci about) it'll have three industrial engines (147ci about). The two place needs another 50ci from original (about) to perform "more normally". So the marriage is not "Frankenstienish". Just have to cut the nose off and split it into three pieces and put it were the weight and balance will allow. Ok so Frank is better looking so what?

#### BBerson

##### Well-Known Member
HBA Supporter
Here is a list of tri-motor airplanes https://en.m.wikipedia.org/wiki/Trimotor
I think it might be less drag to just go with four motors, say two on front and two on rear of dual nacelle pods on the wings or fuselage. (CriCri, for example)

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#### lr27

##### Well-Known Member
Speaking of trimotor that aren't ugly:
-Pander Postjager
-various Dewoitine trimotor
-Spartan Cruiser ( not quite as sleek as the others, but I like it)
-Bellanca 28-92

#### Vigilant1

##### Well-Known Member
I think it might be less drag to just go with four motors, say two on front and two on rear of dual nacelle pods on the wings or fuselage. (CriCri, for example)
I know this is is a trimotor thread, but I'll just say I agree with this. 4 engines, packaged back to back in two nacelles, would be low drag and pretty cool. I'd probably opt to put them on the wing roots. 120 HP for normal ops and 90 HP with an engine out gives a lot of design flexibility, and the wings could fold outboard of the engine nacelles and the fuselage would still be narrow enough to fit in a trailer.
But, about 270 lbs of engines, mounts, and props. Plus, I'm not crazy about the idea of putting people in the path of a thrown blade.

#### blane.c

##### Well-Known Member
HBA Supporter
Here is a list of tri-motor airplanes https://en.m.wikipedia.org/wiki/Trimotor
I think it might be less drag to just go with four motors, say two on front and two on rear of dual nacelle pods on the wings or fuselage. (CriCri, for example)
Every time you add a engine you add weight. The trouble with four engines is that it is expected to fly on two engines so that limits how low you can go on hp and therefore weight. Two engines each side on a pod, pusher in the rear of the pod, tractor in the front of the pod would work well but lose both engines on one pod and you are back to the scenario with two engines only worse now you need more than 125% of the original single engine hp on each side to account for additional drag and weight of two dead engines instead of one. To much of a good thing.

#### cluttonfred

##### Well-Known Member
HBA Supporter
I agree that a four-engine configuration starts to get silly unless the engines are very cheap. Also, tandem engine installations (push me/pull you on the same pylon or nacelle) rarely seem to work out well because of cooling issues. I was looking around for the lowest-powered factory-built trimotor I could find. So far it's the 1920 Caudron C.37 (six passengers on three 80 hp Le Rhône 9C rotariers), but no joy on a photo or drawing.

I do wonder if a twin optimized for single-engine handling and performance might meet your goals with less cost and complexity than a trimotor, perhaps using a propeller brake to stop windmilling or folding propellers.

That Pander Postjager is very pretty!

#### Vigilant1

##### Well-Known Member
...., perhaps using a propeller brake to stop windmilling or folding propellers.
I think someone chimed in with a firm opinion that these little engines wouldn't windmill under the stated conditions. Of course, all bets are off if there's a catastrophic loss of compression, broken crank, etc.

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#### BBerson

##### Well-Known Member
HBA Supporter
Not nesessarily. I would link the pair on each side so only one starter is needed. The simple dog link could be disengaged when needed.

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#### Vigilant1

##### Well-Known Member
I agree that a four-engine configuration starts to get silly unless the engines are very cheap.

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#### lr27

##### Well-Known Member
Revan:
I think you could have a successful airplane kind of like that one. However, even with struts that wing is very thin. I don't know if you could make it strong enough. Also, it appears to have anhedral, which would be bad for the handling, unless there's much more sweep than I think there is. If there is a lot of sweep, you'll need a lot of washout.

What's going on in the middle of the wing? If it's a big notch, that may increase induced drag significantly. You don't have,a,lot of extra power to throw around.

Is everything to scale? The props look small and the pilot looks like he wouldn't fit, but that's just by eyeballing it.

Of course, you have to start someplace, and this basic configuration seems viable to me.

#### lr27

##### Well-Known Member
BTW, I could be completely full of baloney. I don't think so, but then again I wouldn't.

#### REVAN

##### Well-Known Member
Revan:
I think you could have a successful airplane kind of like that one. However, even with struts that wing is very thin. I don't know if you could make it strong enough. Also, it appears to have anhedral, which would be bad for the handling, unless there's much more sweep than I think there is. If there is a lot of sweep, you'll need a lot of washout.

What's going on in the middle of the wing? If it's a big notch, that may increase induced drag significantly. You don't have,a,lot of extra power to throw around.

Is everything to scale? The props look small and the pilot looks like he wouldn't fit, but that's just by eyeballing it.

Of course, you have to start someplace, and this basic configuration seems viable to me.
Yes, the wing has a lot of sweep and a lot of washout.

There is a 'notch' in in the middle of the wing. It is there for reasons that I won't go into at the moment. I'm working on some aerodynamic theories that are incomplete at this time. I know this goes against 'conventional wisdom', but that's part of the reason I'm experimenting here. I think a lot of 'conventional wisdom' in aircraft design may not be warranted, or is sub-optimized. I want to poke around the edges to maybe find some new wisdom.

It is drawn to scale, and the cockpit is actually quite large.

#### BBerson

##### Well-Known Member
HBA Supporter
Three 212cc engines is 636cc. and about 100 pounds. (guessing 30 each)
A single Briggs is 810cc and about 60 pounds.

#### blane.c

##### Well-Known Member
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If coming from simply a power to weight perspective it depends on total weight not empty weight. We all know a part 103 machine is supposed to be under 254lbs empty but then there is the fuel and if you want the whole can of gas and not just part of it that is another 30lbs, some choose to take less to save weight. Then of course there is the pilots weight and whatever they choose to, or not to, wear and bring with them this varies greatly. Going with 170lb human dressed and camera thingy in hand + 30lb fuel and max weight machine we get 454lb total flying weight divide by 27hp = nearly 17lbs per hp and if one engine quits while flying then 18hp (max power) so 25lbs per hp now can the engines survive indefinitely at max power? And how much power is needed to sustain flight dragging around a dead engine? Going with 58% of the total power of 27hp on a triple actually being usable for forward thrust considering most all the variables .58 x 27 = 15 2/3hp at max power is what is actually available to maintain flight not the full 18hp. So can it maintain level flight or is it a better glide ratio? Are you heavier or lighter than 170lbs clothing on and camera in hand? Of course there are other variables using 58% of power with engine out is just a guess it may actually be better or worse than that, pilot technique will be another variable and any number of other factors I can only guess about from here.

Generally though 27hp is considered enough power for many ultralights to fly and some require less power, others need more power it depends on the design.

Are you familiar with AC 103-7 especially starting with page 17? https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_103-7.pdf

#### jedi

##### Well-Known Member
Three 212cc engines is 636cc. and about 100 pounds. (guessing 30 each)
A single Briggs is 810cc and about 60 pounds.
But the Briggs can not direct drive the equivalent 5 foot diameter prop.

.....,,,,,,

Generally though 27hp is considered enough power for many ultralights to fly and some require less power, others need more power it depends on the design.

,......,
This probably refers to a Rotax 227 with a reduction drive allowing a four foot diameter propeller. Low power is ok if high propulsive efficiency is included.

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#### BBerson

##### Well-Known Member
HBA Supporter
But the Briggs can not direct drive the equivalent 5 foot diameter prop.
Sure it can. I have a 5 foot prop on my GX670. The tips are very narrow, about 3/4", and reinforced.

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