# Design Drivers?

### Help Support Homebuilt Aircraft & Kit Plane Forum:

#### Pilot-34

##### Well-Known Member
I’ve always heard that engine design drives aircraft design.

I think that traditionally that’s always meant horsepower but in this forum I believe it’s an outstanding merit in any area.

For instance engine with an amazing specific fuel consumption Could be highly desirable in an aircraft for a long range bush work.

and of course we’re always looking for the lightest weight for the amount of horsepower.

of course price for horsepower is a major influence.

Time between overhaul or reliability is also a major concern

what are your nominations for the greatest engines in any of these areas?

personally I am interested in the engines in the 150 -200 range , I realize many here are interested in a much smaller engine and some in much larger ones, feel free to throw your hat in the ring.

#### dog

##### Well-Known Member
Figure on cost per hour plus overhaul,put that much in the bank for every hour flown,fly and forget about it.
We have a bunch of adequate motors,but nothing
"great",Zoche sure looked like a miracle,as do others
but natch on a plane yet.

#### blane.c

##### Well-Known Member
HBA Supporter
I like the Aeromomentum on paper it makes a lot of sense, liquid cooled for stable temperatures and no co2 in cockpit heat @ 55% power 0.39lbs per hp per hour fuel consumption and low weight.

#### blane.c

##### Well-Known Member
HBA Supporter
Figure on cost per hour plus overhaul,put that much in the bank for every hour flown,fly and forget about it.
We have a bunch of adequate motors,but nothing
"great",Zoche sure looked like a miracle,as do others
but natch on a plane yet.
I have seen no movement on Zoche since the late 1980's or early 1990's.

#### dog

##### Well-Known Member
I have seen no movement on Zoche since the late 1980's or early 1990's.
I got a email BACK from him,once,something,
something,no not yet.

The Chevy LS 5 can be had for reasonable money(compared to the -7 and its forged titanium rods,etc for $40k) #### Vigilant1 ##### Well-Known Member Lifetime Supporter Of possible interest: 1) We had an interesting discussion of$/HP and lb/hp in this thread.
Engines: Cost and weight per HP
Note that the latest estimate for the B&S 810cc engine in the direct drive mode would be about 30hp and 75 lbs.

2) Also, ToddK's 'Wild West of Four Stroke Engines" thread covered some ground that might be of interest.
Wild West of Four Stroke Engines

Last edited:

#### Pilot-34

##### Well-Known Member
Figure on cost per hour plus overhaul,put that much in the bank for every hour flown,fly and forget about it.
We have a bunch of adequate motors,but nothing
"great",Zoche sure looked like a miracle,as do others
but natch on a plane yet.
Huh ? Did you get the wrong thread ?

#### Toobuilder

##### Well-Known Member
HBA Supporter
Log Member
Actually "mission" drives aircraft design. The airframe and powerplant are typically flexible until just before PDR, then locked in. Its true that homebuilders will sometimes select an airframe or (design one) based upon an engine laying around the shop, but thats not "design" in the pure sense.

Also, concerning BSFC, there are very few (if any) standouts when considering basic engine architecture. Sure, a no mixture control, Continental A-40 is not going to be the BSFC champ, but a "current technology" Lycoming with EFI and operated LOP is right up there with some of the best compression ignition engines available. The aircraft engine operating profile is remarkably similar to a stationary engine like one running a pump or genrator - constant RPM and near maximum output for hours on end. Thats not a hard target to optimize.

Last edited:

#### dog

##### Well-Known Member
Huh ? Did you get the wrong thread ?

#### rv7charlie

##### Well-Known Member
Wellll....

You obviously need to decide the mission first. But unless you're talking about cloning an existing design, I suspect that most designers will tell you that the powerplant comes next. IIRC, Barnaby Wainfan has talked about it in some of his Kitplanes articles. One of the problems us alt-engine guys have using existing airframes is that they aren't designed for comparable weights and/or cooling methods. For instance, the RV-x (like most medium size/weight homebuilts these days) is designed around Lyc-like engines. Fitting a water cooled engine is hard, because there's no provision in the airframe for radiator & plumbing. If you were to convert to, say, a boosted Yamaha snowmobile engine (much lighter than a Lyc at the same HP), or one of those vaporware turbines, the nose would have to grow like Pinocchio for CG purposes, and then the vertical stab would have to grow to compensate for all the extra side-area in front of the CG (see almost any certified piston-to-turbine conversion). If you go heavier/higher HP, payload is lost to the engine, and then more payload is often lost to the ballast in the tail, and then more payload is lost to the extra fuel needed.

If that isn't enough evidence, history is littered with designs that failed because the proposed 'in development' powerplant, didn't. Lots of attempted military stuff, but our close-to-home example would be the BD-5.

#### TFF

##### Well-Known Member
Is this an anything other than Lycoming question?

One thing about airplane design that a ground based vehicles don’t have is minimum speed. There is no minimum speed on a car. Coasting in an airplane means you will not be flying quite soon, even in a glider. Flying faster than stall requires power just to get past the zero flying mark.

When thinking of a new design, it is possible to dream up lots of possible airplanes. The design can be the best ever thought of. If you designed it needing an engine that doesn’t exist or you can’t afford to make exist, it’s a dead design. Dream dream dream does not fly.

When one looks at what wars you want to fight to have a flying plane, do you want to fight one or fight two? On the biplane forum there was an old thread on side jobs that payed for your aviation hobby. Lots were National Guard pilot jobs when they were airline pilots, not too much of a stretch, but one I remember best was a guy worked weekends in a coal mine. That is how much he wanted it. It’s all about budget. Do you shrink your dreams to fit what you have now, or do you accept what you want costs X, and it’s going to take extra to get X? That’s airplane design.

#### D Hillberg

##### Well-Known Member
No ballast - Extra range - less pieces

and bunnies

#### Attachments

• 441 KB Views: 17
• 60.1 KB Views: 14
• 7.1 KB Views: 11

#### blane.c

##### Well-Known Member
HBA Supporter
For 150hp to 200+hp Aeromomentum makes a lot of sense to me downscaling hp a bit to around 120hp Rotax of course but super spendy and D-Motor a less expensive though still expensive option. D-Motor was advertising last winter a 150hp supercharged version, not a turbo, but a centrifugal supercharger.

#### Pops

##### Well-Known Member
HBA Supporter
Log Member
Is this an anything other than Lycoming question?

One thing about airplane design that a ground based vehicles don’t have is minimum speed. There is no minimum speed on a car. Coasting in an airplane means you will not be flying quite soon, even in a glider. Flying faster than stall requires power just to get past the zero flying mark.

When thinking of a new design, it is possible to dream up lots of possible airplanes. The design can be the best ever thought of. If you designed it needing an engine that doesn’t exist or you can’t afford to make exist, it’s a dead design. Dream dream dream does not fly.

When one looks at what wars you want to fight to have a flying plane, do you want to fight one or fight two? On the biplane forum there was an old thread on side jobs that payed for your aviation hobby. Lots were National Guard pilot jobs when they were airline pilots, not too much of a stretch, but one I remember best was a guy worked weekends in a coal mine. That is how much he wanted it. It’s all about budget. Do you shrink your dreams to fit what you have now, or do you accept what you want costs X, and it’s going to take extra to get X? That’s airplane design.
When are you going to finish the Beetlemaster ? Engines in stock.

#### aterry1067

##### Member
HBA Supporter
I agree with others, mission drives the design. All design projects are to meet the mission. You wouldn't build a carbon fiber sweep wing retractable low wing for STOL, nor would you build a thick, wide, long aluminum high wing for XC flying. Any powerplant can fit any mission, if the airframe accommodates. The reverse is not true.

#### Pilot-34

##### Well-Known Member
So why is there more than one design permission?

Perhaps I’m guilty of thinking in military terms after all I am from a service background.

it seems like the fastest heaviest longest farthest Have always been engine driven.
More than once a design has failed because it inadequate engines available and then along comes a better engine and the design becomes a winner.
but I straight from the point of the thread.

What do you see as exceptional in its weight or horsepower class?

#### TFF

##### Well-Known Member
A better engine only comes along if it is profitable. Even military needs. The difference is the government throws a bunch of money at a problem up front; civilian is picking up money one person at a time. How many need the engine verses what it will cost happens for both. The only difference is money up front instead of onesies twosies. Which one is easier to start up a company? That is why new engine companies stall. They are looking for government cheese; they don’t care about one or two. Ten thousand or none.

Class winners will be Lycoming and Continental because they are the game. Everything else is fringe “toys”. If you are adamant it’s not going to be a Ly/Con, your choice is pretty much Subaru or Mazda rotary on the light weight side or LS on the heavyweight side. Different approaches. Even if you buy a product, you will be in charge of working through product deficiencies. There are no true bolt and go products. If you want a different engine, you are going to have to be the manufacturer. Brainstorming is not an engine. Lump of metal is.

#### Pilot-34

##### Well-Known Member
A better engine only comes along if it is profitable.
lol hardly better engines come along all the time ,certainly that is what most racing teams hope to do.
But how many tinker in their own garage with their own pet idea?
do they dream of fame and wealth probably but how many achieve it?