# Design Drivers?

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#### TFF

##### Well-Known Member
We wait for a new engine ? If you wait, you may have a design that has to have one. You are the type that needs the latest and greatest. Probably the most prevalent of Internet forums is if the dream is big enough, you don’t have to spend money on something real, because you weren’t going to spend it anyway. That keeps you in the game with no outlay. The people who don’t wait build a plane today.

#### Toobuilder

##### Well-Known Member
HBA Supporter
Log Member
We all don’t...
Indeed. Any aiframe I can reasonably hope to own in my remaining years has a perfectly adequate powerplant to power it "today". That goes for my L-39 or my Taylorcraft or any future want/desire. When it comes down to it, coverting gasoline into thrust is pretty much the same ratio regardles if the valve cover is stamped Lycoming, Chevrolet or Yamaha. Looking for the "game changing" breakthroughs in cost or BSFC? Not going to happen

#### jedi

##### Well-Known Member
General Aviation was built on low cost or surplus engines. Where are the "new" surplus engines from the current military industrial complex. Williams International is one of the suppliers for these engines but it appears this source is still out of reach for EAB buyers. Other smaller engines are likely still in the Middle East.

#### D Hillberg

##### Well-Known Member
In 1992 got a wild hare to put a gas turbine in a helicopter - It was 600 lbs with 145 hp - It worked well
In 1993 Dave & Walter wanted a gas turbine in their Rotorway, We put an M1A1 genset turbine in a Rotorway
Later BJ Schramm put the same power in his helicycle
Mission complete

more turbines in the air

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#### wktaylor

##### Well-Known Member
When war was on the horizon in the later 1930s the US started designs used the Allison engines... which was the most powerful USAAF engine at that time. Normally aspirated, supercharged and turbo-supercharged versions where used when performance dictated. The P-38 needed every HP up to high altitude, efficient-long-range and dash-speed. Since the P-39 was for low-level ground attack... normally aspirated or with a modest supercharger Allison's worked OK at low levels.

The Brits kept the superior Merlin engine development close to their chests. When all hell broke loose, these barriers dissolved in-favor of US co-development and manufacturing of Merlin high performance parts with the vast science, engineering and industrial base of the USA. Generally no secrets were withheld between US and UK in the heat of war... with the notable exception of the Manhattan project [=atom bomb].

#### Pilot-34

##### Well-Known Member
Lol i’ve given the subject a lot of though and I’m even more convinced that engine designs drive aircraft designs.
Would any one of you be building the same airplane you are now if you could’ve bought a 200 hp engine four $12.95? One that flew on 2 gallons per hour?Weighed only 35 pounds ? Let’s face it on this forum the most important component of engine design is price. Last edited: #### BJC ##### Well-Known Member HBA Supporter Let’s face it on this forum the most important component of engine design is price. Yup, until you actually go flying. Then reliability get the top nod from me. BJC #### Toobuilder ##### Well-Known Member HBA Supporter Log Member But by that measure, we can argue that the entire personal transportation industry would be different if we could simply figure out human teleportation. There have been no significant, game changing breakthroughts in powerplant technology in the last 100 years... Only gradual refinement #### Pilot-34 ##### Well-Known Member True but Even here it is still available engines that drive design. It just has a lot to do with the money. #### Saville ##### Well-Known Member Lol i’ve given the subject a lot of though and I’m even more convinced that engine designs drive aircraft designs. Would any one of you be building the same airplane you are now if you could’ve bought a 200 hp engine four$12.95? One that flew on 2 gallons per hour?Weighed only 35 pounds ?
Let’s face it on this forum the most important component of engine design is price.
Amazing that you assume you can speak for the forum.

And you are wrong about what drives design - no matter how much thought you've given it.

#### blane.c

##### Well-Known Member
HBA Supporter
Ah, Shucks and Stuff! If it ain't already mentioned (and even if it has) the primary driver is money closely followed by time. With money you don't care what stinkin' engine you use, ya just use the one you want and enough money can whittle the time down to something a lot more reasonable too. Like the old song says "younger women, faster horses, older whiskey, and more money". (I paraphrase).

#### wktaylor

##### Well-Known Member
Pilot-34... hmmmm…. in this forum... price??... or Value and Utility... and, always... Reliability??

Blane.c… So You would fly yourself and/or your family and friends behind an unreliable engine... Hmmmmm...

Remember what ETOPS really means => Engine(s) Turn(s) Or Passengers Swim

#### blane.c

##### Well-Known Member
HBA Supporter
I personally like a multitude of engines to fly with, the more the better, You can start modifying your flight plan after the first engine falters/fails and reappraise the situation after the second starts acting up ... and so on. You know it is inevitable (an engine problem) so why fly people you care about behind (or in front of) just one? It is the craziest thing in aviation all the safety infinitum and lets hang it all on one prop! Malarkey! It is people talking out of both sides of their mouths.

##### Well-Known Member
There have been no significant, game changing breakthroughts in powerplant technology in the last 100 years... Only gradual refinement
Gas turbines were game changing.

#### Toobuilder

##### Well-Known Member
HBA Supporter
Log Member
Gas turbines were game changing.
Not in the context of "homebuilt airplanes". Despite 7 decades of development, turbines are still essentially an impractical curiosity on homebuilts or those factory airplanes of homebuilt size. The fact that the most popular single engined aircraft available today have piston power indicates the "game" was NOT changed when the turbine showed up.

#### Pilot-34

##### Well-Known Member
Amazing that you assume you can speak for the forum.

And you are wrong about what drives design - no matter how much thought you've given it.
Lol
I don’t speak for the forum but I may speak about it. On the other hand you seem to be quite happy speaking for the universe.
Basically I said follow the money, please explain to me why not to?

#### Topaz

##### Super Moderator
Staff member
Log Member
Lol i’ve given the subject a lot of though and I’m even more convinced that engine designs drive aircraft designs.
Would any one of you be building the same airplane you are now if you could’ve bought a 200 hp engine four $12.95? One that flew on 2 gallons per hour?Weighed only 35 pounds ? Let’s face it on this forum the most important component of engine design is price. No. If I want to build a single-seat light motorglider, that 200hp engine is worthless to me, even if it were free, Free, FREE!!!! Having it wouldn't make me change my design mission, as you seem to assume. You seem to be operating under the assumption that we all want "as much performance as we can afford," and to a certain extent that's true. But performance doesn't necessarily equal cruise speed as you seem to be assuming. Nor does it necessarily mean takeoff run, nor any other particular parameter. For me, soaring performance is key, and the engine choice doesn't affect that. Your basic assumption that the design mission is tailored around the "biggest engine I can find/afford" is what's flawed here. A genuine design process starts with the desired mission. Period. Full stop. Through the process of sizing, the designer gets an idea of the size and weight of the airplane, how much power and fuel it will need to complete the design mission, etc. If you know those things, you can either intelligently select from the available engines or, if you're financially able (like a big company or government project), cause an engine to be designed to your specification. The engine your airplane needs could be any size - whatever suits the design mission. For my motorglider, the optimum engine is 27-32hp with an SFC<0.55. If I then say, "Hey! I suddenly have a 200hp engine for FREE!!!" and chuck the whole project to design a long-range cruising airplane, then how good was my original design mission? It sucked. It wasn't what I really wanted, was it? I should've focused on what I want the airplane to do, developed an appropriate design mission from that, and then figured out how to get the engine that plane needs. The engine is chosen to suit the airplane best adapted (engineered) to the design mission at hand. NOT the other way 'round. #### mcrae0104 ##### Well-Known Member HBA Supporter Log Member The engine is chosen to suit the airplane best adapted (engineered) to the design mission at hand. NOT the other way 'round. I wholeheartedly agree with everything except maybe this last part. If your design parameters could be nearly met by something in the power range of (x-15%)hp through (x+15%)hp, but for some reason, you prefer engine Z, the performance goals of the project can be adapted in order to use that engine. I will say again, it's a two-way street between desired performance parameters on the one hand, and other considerations (initial cost, ongoing costs, reliability, ease of maintenance, or perhaps other factors) on the other hand. Which of course, is not to say that I agree with the the position that one should always choose the most powerful, most advanced thing he possibly can choose. I'd lean more toward reliability, simplicity, and low cost than I would toward something new, flashy, super-efficient, or "advanced." #### Saville ##### Well-Known Member Basically I said follow the money, please explain to me why not to? Your position, and the position I and others have been arguing against, is that you think all design originates with engine selection. Please don't try to change the focus of the debate. Money was not the focus of my debate with you. If you go back and read all my responses, and some others, to your posts it focuses on your assertion - Which. Is. Wrong. - that aircraft design STARTS with the engine. Also you say that the most powerful engine is always selected which is also totally false. You even tried to wiggle out of being wrong by saying that the performance specs of the circulars to which the military designs responded HAD to have assumed the most powerful engine. But of course, as I pointed out, that's not DESIGN...that's SPECIFICATION and we are talking design. So that wriggle failed. You use the fact that some designs used the most powerful engine available as proof, of your position, but it is not proof at all. Topaz encapsulated that with this sentence a couple of posts above [emphasis mine]: "Your basic assumption that the design mission is tailored around the "biggest engine I can find/afford" is what's flawed here. Topaz also encapsulated the opinion that opposes yours, which I've been arguing all along: "A genuine design process starts with the desired mission. Period. Full stop. Above, mcrae0104 responds to you with this: "Which of course, is not to say that I agree with the the position that one should always choose the most powerful, most advanced thing he possibly can choose." I, and others, are responding to your assertion that engine selection drives design, and that the most powerful engine is always used. I am saying that is patently not true. The mission comes first. As one of my myriad of examples, I pointed out that if you were tasked to design an advanced trainer (the mission!) in the late 1930's (which resulted in the AT-6) you HAD TO start with the mission because if you didn't you wouldn't select the proper engine. You had engines ranging from Kinners and Rangers and 65hp engines that powered the J-3 (Continentals?) to R-2800's. How could you POSSIBLY start with the engine given that range of performance UNLESS you KNEW what the mission was? You just can't. After all you started with the task given you - the mission. And I think you are willfully refusing to see that. So now you try a different tack by saying "follow the money". Part of 'the mission" is costs - when you are talking about civilian for sure and to some small degree military. It makes no sense to design a$5,000,000 general aviation trainer, so you wouldn't use
a GE F110-GE-129.

Why?

Because you KNOW the mission and you know that the mission doesn't need nor want that. You know the design would fail. You know that because you KNOW the mission: you know the price is too high, maintenance is too costly, and it's a stupid design for primary students.

But it's one of the most powerful engines out there! So by your logic the F110-GE-129 drives the trainer design.

After giving example after example of why you are wrong I gave up because it became clear to me your mind was shut. I listed a few here as one last attempt to see if your mind has opened a little.

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#### TFF

##### Well-Known Member
Bell XP-77. Even though the US military always wants more more more, there is always attempt for small. The A4 got lucky and was built. Biggest is not always the goal.