# Design Drivers?

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#### Pilot-34

##### Well-Known Member
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. Lol Dang you are good. really good you have me nodding my head in agreement. and I start wondering how can that be. So I reread carefully I didn’t realize is that you’re putting words in my mouth imfact all along people have been arguing with a point that isn’t my point. Heck you even got me arguing off mypoint. (Collective You). Let me quote again part of your first paragraph from the quoted post “ is that you think all design originates with engine selection” Slipping That one little word in there “all” really changes things. re-reading the opening post will actually be a good idea. What I was looking for was discussion of outstanding engines, It’s not the biggest but in every horsepower range. Did Orville and Wilbur need the engines from the Apollo rockets? Of course not but Being able to use relatively light weight for their time engines enabled their mission. Could we have flown the moon missions with the one on the Wright brothers used? The mission was there millennia ago Jules Verne wrote about it but those engines where The key to go in to the moon. Being able to create those engines enable going to the moon the hope that we can create those engines drove a design that drove the creation of those engines. Yes I get it that it’s somewhat circular logic but the available engines drive that circle to improve it. there are lots of missions and aircraft designed for missions that will never be built because the engine needed doesn’t exist yet. But as soon as those engines exist the The designs start to become practical and the aircraft start to be built. Can you see how that means engines drive design ? #### Saville ##### Well-Known Member Lol Dang you are good. really good you have me nodding my head in agreement. and I start wondering how can that be. So I reread carefully I didn’t realize is that you’re putting words in my mouth imfact all along people have been arguing with a point that isn’t my point. Heck you even got me arguing off mypoint. (Collective You). Let me quote again part of your first paragraph from the quoted post “ is that you think all design originates with engine selection” Slipping That one little word in there “all” really changes things. re-reading the opening post will actually be a good idea. What I was looking for was discussion of outstanding engines, It’s not the biggest but in every horsepower range. Did Orville and Wilbur need the engines from the Apollo rockets? Of course not but Being able to use relatively light weight for their time engines enabled their mission. Could we have flown the moon missions with the one on the Wright brothers used? The mission was there millennia ago Jules Verne wrote about it but those engines where The key to go in to the moon. Being able to create those engines enable going to the moon the hope that we can create those engines drove a design that drove the creation of those engines. Yes I get it that it’s somewhat circular logic but the available engines drive that circle to improve it. there are lots of missions and aircraft designed for missions that will never be built because the engine needed doesn’t exist yet. But as soon as those engines exist the The designs start to become practical and the aircraft start to be built. Can you see how that means engines drive design ? Yes I am good. Yes you should be nodding your head in agreement. No, no one is putting words in your mouth: Your contention is that engine selection is the first thing to happen - it's the design driver. And you are wrong. You were NOT looking at every horsepower range or I and several others wouldn't be responding to that. No I do not see how engine selection comes first (AT-6 vs Corsair as I've stated repeatedly) and no matter how you try to wiggle or shade or modify, it won't change the fact that it has been your contention that engine selection comes first and drives the design, and that, the most powerful engine is selected. #### Pilot-34 ##### Well-Known Member No! I am not saying that again you’re putting words in my mouth . Read the title ! Engines drive design. It may help you to read the word drive as enable . Again you’re putting words in my re-read the original title post where I specifically Ask about engines in different horsepower. Last edited: #### Saville ##### Well-Known Member No! I am not saying that again you’re putting words in my mouth . Read the title ! Engines drive design. It may help you to read the word drive as enable . Again you’re putting words in my re-read the original title post where I specifically Ask about engines in different horsepower. Nobody interpreted "drive" = "enable" unitl you did just now. Please stop digging. I knew I shouldn't have responded to you because you've already shown a reluctance to open your mind on this plus you keep trying to swerve, adjust, modify, what you clearly said. So once again I'm done with you on this. And you may have the last word. Cheers #### Toobuilder ##### Well-Known Member HBA Supporter Log Member Seemingly insignificant detail, but makes a big difference. I agree with Saville: "Drive" does not equal "enable" in this context. There are a few examples of engines “driving” design in homebuilts, but that is far more of a marketing scheme than pure “design”. The best known example is the Thorp T-18, which was designed to capitalize on the seemingly endless supply of cheap surplus Lycoming 290 GPU engines. However, it did not define any new “mission” because there were plenty of other 2 place homebuilts available back then. The irony is that almost immediately after first flight, the “simple and cheap” T-18 grew an enclosed cockpit and was fitted with bigger, more expensive engines. Other “marketing” examples include those designed around the “seemingly endless supply of cheap surplus” Russian M-14 radial engines, which ironically are now rare and expensive due to their initial popularity. You also used to be able to get the Eastern Bloc turboprops real cheap too. No more. In my own shop I have a C-85, a 290G, an IO-360, and until recently, a Solar T-62. The little turbine was the closest I came to “driving” a new airplane design or acquisition. I started looking for a derelict Rotorway Exec for a place to hang the Solar. I thought it would be cool. But I really have no need or want for a turbine helicopter in my fleet. My moment of weakness was simply “hobbyist boredom”, not “design”. #### Pilot-34 ##### Well-Known Member Nobody interpreted "drive" = "enable" unitl you did just now. Seemingly insignificant detail, but makes a big difference. I agree with Saville: "Drive" does not equal "enable" in this context. OK I’m curious in the context I used the word drive how would you have defined it? #### D Hillberg ##### Well-Known Member Mission above all. The wrights went and built their own engine Glen Curtis said hold my beer Globally you had 7 odd others trying to do the powered flight thingy The Wrights sued Curtis Smithsonian Wright contract 1947 , Lots of powered flight in the day. "There can only be one!" History rewritten Mission is the main driver the rest are accommodations - Mission, materials, powerplant Whine about it all you want. "Glider" "Troop Glider" "Big honking Tank carrying GLIDER" where's THE ENGINE? #### Pilot-34 ##### Well-Known Member You can talk mission all you want but for the love of all good can’t we talk engines ? On another thread that I’m involved in I suddenly realized it’s the engine that’s driving my quest for an airplane. The mission is easy, get me and my fat friends up in the air and go for a ride. Be able to take me and my fat friends hunting and camping and fishing, Now that’s a pretty plain Jane middle of the road mission right so how can the engine be driving it ? Price. There are some great deals on Airframes floating around. Toobuilder Seems to have one that would be great for my mission and quite honestly the build quality is probably far better than I would do it myself. So why am I not trying to talk his plane down out of his rafters? The engine. It doesn’t have an engine and and I’m not sure I can afford build and hang the right engine. Last edited: #### Topaz ##### Super Moderator Staff member Log Member 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." Tweaking some of the mission parameters to suit an after-sizing engine choice is part of the trade-study effort made in finalizing a conceptual design. That's a very different thing than chucking major parameters of the design mission just because a more-powerful engine became available. You and I are in agreement on this, completely. This is nuance, not substantive difference. #### Topaz ##### Super Moderator Staff member Log Member You can talk mission all you want but for the love of all good can’t we talk engines ? On another thread that I’m involved in I suddenly realized it’s the engine that’s driving my quest for an airplane. The mission is easy, get me and my fat friends up in the air and go for a ride. Be able to take me and my fat friends hunting and camping and fishing, Now that’s a pretty plain Jane middle of the road mission right so how can the engine be driving it ? Price. There are some great deals on A-frames floating around. Toobuilder Seems to have one that would be great for my mission and quite honestly the build quality is probably far better than I would do it myself. So why am I not trying to talk his plane down out of his rafters? The engine. It doesn’t have an engine and and I’m not sure I can afford build and hang the right engine. Okay, this is getting out of hand. You're the one who started this thread insisting (or wanting to discuss whether) engine choice drives design. Your words. Now, having been backed into a corner on that argument on multiple fronts, you're trying to move the goalposts. Which is verging on trolling the way you're doing it. You asked a question and then started arguing with everyone who answered, out of some hidden personal situation you just now revealed. If you're done with this topic and want to start another one on "discussing engines," please do so. Don't start a thread about engines driving design and then insist you "only wanted to discuss engines" when that goes awry. This one will be closed if it continues to be an exercise in trolling. Please PM me if you have any questions. #### TFF ##### Well-Known Member There is no magic. If you need a$2000 engine and that will be forever the price you will pay, there are two choices, used Lycoming 235 or 290G. That’s pretty much it until they dry up. At $4000 you might get a runner O-320, but pretty much you will be above$6000 for anything else.

You can look into the Yamahas, you might find some Blanton drive from the 70s. You might find a free engine on Craigs list, but none of it is in the same ballpark of known to people who will go fly.

Nothing keeping you from putting a V8 on a Bearhawk except you. Nothing keeping you from putting a V6 on a Cub but you. You can dream. Lots of dreams out there, but people who build flying airplanes accept the limited options. They don’t have the time to dream, they got an airplane to build. We all would love to see a build thread of you with a junkyard Chevy V8 machining parts and overhauling it and a gearbox just like you want. You would never see so much applause. The applause is from just doing it. Taking the chance you might be building a grenade. Those people are honored. Off the shelf, no one will sweat for you. It’s got to be you either buying or building. That goes for engine and airframe.

#### Pilot-34

##### Well-Known Member
Lol you have no idea how often I say “ there is no magic”

But there is weirdness ,one of the things I’ve always seen as weird is how a complete airplane or car can often sell for less than just the engine. wouldn’t you think that would cost more if it’s in an air frame you can crank it up take it for a fly reassure yourself how it sounds even run a compression test on.
Yet that engine on a pallet wrapped in plastic will probably cost you more!
Weird huh ?

#### blane.c

##### Well-Known Member
HBA Supporter
Lol you have no idea how often I say “ there is no magic”

But there is weirdness ,one of the things I’ve always seen as weird is how a complete airplane or car can often sell for less than just the engine. wouldn’t you think that would cost more if it’s in an air frame you can crank it up take it for a fly reassure yourself how it sounds even run a compression test on.
Yet that engine on a pallet wrapped in plastic will probably cost you more!
Weird huh ?
I've always thought so but there is that you don't have to remove the one on the pallet and then there is the carcass to get rid of too.

#### llemon

##### Well-Known Member
How is a 0 hour engine costing more than a nth hour engine weird?

#### Pilot-34

##### Well-Known Member
I don’t think we were talking about 0 hour engines, just those already removed from a vehicle. Comparable in other words.

#### Toobuilder

##### Well-Known Member
HBA Supporter
Log Member
...There are some great deals on Airframes floating around. Toobuilder Seems to have one that would be great for my mission and quite honestly the build quality is probably far better than I would do it myself.
So why am I not trying to talk his plane down out of his rafters?
The engine.
It doesn’t have an engine and and I’m not sure I can afford build and hang the right engine.
PA-22. Perfect airplane to illustrate this point. My project is a slightly modified PA-22. This airframe had engine choices ranging from the Lyc 0-235 as the two seat trainer (Colt), the 150 HP family truckster (Tri-Pacer), 160 HP as the seaplane, a Ford V-6 as the low cost auto conversion experiment (Javelin), up to the big bore Continental 520's as the hard core back country workhorse (Bushmaster). These are all the essentially same airframes with a vast disparity in engines. Why the difference in engines?

Mission requirements.

#### Vigilant1

##### Well-Known Member
Confession: For both the Beetlemaster idea and the Micromaster idea, I think the engines themselves (being available and inexpensive) together with the attraction of a centerline thrust twin (hmm, can we use those engines to get some inexpensive, safe redundancy?) were factors that we considered at the same time as the mission. As it happened, the layout and engines work pretty well for the type of flying in which some of us were interested (two-place and single place touring, respectively) so it worked out. If the objective was different (STOL, high speed,, big payload, etc) those engines and the centerline layout wouldn't make sense.

Of course, no one has actually built either of these concepts, so maybe it proves nothing.

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#### rv7charlie

##### Well-Known Member
For Topaz's benefit, I don't see any of this as trolling. If anything, it's people talking past each other, because of fuzzily defined terms and semantics issues causing 'failure to communicate'. So I hope the thread isn't locked. I went back and re-read the opening post, and it started with a premise, and then asked for suggestions based on various criteria. Answers fairly quickly devolved into challenging the premise, instead of simply answering the question. Nothing wrong with mentioning that you disagree with the premise, but that wasn't really the question.

I do think that there's a big difference between defining a mission and designing an aircraft. If defining a mission designed the a/c, then there wouldn't be both a C172 & a PA28 (or dozens of other brands/models).

As a mental exercise, design us a C172 (or PA28) replacement, with no information on engine HP, weight, fuel efficiency, etc. We know the mission; carry 4 adults with minimal baggage and not much range, or a couple of adults with some baggage and some range.

Now, defend your choices on fuselage length, fuselage shape, wing position relative to the tail & nose of the fuselage, wing size, fuel capacity, etc. Remember, you have no information whatsoever on powerplant; not even powerplant type (steam, gasoline, diesel, piston, flat or round or inline or inverted or upright, rotary, turbine, prop, jet; NOTHING).

#### Toobuilder

##### Well-Known Member
HBA Supporter
Log Member
The C-172 and PA-28 satisfy essentially the same "mission", but each have various market biases that hope to capitalize on sales in a free market. Nothing wrong with that. Look at the "omni vision" rear windows and swept tails of later model C-172s. Both features HURT performance, but boosted sales. Also look at the L-1011 and DC-10:. Both were targeting the exact same mission, but each had a different "spin" on marketing. Arguably, the technologically superior Lockheed product failed due to poor product management (and engine choice).

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