Design Drivers?

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blane.c

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Hard to R&D a new IC engine for small planes when so much noise is being made about electric, you could be R&D-ing a Dodo bird.
 

rv7charlie

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In addition to TFF's list, in the 150-200 hp 'up&coming' category I'd be looking at the Yamaha sled engines. Haven't seen many on faster a/c yet, but there seems to be dozens, if not hundreds flying on trikes and light STOL a/c.

If picking between Sube & rotary, the rotary can be significantly lighter in that HP range, but will be a bit tougher conversion, for several different reasons.

I'm a bit confused on the original question; is this about chicken/egg, or just about a good non-Lyc choice in the 150-200 HP range?
 

Riggerrob

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"
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. ...
"

Shoving any old engine into a frame is merely "hot-rodding."
 

TFF

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Hot Rodding skills will do you well once you have a stack of parts, but a lot of non standard is unknown with each and every engine installation. The idiot proof installation does not exist. That’s why there are no hard answers yet.

WW2 planes started out pretty much like us now. Limited engines available so the design has to use one. Almost every design was designed in the depression. All you got is all you got; limited funds. During the war, money is there to win. Winning means new development and spending money on any idea to win. Win or change your national anthem.

Essentially we design under depression terms. We have what we have. There is a few that will do something different on ability, but it’s only for those who have it. The ones with ability don’t count ability, when it’s the only thing that really counts.
 

wsimpso1

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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.
Not quite. Wing thickness (in %) may be driven by mission but not by speed part of mission. For instance, Boomerang does not look particularly thick, but it’s wing is 17% thick. Burt kept making it thicker until range (fuel capacity) was met. I think we can agree that the Boomerang qualifies as long range and fast…

Cruise drag in laminar flow foils appears to be min around 12%, while drag penalty is tiny in most families to 15%. Hardly thin.

Billski
 

Riggerrob

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It seems like when I read the history of a lot of World War II aircraft that they will not the airplane that we think of until some particular higher horsepower engine or modification came along.
Digressing into history here ....
Yes, all combatants started World War 2 with 1,000 horsepower engines and airframes designed during the 1930s. The Rolls-Royce Merlin engine started the war with barely 1,000 horsepower, but later variants produced 2,000 hp. by the end of the war. Some airframes were continually up-graded during the war (e.g. Spitfire and Messerschmitt 109) while others were introduced as new, more powerful engines came available (e.g. Griffin, large radials and jets).
The most popular American example was North American Aviation's P-51 Mustang which was designed - during the war - for the British Purchasing Commission. P-51A proved among the fastest at low-altitudes for photo recce and ground attack, but was limited by the super-charger in its Allison engine. After a Brit suggested installing a RR Merlin engine, the Mustang came into its own as a medium-altitude, long-range escort fighter. Merlin's principle advantage was a series of up-graded super-chargers that improved high-altitude performance. Every combatant struggled to produce improved super or turbo-chargers, but only a few succeeded.
 

Pilot-34

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I’ve been thinking about this a lot in this thread.
Would it be fair to say mission drives design but engines drive success ?

I’ve noticed it works the other way too a design that fails at its design mission may succeed at another.
In particular I’m thinking of the P-39
 

Riggerrob

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In the hands of Soviet pilots, P-39 Airacobras downed huge numbers of notzis. Mind you, the Soviet Air Force focussed on supporting ground troops and most of their air battles were fought at low altitudes.
 

Saville

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Not quite. Wing thickness (in %) may be driven by mission but not by speed part of mission.

Billski

Not sure I agree with that. If you are designing the Spitfire you are designing a point defense interceptor whose speed has to be competitive with the possible opposition.

Wing thickness is going to play a huge part in final speed but is traded with climb rate in this particular case.

Speaking on the general topic, it is my opinion that what comes first is the government circular which stipulates performance requirements. OR a company decides to work towards a design which encapsulates certain performance parameters (say for a civilian trainer).

Take the B-17 for example:

The governments said: (from Wiki) carry a "useful bombload" at an altitude of 10,000 ft for 10 hours with a top speed of at least 200 mph They also desired, but did not require, a range of 2,000 mi (3,200 km) and a speed of 250 mph.

Designers at different companies went to work. Engine selection and number of engines came next - after the performance specs.

Some, like Douglas and Martin, selected 2 engines. Boeing chose 4.

The government didn't specify engine, horsepower or numbers of engines. The designers made their choices in order to achieve the performance.

In my opinion the mission always comes first. it has to because otherwise you don't know what you are designing. I'm sure the engines come soon thereafter, but even there you get to decide - liquid cooled or air cooled before you select.

As for the P-39, my understanding was that it was initially designed to have a turbo-charger for high altitude performance but after a prototype the idea was discarded.
 

TFF

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We don’t have a nuclear airplane because of the danger. Sub plane, somewhat done, but missiles worked better. The military can throw money at the engine and the airframe if they want it that much. Your Congressman will see to it. You can too, but most run out before it ever runs. Cost of development then cost of production. The only way to build something “cheaper” than a Lycoming or Continental is if you have the machining and problem solving talent along with an auto engine and some sort of gearbox. It’s cheaper because you don’t pay yourself for your talent. If you have the talent, it’s a personal chess game, done for the fun of it. If you don’t have the talent, it’s a different game called don’t drowned.
 

Toobuilder

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But what good is it to design anything without a engine capable of driving it ?
Actually, the opposite is usually true - there are multiple engines capable of satisfying the mission. This was the case for my last design team. The Government stated the performance goals and we the designers had to do a very painful trade study to weed through the engine choices until we found the one that gave us a favorable market advantage. In this case we found an engine that not only gave us some margin back in the payload, but also was a logistics win because it was the same engine used in an already large fleet owned by the government.

In the case of a "paper" engine, that's obviously a higher risk. Sometimes the promising new technology pans out and the airframe is a success; sometimes it is initially flown with known substandard engines, betting on the new tech that never appears - resulting in a mediocre airplane that never meets mission requirements, or sometimes the airframe dies on the vine because the new tech just isn't going to happen.
 

Saville

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But what good is it to design anything without a engine capable of driving it ?
As in my B-17 example you can choose to use more than one available engine...like 2 (Martin and Douglas) or 4 (Boeing).

Also you don't KNOW that there is no engine capable of delivering the performance specs until you at least do a preliminary design. And again it's the performance requirements that are the goal, not engine use.

I don't think you can just sit there and say, "Ok I have an Allison V-1710 so that means I can get 385mph at 15,000. So I'll design a plane to do that."
 
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